A theory for cerebral neocortex

Abstract

It is proposed that the learning of many tasks by the cerebrum is based on using a very few fundamental techniques for organizing information. It is argued that this is made possible by the prevalence in the world of a particular kind of redundancy, which is characterized by a 'Fundamental Hypothesis'. This hypothesis is used to found a theory of the basic operations which, it is proposed, are carried out by the cerebral neocortex. They involve the use of past experience to form so-called 'classificatory units' with which to interpret subsequent experience. Such classificatory units are imagined to be created whenever either something occurs frequently in the brain's experience, or enough redundancy appears in the form of clusters of slightly differing inputs. A (non-Bayesian) information theoretic account is given of the diagnosis of an input as an instance of an existing classificatory unit, and of the interpretation as such of an incompletely specified input. Neural models are devised to implement the two operations of diagnosis and interpretation, and it is found that the performance of the second is an automatic consequence of the model's ability to perform the first. The discovery and formation of new classificatory units is discussed within the context of these neural models. It is shown how a climbing fibre input (of the kind described by Cajal) to the correct cell can cause that cell to perform a mountain-climbing operation in an underlying probability space, that will lead it to respond to a class of events for which it is appropriate to code. This is called the 'spatial recognizer effect'. The structure of the cerebral neocortex is reviewed in the light of the model which the theory establishes. It is found that many elements in the cortex have a natural identification with elements in the model. This enables many predictions, with specified degrees of firmness, to be made concerning the connexions and synapses of the following cortical cells and fibres: Martinotti cells; cerebral granule cells; pyramidal cells of layers III, V and II; short axon cells of all layers, especially I, IV and VI; cerebral climbing fibres and those cells of the cortex which give rise to them; cerebral basket cells; fusiform cells of layers VI and VII. It is shown that if rather little information about the classificatory units to be formed has been coded genetically, it may be necessary to use a technique called codon formation to organize structure in a suitable way to represent a new unit. It is shown that under certain conditions, it is necessary to carry out a part of this organization during sleep. A prediction is made about the effect of sleep on learning of a certain kind.