Human ‘parietal reach region’ encodes visual stimulus coordinates, not movement direction, during reversing prism adaptation

Abstract

Previous investigations suggest that posterior parietal cortex, in particular the ‘parietal reach region’ (PRR) encodes reach-related activity in gaze-centered coordinates (Batista et al., Science, 1999; Medendorp et al., J. Neurosci., 2003). However it is not known if PRR encodes the visual direction of the goal relative to gaze, or the direction of movement relative to gaze. To dissociate these two variables and correlate them with cortical activation, we imaged subjects using 4-T fMRI with a ‘surface coil’ placed over the posterior cortex while they pointed to remembered leftward and rightward targets; before, during, and after adaptation to left-right reversing prisms. 6 subjects were tested using a block design in which they first fixated and pointed to a central target, and then a pointing target was briefly flashed, followed by a 2.5s memory delay with a visual mask, and finally the pointing movement. 9 subjects were also tested in an event-related paradigm with the memory interval extended to 10 seconds. The control task revealed a network of cortical areas putatively labeled V2/V3, V7, LIP (Lateral Intraparietal Area), and PRR that were more active for contralateral than ipsilateral pointing. After prism adaptation, this reversed, so that lateralized activation in all these areas stayed linked to visual input and was dissociated from motor output. Another area in the supramarginal gyrus showed the opposite motor (or world) -fixed pattern of activation. Event-related recordings showed that PRR held a sustained contralateral activation during the memory interval leading up to pointing, which again reversed following prism adaptation. Apparently, the learned transformation from visual to motor coordinates in the reversing prism task was implimented ‘downstream’ from PRR. These data show that PRR does not encode the kinematics of movement direction, but rather the goal of the intended movement in visual coordinates.

Footnotes

 Canadian Institutes for Health Research