Preparatory neuronal activity for reaching: movement planning, target location, and attentional signals converge in macaque medial posterior parietal cortex

摘要

Posterior parietal cortex (PPC) in humans and monkeys contains many reach-related areas, and V6A is one of them. Although several fMRI studies in humans found reach planning activations in regions putatively homolog to monkey V6A (Galati et al., 2011; Gallivan et al., 2011; Bernier et al., 2012), the involvement of this area in encoding reach intentions has never been explicitly demonstrated. Here, we addressed this issue by comparing the activity of single cells during the instructed delay period in a reaching and a fixation task, performed in separate blocks. In these tasks, the animals were fixating targets arranged in 3D space and waited for a Go cue to reach (reaching task), or release a button (fixation task). We found different types of cells: cells modulated by the fixation of a target regardless of whether it was reached or not (25%), cells modulated only by reaching preparation (17%), and cells influenced by both signals (44%). Reach preparation cells often showed a congruent spatial tuning during movement execution. Most cells showed lower delay activity before reaches compared to before button releases, whereas a minority showed the opposite effect. The timing of excitation, but not that of inhibition, was time-locked to reach onset. We propose that inhibition could derive from changes of attention/alertness associated with preparing a reaching movement, whereas excitation reflects reach planning. In agreement with human fMRI studies, cells involved in reach planning are less in V6A than in nearby parietal reach region (PRR), suggesting common neuronal mechanisms for reach planning in monkey and human medial PPC. In V6A, reach planning signals coexist with target location and attentional signals. We propose that before reaches, while PRR encodes the intention to reach, V6A is more involved in processing various inputs to localize reach targets in the 3D space.