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Project AGI

Building an Artificial General Intelligence

Wednesday, 18 May 2016

Reading list - May 2016

Digit classification error over time in our experiments. The image isn't very helpful but it's a hint as to why we're excited :)

Project AGI

A few weeks ago we paused the "How to build a General Intelligence" series (part 1, part 2, part 3, part 4). We paused it because the next article in the series requires us to specify everything in detail, and we need working code to do that.

We have been testing our algorithm on a variety of MNIST-derived handwritten digit datasets, to better understand how well it generalizes its representation of digit-images and how it behaves when exposed to varying degrees of predictability. Initial results look promising: We will post everything here once we've verified them and completed the first batch of proper experiments. The series will continue soon!

Deep Unsupervised Learning

Our algorithm is a type of Online Deep Unsupervised Learning, so naturally we're looking carefully at similar algorithms.

We recommend this video of a talk by Andrew Ng. It starts with a good introduction to the methods and importance of feature representation and touches on types of automatic feature discovery. He looks at some of the important feature detectors in computer vision, such as SIFT and HoG and shows how feature detectors - such as edge detectors - can emerge from more general pattern recognition algorithms such as sparse coding. For more on sparse coding see Shakir's excellent machine learning blog.

For anyone struggling to intuit deep feature discovery, I also loved this post on yCombinator which nicely illustrates how and why deep networks discover useful features, and why the depth helps.

The latter part of the video covers Ng's latest work on deep hierarchical sparse coding using Deep Belief Networks, in turn based on AutoEncoders. He reports benchmark-beating results on video activity and phoneme recognition with this framework. You can find details of his deep unsupervised algorithm here:

http://deeplearning.stanford.edu/wiki

Finally he presents a plot suggesting that training dataset size is a more important determiner of eventual supervised network performance than algorithm choice! This is a fundamental limitation of supervised learning where the necessary training data is much more limited than in unsupervised learning (in the latter case, the real world provides a handy training set!)

Effect of algorithm and training set size on accuracy. Training set size more significant. This is a fundamental limitation of supervised learning.

Online K-sparse autoencoders (with some deep-ness)

We've also been reading this paper by Makhzani and Frey about deep online learning with auto-encoders (a type of supervised learning neural network that is used in an unsupervised way to reconstruct its input, often known as semi-supervised learning). Actually we've struggled to find any comparison of autoencoders to earlier methods of unsupervised learning both in terms of computational efficiency and ability to cover the search space effectively. Let us know if you find a paper that covers this.

The Makhzani paper has some interesting characteristics - the algorithm is online, which means it receives data as a stream rather than in batches. It is also sparse, which we believe is desirable from a representational perspective. 

One limitation is that the solution is most likely unable to handle changes in input data statistics (i.e. non-stationary problems). The reason this is an important quality is that in any arbitrarily deep network the typical position of a vertex is between higher and lower vertices. If all vertices are continually learning, the problem being modelled by any single vertex is constantly changing. Therefore, intermediate vertices must be capable of online learning of non stationary problems otherwise the network will not be able to function effectively. In Makhzani and Frey, they instead use the greedy layerwise training approach from Deep Belief Networks. The authors describe this approach:

"4.6. Deep Supervised Learning Results The k-sparse autoencoder can be used as a building block of a deep neural network, using greedy layerwise pre-training (Bengio et al., 2007). We first train a shallow k-sparse autoencoder and obtain the hidden codes. We then fix the features and train another ksparse autoencoder on top of them to obtain another set of hidden codes. Then we use the parameters of these autoencoders to initialize a discriminative neural network with two hidden layers."

The limitation introduced can be thought of as an inability to escape from local minima that result from prior training. This paper by Choromanska et al tries to explain why this happens.

Greedy layerwise training is an attempt to work around the fact that deep belief networks of Autoencoders cannot effectively handle nonstationary problems.

For more information here's some papers on deep sparse networks built from autoencoders:

Variations on Supervised Learning - a Taxonomy

Back to supervised learning, and the limitation of training dataset size. Thanks to a discussion with Jay Chakravarty we have this brief taxonomy of supervised learning workarounds for insufficient training datasets:

Weakly supervised learning: [For poorly labelled training data] where you want to learn models for object recognition under weak supervision - you have say object labels for images, but no localization (e.g. bounding box) for the object in the image (there might be other objects in the image as well). You would use a Latent SVM to solve the problem of localizing the objects in the images, and at the same time learning a classifier for it.

Another example of weakly supervised learning is that you have a bag of positive samples mixed up with negative training samples, but also have a bag of purely negative samples - you would use Multiple Instance Learning for this.

Cross-modal adaptation: where one mode of data supervises another - e.g. audio supervises video or vice-versa.

Domain adaptation: model learnt on one set of data is adapted, in unsupervised fashion, to new datasets with slightly different data distributions.

Transfer learning: using the knowledge gained in learning one problem on a different, but related problem. Here's a good example of transfer learning, a finalist in the NVIDIA 2016 Global Impact Award. The system learns to predict poverty from day and night satellite images, with very few labelled samples.

Full paper:

http://arxiv.org/pdf/1510.00098v2.pdf

Interactive Brain Concept Map

We enjoyed this interactive map of the distribution of concepts within the cortex captured using fMRI and produced by the Gallant Lab (source papers here).

Using the map you can find the voxels corresponding to various concepts, which although maybe not generalizable due to the small sample size (7) gives you a good idea of the hierarchical structure the brain has produced, and what the intermediate concepts represent.

Thanks to David Ray @ http://cortical.io for the link.

Interactive brain concept map

OpenAI Gym - Reinforcement Learning platform

We also follow the OpenAI project with interest. OpenAI have just released their "Gym" - a platform for training and testing reinforcement learning algorithms. Have a play with it here:

https://openai.com/blog/openai-gym-beta/

According to Wired magazine, OpenAI will continue to release free and open source software (FOSS) for the wider impact this will have on uptake. There are many companies now competing to win market share in this space.

The Talking Machines Blog


We're regular readers of this blog and have been meaning to mention it for months. Worth reading.

How the brain generates actions

A big gap in our knowledge is how the brain generates actions from its internal representation. This new paper by Vicente et al challenges the established (rather vague) dogma on how the brain generates actions.

“We found that contrary to common belief, the indirect pathway does not always prevent actions from being performed, it can actually reinforce the performance of actions. However, the indirect pathway promotes a different type of actions, habits.”

This is probably quite informative for reverse-engineering purposes. Full paper here.

Hierarchical Temporal Memory

HTM is an online method for feature discovery and representation and now we have a baseline result for HTM on the famous MNIST numerical digit classification problem. Since HTM works with time-series data, the paper compares HTM to LSTM (Long-Short-Term Memory), the leading supervised-learning approach to this problem domain.

It is also interesting that the paper deals with adaptation to sudden changes in the input data statistics, the very problem that frustrates the deep belief networks described above. 

Full paper by Cui et al here.

For a detailed mathematical description of HTM see this paper by Mnatzaganian and Kudithipudi.

Wednesday, 23 March 2016

Reading list: Assorted AGI links. March 2016

A Minecraft API is now available to train your AGIs

Our News

We are working hard on experiments, and software to run experiments. So this week there is no normal blog post. Instead, here’s an eclectic mix of links we’ve noticed recently.

First, AlphaGo continues to make headlines. Of interest to Project AGI is Yann LeCun agreeing with us that unsupervised hierarchical modelling is an essential step in building intelligence with humanlike qualities [1]. We also note this IEEE Spectrum post by Jean-Christophe Baillie [2] which argues, as we did [3], that we need to start creating embodied agents.

Minecraft 

Speaking of which, the BBC reports that the Minecraft team are preparing an API for machine learning researchers to test their algorithms in the famous game [4]. The Minecraft team also stress the value of embodied agents and the depth of gameplay and graphics. It sounds like Minecraft could be a crucial testbed for an AGI. We’re always on the lookout for test problems like these.

Of course, to play Minecraft well you need to balance local activities - building, mining etc. - with exploration. Another frontier, beyond AlphaGo, is exploration. Monte-Carlo Tree Search (as used in AlphaGo) explores in more limited ways than humans do, argues John Langford [5].

Sharing places with robots 

If robots are going to be embodied, we need to make some changes. Wired magazine says that a few small changes to the urban environment and driver behaviour will make the rollout of autonomous vehicles easier [6]. It’s important to meet the machines halfway, for the benefit of all.

This excellent paper on robotic grasping also caught our attention [7]. A key challenge in this area is adaptability to slightly varying circumstances, such as variations in the objects being grasped and their pose relative the the arm. General solutions to these problems will suddenly make robots far more flexible and applicable to a greater range of tasks.

