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

Abstract

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.

Visual and proprioceptive recalibrations after exposure to a visuomotor rotation

“…Both the mean shift of the sensed position of the hand and the intra‐individual variability were larger with the direct than with the indirect procedure. This is a typical difference that we have also seen in studies of multisensory integration (Rand & Heuer, , , , ). The indirect measure would reflect the full adaptive shift of the sensed position of the hand only under the assumption of pure vector coding (e.g., Vindras & Viviani, ).…”

“…The experimental setting was quite similar to the one used in our previous studies (Rand & Heuer, , , , , Figure a). In brief, seated participants held a stylus with their right hand and made a sequence of three movements on a digitizer (Wacom Intuos 4 XL, 133 Hz sampling rate), the first from an initial position to a start position, the second a center‐out movement to a memorized target and the third a return movement back to the remembered start position.…”

“…This was the angle between the line connecting the start position with the end position of the center‐out movement (Figure d, Line A) and the line connecting the end position of the center‐out movement with the end position of the return movement (Line B). This angle reflects the angular deviation of the sensed position of the hand from its physical position at the end of the center‐out movement and thus at the start of the return movement (angle α′ in Figure d), which should result from proprioceptive recalibration similar to how it is produced by concurrent rotated visual feedback (e.g., Rand & Heuer, , , , ). When the sensed position deviated in the counterclockwise or clockwise direction from the actual position of the hand, the angular deviation (α′) had a positive or negative sign, respectively.…”

“…In addition to the direct (or explicit) measure, we use an indirect measure in the present study. This is because we have identified some variables that differently affect these measures in our previous studies on multisensory integration (Rand & Heuer, , , , ). For example, aging (Rand & Heuer, ), an enhanced proprioceptive information at the endpoint of the center‐out movement (Rand & Heuer, ), and a preceding adaptation to a visuomotor rotation (Rand & Heuer, ) are among the variables that differentially affect the two types of measure.…”

“…The return movement should be on average in the direction opposite the center‐out movement. However, systematic deviations from this direction can be observed when the sensed position of the hand at the start of the (return) movement is laterally displaced relative to its physical position, for example, by the influence of prismatically distorted visual information (Mikula, Gaveau, Pisella, Khan, & Blohm, ; Rossetti, Desmurget, & Prablanc, ), by rotated visual feedback in cursor control (Rand & Heuer, , , , ) or probably by proprioceptive recalibration after prolonged exposure to a visuomotor rotation (as we test in the present experiment).…”