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Geoffrey P. Bingham, Mark Mon-Williams; Visually guided reaching allows both slope and intercept of distance functions to be recalibrated without awareness. Journal of Vision 2004;4(8):22. doi: 10.1167/4.8.22.
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Inaccuracy and imprecision in distance perception have been found in many verbal judgment and reaching studies. This implies that one might see an object in one position while reaching to touch it in a different location without conscious awareness of this discrepancy. On the other hand, visual or haptic feedback from reaching has been found to yield accurate calibration of distance perception. The fact that there is flexibility in the relationship between vision and the kinesthetically controlled motor system raises the question as to how labile is this relation? We used distorted haptic feedback from reaches to alter the calibration of perceived distance and to progressively dissociate the visual location from the location to which participants reached to touch the visually presented object. Experiments in virtual and actual environments found alike that a dissociation of about 8cm could be imposed upon participants without their awareness. We investigated the generalization of such calibration across reach space. If a target at one distance was recalibrated to a shorter distance than its actual visual location, would this change the slope of a function relating visual distances to reach distances so as to preserve the zero intercept? The answer to this question was negative. The results of the experiment showed that all distances (including the intercept) changed approximately by the amount of the recalibrated target. Next we sought to change both slope and intercept by calibrating two targets using distorted feedback to a near target and accurate feedback to a far target. The result (tested with a middle target) was a change in slope and intercept. Participants remained unaware of these changes. This demonstrates that calibration is essential to the perception of distance used to guide reaching and that recalibration can occur without conscious awareness.
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