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Gunnar Blohm, William C. Gaetz, Herbert C. Goltz, Joseph F. X. DeSouza, Sonya Bells, Douglas O. Cheyne, J. Douglas Crawford; Cortical oscillations in human posterior parietal cortex during visually-guided reach planning. Journal of Vision 2009;9(8):1150. doi: 10.1167/9.8.1150.
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Planning reaching or pointing movements requires a number of processing steps involving different brain areas. One important step consists of transforming visual information into motor plans that are appropriate for movement control. A key brain region involved in this process is the posterior parietal cortex (PPC). Here we use magnetoencephalography (MEG) to investigate the spatio-temporal coding of PPC during reach planning on a millisecond time scale.
Human subjects sat upright, fixating a central white cross. After 500ms, a green or red dot was briefly presented right or left of fixation. The color of the dot indicated the task, i.e. to point towards (pro) or to the mirror opposite location (anti) of the target. Pro- and anti-trials required opposite motor output following identical visual stimulation, which allowed distinction between visual and motor coding. Subjects waited for the fixation cross to dim (1500ms later) before making a wrist-only movement. We used three different forearm/wrist postures and the left or right hand (in separate blocks of trials) for pointing. A beamformer-based spatial filtering algorithm was employed to reconstruct brain activity from the MEG recordings.
Comparing pro- and anti-trials revealed that PPC coded visual target location in retinal coordinates until ∼150ms after target onset. Between 150–300ms after target onset, a transformation of the early representation of the target from visual coordinates into extrinsic (spatial) motor coordinates occurred in PPC. We also observed posture- and hand-dependency of the activity in PPC. Finally, we observed PPC activation before and during movement execution, indicating that PPC also plays a role in visuomotor memory and/or online motor execution.
In summary, our results indicate that PPC is involved in a dynamical network transforming visual signals into representations that are appropriate for the required motor output.
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