Hierarchical Quilted Self-Organizing Maps & Distributed Representations

Last week I also rediscovered this older paper on Hierarchical-Quilted Self-Organizing Maps (HQSOMs) [8].This is close to our hearts because we originally believed this type of representation was the right approach for AGI. With the success of Deep Convolutional Networks (DCNs) it’s worth looking back and noticing the similarities between the two. While HQSOM is purely unsupervised learning, (a plus, see comment from Yann LeCun above) DCNs are trained by supervised techniques. However, both methods use small, overlapping, independent units - analogous to biological cortical columns - to classify different patches of the input. The overlapping and independent classifiers lead to robust and distributed representations, which is probably the reason these methods work so well.

Distributed representation is one of the key features of Hawkins’ Hierarchical Temporal Memory (HTM). Fergal Byrne has recently published an updated description of the HTM algorithm [9] for those interested.

We at Project AGI believe that a grid-like “region” of columns employing a “Winner-Take-All” policy [10], with overlapping input receptive fields, can produce a distributed representation. Different regions are then connected together into a tree-like structure (acyclic). The result is a hierarchy. Not only does this resemble the state-of-the-art methods of DCNs, but there’s a lot of biological evidence for this type of representation too. This paper by Rinkus [11] describes columnar features arranged into a hierarchy, with winner-take-all behaviour implemented via local inhibition.

Rinkus says: “Saying only that a group of L2/3 units forms a WTA CM places no a priori constraints on what their tuning functions or receptive fields should look like. This is what gives that functionality a chance of being truly generic, i.e., of applying across all areas and species, regardless of the observed tuning profiles of closely neighboring units.”

Reinforcement Learning 

But unsupervised learning can’t be the only form of learning. We also need to consider consequences, and so we need reinforcement learning to take account of these. As Yann said, the “cherry on the cake” (this is probably understating the difficulty of the RL component, but right now it seems easier than creating representations).

Shakir’s Machine Learning blog has a great post exploring the biology of reinforcement learning [12] within the brain. This is a good overview of the topic and useful for ML researchers wanting to access this area.

But regular readers of this blog will remember that we’re obsessed with unfolding or inverting abstract plans into concrete actions. We found a great paper by Manita et al [13] that shows biological evidence for the translation and propagation of an abstract concept into sensory and motor areas, where it can assist with perception. This is the hierarchy in action.

Long-Short-Term Memory (LSTM)

One more tack before we finish. Thanks to Jay for this link to NVIDIA’s description of LSTMs [14], an architecture for recurrent neural networks (i.e. the state can depend on the previous state of the cells). It’s a good introduction, but we’re still fans of Monner’s Generalized LSTM [15].

Fun thoughts

Now let’s end with something fun. Wired magazine again, describing watching AlphaGo as our first taste of a superhuman intelligence [16]. Although this is a “narrow” intelligence, not a general one, it has qualities beyond anything we’ve experienced in this domain before. What’s more, watching these machines can make us humans better, without any nasty bio-engineering:

“But as hard as it was for Fan Hui to lose back in October and have the loss reported across the globe—and as hard as it has been to watch Lee Sedol’s struggles—his primary emotion isn’t sadness. As he played match after match with AlphaGo over the past five months, he watched the machine improve. But he also watched himself improve. The experience has, quite literally, changed the way he views the game. When he first played the Google machine, he was ranked 633rd in the world. Now, he is up into the 300s. In the months since October, AlphaGo has taught him, a human, to be a better player. He sees things he didn’t see before. And that makes him happy. “So beautiful,” he says. “So beautiful.”

References

[1] https://www.facebook.com/yann.lecun/posts/10153426023477143

[2] http://spectrum.ieee.org/automaton/robotics/artificial-intelligence/why-alphago-is-not-ai

[3] http://blog.agi.io/2016/03/what-after-alphago.html

[4] http://www.bbc.com/news/technology-35778288

[5] http://cacm.acm.org/blogs/blog-cacm/199663-alphago-is-not-the-solution-to-ai/fulltext

[6] http://www.wired.com/2016/03/self-driving-cars-wont-work-change-roads-attitudes/

[7] http://arxiv.org/pdf/1603.02199v1.pdf

[8] http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.84.1401&rep=rep1&type=pdf

[9] http://arxiv.org/pdf/1509.08255v2.pdf

[10] https://en.wikipedia.org/wiki/Winner-take-all_(computing)

[11] http://journal.frontiersin.org/article/10.3389/fnana.2010.00017/full

[12] http://blog.shakirm.com/2016/02/learning-in-brains-and-machines-1/

[13] https://www.researchgate.net/profile/Masanori_Murayama/publication/277144323_A_Top-Down_Cortical_Circuit_for_Accurate_Sensory_Perception/links/556839e008aec22683011a30.pdf

[14] https://devblogs.nvidia.com/parallelforall/deep-learning-nutshell-sequence-learning/

[15] http://www.overcomplete.net/papers/nn2012.pdf

[16] http://www.wired.com/2016/03/sadness-beauty-watching-googles-ai-play-go/




Thursday, 10 March 2016

What's after AlphaGo?

What's AlphaGo?

AlphaGo is a system that can play Go at least as well as the best humans. Go was widely cited as the hardest (and only remaining) game at which humans could beat machines, so this is a big deal. AlphaGo has just defeated a top-ranked human expert.

Why is Go hard?

Go is hard because the search-space of possible moves is so large that tree search and pruning techniques, such as those used to beat humans at Chess, won't work - or at least, they won't work well enough, with a feasible amount of memory, to play Go better than the best humans. 

Instead, to play Go well, you need to have "intuition" rather than brute search power: To look at the board and spot local (or gross) patterns that represent opportunities or dangers. And in fact, AlphaGo is able to play in this way. It beat the next best computer algorithm "Pachi" 85% of the time without any tree search - just predicting the best action based on its interpretation of the current state. The authors of the AlphaGo Nature paper say:

“During the match against Fan Hui, AlphaGo evaluated thousands of times fewer positions than Deep Blue did in its chess match against Kasparov; compensating by selecting those positions more intelligently, using the policy network, and evaluating them more precisely, using the value network—an approach that is perhaps closer to how humans play.”

How does AlphaGo work?

AlphaGo is trained by both supervised and reinforcement learning. Supervised learning feedback comes from recordings of moves in expert games. However, these are finite in size and used naively, would lead to overfitting

Instead, in AlphaGo a Supervised Learning deep neural network learns to model and predict expert behaviour in the recorded games, via conventional deep learning techniques. Then, a reinforcement learning network is used to generate reward data for novel games that AlphaGo plays against itself! This mitigates the limited size of the supervised learning dataset.

Of course, AlphaGo also wants the play better than the best play observed in the training data. To achieve this, the reinforcement learning network is further trained by playing pairs of them (networks) against each other - mixing the pairs up to prevent policies overfitting each other. This is a really clever feature because it allows AlphaGo to go beyond its training data.

Note also that the neural networks cannot possibly fully represent a sufficiently deep tree of board outcomes within their limited set of weights. Instead, the network has to learn to represent good and bad situations with limited resources. It has to form its own representation of the most salient features, during training.

The neural networks function without pre-defined rules specific to Go; instead they have learned from training data collected from many thousands of human and simulated games.

Key advances

AlphaGo is an important advance because it is able to make good judgments about play situations based on a lossy interpretation in a finitely-sized deep neural network.

What’s more, Go wasn’t simply taught to copy human experts - it went further, and improved, by playing against itself.

So, what doesn't it do?

The techniques used in deep neural networks have recently been scaled to work effectively on a wide range of problems. In some subject areas, narrow AIs are reaching superhuman performance. However, it is not clear that these techniques will scale indefinitely. Problems such as vanishing gradients have been pushed back, but not necessarily eliminated.

Much greater scale is needed to get intelligent agents into the real world without them being immediately smashed by cars or stuck in holes. But already, it is time to consider what features or characteristics constitute an artificial general intelligence (AGI), beyond raw intelligence (which AIs now have).

AlphaGo isn't a general intelligence; it's designed specifically to play Go. Sure, it's trained rather than programmed manually, but it was designed for this purpose. The same techniques are likely to generalize to many other problems, but they'll need to be applied thoughtfully and retrained.

AlphaGo isn't an Agent. It doesn't have any sense of self, or intent, and its behaviour is pretty static - its policies would probably work the same way in all similar situations, learning only very slowly. You could say that it doesn't have moods, or other transient biases. Maybe this is a good thing! But this also limits its ability to respond to dynamic situations.

AlphaGo doesn't have any desire to explore, to seek novelty or to try different things. AlphaGo couldn't ever choose to teach itself to play Go because it found it interesting. On the other hand, AlphaGo did teach itself to play Go… 

All in all, it's a very exciting time to study artificial intelligence!

by David Rawlinson & Gideon Kowadlo

Tuesday, 9 February 2016

Some interesting finds: Acyclic hierarchical modelling and sequence unfolding

This week we have a couple of interesting links to share.

