No visual-proprioceptive re-weighting after removal of vision

Abstract

When the target of a reach is on one’s body (e.g., a bug to swat), visual and proprioceptive information about both the effector and the target can be used to plan the movement. If vision is removed, proprioceptive information should receive increased weighting to compensate for the decaying visual representation. We investigated whether the rate of re-weighting was equivalent for the effector and the target. In our experiment participants received visual information (projected dots representing the unseen left and right index fingers) that was shifted (near/far) with respect to the fingers’ veridical locations. Zero, two, or five seconds after the dots were extinguished, participants rapidly aimed with a stylus to their left index finger, which was located directly under the reaching surface. The visual manipulation influenced reaching, but the direction of the effect differed across participants: some participants aimed high (low) when the visual information about the hand and target was below (above) the veridical positions, which suggests preferential coding of visual information about the effector, while some participants aimed low (high) when the visual information was below (above) the veridical positions, which suggests preferential coding of visual information about the target. More surprising was the absence of any effect of the delay manipulation; the effect of the visual manipulation was constant across all 3 delays for all participants. Moreover, we saw no increase in variable error as delay increased. These results suggest that a reliable visual position estimate persisted for at least 5 seconds. In the absence of intervening hand and/or eye movements (in our study participants maintained fixation throughout the trial and their hands were stationary until the onset of the reach), visual memory for hand location may be as robust as real-time proprioception. Meeting abstract presented at VSS 2013