Binocular advantage for prehension movements performed in visually enriched environments requiring visual search

Abstract

The purpose of this study was to examine the role of binocular vision during a prehension task performed in a visually enriched environment where the target object was surrounded by distractors/obstacles. Fifteen adults reached and grasped for a cylindrical peg while eye movements and upper limb kinematics were recorded. The complexity of the visual environment was manipulated by varying the number of distractors and by varying the saliency of the target. Gaze behavior (i.e., the latency of the primary gaze shift and frequency of gaze shifts prior to reach initiation) was comparable between viewing conditions. In contrast, a binocular advantage was evident in performance accuracy. Specifically, participants picked up the wrong object twice as often during monocular viewing when the complexity of the environment increased. Reach performance was more efficient during binocular viewing, which was demonstrated by shorter reach reaction time and overall movement time. Reaching movements during the approach phase had higher peak velocity during binocular viewing. During monocular viewing reach trajectories exhibited a direction bias during the acceleration phase, which was leftward during left eye viewing and rightward during right eye viewing. This bias can be explained by the presence of esophoria in the covered eye. The grasping interval was also extended by ~20% during monocular viewing; however, the duration of the return phase after the target was picked up was comparable across viewing conditions. In conclusion, binocular vision provides important input for planning and execution of prehension movements in visually enriched environments. Binocular advantage was evident, regardless of set size or target saliency, indicating that adults plan their movements more cautiously during monocular viewing, even in relatively simple environments with a highly salient target. Nevertheless, in visually-normal adults monocular input provides sufficient information to engage in online control to correct the initial errors in movement planning.

Keywords: binocular vision; eye-hand coordination; phoria; reaching and grasping movements; visual search.

Figure 1

Schematic diagram showing a bird's eye view of the workspace used in the experiment—shown here is set size 6, high salience condition. At the beginning of each trial the hand was located at home position. The black circle represents the fixation point where the participant was required to fixate at the initiation of each trial. When the fixation point disappeared, the criterion (a circle with a diameter matching one of the pegs in the workspace) was displayed at this location. On each trial the participant was presented with three or six pegs (locations are represented by the circles in the diagram). One of the pegs matched the diameter of the criterion, and was defined as the target for that trial. Participants were instructed to reach and grasp the target as quickly as possible, and to place it on top of the criterion.

Figure 2

Finger and gaze position along the depth direction (z-axis) and corresponding velocity representing a single trial during binocular viewing. The discrimination interval is defined from the onset of the criterion to the onset of the first gaze shift. The three components of the prehension movement were identified based on the kinematic data as shown in the figure (see text for details).

Figure 3

Temporal performance measures for reaching and grasping. (A) Mean reach reaction time across the experimental conditions. There was a significant main effect of viewing condition, set size, and target salience (p < 0.05). (B) Mean total movement time across the experimental conditions. There was a significant main effect of viewing condition and set size (p < 0.05). Error bars show ±1 standard error of the mean.

Figure 4

Mean peak velocity during the approach phase along the main direction of movement (z-axis). Peak velocity was significantly higher during binocular viewing and in the small set size condition (^*p < 0.05).

Figure 5

Mean reach angle difference between monocular and binocular viewing during the approach trajectory. A significant bias in the reach trajectory was found during the acceleration phase: 100 ms and 50 ms before reach peak velocity (i.e., at PV-100 and PV-50). (^*p < 0.05).

Figure 6

Mean duration of the grasping phase. The grasping interval was significantly shorter during binocular viewing and in the small set size condition (^*p < 0.05).