Abstract
The introduction of non-target objects into a workspace leads to temporal and spatial adjustments of reaching trajectories towards a target. If the non-target is obstructing the path of the hand towards the target, the reach is adjusted such that collision with the non-target, or obstacle, is avoided. Little is known about the influence of features which are irrelevant for the execution of the movement on avoidance movements, like color similarity between target and non-target objects. In eye movement studies the similarity of non-targets has been revealed to influence oculomotor competition. Because of the tight neural and behavioral coupling between the gaze and reaching system, our aim was to determine the contribution of similarity between target and non-target to avoidance movements. We performed 2 experiments in which participants had to reach to grasp a target object while a non-target was present in the workspace. These non-targets could be either similar or dissimilar in color to the target. The results indicate that the non-spatial feature similarity can further modify the avoidance response and therefore further modify the spatial path of the reach. Indeed, we find that dissimilar pairs have a stronger effect on reaching-to-grasp movements than similar pairs. This effect was most pronounced when the non-target was on the outside of the reaching hand, where it served as more of an obstacle to the trailing arm. We propose that the increased capture of attention by the dissimilar obstacle is responsible for the more robust avoidance response.
Highlights
During our everyday activities we reach towards and grasp many objects
Further investigation using Bonferroni-corrected ttests revealed that participants reacted significantly slower, t (9) = 22.39, p,.025, in the dissimilar conditions (M = 351 ms, SD = 47) than in the similar conditions (M = 333 ms, SD = 53) when the non-target was presented on the outside of the arm, whereas there was no significant difference in reaction time between the different similarity conditions when non-targets were presented on the inside of the arm. These parameters imply that the movement trajectory as a whole was affected by similarity, as can be seen in Figure 2, at least as long as the non-target is on the outside of the arm. The purpose of this experiment was to uncover the role of colordefined similarity between target and non-target by investigating hand reaching trajectories in an obstacle avoidance paradigm
The results support this hypothesis as the trajectories of the hand are different when target and non-target have the same color compared to when they have different colors. This effect is a replication of Kritikos and colleagues [9] who found an analogous effect for size and orientation and Ludwig and Gilchrist [16] who report a similarity effect of visually presented targets and distractors for eye movements. This ‘similarity’ effect extends to color-defined similarity in physical objects and we have support for an additional modifier of the hand trajectory in obstacle avoidance beyond the avoidance of a collision
Summary
During our everyday activities we reach towards and grasp many objects. these objects are seldom the only items present in our direct surroundings, we are able to steer our hand toward them and evade any obstacles that are present. Evidence in line with this account comes from many studies in which an increase in movement time is observed when a non-target stimulus is placed in the workspace [1,2,3,4,5,6], suggesting that the movement is slowed down to increase spatial accuracy and avoid potential collisions These adjustments are not a general response to the presence of non-target objects [2], on the contrary, the effect is specific to the layout of the workspace, in that non-target objects only elicit an avoidance response when the preferred distance to them is too small. Chapman and Goodale have noted that the obstacle avoidance system is sensitive to changes in obstacle size and obstacle location [7]
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