From our experiments with generative hierarchical models, we claimed that the model produced by feed-forward processing should not have loops. Now we have discovered a paper by Bengio et al titled "Towards biologically plausible deep learning" [1] that supports this claim. The paper looks for biological mechanisms that mimic key features of deep learning. Probably the credit assignment problem is the most difficult feature to substantiate - ensuring each weight is updated correctly in response to its contribution to the overall output of the network - but the paper does leave me thinking it's plausible.

Anyway the reason I'm talking about it is this quote:

"There is strong biological evidence of a distinct pattern of connectivity between cortical areas that distinguishes between “feedforward” and “feedback” connections (Douglas et al., 1989) at the level of the microcircuit of cortex (i.e., feedforward and feedback connections do not land in the same type of cells). Furthermore, the feedforward connections form a directed acyclic graph with nodes (areas) updated in a particular order, e.g., in the visual cortex (Felleman and Essen, 1991)."

This says that the feedforward modelling process (which we believe is constructing a hierarchical model) is a directed acyclic graph (DAG) - which means it does not have loops, as we predicted. Secondly, it is another source claiming that the representation produced is hierarchical (in this case, a DAG). The cited work is a much older paper - "Distributed hierarchical processing in the primate cerebral cortex" [2]. We're still reading, but there's a lot of good background information here.

The second item to look at this week is a demo by Felix Andrews featuring temporal pooling [3] and sequence unfolding. "Unfolding" means transforming the pooled sequence representation back into its constituent parts - i.e. turning a sequence into a series of steps.

Felix demonstrates that high-level sequence selection can successfully be used to track and predict through observation of the corresponding lower-level sequence. This is achieved by causing the high-level sequence to predict all steps, and then tracking through the predicted sequence using first-order predictions in the lower level. Both levels are necessary - the high level prediction provides guidance for the low-level to ensure it predicts correctly through forks. The low level prediction keeps track of what's next in the sequence.

[1] "Towards Biologically Plausible Deep Learning" Yoshua Bengio, Dong-Hyun Lee, Jorg Bornschein and Zhouhan Lin (2015) http://arxiv.org/pdf/1502.04156v2.pdf

[2] "Distributed hierarchical processing in the primate cerebral cortex" Felleman DJ, Van Essen DC (1991) http://www.ncbi.nlm.nih.gov/pubmed/1822724

[3] Felix Andrews HTM temporal pooling and sequence unfolding demo http://viewer.gorilla-repl.org/view.html?source=gist&id=95da4401dc7293e02df3&filename=seq-replay.clj

Wednesday, 3 February 2016

Intuition over reasoning for AI


By Gideon Kowadlo

I’m reading a fascinating book called The Righteous Mind, by Jonathan Haidt. It’s one of those reads that can fundamentally shift the way that you see the world. In this case, the human world, everyone around you, and yourself.

A central idea of the book is that our behaviour is mainly dictated by intuition rather than reasoning and that both are aspects of cognition.

Many will be able to identify in themselves and others, the tendency to act first and rationalise later - even though it feels like the opposite. But more than that, our sense of morality arises from intuition and it enables us to act quickly and make good decisions.

A compelling biological correlate is the ventromedial prefrontal cortex. The way it enables us to use emotion/intuition for decision making is described well in this passage:

Damasio had noticed an unusual pattern of symptoms in patients who had suffered brain damage to a specific part of the brain - the ventromedial (i.e., bottom-middle) prefrontal cortex (abbreviated vmPFC; it’s the region just behind and above the bridge of the nose). Their emotionality dropped nearly to zero. They could look at the most joyous or gruesome photographs and feel nothing. They retained full knowledge of what was right and wrong, and they showed no deficits in IQ. They even scored well on Kohlberg’s tests of moral reasoning. Yet when it came to making decisions in their personal lives and at work, they made foolish decisions or no decisions at all. They alienated their families and their employers, and their lives fell apart.

Damasio’s interpretation was that gut feelings and bodily reactions were necessary to think rationally, and that one job of the vmPFC was to integrate those gut feelings into a person’s conscious deliberations. When you weigh the advantages and disadvantages of murdering your parents … you can’t even do it, because feelings of horror come rushing in through the vmPFC.

But Damasio’s patients could think about anything, with no filtering or coloring from their emotions. With the vmPFC shut down, every option at every moment felt as good as every other. The only way to make a decision was to examine each option, weighting the pros and cons using conscious verbal reasoning. If you’ve ever shopped for an appliance about which you have few feelings - say, a washing machine - you know how hard it can be once the number of options exceeds six or seven (which is the capacity of our short-term memory). Just imagine what your life would be like if at every moment, in every social situation, picking the right thing to do or say became like picking the best washing machine among ten options, minute after minute, day after day. You’d make foolish decisions too.

Our aim has always been to build a general reasoning machine that can be scaled up. We aren’t interested in building an artificial human, which carries the legacy of a long evolution through many incarnations.

This is the first time I’ve considered the importance of building intuition into the algorithm as a fundamental component. ‘Gut’ reactions are not to be underestimated. It may be the only way to make effective AGI, not to mention the need to create ‘pro-social’ agents with which we can interact in daily life.

It is possible though, that this is an adaption to the limitations of our reasoning, rather than a fundamentally required feature. If the intelligence was implemented in silicon and not bound by 'cognitive effort' in the same way that we are, it could potentially select favourable actions efficiently based on intellectual reasoning, without the ‘intuition’.

This is fascinating to think about in terms of human intelligence and behaviour. It raises exciting questions about the nature of intelligence itself and the relationship between cognition and both reasoning and intuition. We’ll be sure to consider these questions as we continue to develop an algorithm for AGI.

Addendum

From a functional perspective the vmPFC appears to be a separate parallel ‘component’ that is richly connected to many other brain areas.

"The ventromedial prefrontal cortex is connected to and receives input from the ventral tegmental area, amygdala, the temporal lobe, the olfactory system, and the dorsomedial thalamus. It, in turn, sends signals to many different brain regions including; The temporal lobe, amygdala, the lateral hypothalamus, the hippocampal formation, the cingulate cortex, and certain other regions of the prefrontal cortex.[4] This huge network of connections affords the vmPFC the ability to receive and monitor large amounts of sensory data and to affect and influence a plethora of other brain regions, particularly the amygdala." 
 Wikipedia Ventromedial prefrontal cortex

Sunday, 24 January 2016

How to build a General Intelligence: An interpretation of the biology

Figure 1: Our interpretation of the Thalamocortical system as 3 interacting sub-systems (objective, subjective and executive). The structure of the diagram indicates the dominant direction of information flow in each system. The objective system is primarily concerned with feed-forward data flow, for the purpose of building a representation of the actual agent-world system.  The executive system is responsible for making desired future agent-world states a reality. When predictions become observations, they are fed back into the objective system. The subjective system is a circular because its behaviour depends on internal state as much as external. The subjective system builds a filtered, subjective model of observed reality, that also represents objectives or instructions for the executive. This article will describe how this model fits into the structure of the Thalamocortical system.

Authors: David Rawlinson and Gideon Kowadlo

This is part 4 of our series on how to build an artificial general intelligence (AGI).

  • Part 1: An overview of hierarchical general intelligence
  • Part 2: Reverse engineering (the physical perspective - cells and layers - and the logical perspective - a hierarchy).
  • Part 3: Circuits and pathways; we introduced our canonical cortical micro-circuit and fitted pathways to it.

In this article, part 4, we will try to interpret all the information provided so far. We will try to fit what we know about the biological general intelligence to our theoretical expectations.

Systems

We believe cortical activity can be usefully interpreted as 3 integrated systems. These are:

  • Objective system
  • Subjective system
  • Executive system

So, what are these systems, why are they needed and how do they work?

Objective System

We theorise that the purpose of the objective system is to construct a hierarchical, generative model of both the external world and the actual state of the agent. This includes internal plans & goals already executed or in progress. From our conceptual overview of General Intelligence we think that this representation should be distributed, compositional and therefore robust and able to immediately model novel situations instantly and meaningfully.

The objective system models varying timespans depending on the level of abstraction, but events are anchored to the current state of the world and agent. Abstract events may cover long periods of time - for example, “I made dinner” might be one conceptual event.

We propose that the objective system is implemented by pyramidal cells in layers 2/3 and by spiny excitatory cells in layer 4. Specifically, we suggest that the purpose of the spiny excitatory cells is primarily dimensionality reduction, by performing a classifier function, analogous to the ‘Spatial Pooling’ function of Hawkins’ HTM theory. This is supported by analysis of C4 spiny stellate connectivity: “... spiny stellate cells act predominantly as local signal processors within a single barrel...”. We believe the pyramidal cells are more complex and have two functions. First, they perform dimensionality reduction by requiring a set of active inputs on specific apical (distal) dendrite branches to be simultaneously observed before the apical dendrite can output a signal (an action potential). Second, they use basal (proximal) dendrites to identify the sequential context in which the apical dendrite has become active. Via a local competitive process, pyramidal cells learn to become active only when observing a set of specific input patterns in specific historical contexts.

