A FORMALIZATION OF THE NEURAL ASSEMBLY CONCEPT .1. CONSTRAINTS ON NEURAL ASSEMBLY SIZE

Hebb proposed the concept of a neural assembly distributed across cort ical tissue as a model for representation of information in the cerebr al cortex. Later developments of the concept highlight the need for ov erlapping membership between independent assemblies, and the spread of activity throughout the assembly once it is activated above a critica l level (ignition). Formalisation of the neural assembly concept, espe cially in relation to quantitative data from the real cortex, is at a very early stage. We consider two constraints on neural assembly size: (1) if a neural assembly is too small the fraction of its neurons tha t need to be active to ignite the whole assembly becomes unrealistical ly large; (2) if assemblies in a block of cortical tissue become too l arge then the block becomes 'unsafe', that is, unwanted spread from an active assembly to overlapping ones becomes inevitable. We consider v ariations in three parameters: neuronal firing threshold; connection d ensity; and the total number of assemblies stored in the block of cort ical tissue. Given biologically plausible values for these parameters we estimate maximum assembly size compatible with ignitability of indi vidual assemblies, low probability of unwanted spread to overlapping a ssemblies, and safe operation of the block as a whole.