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Craig Chapman, Melvyn Goodale; Missing in action: Obstacle avoidance while reaching. Journal of Vision 2007;7(9):566. doi: 10.1167/7.9.566.
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When reaching to objects, our hand and arm rarely collide with non-target objects, even if our workspace is cluttered. The simplicity of these actions hides what must be a relatively sophisticated obstacle avoidance system. Recent studies on patients with optic ataxia and visual form agnosia have demonstrated that obstacle avoidance is an automatic process, likely governed by the dorsal stream (Schindler et al., 2004, Nature Neuroscience, 7(7):779–784, Rice et al., 2006, Experimental Brain Research, 174(1):176–188). The current study sought to quantify how normal participants react to changes in the size and position of non-target objects in and around their workspace. In the first experiment, 13 right-handed subjects performed reaches to a target strip in the presence of two non-target objects, which varied in depth and horizontal configuration. We found that objects with horizontal alignments that were asymmetric about midline created systematic deviations in reach trajectory away from midline, with participants seeming to maximize the distance away from the two objects. These deviations were significantly greater for objects nearer in depth and nearly disappeared when the objects were placed beyond the target strip, suggesting the obstacle avoidance system is sensitive to the depth at which an object becomes an obstacle. In a second experiment, we varied the height of the two objects, as well as the depth. Object pairs could now be both tall, both short, or one short and one tall (with the tall on the right or on the left). We replicated the first experiment, extending the finding to include sensitivity to the size of the objects. Here the deviations induced by short objects, while still significant, were significantly less than the tall-object deviations. Taken together, these experiments indicate a sophisticated obstacle avoidance system that is extremely sensitive and conservative in evaluating potential obstacles and deviating reach accordingly.
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