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Hiroshi Ashida; ‘Representational momentum’ in reaching action. Journal of Vision 2002;2(7):59. doi: 10.1167/2.7.59.
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Purpose: Perceptual judgment of the final position of a moving target is shifted in the direction of motion (‘representational momentum’, RM). This illusion has been considered as a memory shift at a cognition level (Hubbard, 1995, Psychonomic Bulletin & Review), but it can be more related to visuomotor coordination as automatic compensation of processing delays. RM was measured with perceptual and action tasks to test this idea.
Method: Stimuli were presented on a CRT equipped with a touch panel. A target disk moved horizontally and the observers pointed the position where it disappeared either by adjusting a screen cursor (perceptual task) or by touching the screen with a pen (action task). For the action task, an open loop condition was also tested in which an LCD goggle prevented the observers from seeing the screen and their hand while responding. Target motion was observed either with free viewing or with fixation.
Results: With free viewing, similar amount of shift in the motion direction were found both for perception and action. With fixation, however, the shift was greatly reduced except under the open loop condition that yielded similar shifts regardless of fixation.
Discussion: RM found for an open loop reaching action suggests that the origin of this illusion lies in rather peripheral level. Pursuit eye movement seems necessary for perceptual RM (cf. Kerzel, 2000, Vision Res.), but it cannot be the primal cause because it does not affect RM for open loop reaching. Frame of reference is rather crucial. With fixation, positional coding against the background would suppress RM, while such information is not readily available under eye tracking or open loop pointing. More robust RM for action is consistent with our previous result of larger positional bias by action than by perception for a drifting Gabor patch (Yamagishi, Anderson, & Ashida, 2001, Proc. R. Soc. Lond. B), supporting separate pathways for vision and action (Milner & Goodale, 1985).
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