Interaction of planning regions in cortex

To what extent do parietal and frontal areas involved in action planning interact as a monkey plans a movement? This report seeks an answer using the timing relationships between action potentials, local field potentials (LFPs) and behavioral events as a monkey plans reaches and eye movements to remembered targets. Both parietal reach region (PRR) and dorsal premotor cortex (PMd) show similar profiles of activity characteristic of action planning. In some cases, both premotor and intraparietal areas show decision-making activity far earlier than previously anticipated, even before the onset of the trial. However, despite their similarities in action planning, PMd responds tens of milliseconds sooner to targets and movement instructions. These results suggest that PMd precedes PRR, apparently contrary to a common heuristic about the chain of processing from sensation to action. On the other hand, during periods of steady state, as the monkey anticipates information or plans a movement, the apparent directionality of fronto-parietal interaction may reverse. Coherent phase-locking between action potentials and local field potentials (LFPs), which has been implicated in directional influence between brain regions, is highly significant from PRR to PMd, but not vice-versa. Spikes in PRR cohere with LFPs in PMd between 15-25 Hz, whereas spikes in PMd do not cohere with LFPs in PRR at any frequency. This uni-directional spike-LFP coherence varies over the course of the trial, achieving a peak in magnitude and frequency, on average, during the planning period. The phase-locking component of the coherence shows weak but significant variation according to the particular action being planned. The cross-cortical coherence also varies significantly with cortical anatomy. Coherence is stronger between spikes in PRR and LFPs in its anatomical target PMd than between PRR and other recording areas within and beyond the arcuate sulcus (associated with saccades, and not known to be connected with PRR). The asymmetry of spike-LFP coherence, its task-dependence, and variation over cortical territory add to a growing body of knowledge implicating the intraparietal sulcus as the center of a network of beta-band activity characteristic of action planning. This highly specific beta-band oscillation links frontal and parietal planning regions at the single cell level. Overall, these results suggest an interplay between premotor and parietal regions, with influence shifting back and forth according to the phase of behavior.