The effects of visual control and distance in modulating peripersonal spatial representation

In the presence of vision, finalized motor acts can trigger spatial remapping, i.e., reference frames transformations to allow for a better interaction with targets. However, it is yet unclear how the peripersonal space is encoded and remapped depending on the availability of visual feedback and on the target position within the individual’s reachable space, and which cerebral areas subserve such processes. Here, functional magnetic resonance imaging (fMRI) was used to examine neural activity while healthy young participants performed reach-to-grasp movements with and without visual feedback and at different distances of the target from the effector (near to the hand–about 15 cm from the starting position–vs. far from the hand–about 30 cm from the starting position). Brain response in the superior parietal lobule bilaterally, in the right dorsal premotor cortex, and in the anterior part of the right inferior parietal lobule was significantly greater during visually-guided grasping of targets located at the far distance compared to grasping of targets located near to the hand. In the absence of visual feedback, the inferior parietal lobule exhibited a greater activity during grasping of targets at the near compared to the far distance. Results suggest that in the presence of visual feedback, a visuo-motor circuit integrates visuo-motor information when targets are located farther away. Conversely in the absence of visual feedback, encoding of space may demand multisensory remapping processes, even in the case of more proximal targets

Cognitive allocentric representations of visual space shape pointing errors

Subjects reached in three-dimensional space to a set of remembered targets whose position was varied randomly from trial to trial, but always fell along a "virtual" line (line condition). Targets were presented briefly, one-by-one and in an empty visual field. After a short delay, subjects were required to point to the remembered target location. Under these conditions, the target was presented in the complete absence of allocentric visual cues as to its position in space. However, because the subjects were informed prior to the experiment that all targets would fall on a straight line, they could conceivably imagine each point target as belonging to a single rigid object with a particular geometry and orientation in space, although this virtual object was never explicitly shown to the subjects. We compared the responses to repeated measurements of each target with those measured for targets presented in a directionally neutral configuration (sphere condition), and used the variable errors to infer the putative reference frames underlying the corresponding sensorimotor transformation. Performance in the different tasks was compared under two different lighting conditions (dim light or total darkness) and two memory delays (0.5 or 5 s). The pattern of variable errors differed significantly between the sphere condition and the line condition. In the former case, the errors were always accounted for by egocentric reference frames. By contrast the errors in the line condition revealed both egocentric and allocentric components, consistent with the hypothesis that target information can be defined concurrently in both egocentric and allocentric frames of reference, resulting in two independent coexisting representations