Reaching or pointing to targets in the environment requires a complex transformation of visual signals into purposeful muscle activations that move the arm. This has been shown to involve a large number of cortical regions. Here, we investigate how the brain performs delayed visually-guided pointing to a viewed object, addressing two major questions that have remained unanswered, 1) what areas specifically transform visual signals into motor commands suitable to drive the arm and 2) how does this transformation occur in a large cortical network on a millisecond time scale. To shed new light onto these issues, we used magnetoencephalographic (MEG) recordings and reconstructed brain activity with high spatio-temporal resolution during a delayed visually guided pointing task.
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 coordinates. Subjects waited for the fixation cross to dim (1500ms later) before making a wrist-only movement. In order to also distinguish between an intrinsic (muscle-based) and an extrinsic (spatial) motor code, we used three different forearm/wrist postures to perform the pointing. A beamformer-based spatial filtering algorithm (event-related Synthetic Aperture Magnetometry) was employed to reconstruct brain activity from the MEG recordings.
Comparing pro- and anti-trials revealed that the transformation of the early visual representation of the target from visual coordinates into extrinsic (spatial) motor coordinates occurred in the posterior parietal cortex (PPC) between 150ms and 300ms after the target/cue onset. This transformation of lateralized PPC activation involved a large dynamic network of occipital, parietal and pre-frontal areas. Investigating movement-related activity in the primary motor cortex (M1) revealed modulation of beta-band activity depending on movement direction and wrist posture. This modulation was consistent with an intrinsic motor code.
To summarize, we have characterized the spatio-temporal properties of the complete visuomotor transformation for delayed visually guided pointing. At an initial stage, visual inputs are transformed into motor goals in extrinsic coordinates in PPC. Between PPC and M1, these extrinsic motor plans are further converted into intrinsic coordinates to activate the arm muscles.

Supported by: Marie Curie Fellowships (EU), CIHR (Canada)