Distributed sensitivity for movement amplitude in directionally tuned neuronal populations.

Neurons in macaque primary motor cortex and dorsal premotor cortex are tuned to movement direction. In humans, neuronal populations tuned to movement direction have recently been described using multivoxel pattern analysis and functional magnetic resonance imaging adaptation. It is unclear, however, to what extent directionally tuned neuronal populations are sensitive to movement amplitude. Here we used functional magnetic resonance imaging adaptation to determine whether directionally tuned neuronal populations are modulated by movement amplitude. In different blocks, participants were adapted to small- or large-amplitude hand-reaching movements. On occasional test trials, we parametrically varied the angular difference between adaptation and test direction and the congruency between adapted and tested amplitude (same or different). We predicted that the blood oxygen level-dependent signal in directionally tuned regions should be adapted in proportion to the angular difference between adaptation and test direction. Directionally tuned regions insensitive to movement amplitude should show a transfer of adaptation from the adapted to the nonadapted amplitude. In contrast, regions sensitive to the specific combination of movement direction and amplitude should show directional tuning only for the adapted amplitude. We identified a network of parietal and frontal regions tuned to movement direction. We found that parietal areas contain neuronal populations sensitive to specific combinations of movement direction and amplitude, while frontal areas show transfer from the adapted to the nonadapted amplitude during small-amplitude movements after adaptation to large amplitude, but not vice versa. Our results thus imply different processing of movement amplitude in directionally tuned frontal and parietal areas.