Dissociating explicit and implicit measures of sensed hand position in tool use: Effect of relative frequency of judging different objects.

In a cursor-control task, the sensed positions of cursor and hand are biased toward each other. We previously found different characteristics of implicit and explicit measures of the bias of sensed hand position toward the position of the cursor, suggesting the existence of distinct neural representations. Here we further explored differences between the two types of measure by varying the proportions of trials with explicit hand-position (H) and cursor-position (C) judgments (C20:H80, C50:H50, and C80:H20). In each trial, participants made a reaching movement to a remembered target, with the visual feedback being rotated randomly, and subsequently they judged the hand or the cursor position. Both the explicitly and implicitly measured biases of sensed hand position were stronger with a low proportion (C80:H20) than with a high proportion (C20:H80) of hand-position judgments, suggesting that both measures place more weight on the sensory modality relevant for the more frequent judgment. With balanced proportions of such judgments (C50:H50), the explicitly assessed biases were similar to those observed with a high proportion of cursor-position judgments (C80:H20), whereas the implicitly assessed biases were similar to those observed with a high proportion of hand-position judgments (C20:H80). Because strong weights of cursor-position or hand-position information may be difficult to increase further but are easy to reduce, the findings suggest that the implicit measure of the bias of sensed hand position places a relatively stronger weight on proprioceptive hand-position information, which is increased no further by a high proportion of hand-position judgments. Conversely, the explicit measure places a relatively stronger weight on visual cursor-position information.