Online visual feedback of the hand suppresses gaze-dependent overshoots in memory-guided reach

Abstract

Reaching movements in the dark overshoot memorized visual targets relative to the gaze direction held just before reach onset, even if a saccade intervenes between target presentation and reach onset. The latter pattern has been cited as evidence for the updating of memorized visual target positions in a gaze-centered reference frame. However, the exact origin of the gaze-dependent overshoot is not known. Because the reach errors depend on the visual target eccentricity, all previous studies (e.g., Henriques et al. 1998; McGuire & Sabes 2009) have assumed that it reflects biases in target‑related inputs to the visuomotor transformation. An alternative possibility, as of yet untested, is that the error is associated with biases in hand-related signals. Here, we tested this hypothesis through its prediction that visual feedback of the hand during the reaching movement should greatly reduce or even abolish the gaze-dependent overshoot. Six subjects sat in the dark in front of a screen behind which target and fixation LEDs were mounted 10 deg. of visual angle apart, while their eye and finger movements were recorded using EyeLink II and Optotrak, respectively. All subjects showed the typical gaze-dependent overshoot reported before (~±2 deg. at ±10 deg. retinal target eccentricity; P<0.005). The overshoot disappeared when they could see their finger during the reach (~±0.2 deg. at ±10 deg. retinal target eccentricity; P>0.37). This effect was most parsimoniously explained by a reach planning model that included biases in the transformation of proprioceptive signals into visual coordinates, for the purpose of calculating a reach vector in visual coordinates (further predictions of this model are currently under investigation). This is the first demonstration that overshoots to remembered visual targets can be suppressed entirely in healthy human subjects. This finding is consistent with our hypothesis that the overshoot arises within hand-related inputs into the visuomotor transformation.

Meeting abstract presented at VSS 2012