A formalisation of the neural assembly concept 1. Constraints on neural assembly size |
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Authors: | JR Wickens R Miller |
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Institution: | (1) Department of Anatomy & Structural Biology, University of Otago Medical School, P.O. Box 913, Dunedin, New Zealand , NZ |
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Abstract: | Hebb proposed the concept of a neural assembly distributed across cortical tissue as a model for representation of information
in the cerebral cortex. Later developments of the concept highlight the need for overlapping membership between independent
assemblies, and the spread of activity throughout the assembly once it is activated above a critical level (ignition). Formalisation
of the neural assembly concept, especially 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 that
need to be active to ignite the whole assembly becomes unrealistically large; (2) if assemblies in a block of cortical tissue
become too large then the block becomes ‘unsafe’, that is, unwanted spread from an active assembly to overlapping ones becomes
inevitable. We consider variations in three parameters: neuronal firing threshold; connection density; and the total number
of assemblies stored in the block of cortical tissue. Given biologically plausible values for these parameters we estimate
maximum assembly size compatible with ignitability of individual assemblies, low probability of unwanted spread to overlapping
assemblies, and safe operation of the block as a whole.
Received: 7 March 1997 / Accepted in revised form: 1 July 1997 |
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