Vision for spatial perception and vision for action: a dissociation between the left-right and near-far dimensions.

Neuropsychological and psychophysical studies have suggested that two distinct visual sub-systems are responsible for perception and action. One of the main psychophysical arguments for this is based on visual illusion such as the Induced Roelofs Effect (IRE), where the location of a visual target presented with an off-centre frame is misperceived when evaluated verbally, but not with a reaching response. This dissociated effect suggests the existence of two independent representations of visual space devoted, respectively, to categorisation and to egocentric localisation of reachable objects. These "cognitive" and "sensorimotor" representations have been assumed to be produced through specific anatomical pathways stemming from the primary visual cortex (respectively, the ventral and dorsal streams). To account for the dissociation found with the IRE, it has been suggested that only the cognitive system is sensitive to contextual information. However this view has been challenged by recent psychophysical studies demonstrating the influence of environmental cues on distance perception and the guiding of movement. In the present study, the IRE is re-evaluated but the near-far and right-left dimensions were dissociated. In agreement with previous findings, our results showed that the IRE in the right-left dimension gives rise to a perceptual misperception of target position with no effect on motor performance. Conversely, when the IRE was induced in the near-far dimension a misperception of the target position affected both perceptual and motor responses. This dissociation indicates that the spatial constraints of the task, and not only the nature of the response, interfere with sensitivity to contextual information leading to visual illusions. It is thus likely that the action system (imputed to the dorsal stream) can be sensitive to contextual information, at least when depth processing is emphasised.