Effect of single-pulse TMS over Dorsal Posterior Parietal Cortex in memory-guided pointing before and after reversing prism adaptation

Michael Vesia^1,3*, Gunnar Blohm ¹, Jachin A. Monteon ¹, Lauren E. Sergio ^1,2,3, J. D. Crawford ^1,2,3

Centre for Vision Research ¹ and Departments of, Psychology ², and Kinesiology and Health Science ³, York University, Toronto, Canada.

The planning and execution of a simple visually-guided movement requires a series of visuomotor transformations by the nervous system. Specifically, the brain initially codes the target in eye-fixed retinal coordinates, but the final output command is coded in muscle coordinates.  Previous studies suggest that the posterior parietal cortex (PPC) encodes reach-related activity in gaze-centered coordinates (Batista et al., Science, 1999; Medendorp et al., J. Neurosci., 2003). Moreover, it has recently been suggested that following reversing prism adaptation, PPC activity remains tied to the reversed visual input rather than the direction of motor output (Fernandez-Ruiz et al. Soc. Neurosci. Abst. 2004).  To further examine this question, we applied transcranial magnetic stimulation (TMS) over the right dorsal PPC (P4 in EEG coordinate system) in 3 subjects during the memory interval between subjects seeing and pointing toward targets presented 5 deg to the left or right of a central fixation target.  A Magstim 200 stimulator (55% output) was used to deliver single magnetic pulses through a figure-eight coil. After 20 trials (left, right, with and without TMS), subjects underwent adaptation in the same task while wearing left-right Dove reversing prisms.  When subjects obtained 10 correct trials in a row, an equal block of TMS/non-TMS trials were recorded.  The TMS pulse produced a significant leftward (contralateral) shift for both the leftward and rightward pointing target. After prism adaptation, the pointing direction for a given remembered target reversed, but the direction of TMS shift did not.  These results suggest that a brief TMS pulse modifies the output of the PPC in motor coordinates (perhaps through the unadapted pathway), rather than modifying the visual coordinates of the memory representation upstream from the adapted visual-motor reversal.

Supported by: CIHR