The output of pyramidal cells in C2/3 is routed via the Feed-Forward Direct pathway to a “higher” or more abstract cortical region, where it enters in C4 (or in some parts of the Cortex, C2/3 directly). In this “higher” region, the same classifier and context recognition process is repeated. If C4 cells are omitted, we have less dimensionality reduction and a greater emphasis on sequential or historical context.

We propose these pyramidal cells only output along their axons when they become active without entering a “predicted” state first. Alternatively, interneurons could play a role in inhibiting cells via prediction to achieve the same effect. If pyramidal cells only produce an output when they make a False-Negative prediction error (i.e. they fail to predict their active state), output is equivalent to Predictive Coding (link, link). Predictive Coding produces an output that is more stable over time, which is a form of Temporal Pooling as proposed by Numenta.

To summarize, the computational properties of the objective system are:

  1. Replace simultaneously active inputs with a smaller set of active cells representing particular sub-patterns, and
  2. Replace predictable sequences of active cells with a false-negative error coding to transform the output into a simpler sequence of prediction errors

These functions will achieve the stated purpose of incrementally transforming input data into simpler forms with accumulating invariances, while propagating (rather than hiding) errors, for further analysis in other Columns or cortical regions. In combination with a tree-like hierarchical structure, higher Columns will process data with increasing breadth and stability over time and space.

The Feed-Forward direct pathway is not filtered by the Thalamus. This means that Columns always have access to the state of objective system pyramidal cells in lower columns. This could explain the phenomenon that we can process data without being aware of it (aka “Blindsight”); essentially the objective system alone does not cause conscious attention. This is a very useful quality, because it means the data required to trigger a change in attention is available throughout the cortex. The “access” phenomenon is well documented and rather mysterious; the organisation of the cortex into objective and subjective systems could explain it.

Another purpose of the objective system is to ensure internal state cannot become detached from reality. This can easily occur in graphical models, when cycles form that exclude external influence. To prevent this, we believe that the roles of feed-forward and feed-back input must be separated to break the cycles. However, C2/3 pyramidal cells’ dendrites receive both feed-forward (from C4) and feed-back input (via C1).

One way that this problem might be avoided is by different treatment of feed-forward and feed-back input, so that the latter can be discounted when it is contradicted by feed-forward information. There is evidence that feed-forward and feedback signals are differently encoded, which would make this distinction possible.

We speculate that the set of states represented by the cells in C2/3 could be defined only using feed-forward input, and that the purpose of feedback data in the objective system is restricted to improved prediction, because feedback contains state information from a larger part of the hierarchy (see figure 2).

Figure 2: The benefit of feedback. This figure shows part of a hierarchy. The hierarchy structure is defined by the receptive fields of the columns (shown as lines between cylinders, left). Each Column has receptive fields of similar size. Moving up the hierarchy, Columns receive increasingly abstract input with a greater scope, being at the top of a pyramid of lower Columns whose receptive fields collectively cover a much larger area of input. Feedback has the opposite effect, summarizing a much larger set of Column states from elsewhere and higher in the hierarchy. Of course there is information loss during these transfers, but all data is fully represented somewhere in the hierarchy.
So although the objective system makes use of feedback, the hierarchy it defines should be predominantly determined by feed-forward information. The feed-forward direct pathway (see figure 3) enables the propagation of this data and consequently the formation of the hierarchy.

Figure 3: Feed-Forward Direct pathway within our canonical cortical micro-circuit. Data travels from C4 to C2/3 and then to C4 in a higher Column. This pattern is repeated up the hierarchy. This pathway is not filtered by the Thalamus or any other central structure, and note that it is largely uni-directional (except for feedback to improve prediction accuracy). We propose this pathway implements the Objective System, which aims to construct a hierarchical generative model of the world and the agent within it.

Subjective System

We think that the subjective system is a selectively filtered model of both external and internal state including filtered predictions of future events. We propose that filtering of input constitutes selective attention, whereas filtering of predictions constitutes action selection and intent. So, the system is a subjective model of reality, rather than an objective one, and it is used for both perception and planning simultaneously.

The time span encompassed by the system includes a subset of both present and future event-concepts, but as with the objective system, this may represent a long period of real-world time, depending on the abstraction of the events (for example, “now” I am going to work, and “next” I will check my email [in 1 hour’s time]).

It makes good sense to have two parallel systems, one filtered (subjective) and one not (objective). Filtering external state reduces distraction and enhances focus and continuity. Filtering of future predictions allows selected actions to be maintained and pursued effectively, to achieve goals.

In addition to events the agent can control, it is important to be aware of negative outcomes outside the agent’s control. Therefore the state of the subjective system must include events with both positive and negative reward outcomes. There is a big difference between a subjective model and a goal-oriented planning model. The subjective system should represent all outcomes, but preferentially select positive outcomes for execution.

The subjective system represents potential future states, both internal and external. It does not necessarily represent reality; it represents a biased interpretation of intended or expected outcomes based on a biased interpretation of current reality! These biases and omissions are useful; they provide the ability to “imagine’ future events by serially “predicting” a pruned tree of potential futures.

More speculatively, differences between the subjective and objective systems may be the cause of phenomena such as selective awareness and “access” consciousness.

Figure 4: Feed-Forward Indirect pathway, particularly involved in the Subjective system due to its influence on C5. The Thalamus is involved in this pathway, and is believed to have a gating or filtering effect. Data flows from the Thalamus to C4, to C2/3, to C5 and then to a different Thalamic nuclei that serves as the input gateway to another cortical Column in a different region of the Cortex. We propose that the Feed-Forward Indirect pathway is a major component of the subjective system.
Figure 5:  The inhibitory micro-circuit, which we suggest makes the subjective system subjective! The red highlight shows how the Thalamus controls activity in C5 by activating inhibitory cells in C4. The circuit is completed by C5 pyramidal cells driving C6 cells that modulate the activity of the same Thalamic nuclei that selectively activate C5.
The subjective system primarily comprises C5 (where subjective states are represented) and the Thalamus (which controls subjectivity), but it draws input from the objective system via C2/3. The latter provides context and defines the role and scope (within the hierarchy) of C5 cells in a particular column. Between each cortical region (and therefore every hierarchy level), input to the subjective system is filtered by the Thalamus (figure 5). This implements the selection process. The Feed-Forward Indirect pathway includes these Thalamo-Cortical loops.

We suggest the Thalamus implements selection within C5 using special cells in C4 that are activated by axons (outputs) from the Thalamus (see figure 6). These inhibitory C4 cells target C5 pyramidal cells and inhibit them from becoming active. Therefore, thalamic axons are both informative (“this selection has been made”) and executive (the axon drives inhibition of selected C5 pyramidal cells).

Figure 6: Thalamocortical axons (afferents) are shown driving inhibitory cells in C4 (leftmost green cell) that in turn inhibit pyramidal cells in C5 (red). They also provide information about these selections to other layers, including C2/3. When a selection has been made, it becomes objective rather than subjective, hence provision of a copy to C2/3. Image source.

Note that selection may be a process of selective dis-inhibition rather than direct control: Selection alone may not be enough to activate the C5 cells.  Instead, C5 pyramidal cells likely require both selection by the Thalamus, and feed-forward activation via input from C2/3. The feed-forward activation could occur anywhere within a window of time in which the C5 cell is “selected”.  This would relax timing requirements on the selection task, making control easier; you only need to ensure that the desired C5 cell is disinhibited when the right contextual information arrives from other sources (such as C2/3). This also ensures C5 cell activation fits into its expected sequence of events and doesn’t occur without the right prior context.

C5 also benefits from informational feedback from higher regions and neighbouring cells that help to define unique contexts for the activation of each cell.

We suggest that C5 pyramidal cells are similar to C2/3 pyramidal cells but with some differences in the way the cells become active. Whereas C2/3 cells require both matching input via the apical dendrites and valid historical input to the basal dendrites to become active, C5 cells additionally need to be disinhibited for full activation to occur.

As mentioned in the previous article, output from C5 cells sometimes drives motors very directly, so full activation of C5 cells may immediately result in physical actions. We can consider C5 to be the “output” layer of the cortex. This makes sense if the representation within C5 includes selected future states.

Management of C5 activity will require a lot of inhibition; we would expect most of the input connections to C5 to be inhibitory because in every context, for every potential outcome, there are many alternative outcomes that must be inhibited (ignored). At any given time, only a sparse set of C5 cells would be fully active, but many more would be potentially-active (available for selection).

Given predictive encoding and filtering inhibition, it would be common for few pyramidal cells to be active in a Column at any time. Separately, we would expect objective C2/3 pyramidal activity to be more consistent and repeatable than subjective C5 pyramidal activity, given a constant external stimulus.

