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B. Bridgeman, P. Thiem; Limits of the sensorimotor visual system. Journal of Vision 2001;1(3):192. doi: https://doi.org/10.1167/1.3.192.
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© ARVO (1962-2015); The Authors (2016-present)
When a small fixed target is presented inside a frame that is offset to one side, normal humans perceive the target to be deviated in the direction opposite the frame's offset (the Roelofs effect). They can still jab the target accurately, however, even though it is perceptually mislocalized. This dissociation indicates that motor coordinates are coded in a ‘sensorimotor’, possibly dorsal, pathway containing visual information that can be inconsistent with perceived information in a ‘cognitive’, possibly ventral pathway. Lack of a Roelofs effect indicates use of information in the sensorimotor pathway, independent from perception. We ask whether the sensorimotor pathway can handle a transformation of target position, in an anti-jabbing task analogous to anti-saccade tasks: the observer jabs a position symmetrically opposite the target's position, relative to the midline of the head. A 1í target and a 38í wide frame appeared together, remained for 1 sec, and disappeared together. The target could appear 3 deg left or right of center; the frame could appear centered or 6 deg left or right. Observers were to jab the symmetrically opposite position as soon as the target disappeared. The result was a large and consistent Roelofs effect (p<0.01) for an open-loop motor task, indicating that information from the cognitive pathway must be used to perform this task. From this and similar experiments we conclude that the sensorimotor system requires that a motor response be physically congruent with a target position; otherwise, the more flexible but illusion-prone cognitive system must provide the required spatial information.
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