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Abstract:
The point spread in visual cortex (the area of the cortex activated by a minimal visual stimulus) measured by optical recording is very much larger than the point spread measured by spike activity; it develops more slowly and lasts for a longer time (Grinvald et al, 1994 Journal of Neuroscience 14 2545 -- 2568). It is considered to reflect mainly subthreshold activity of neuronal arborisations. It is shown how this subthreshold activity may contribute to information processing: the sudden onset of a visual cue is known to trigger visual attention, which then enhances visual processing in the zone near the cue. The most likely physiological basis of attention is a subthreshold wave of negativity. This wave develops more slowly than signals leading to perception, and spreads outward from the cortical region associated with the cue, into regions associated with other retinal sites. Such slow potentials are known to influence behavioural responses: negativity reduces neuronal thresholds and hence shortens response latencies (Birbaumer et al, 1990 Physiological Reviews 70 1 -- 41). As examples, the line motion illusion and the generation of express saccades are discussed on the basis of this concept.