Executive System

So far we have defined a mechanism for generating a hierarchical representation and a mechanism for selectively filtering activity within that representation. In our original conceptual look at general intelligence, we also desired that filtering predictions would be equivalent to action selection. But if we have selected predictions of future actions at various levels of abstraction within the hierarchy, how can we make these abstract prediction-actions actually happen?

The purpose of the executive system is to execute hierarchical plans reliably. As previously discussed, this is no trivial matter due to problems such as vanishing agency at higher hierarchy levels. If a potential future outcome represented within the subjective system is selected for action, the job of the executive system is to make it occur.

We know that we want abstract concepts at high levels within the hierarchy to be faithfully translated into their equivalent patterns of activity at lower levels. Moving towards more concrete forms would result in increasing activity as the incremental dimensionality reduction of the feed-forward hierarchy is reversed.

Figure 7: Differences in dominant direction of data flow between objective and executive systems. Whereas the Objective system builds increasingly abstract concepts of greater breadth, the Executive system is concerned with decomposing these concepts into their many constituent parts. so that hierarchically-represented plans can be executed.
We also know that we need to actively prioritize execution of a high level plan over local prediction / action candidates in lower levels. So, we are looking for a cascade of activity from higher hierarchy levels to lower ones.

Figure 8: One of two Feed-Back direct pathways. This pathway may well be involved in cascading control activity down the hierarchy towards sensors and motors. Activity propagates from C6 to C6 directly; C6 modulates the activity of local C5 cells and relevant Thalamic nuclei that activate local C5 cells by selective disinhibition in conjunction with matching contextual information from C2/3.
It turns out that such a system does exist: The feed-back direct pathway from C6 to C6. Cortex layer 6 is directly connected to Cortex layer 6 in the hierarchy levels immediately below. What’s more, these connections are direct, i.e. unfiltered (which is necessary to avoid the vanishing agency problem). Note that C5 (the subjective system) is still the output of the Cortex, particularly in motor areas. C6 must modulate the activity of cells in C5, biasing C5 to particular predictions (selections) and thereby implementing a cascading abstract plan. Finally, C6 also modulates the activity of Thalamic nuclei that are responsible for disinhibiting local C5 cells. This is obviously necessary to ensure that the Thalamus doesn’t override or interfere with the execution of a cascading plan already selected at a higher level of abstraction.

Our theory is that ideally, all selections originate centrally (e.g. in the Thalamus). When C5 cells are disinhibited and then become predicted, an associated set of local C6 cells is triggered to make these C5 predictions become reality.

These C6 cells have a number of modulatory outputs to achieve this goal:


Executive Training


No, this is not a personal development course for CEOs. This section checks whether C6 cells can learn to replay specific action sequences via C5 activity. This is an essential feature of our interpretation, because only C6 cells participate in a direct, modulatory feedback pathway.

We propose that C6 pyramidal neurons are taught by historical activity in the subjective system. Patterns of subjective activity become available as “stored procedures” (sequences of disinhibition and excitatory outputs) within C6.

Let’s start by assuming that C6 pyramidal cells have similar functionality to C2/3 and C5 pyramidal cells, due to their common morphhology. Assume that C5 cells in motor areas are direct outputs, and when active will cause the agent to take actions without any further opportunity for suppression or inhibition (see previous article).

In other cortical areas, we assume that the role of C5 cells is to trigger more abstract “plans” that will be incrementally translated into activity in motor areas, and therefore will also become actions performed by the agent.

To hierarchically compose more abstract action sequences from simpler ones, we need activity of an abstract C5 cell to trigger a sequence of activity in more concrete C5 cells. C6 cells will be responsible for linking these C5 cells. So, activating a C6 cell should trigger a replay of a sequence of C5 cell activity in a lower Column. How can C6 cells learn which sequences to trigger, and how can these sequences be interpreted correctly by C6 cells in higher hierarchy levels?

C6 pyramidal cells are mostly oriented with their dendrites pointing towards the more superficial cortex layers C1,...,C5 and their axons emerging from the opposite end. Activity from C5 to C6 is transferred via axons from C5 synapsing with dendrites from C6. Given a particular model of pyramidal cell learning rules, C6 pyramidal cells will come to recognize patterns of simultaneous C5 activity in a specific sequential context, and C6 interneurons will ensure that unique sets of C6 pyramidal cells respond in each context.

So how will these C6 cells learn to trigger sequences of C5 cells? We know that the axons of C6 cells bend around and reach up into C5, down to the Thalamus and directly to hierarchically-lower C6 cells. At all targets they can be excitatory or inhibitory.

All we need beyond this, is for C6 axons to seek out axon target cells that become active immediately after the originating C6 cell is stimulated by active C5 cells. This will cause each C6 cell to trigger C5 and C6 cells that are observed to be activated afterwards. Note that we require the C6 cells themselves be organised into sequences (technically, a graph of transitions).

Target seeking by axons is known as “Axon Guidance” and C6 pyramidal cells’ axons do seem to target electrically active cells by ceasing growth when activity is detected. We have not yet found biological evidence for the predicted timing.

C6 axons can also target C4 inhibitory cells (evidence) and Thalamic cells, which again is compatible with our interpretation, as long as they are cells that become active after the originating C6 cell. If we want to “replay” some activity that followed a particular C6 cell, then all the cells described above should be excited or inhibited to ensure that the same events occur again. Activating a C6 cell directly should reproduce the same outcome as incidental activation of the C6 cell via C5 - a chain of sequential inhibition and promotion will result. Note that the same learning rule could work to discover all axon targets mentioned.

Collectively, the C6 cells within a Column will become a repertoire of “stored procedures” that can be triggered and replayed by a cascade of activity from higher in the hierarchy or by direct selection via C5. C6 cells would behave the same way whether activated by local C5 cells, or by C6 cells in the hierarchy level above. This allows cascading, incremental execution of hierarchical plans.

C6 cells do not need to replace sequences of C5 cell activity with a single C6 cell (i.e. label replacement for symbolic encoding), but they do need to collectively encode transitions between chains of C5 cells, individually trigger at least 1 C5 cell and collectively allow a single C6 cell to trigger a sequence of C6 cells in both the current and lower hierarchy regions.

C6 interneurons can resolve conflicts when multiple C6 triggers coincide within a column. We can expect C6 interneurons to inhibit competing C6 pyramidal cells until the winners are found, resulting in a locally consistent plan of action.

As with layers C2/3 and C5, C6 inhibitory interneurons will also support training C6 pyramidal cells for collective coverage of the space of observed inputs, in this case from C5 and C2/3.

Bootstrapping

Now we are only left with a bootstrapping problem: How can the system develop itself? Specifically, how do the sequences of C5 activity come to be defined so that they can be learned by C6?

We suggest that conscious choice of behaviour via the Thalamus is used to build the hierarchical repertoire from simple primitive actions to increasingly sophisticated sequences of fine control. Initially, thalamic filtering of C5 state would be used to control motor outputs directly, without the involvement of C6. Deliberate practice and repetition would provide the training for C6 cells to learn to encode particular sequences of behaviour, making them part of the repertoire available to C6 cells in hierarchically “higher” Columns.

Initially, concentration is needed to perform actions via direct C5 selections; these activities need to be carefully centrally coordinated using selective attention. However, when C6 has learnt to encode these sequences, they become both more reliable and require less effort to execute, requiring only a trigger to one C6 cell.

After training, only minimal thalamic interventions are needed to execute complex sequences of behaviour learned by C6 cells. Innovation can continue by combining procedures encoded by C6 with interventions via the Thalamus, that can still excite or inhibit C5 cells. However, in most other cases C6 training is accelerated by the independence of Columns: When a C6 cell learns to control other cells within the Column, this learning remains valid no matter how many higher hierarchy levels are placed on top. By analogy, once you’ve learned to drink from a cup, you don’t need to relearn that skill to drink in restaurants, at home, at work etc.

As C6 learns and starts to play a role in the actions and internal state of the agent, it becomes important to provide the state of C6 to the objective and subjective systems as contextual input.

Axons from C6 to other, hierarchically lower Columns take two paths: To C6, and to C1. We propose that the copy provided to C1 is used as informational feedback in C2/3 and C5 pyramidal cells (these axons synapse with Pyramidal cell Apical dendrites). We suggest the copy to C6 allows C6 cells to execute plans hierarchically, by delegating execution to a number of more concrete C6 cells. Therefore, the feedback direct pathway from C6 to C6 is part of the executive system. These axons should synapse on cell bodies, or nearby, to inhibit or trigger C6 activation artificially (rather than via C5).

Interpretation of the Thalamus

Rather than as merely a relay, we propose that a better concept of the Thalamus is as a control centre. It’s job is to centrally control cortical activity in C5 (the subjective system). Abstract activity in C5 is propagated down the hierarchy by C6, and translated into its concrete component states, eventually resulting in specific motor actions. Therefore, via this feedback pathway the filtering performed by the Thalamus assumes an executive role also.

