On the relation between nontarget object location and avoidance responses.

The presence of nontarget objects influences kinematic parameters of reaches toward target objects. In previous studies, several different nontarget positions have been used. Taken together, these studies suggest that when the horizontal or vertical distance to nontargets is decreased, avoidance responses are more pronounced. Furthermore, responses to nontarget objects are asymmetrical across workspace, i.e., responses in the presence of equidistant nontargets on the inside and the outside of the reaching arm are different. However, these studies have provided a coarse overall picture of the effect of nontarget location. Therefore, the aim of this experiment was to systematically map the avoidance responses across the workspace in order to determine in detail the relation between nontarget position and the avoidance response. Specifically, we were interested in the contribution of four parameters to the reaching response: the nontarget's horizontal and vertical position, its distance from the starting position, and its angle with the vertical midline of the workspace. Participants were asked to perform reaches towards physical targets while nontargets were present in 1 of 24 different positions. Our results replicate horizontal and vertical effects of the nontarget object on reaching behavior. We also replicate stronger avoidances of nontargets on the outside of the reaching limb compared to nontargets on the inside. Furthermore, our results provide a detailed overview of the interaction between these factors and demonstrate that there is a "hot" region qua nontarget positions that prompt the strongest responses. Lastly, our results provide evidence that support a fine-grained spatial resolution of nontarget motor representation.