The effect of viewing the static hand prior to movement onset on pointing kinematics and variability

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Abstract
Pointing accuracy and arm movement kinematics of six human subjects were measured in three conditions where the hand was never visible during the ongoing movement: (1) in the dark; (2) the static hand was seen in peripheral vision prior to target presentation, but not during the reaction time (H−T); (3) the static hand was seen in peripheral vision until movement onset (H+T). It was shown that: (1) viewing the hand prior to movement decreased pointing variability as compared to the dark condition. (2) Viewing simultaneously hand and target (H+T) further decreased pointing variability as compared to the H−T condition. This effect was proportional to the reaction time. (3) A lengthening of the deceleration phase was observed for movements performed in the H + T condition, as compared to the other two conditions. (4) A negative correlation between variability and the first part of the deceleration phase was observed in the H + T condition, but neither in the H-T condition nor in the dark. These results suggest that the decrease in pointing variability observed in the H + T condition is due to a feedback based on kinesthetic reafference. Better encoding of the initial position of the hand relative to the target (as in H + T) would allow a calibration of arm position sense, which is used to drive the hand toward the target during the deceleration phase.