Waiting for a hand: saccadic reaction time increases in proportion to hand reaction time when reaching under a visuomotor reversal

Abstract

Although eye movement onset typically precedes hand movement onset when reaching to targets presented in peripheral vision, arm motor commands appear to be issued at around the same time, and possibly in advance, of eye motor commands. A fundamental question, therefore, is whether eye movement initiation is linked or yoked to hand movement. We addressed this issue by having participants reach to targets after adapting to a visuomotor reversal (or 180° rotation) between the position of the unseen hand and the position of a cursor controlled by the hand. We asked whether this reversal, which we expected to increase hand reaction time (HRT), would also increase saccadic reaction time (SRT). As predicted, when moving the cursor to targets under the reversal, HRT increased in all participants. SRT also increased in all but one participant, even though the task for the eyes-shifting gaze to the target-was unaltered by the reversal of hand position feedback. Moreover, the effects of the reversal on SRT and HRT were positively correlated across participants; those who exhibited the greatest increases in HRT also showed the greatest increases in SRT. These results indicate that the mechanisms underlying the initiation of eye and hand movements are linked. In particular, the results suggest that the initiation of an eye movement to a manual target depends, at least in part, on the specification of hand movement.

Keywords: eye-hand coordination; humans; motor adaptation; reaching movements; saccadic reaction time.

Figure 1

Apparatus used to measure gaze and hand movements and to present visual feedback about targets and cursor position. While seated, participants used their right hand to grasp the handle of a light-weight manipulandum that measured the position of the hand in three-dimensions. The handle of the manipulandum was supported by an “air-sled” that rode across a horizontal glass surface on a cushion of air. With this support, the hand was effectively constrained to move in a horizontal plane. A video projector (not shown), above and to the right of the participant, projected targets, and cursor onto a projection screen via a 45° mirror. Participants viewed the cursor targets in a semi-silvered mirror located halfway between the projection screen and the plane of hand movement and could not see their hand. Thus, the targets and cursor appeared in the same plane as the hand.

Figure 2

Schematic outline of the four phases that participants completed in both Experiment 1 and Experiment 2. In the first two phases, training and test 1, the mapping between position of the hand and the cursor controlled by the hand was veridical. In contrast, in the last two phases the cursor position was rotated 180° about the start position such that a rightward hand movement produced a leftward cursor movement. During training and adaptation, participants made only coordinated eye and cursor movements to the target. During the two test phases, participants made both eye only movements and eye plus cursor movements with these two trial types presented either in blocks (Experiment 1) or randomly interleaved (Experiment 2). In the first test phase in Experiment 1, participants also completed a block of trials in which they had to move the cursor away from the target while shifting gaze to the target (dashed box).

Figure 3

Mean reaction time as a function of target amplitude for eye movements (SRT) in Experiment 1 (A) and Experiment 2 (C) and hand movements (HRT) for both experiments (B,D). Hollow bars represent eye movement only conditions and filled bars represent eye plus hand movement conditions. The narrow bars represents mean reaction time for anti-hand movement trials. The vertical lines represent 1 SE.

Figure 4

The relationship between Δ SRT and Δ HRT for each participant in Experiment 1 (filled symbols) and Experiment 2 (open symbols). Δ represents the effect of reversal adaptation, i.e., the difference between pre- and post-adaptation reaction time when the eyes and hand moved concurrently. A positive value indicates the reaction time was longer after adaptation. Different symbols are used for each of the six participants. Separate regression lines are shown for each experiment. However, these two lines have similar slopes and intercepts.

Figure 5

Mean predicted velocity as a function of target amplitude for eye movements in Experiment 1 (A) and Experiment 2 (C) when eyes moved without hand movements (hollow bars) and with hand movements (filled bars) before and after the reversal adaptation. Hand movements in both experiments (B,D) are also shown for pre- and post-adaptation. Anti-hand movement condition in Experiment 1 is shown by the narrow bars. The vertical lines represent 1 SE.

Figure 6

HRT and SRT as a function of trial during the adaptation periods of Experiment 1 (filled circles) and Experiment 2 (open circles). Each point represents the mean across participants where the reaction time for each participant is the average across 10 successive trials. The figure also shows, for each experiment, HRT and SRT for eye + hand trials during the pre-adaptation and the post-adaptation phases. Each point represents the mean across participants. The vertical lines represent 1 SE.