Active collisions in altered gravity reveal eye-hand coordination strategies

Abstract

Most object manipulation tasks involve a series of actions demarcated by mechanical contact events, and gaze is usually directed to the locations of these events as the task unfolds. Typically, gaze foveates the target 200 ms in advance of the contact. This strategy improves manual accuracy through visual feedback and the use of gaze-related signals to guide the hand/object. Many studies have investigated eye-hand coordination in experimental and natural tasks; most of them highlighted a strong link between eye movements and hand or object kinematics. In this experiment, we analyzed gaze strategies in a collision task but in a very challenging dynamical context. Participants performed collisions while they were exposed to alternating episodes of microgravity, hypergravity and normal gravity. First, by isolating the effects of inertia in microgravity, we found that peak hand acceleration marked the transition between two modes of grip force control. Participants exerted grip forces that paralleled load force profiles, and then increased grip up to a maximum shifted after the collision. Second, we found that the oculomotor strategy adapted visual feedback of the controlled object around the collision, as demonstrated by longer durations of fixation after collision in new gravitational environments. Finally, despite large variability of arm dynamics in altered gravity, we found that saccades were remarkably time-locked to the peak hand acceleration in all conditions. In conclusion, altered gravity allowed light to be shed on predictive mechanisms used by the central nervous system to coordinate gaze, hand and grip motor actions during a mixed task that involved transport of an object and high impact loads.

Figure 1. Typical collisions in microgravity.

Records of a single collision upwards (left column) and downwards (right column). The following traces are shown as a function of time: vertical position, velocity and acceleration of the object, load and grip forces, and the vertical gaze position. The left, middle and right cursors are aligned with hand onset (H_onset), peak acceleration (PA) and contact with the target (T_contact) respectively. PV: peak velocity; LF_PA: load force at peak acceleration; GF_M: peak grip force; S_onset, S_land and S_back: saccade to and arrival on the target, saccade onset back to home position.

Figure 2. Grip force and load force in the transport and collision phases.

Grip force and load force were coupled in the transport phase (left column) but not in the collision phase (right column). Forces are plotted in the two directions (up vs. down) and in three gravity phases (0 g, 1 g and 1.8 g). During transport, both load force and grip force are plotted at peak hand acceleration. At collision, load force is the magnitude of impact and grip force was recorded at contact (8 ms before peak load force), to avoid any artefact. Closed and open disks represent upward and downward movements respectively. Error bars represent between participants SE.

Figure 3. Grip force adjustment over time.

(Left) Grip force in function of time to contact in upward (closed disks) and downward (open disks) movements in 0 g. The means and SE of grip forces are reported for the following occurrences: hand onset (H_onset), peak acceleration (PA), peak velocity (PV), time of contact (T_contact) and grip force maximum (GF_M). The horizontal SEs quantify the variability in the occurrences of these events. The vertical dotted line is positioned at contact. (Right) Delay between peak grip forces and contact in upward (black bars) and downward (open bars) trials in 0 g, 1 g and 1.8 g. All values are positive which means that peak grip forces lag the collision. Error bars are between participants SD. Asterisk denotes significant difference (p<0.05).

Figure 4. Durations of target fixations before and after target contact.

Saccades landed on the target 241.1±84.1 ms before the collision but left later in upward compared to downward trials (black bars vs. open bars) and in unknown gravity fields (0 g and 1.8 g vs. 1 g). The vertical dashed line denotes time of contact. Error bars are between participants SD.

Figure 5. Constant latency across direction and gravitational conditions between times of peak hand acceleration and gaze saccade onset to the target.

Relative latencies between peak hand acceleration and hand onset (left panel), gaze onset and hand onset (middle panel) and between gaze onset and peak hand acceleration (right panel) in the three gravity fields. Open and closed disks denote downward and upward trials, respectively. Error bars represent between participants SE. The horizontal dashed line in the right panel is positioned at mean latency (132.6 ms).

Figure 6. Decrease of variability of latency between times of peak hand acceleration and gaze saccade onset to the target across parabolas.

Mean gaze latencies relative to peak hand acceleration (A) and standard deviations of mean latencies (B) across the 10 parabolas. The horizontal dashed line in the top panel is positioned at mean latency (132.6 ms). Panel B reports a strong decrease in variability within three parabolas.