We believe that filtering predictions of oneself performing an action or experiencing a reward is the mechanism by which objectives and plans are selected. We believe there is only one representation of the world in our heads. There is no separate “goal-oriented” or “action-based” representation. This means that filtering predictions is the mechanism of behaviour generation. Note that in a hierarchical system, you can simultaneously select novel combinations of predictions to achieve innovation without changing the hierarchical model.

Our interpretation of the Thalamus depends on some theoretical assumptions about how general intelligence works. Crucially, we believe there is no difference between selective awareness of externally-caused and self-generated events, except some of the latter have agency in the real world via the agent’s actions. This means that selective attention and action selection can both be consequences of the same subjective modelling process.

But where does selection actually occur?

For a number of practical reasons, action and attentional selection should be centralized functions. For one thing, the reward criteria for selecting actions are of much smaller dimension than the cortical representations - for example, the set of possible pain sensations are far more limited than the potential external causes of pain. We essentially need to compare the reward of all potential actions against each other, rather than an absolute scale.

It is also important that conflicts between items competing for attention or execution are resolved so that incompatible plans are replaced by a single clear choice. Conflict resolution is difficult to do in a highly parallel & distributed system; instead, it is preferable to force all alternatives to compete against each other until a few clear winners are found.

Finally, once an action or attentional target is selected, it should be maintained for a long period (if still relevant), to avoid vacillation. (See Scholarpedia for a good introduction to the difficulties of conflict resolution and the importance of sticking to a decision for long enough to evaluate it).

We believe the Thalamus plays this role via its interactions with the Cortex. It interacts with the Cortex in two ways. First, the Thalamus selectively dis-inhibits particular C5 cells, allowing them to become active when the right circumstances are later observed objectively (i.e. via C2/3, which is not subjective).

Second, the Thalamus must also co-operate with the Feed-Back cascade via C6.  While the Thalamus generates new selections by controlling C5, it must also permit the execution of existing, more abstract Thalamic selections by allowing cascading feedback activity to override local selections. Together, these mechanisms ensure that execution of abstract plans is as easily accomplished as simpler, concrete actions.

Interpretation of the Basal Ganglia

The Basal Ganglia are involved in so many distinct functions that they can’t be fully described within this article. They consist of a set of discrete structures located adjacent to the Thalamus.

In our model, selection is implemented by the Thalamus manipulating the subjective system within the Cortex. We propose that the selections themselves are generated by the Basal Ganglia, which then controls the behaviour of the Thalamus.

Crucially, we believe the Striatum within the Basal Ganglia uses reward values (such as pleasure and pain) to make adaptive selections. In other words, the Basal Ganglia are responsible for picking good actions, biasing the entire Thalamo-Cortical system towards futures that are expected to be more pleasant for the agent.

However, to make adaptive choices it is necessary to have accurate context and predictions (candidate actions). The hierarchical model defined within the Cortex is an efficient and powerful source for this data, and in fact, this pathway (Cortex → Basal Ganglia → Thalamus → Cortex) does exist within the brain (see figure 9 below).

Thanks to studies of relevant disorders such as Parkinson’s and Huntingdon’s, it is known that this pathway is associated with behaviour initiation and selection based on adaptive criteria.

Figure 9: Pathways forming a circuit from Cortex to Basal Ganglia to Thalamus and back to Cortex. Image source.

Lifecycle of an idea

Using our interpretation of biological general intelligence, we can follow the lifecycle of an idea from the conception to execution. Lets walk through the theorized response to a stimulus, resulting in an action.

Although the brain is operating constantly and asynchronously, we can define the start of our idea as some sensory data that arrives at the visual cortex. In this example, it’s an image of an ice-cream in a shop.

Objective Modelling

Sensor data propagates unfiltered up the Feed-Forward Direct pathway, activating cells in C4 and C2/3 in numerous cortical areas as it is transformed into its hierarchical form. The visual stimuli become a rich network of associated concepts, including predictions of near-future outcomes, such as experiencing the taste of ice-cream. These concepts represent an objective external reality and are now active and available for attention.

Subjective Prediction

Activity within the Objective system triggers activity in the Subjective system. Some C5 cells become “predicted”, but are inhibited by the Thalamus. These cells represent potential future actions and outcomes. Things that, from experience, we know are likely to occur after the current situation.

The Cortex projects data from C2/3 to the Striatum where it is weighted according to reward criteria. A strong response to the flavour of the frozen treat percolates through the Basal Ganglia and manipulates the activity of the Thalamus.

Between the Thalamus and the Cortex, an iterative negotiation takes place resulting in the selection (via dis-inhibition) of some C5 cells. The Basal Ganglia have learned which manipulations of the Thalamus maximize the expected Reward given the current input from Cortex.

The way that the Thalamus stimulates particular C5 cells is somewhat indirect. The path of activity to “select” C5 cells in layer n is C5[n-1] →  Thalamus → C4[n] → C5[n]. The signal is re-interpreted at each stage of this pipeline - that is, connections do not carry a specific meaning from point to point. Therefore, you can’t just adjust one “wire” to trigger a particular C5 cell. Rather, you must adjust the inhibition of input to many C4 → C5 cells until you’ve achieved the conditions to “select” a target C5 cell. Many target C5 cells might be simultaneously selected.

In addition to requiring disinhibition, C5 cells also wait for specific patterns of cell activity in C2/3 prior to becoming “predicted”. This means that it’s very difficult to select a C5 cell that is not “predicted”; it simply doesn’t have the support to out-compete its neighbours in the column and become “selected”. This prevents unrealistic outcomes being “selected”, or output commencing, before the right circumstances have arrived to match the expectation.

Eventually, a subset of C5 cells become “predicted” and “selected”, representing a subjective model of potential futures for the agent in the world. In this case, the anticipated future involves eating ice-cream.

Execution

When C5 cells become active, they in turn drive C6 pyramidal cells that are responsible for causing the future represented by “contextual, selected & predicted” C5 cells. In this case, C6 cells are charged with executing the high-level plan to “buy some ice-cream and eat it”.

The plan is embodied by many C5 cells, distributed throughout the hierarchy; each represents a subset of the “qualia” relating to the eating of ice-cream. C6 cells begin to interpret these C5 cells into concrete actions, via the C6-C6 Feed-Back Direct pathway. Crucially, they no longer require the Thalamus to modulate the input that makes C5 cells “selected”. Instead, C6 cells stimulate C5 and C6 cells in hierarchically-lower Columns directly, moving them to “selected” status and allowing them to become active as soon as the corresponding Feed-Forward evidence arrives to match.

C6 cells also modulate relay cells in the Thalamus, guiding the Thalamus to disinhibit C5 cells in lower hierarchy regions. This helps to ensure the parts of the decomposed plan are executed as intended. In turn, these newly selected “lower” C5 cells drive associated C6 cells, and the plan cascades down the hierarchy.

Note that the plan is also flowing in the “forward” direction, as it incrementally becomes reality rather than expectation. As motor actions take place, they are sensed and signalled through the Feed-Forward pathways. When C5 cells become “selected”, this information becomes available to higher columns in the hierarchy, if not filtered. This also helps the Feed-Forward Indirect pathway and C6 cells to keep track of activity and execute the plan in a coordinated manner.

At the lowest levels of the hierarchy, the plan becomes a sequence of motor activity, which is activated by C5 cells directly, and also by other brain components that are not covered by our general intelligence model.

A few moments later, the ice-cream is enjoyed, triggering a release of Dopamine into the Striatum and reinforcing the rewards associated with recent active Cortical input. Delicious!

Summary

In the previous articles we explored the characteristics of a general intelligence and looked at some of the features we expected it to have. In part 2 and part 3 we reviewed some relevant computational neuroscience research. In this article we’ve described our interpretation of this background material.

We presented a model of general intelligence built from 3 interacting systems - Objective, Subjective and Executive. We described how these systems could learn and bootstrap via interaction with the world, and how they could be implemented by the anatomy of the brain. As an example, we traced an experience from sensation, through planning and to execution.

Let’s assume that our understanding of biology is approximately correct. We can use this as inspiration to build an artificial general intelligence with a similar architecture and test whether the systems behave as described in these articles.

The next article in this series will look specifically at how these concepts could be implemented in software, resulting in a system that behaves much like the one described here.

Tuesday, 22 December 2015

How to build a General Intelligence: Circuits and Pathways

Figure 1: Our headline image is from the Cognitive Consilience: An atlas of key pathways cross-referenced to supporting literature articles. The complexity and variety of routing within the brain can be appreciated with this beautiful illustration. Note in particular the specialisation of cortical cells and the way this affects their interactions with other cells in the cortex and elsewhere in the brain. Explore this fantastic resource yourself.

By David Rawlinson and Gideon Kowadlo

This is part 3 of our series “how to build an artificial general intelligence” (AGI). Part 1 was a theoretical look at General Intelligence (follow the link if you don’t know what General Intelligence is).

