Neurons in the central mesencephalic reticular formation related to the control of eye and head movements

The central mesencephalic reticular formation (cMRF), a structure known to participate in the control of gaze movements, has been hypothesized to play a role in the conversion of the spatial signals encoding desired movement into the temporal signals used to drive the eye and head movements. Single neurons were recorded extracellularly from the cMRF of four alert rhesus monkeys performing head unrestrained gaze shifts to visual targets. Two populations of neurons are described. The first discharged \u3c25 ms before the saccadic component of gaze shifts, predominantly to the side contralateral to the recording. They demonstrated widely-tuned movement fields and most discharged before the horizontal component of contralateral gaze shifts of all amplitudes. Their discharge was well correlated with gaze shift duration and velocity. These neurons have been hypothesized to participate in the conversion of the spatially encoded target selection signal from the superior colliculus into a temporal signal required by pre-motor neurons in the pons. A computational simulation of this hypothesis was able to replicate the physiological data and supports this hypothesis. In contrast, a distinct population of cMRF neurons discharged after gaze shift onset. These neurons discharged during the head component of gaze shifts, with their peak discharge occurring near the time of peak head velocity. Their discharge was well correlated with head movement amplitude, but not head velocity or acceleration. There was no correlation between their discharge and the saccadic component of the gaze movement. These neurons are hypothesized to play a role in stopping head movements following rapid gaze shifts. Such a population of neurons has not previously been described in the primate. As a whole, the cMRF plays a critical role in gaze control and a model of its overall function, based on previous reports and the physiological data presented here, is proposed.