Stimulus Location is Processed Faster than Stimulus Colour

Abstract

There is a convergence of anatomical, electrophysiological, neuropsychological, and psychophysical data to support the dissociation of visual pathways into two main streams projecting from occipital to frontal cortex via the posterior parietal lobe (dorsal route) and via the inferotemporal lobe (ventral route). It is usually assumed that the dorsal route provides information that is useful for driving an action toward the stimulus (ie metric properties, such as localisation), whereas the ventral route extracts information useful for identifying it (ie intrinsic properties, such as colour). It is known that pointing movements can be reoriented to a novel target location within a short delay (about 110 ms), even when the target jump cannot be detected because of saccadic suppression. Electrophysiological studies have suggested that inputs to the dorsal pathway have a latency shorter than inputs to the ventral pathway.

We compared latencies of visuomotor processing for colour and location during a pointing task. Target location and/or colour were altered upon movement onset. Instructions were to correct movement direction or to interrupt the movement according to the target change. We found that in both cases colour processing was slower (by about 100 ms) than location processing of the same target. Performance observed for identical movement speed was always higher for location responses whereas movement duration spontaneously chosen by subjects was longer when they had to process colour. Strikingly, corrections were also observed with the interruption instruction. We conclude that (1) colour is processed more slowly than location, and (2) automatic corrections can be observed prior to response inhibition for fast movements.