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Urs Kleinholdermann, Volker H. Franz; Spatial bias, spatial uncertainty and illusion effects in antigrasping. Journal of Vision 2011;11(11):969. doi: 10.1167/11.11.969.
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In a virtual reality (VR) setup we let participants grasp bar stimuli with a precision grip. In an orthograsp condition physical and VR stimuli were at corresponding locations. In an antigrasp condition physical stimuli were located opposite to stimuli displayed in VR, similar to the well established anti-saccade paradigm.
In three experiments we varied the amount of spatial uncertainty of stimulus position by changing stimulus size and permitting auditory location cues. In two of the experiments we used Mueller-Lyer type illusory displays. Here we also measured the perceptual effect of the illusion with a size estimation task.
We found that participants exhibited a movement bias towards the location of the visually displayed stimulus in antigrasping. Antigrasping in interaction with spatial uncertainty also had a diminishing effect on maximum grip aperture (MGA) scaling. Other kinematic parameters - reaction time, movement time, size and timing of MGA - showed no significant difference between ortho- and antigrasping. Presenting the Mueller-Lyer stimuli had a highly significant effect on perceptual size estimates and on MGA size in anti- and orthograsping.
We conclude that spatial movement biases are present in antigrasping that are analogous to those found in antisaccades. Antigrasping seems to impose higher demands on the visuo-motor transformations from perception to action which become evident when the task gets more difficult.
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