Abstract
Planning an accurate reach involves the transformation of the neural representation of target location in sensory coordinates into a command for hand motion in motor coordinates. Although imaging techniques such as fMRI reveal the cortical topography of such transformations, and neurophysiological recordings provide local dynamics, we do not yet know the real-time dynamics of sensorimotor transformations at the whole brain level. We used high spatiotemporal resolution magnetoencephalography (MEG) during a pro-/anti-reaching task to determine (1) which brain areas are involved in transforming visual signals into appropriate motor commands for the arm, and (2) how this transformation occurs on a millisecond time scale, both within and across the regions involved. We performed time-frequency response analysis and identified 16 bilateral brain regions using adaptive hierarchical clustering (Alikhanian et al. 2013). We then computed sensory, motor, and sensorimotor indices for direction coding based on hemispherically lateralized de/synchronization in the α (7-15Hz) and β (15-35Hz) bands. Importantly, we found a visuomotor progression both within and across these areas, from pure sensory codes in some ‘early’ areas (V1/2, V3/3a and SPL), to a temporal transition from sensory to motor coding in the majority of parietal-frontal areas (SPOC, AG, POJ, mIPS, VIP, IPL, STS, S1, M1, SMA, PMd and FEF), to a pure motor code (PMv only), in both the α and β bands. Further, the timing of these transformations revealed a pro/anti cue influence that proceeded from frontal to posterior cortex. These data directly demonstrate a progressive, real-time transformation both within and across the entire occipital-parietal-frontal reach network that follows specific rules of spatial distribution and temporal order.