Early coding of reaching in the parietooccipital cortex

Neural activity was recorded in the parietooccipital cortex while monkeys p erformed different tasks aimed at investigating visuomotor interactions of retinal, eye, and arm-related signals on neural activity. The tasks were ar m reaching I) to foveated targets; 2) to extrafoveal targets, with constant eye position; 3) within an instructed-delayed paradigm, under both light a nd darkness; 4) saccadic eye movements toward, and static eye holding on pe ripheral targets; and 5) visual fixation and stimulation. The activity of m any cells was modulated during arm reaction (68%) and movement time (58%), and during static holding of the arm in space (64%), when eye position was kept constant. Eye position influenced the activity of many cells during ha nd reaction (45%) and movement time (51%) and holding of hand static positi on (69%). Many cells (56%) were also modulated during preparation for hand movement, in the delayed reach task. Modulation was present also in the dar k in 59% of cells during this epoch, 51% during reaction and movement time, and 48% during eye/hand holding on the target. Cells (50%) displaying ligh t-dark differences of activity were considered as related to the sight and monitoring of hand motion and/or position in the visual field. Saccadic eye movements modulated a smaller percentage (25%) of cells than eye position (68%). Visual receptive fields were mapped in 44% of the cells studied. The y were generally large and extended to the periphery of the tested (30 degr ees) visual field. Sixty-six percent of cells were motion sensitive. Theref ore the activity of many neurons in this area reflects the combined influen ce of visual, eye, and arm movement-related signals. For most neurons, the orientation of the preferred directions computed across different epochs an d tasks, therefore expression of all different eye- and hand-related activi ty types, clustered within a limited sector of space, the field of global t uning. These spatial fields might be an ideal frame to combine eye and hand signals, thanks to the congruence of their tuning properties. The relation ships between cell activity and oculomotor and visuo-manual behavior were t ask dependent. During saccades, most cells were recruited when the eye move d to a spatial location that was also target for hand movement, whereas dur ing hand movement most cells fired depending on whether or not the animal h ad prior knowledge about the location of the visual targets.