The visual cortical association field: A Gestalt concept or a psychophysiological entity?

The receptive field of a visual neurone is classically defined as the regio n of space (or retina) where a visual stimulus evokes a change in its firin g activity. Intracellular recordings in cat area 17 show that the visually evoked synaptic integration field extends over a much larger area than that established on the basis of spike activity. Synaptic depolarizing (dominan t excitation) decreases in strength for stimuli that are hashed at increasi ng distances away from the centre of the discharge field, while their onset latency increases. A detailed spatio-temporal analysis of these electrophy siological data shows that subthreshold synaptic responses observed in the 'silent' surround of cortical receptive fields result from the intracortica l spread of activation waves carried by slowly conducting horizontal axons within primary Visual cortex. They also predict that a perceptual facilitat ion may occur when feedforward activation produced by the motion signal in the retina travels in phase in the primary Visual cortex with the visually induced spread of horizontal activation. A psychophysical correlate has bee n obtained in humans, showing that apparent motion produced by a sequence o f co-linear Gabor patches, known to preferentially activate VI orientation selective cells, are perceived by human observers as much faster than non c o-linear sequences of the same physical speed. (C) 2000 Elsevier Science Lt d. Published by Editions scientifiques et medicales Elsevier SAS.