We believe that the Thalamo-Cortical system is the origin of General Intelligence in people. In Part 2 we presented very broadly how the Thalamo-Cortical system is structured and organised. We applied some core concepts, such as hierarchy, to help us describe the system.

We also looked at the cellular structure of the Cortex and in particular introduced Pyramidal cells.

This article is again about what we can learn from reverse-engineering the Thalamo-Cortical system, but this time from its connectivity, which we present in terms of circuits and pathways.

Pathways and Circuits

A pathway is a gross pattern of sequential connectivity between brain regions - for example, if part A is highly connected to part B, and activity in A is followed by activity in B, we say there exists a pathway between A and B. Cells in the Thalamo-Cortical system are connected to each other in quite restricted and specific ways, so these pathways are quite informative.

Circuits are more specific and precise details of both connectivity and functional interaction between neurons. In computational neuroscience there exists a concept called the Canonical Cortical Micro-Circuit. The specifics of this circuit are not widely agreed, because (a) the Cortex is complex and (b) many of the evidence-gathering exercises are statistical observations (e.g. “X% of outputs from A and Y% of output from B projects to region C”) which may obscure fundamental functional or topological features. For example, outputs from A and B may project to cells with exclusive roles, but physically co-located in C. Statistical, regional approaches will not capture such distinctions.

In the neuroscience literature there’s a frustrating habit of selectively reporting supporting details while ignoring others. Perhaps this is simply because it's impossible to describe any part exhaustively. In particular, there is a lot of contradictory information about Cortical circuits. But the research can still shed some light on what is happening. Just don’t expect all sources to be consistent or complete!

Key Cortical Pathways

There are several widely-cited and well established cortical pathways (i.e. routes with at least one end in the Cortex). To understand these, it is important to remember both the physical and logical structure of the Cortex as described in the previous article. Physically, the cortex is made of layers, and logically, Columns within the Cortex form a hierarchy.

The hierarchy defines a structure made of Columns, and determines which Columns interact. Pathways describe the patterns of interaction between cells within a Column, and between Columns. We assume that all Columns are functionally identical prior to training.

Cells within Columns are usually identified by both the physical location of cell bodies within particular Layers in the Column, and by the morphology (shape) of the cell. Data flow to and from Cortical cells is largely restricted to a handful of core pathways that begin and terminate in particular cell types in specific cortical layers.

There are many descriptions of cortical pathways and circuits in the literature. We will first introduce just 4 well-established cortical-cortical pathways, and then some thalamo-cortical pathways. Note that although the existence of these pathways is unambiguous, their purpose and function is poorly understood. They appear to be consistent across various somatosensory regions of the cortex, especially in comparison to variations in other brain tissues.

Hawkins’ Hierarchical Temporal Memory (HTM) introduces 3 of the 4 pathways in a single, coherent scheme and relates them to a general intelligence algorithm. We will borrow this terminology and describe them in detail below. Their names describe the direction of data flow and the routing used:

Feed-Forward Direct Pathway: C2/3 → C4 → C2/3
Feed-Forward Indirect Pathway: C5 → Thalamus → C4 → C2/3
Feed-Back Direct Pathway #1: from C6 → C1

We are also interested in a second Feed-Back Direct “pathway” implemented by cortically projecting C6 pyramidal cells with axons that terminate in both C6 and C1 in hierarchically lower regions (see here for a diagram).

Feed-Back Direct Pathway #2: from C6 → C6

Note that cells in all cortical layers (except, perhaps, C4) receive input via their dendrites in C1. In other words, feedback from C6 to C1 is then used as input to many layers. Feedback from C6 to C6 is generally not input for other layers.

In neuroscience, Feed-Forward usually means the flow of data away from external sources such as sensors (towards greater abstraction, if you believe in a cortical hierarchy). Feed-Back means the opposite - data flow towards regions that have direct interaction with external sensors and motors.

Direct pathways are so-called because data is routed directly from one cortical column or region to another, without a stop along the way. Indirect pathways are routed via other structures. The  “Feed-Forward-Indirect” pathway described by Hawkins is routed via the Thalamus.

Figure 2, derived from a Hawkins/Numenta publication, shows graphically how information flows between columns and between layers within columns, as part of these 3 pathways according to the HTM theory. As mentioned before, the community is welcome to contribute by updating and adding to the figure.

Hawkins assigns specific roles to these pathways, but we will be re-interpreting them in the next article.

Figure 2: Routing of 3 core pathways, based on a diagram from the HTM/CLA White Paper. Note the involvement of specific cortical layers with each pathway, and the central role of the Thalamus. The names of the pathways indicate direct (cortex-to-cortex) and indirect (cortex-thalamus-cortex) variants, with direction being either forward (away from external sensors and motors, towards increasing abstraction) or backward (towards more concrete regions dealing with specific sensor/motor input). 

The role of the Thalamus

Let’s recap: The Cortex is composed of Columns, organised into a hierarchy. Cells pass messages directly to other Columns that are higher or lower in the hierarchy. Messages may also be transmitted indirectly between Columns, via the Thalamus.

The Thalamus is often viewed as having a gating or relaying function. The Thalamus is particularly associated with control of attention.

This section will describe indirect pathways involving the Thalamus. Figure 3 is a reproduction of a figure from Sherman and Guillery (2006) that has two new features of interest. These authors use the terminology “first order” to denote cortical regions receiving direct sensor input and “higher order” to denote cortical regions receiving input from “first order” cortical regions. This corresponds with the notion of hierarchy levels 1 and 2.

The Thalamus is a significant part of the “Feed-Forward Indirect” pathway. This pathway originates at Cortex layer 5 and propagates to a nucleus in the Thalamus. There, the nucleus may react by transmitting a (presumably corresponding) signal to one or more other Cortical Columns, in a different region. In some theories of cortical function, the target Column is conceptually “higher” in the hierarchy. The Thalamic input enters the Cortex via Thalamic axons terminating in Cortex layer 4 and is then propagated to Cortex Layer 5 where the pathway begins again.

Figure 3 also shows Cells in Columns in Cortex layer 6 fairly accurately form reciprocal modulatory connections to Thalamic nuclei that provide input to the Column via C4 and C5! Therefore, a Column within the Cortex has influence on data that it receives from the Thalamus. In effect, the Cortex is not a passive recipient but works with the Thalamus to control its input. The figure also depicts C6 cells projecting to C6 in lower regions (our second feedback pathway).


Figure 3: Pathways between cortical columns in different regions, showing layer involvement in each pathway and the role of the Thalamus. Sherman and Guillery use the terminology “first order” to denote cortical regions receiving direct sensor input and “higher order” to denote cortical regions receiving input from lower (e.g. “first order”) cortical regions. This corresponds with the notion of hierarchy levels 1 and 2. Note that in addition to the 3 pathways shown in the previous figure, we see additional direct feedback pathways and reciprocal feedback from Cortex layer 6 to the Thalamic nuclei that stimulate the cortical region. Image source.

Motor output

At this point it is interesting to look at how the Cortex can influence or control behaviour, particularly the generation of motor output. There are two pathways that allow the cortex to influence or control behaviour:

Cortical Control: Basal Ganglia → Thalamus → Cortex → Motors
Cortical Influence: Cortex → Basal Ganglia → Motors

Note that in both cases, the origin of action selection is the Basal Ganglia. In the first case, the Basal Ganglia control signals emitted by the Thalamus, with these signals in turn affecting activity within Cortex layer 5 (C5). C5, particularly in motor areas, has been studied in detail. 10-15% of the cells in these areas are very large pyramidal neurons known as Betz cells, that can be observed to drive muscles very directly with few synapses in between. These cells are more prevalent in primates and are especially important for control of the hands. This makes sense given that manual tasks are typically more complex and require greater dexterity than movements by other parts of the body. The human Cortex is believed to be crucial for innovative and sophisticated manual tasks such as tool-making.

Within the Cortical layers, C5 seems to be uniquely involved in motor output. Figure 4 shows some of the ways output from Pyramidal cells in C5 project output to areas of the brain associated with motor output and control. In contrast, pyramidal cells in C2/3 predominantly project to other areas of the cortex and are not directly involved in control.

Figure 4: Pyramidal cells in C5 project output to areas of the brain associated with motor output and control. In contrast, pyramidal cells in C2/3 predominantly project to other areas of the cortex and are not directly involved in control. Image source.
The second way that the Cortex can influence motor output is via the Basal Ganglia. In this case, we propose that the Cortex might provide contextual information to assist the Basal Ganglia in its direct control outputs, but we found no evidence that the Cortex is able to exert control over the Basal Ganglia.

We suggest Cortical influence over the Basal Ganglia is less interesting from a General Intelligence perspective, because the hierarchical representations formed within the Cortex are not exploited, and execution is performed by more ancient brain systems not associated with General Intelligence qualities.

