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Jennifer Anderson, Michael Levine, J. Jason McAnany; Prestidigitation: Easier to fool the eye than the hand. Journal of Vision 2008;8(6):1051. doi: 10.1167/8.6.1051.
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Processing in the dorsal and ventral visual pathways has been studied in brain-damaged subjects. We sought to understand processing in these visual pathways in the normal human brain. To do this, we asked subjects to respond in two different ways to the same visual stimuli. A target that emerged from the top or the bottom of the display traversed a field of leftward or rightward moving distracters, which induced an illusory deflection of the target (the Duncker illusion). In the hand-eye condition, the display was arranged such that the subject's hand was on the same virtual plane as the target and distracters. The subject's goal was to “stab” the target with a stylus before it reached a horizontal bar spanning the center of the display; we believe the dorsal pathway mediates such hand-eye coordination. In the cognitive condition, the subject saw the same display and decided where the target would have intercepted the central bar. The subject indicated a choice on a virtual keypad that appeared after the display concluded; we believe the ventral pathway mediates such cognitive tasks. We found the effect of illusory motion in the cognitive condition but not in the hand-eye condition. In the cognitive condition, when the distracters were moving leftwards the subject reported the target to be headed to the right of the actual location, and vice versa. In the hand-eye condition, the motion of the distracters did not affect the subject's response. These results suggest a way to examine the two visual pathways separately in the normal human brain. By manipulating properties of the stimuli and distracters we are learning more about normal visual processing.
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