Spatial characteristics of neurons in the central mesencephalic reticular formation (cMRF) of head-unrestrained monkeys.

Prior studies of the central portion of the mesencephalic reticular formation (cMRF) have shown that in head-restrained monkeys, neurons discharge prior to saccades. Here, we provide a systematic analysis of the patterns of activity in cMRF neurons during head unrestrained gaze shifts. Two types of cMRF neurons were found: presaccadic neurons began to discharge before the onset of gaze movements, while postsaccadic neurons began to discharge after gaze shift onset and typically after the end of the gaze shift. Presaccadic neuronal responses were well correlated with gaze movements, while the discharge of postsaccadic neurons was more closely associated with head movements. The activity of presaccadic neurons was organized into gaze movement fields, while the activity of postsaccadic neurons was better organized into movement fields associated with head displacement. We found that cMRF neurons displayed both open and closed movement field responses. Neurons with closed movement fields discharged before a specific set of gaze (presaccadic) or head (postsaccadic) movement amplitudes and directions and had a clear distal boundary. Neurons with open movement fields discharged for gaze or head movements of a specific direction and also for movement amplitudes up to the limit of measurement (70 degrees). A subset of open movement field neurons displayed an increased discharge with increased gaze shift amplitudes, similar to pontine burst neurons, and were called monotonically increasing open movement field neurons. In contrast, neurons with non-monotonically open movement fields demonstrated activity for all gaze shift amplitudes, but their activity reached a plateau or declined gradually for gaze shifts beyond specific amplitudes. We suggest that presaccadic neurons with open movement fields participate in a descending pathway providing gaze signals to medium-lead burst neurons in the paramedian pontine reticular formation, while presaccadic closed movement field neurons may participate in feedback to the superior colliculus. The previously unrecognized group of postsaccadic cMRF neurons may provide signals of head position or velocity to the thalamus, cerebellum, or spinal cord.