For the rest of this article series, we will ignore control pathways that do not involve the Cortex,  and will focus on direct control output from Cortex layer 5.

Action Selection

It is widely believed that action selection occurs within the flow of information from Cortex through the Basal Ganglia, a group of deep, centralised brain structures adjacent to the Thalamus. There are a number of theories about how this occurs, but it is generally believed to involve a form of Reinforcement Learning used to select ideas from the options presented by the Cortex, with competitive mechanisms for clean switching and conflict resolution.

A major output of the Basal Ganglia is to the Thalamus; one prevailing theory of this relationship is that the Basal Ganglia controls the gating or filtering function performed by the Thalamus, effectively manipulating the state of the Cortex in consequence. The full loop then becomes Cortex → Basal Ganglia → Thalamus → Cortex (see Wikipedia for a good illustration, or figure 5).

As discussed above, this article will focus on motor output generated directly by the Cortex.
Figure 5: Pathways forming a circuit from Cortex to Basal Ganglia to Thalamus and back to Cortex. Image Source.

Canonical Cortical Circuit

We now have the all the background information needed to define a “Canonical Cortical micro-Circuit” at a cellular level. All the information presented so far has been relatively uncontroversial, but this circuit is definitely our interpretation, not an established fact. However, we will present some evidence to (inconclusively) support our interpretation.
Figure 6: Our interpretation of the canonical cortical micro-circuit. Only a single cortical region or Column is shown. Arrow endings indicate the type of connection - driver, modulator or inhibitor. The numbers 2/3, 4, 5, and 6 refer to specific cortical layers. Each shape represents a set of cells of a particular type, not an individual cell. Self-connections and connections within each set are not shown, but often exist. Shapes T and B refer to Thalamus and Basal Ganglia, not broken down into specific cell layers or types. Data enters the diagram at 4 points, labelled A-D, but does not exit; in general the system forms a circuit not a linear path. Note that shape T occurs twice, because the circuit receives data from only one part of the Thalamus but projects to two areas in forward and backward directions.

Diagram Explanation

We will use variants of the diagram shown in figure 6 to explain our interpretation of cortical function. In this diagram, only a single Cortical region or Column (used interchangeably here) is shown. In later diagrams, we will show 3 hierarchy levels together so the flow of information between hierarchy levels is apparent.

In these diagrams, shapes represent a class of Neurons within a specific Cortical Layer. The numbers 2/3, 4, 5 and 6 refer to the Cortical layers in which these cell classes occur. The shapes labelled T and B refer to the Thalamus and Basal Ganglia (internal cell types and layers are not shown). Arrows on the diagram show the effect of each connection, either driving (providing information or input that causes another cell to become active), modulation (stimulating or inhibiting the activity of a target cell) or inhibition (exclusively inhibiting the activity of a target cell).

If you want more detail on the thalamic end of the thalamocortical circuitry, an excellent source is this paper by Sherman.

There are many interneurons (described in the previous article) that are not shown in this diagram. We chose to omit these because we believe they are integral to the function of a layer of pyramidal cells within a Column, rather than an independent system. Specifically, we suggest that inhibitory interneurons implement local self-organising and local competitive functions (e.g. winner-take-all), ensuring sparse activation of the cell types represented by shapes in our diagram (C2/3, C4, C5, and C6). The self-organising behaviour also ensures that cells within each column optimise coverage of observed input patterns given a finite cell population. Inclusion of the interneurons would clutter the diagram without adding much explanatory value.

We also omit self-connections within a class of cells represented by a shape. These self-connections likely provide context and contribute to learning and exclusive activity within the class, but don’t make it easier to understand circuits in terms of cortical layers and hierarchy levels.

Excitatory Circuit

Figure 7 shows a multilevel version of the cortical circuit, similar to the multi-level figure from Sherman and Guillery (figure 3). We can now understand where the inputs to the circuit come from, in terms of other layers and external Sensors (S) and Motors (M). Note that Motors are driven directly from C5.

Figure 7: The cortical micro-circuit across several levels of Cortex with involvement of Thalamus and Basal Ganglia. The red highlight shows a single excitatory ‘circuit’. See text for details.
The red path in figure 7 shows our excitatory “canonical circuit”: Data flows from the Thalamus to spiny stellate (star-shape in figures) cells in C4 (see source and source), from where it propagates to pyramidal cells in C2/3, and then to pyramidal cells in C5. C6 is known as the multiform layer, but also contains many pyramidal cells of unusual proportions and orientations. C6 cells are driven by C5, and in turn modulate the Thalamus. Note that C6 cells within a region modulate the same Thalamic nuclei that provide input to that region of Cortex.

Inhibitory Circuit

A second, inhibitory circuit exists alongside our excitatory circuit. In addition to providing input to the Cortex via C4, axons from the Thalamus also drive inhibitory Parvalbumin-expressing (PV) neurons in C4 (shown as circles in the diagram). These inhibitory neurons make up a large fraction of all the cells in C4, and inhibit pyramidal cells in C5 (see source or source ).

This means that the input from the Thalamus can be both informative and executive. It is executive in that it actually manipulates the activity of layer 5 within the Cortex, and informative by providing a copy of the signal driving the manipulation to C4. Figure 8 shows our inhibitory circuit. We believe this circuit is of critical importance because it provides a mechanism for the Thalamus to centrally manipulate the state of the Cortex, specifically layer 5 and 6 pyramidal cells. This hypothesis will be expanded in the next article.

Figure 9 catalogues inhibitory cells, notably showing the cells used in our inhibitory circuit.
Figure 8: The inhibitory micro-circuit. The red highlight shows how the Thalamus controls activity in C5 within a Column by activating inhibitory cells in C4. The circuit is completed by C5 pyramidal cells driving C6 cells, which in turn modulate the activity of the same Thalamic nuclei that selectively activates C5. Each shape denotes a population of cells of a specific type within a single Column, excluding ‘T’ and ‘B’ that refer to the Thalamus and Basal Ganglia respectively.
Figure 9: Inhibitory interneurons in the Cortex. Of particular interest are the “PV” cells that are driven by axons from the Thalamus terminating in layer 4 and in turn inhibit pyramidal cells in layer 5. Image source

Pathways and the Canonical Circuits

Now let’s look at how pathways emerge from our cortical micro-circuit. Figures 10, 11, 12 show the Feed-Forward Direct, Feed-Forward Indirect and first Feed-Back pathways respectively. We also include another direct, Feed-Back pathway terminating at C6 (figure 13). Feed-back direct pathways terminating at C1, where many fibres are intermingled, are harder to interpret than feedback terminating directly at C6. Pyramidal neurons from many layers have dendrites in C1.
Figure 10: Feed-Forward Direct pathway within our canonical cortical micro-circuit.
Figure 10 highlights the Feed-Forward direct pathway. Signals propagate from C4 to C2/3 and then to C4 in a higher Column. This pattern is repeated up the hierarchy. This pathway is not filtered by the Thalamus or any other central structure. Although activity from C2/3 propagates to C5, it does not ascend the hierarchy via this route: C5 in one Column does not directly connect to C5 in a higher Column, only via an indirect pathway (see below).

Figure 11: Feed-Forward Indirect pathway.
Figure 11 highlights the Feed-Forward Indirect pathway. The Thalamus is involved in this pathway, and may have a gating or filtering effect. Data flows from the Thalamus to C4, to C2/3, to C5 and then to a different Thalamic nucleus that serves as the input gateway to another cortical Column in a different region of the Cortex.

Figure 12: The first of two Feed-Back Direct pathways.
Figure 12 highlights the first type of Feed-Back Direct pathway. This pathway may be more concerned with provision of broader and more abstract (i.e. hierarchically higher) contextual information to be used in the Feed-Forward pathways for better prediction. This suggestion is supported by evidence that axons from C6 via C1 synapse with apical dendrites of pyramidal cells in C2/3, C5 and C6, in hierarchically lower regions.

Figure 13 highlights the second of two Feed-Back Direct pathways. This pathway might be involved in cascading control activity down the hierarchy towards sensors and motors - the next article will expand on this idea. Activity propagates from C6 to C6 directly. C6 modulates the activity of local C5 cells and relevant Thalamic nuclei that drive local C5 cells. Note that connections from a Column to the Thalamus are reciprocal; feedback from C6 to the Thalamus targets the same nuclei that project axons to C4.
Figure 13: The second of two Feed-Back Direct pathways.

Summary

We’ve presented some additional, detailed perspectives on the organisation and function of circuits and pathways within the Thalamo-Cortical system and presented our interpretation of the canonical cortical micro-circuit.

So what’s the point of all this information? What do these circuits and pathways do, and why are they connected this way? How do they work?

It might seem that we’ve stopped short of really trying to interpret all this information and that’s because we are, indeed, holding back. After having spent so much time presenting background information, the next article finally attempts to understand why the thalamocortical system is connected in the ways described here, and how this system might give rise to general intelligence.