Simultaneous order in nervous nets from a functional standpoint.

A nervous net may be studied by an external observer who notes, for example, such features as the spatial lay-out of receptive fields, the existence of somatotopic maps, etc. It is not often acknowledged that many of such features have no functional relevance to the machine itself. Thus somatotopy from the sensor to the motor apparatus may appear important (and in certain respects it is), but it is irrelevant to the function of the organism as an abstract machine. For the abstract machine only functional relations, but not spatial relations per se, count. It is shown how, proceeding from purely functional relations (cross-correlations of signals), a simultaneous order of neural elements may be constructed. Such an order has an objective existence for the abstract machine, not merely for an external observer. It is argued that sensory modalities and the cohesion within modalities (e.g. the visual field) must be understood in such a functional manner. Thus the two-dimensionality of the visual field is objectively present in the cross-correlation structure of optic nerve signals, it exists independently from an external observer's description of the retina as a two-dimensional receptor array.