The visuomotor system resists the horizontal-vertical illusion

The effect of the horizontal-vertical illusion on the visual and visuomotor systems was investigated. Participants (N = 8) viewed horizontal and verti cal lines in an inverted-T stimulus and judged whether the two line segment s were the same or different lengths. Participants also reached out and gra sped either the vertical or the horizontal line segment of the stimulus. Pe rceptually, participants succumbed to the illusion; that is, they judged Ts of equal horizontal and vertical line lengths to be different and Ts of un equal line lengths to be the same. When reaching toward the same stimuli, h owever, the size of their grip aperture was scaled appropriately for the va rious line lengths. Thus, whereas the perceptual system succumbed to the il lusion, the visuomotor system did not. Those results support a model propos ed by M. A. Goodale and A. D. Milner (1992), who posited separate cortical pathways for visual perception and visually guided action.