Proposed Mechanism for Layer 4 Sensorimotor Prediction

  • May 09 / 2014
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Cortical Learning Algorithm, General Interest

Proposed Mechanism for Layer 4 Sensorimotor Prediction

Jeff Hawkins has recently talked about a sensorimotor extension for his Cortical Learning Algorithm (CLA). This extension involves Layer 4 cells learning to predict near-future sensorimotor inputs based on the current sensory input and a copy of a related motor instruction. This article briefly describes an idea which can explain both the mechanism, and several useful properties, of this phenomenon. It is both a philosophical and a neuroscientific idea, which serves to explain our experience of cognition, and simultaneously explains an aspect of the functioning of the cortex.

In essence, Jeff’s new idea is based on the observation that Layer 4 cells in a region receive information about a part of the current sensory (afferent, feedforward) inputs to the region, along with a copy of related motor command activity. The idea is that Layer 4 combines these to form a prediction of the next set of sensory inputs, having previously learned the temporal coincidence of the sensory transition and the effect of executing the motor command.

One easily visualised example is that a face recognising region, currently perceiving a right eye, can learn to predict seeing a left eye when a saccade to the right is the motor command, and/or a nose when a saccade to the lower right is made, etc. Jeff proposes that this is used to form a stable representation of the face in Layer 3, which is receiving the output of these Layer 4 cells.

The current article claims that the “motor command” represents either a real motor command to be executed, which will cause the predicted change in sensory input, or else the analogous “change in the world” which would have the same transitional sensory effect. The latter would represent, in the above example, the person whose face is seen, moving her own head in the opposite direction, and presenting an eye or nose to the observer while the observer is passive.

In the case of speech recognition, the listener uses her memory of how to make the next sound to predict which sounds the speaker is likely to make next. At the same time, the speaker is using his memory of the sound he expects to make to perform fine control over his motor behaviour.

Another example is the experience of sitting on a stationary train when another train begins to move out of the station. The stationary observer often gets the feeling that she is in fact moving and that the other train is not (and a person in the other train may have the opposite perception – that he is stationary and the first person’s train is the one which is moving).

The colloquial term for this idea is the notion of a “mirror cell”. This article claims that so-called “mirror cells” are pervasive at all levels of cortex and serve to explain exactly why every region of cortex produces “motor commands” in the processing of what is usually considered pure sensory information.

In this way, the cortex is creating a truly integrated sensorimotor model, which not only contains and explains the temporal structure of the world, but also stores and provides the “means of construction” of that temporal structure in terms of how it can be generated (either by the action of the observer interacting with the world, or by the passive observation of the external action of some cause in the world).

This idea also provides an explanation for the learning power of the cortex. In learning to perceive the world, we need to provide – literally – a “motivation” for every observed event in the world, as either the result of our action or by the occurrence of a precisely mirrored action caused externally. At a higher cognitive level, this explains why the best way to learn anything is to “do it yourself” – whether it’s learning a language or proving a theorem. Only when we have constructed both an active and a passive sensorimotor model of something do we possess true understanding of it.

Finally, this idea explains why some notions are hard to “get” at times – this model requires a listener or learner not just to imagine the sensory perception or cognitive “snapshot” of an idea, but the events or actions which are involved in its construction or establishment in the world.

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