Hand-related rather than goal-related source of gaze-dependent errors in memory-guided reaching

Mechanisms for visuospatial cognition are often inferred directly from errors in behavioral reports of remembered target direction. For example, gaze-centered target representations for reach were first inferred from reach overshoots of target location relative to gaze. Here, we report evidence for the hypothesis that these gaze-dependent reach errors stem predominantly from misestimates of hand rather than target position, as was assumed in all previous studies. Subjects showed typical gaze-dependent overshoots in complete darkness, but these errors were entirely suppressed by continuous visual feedback of the finger. This manipulation could not affect target representations, so the suppressed gaze-dependent errors must have come from misestimates of hand position, likely arising in a gaze-dependent transformation of hand position signals into visual coordinates. This finding has broad implications for any task involving localization of visual targets relative to unseen limbs, in both healthy individuals and patient populations, and shows that response-related transformations cannot be ignored when deducing the sources of gaze-related errors.