Neuronal activity in prepositus nucleus correlated with eye movement in the alert cat.

1. In nine alert chronically prepared cats the activity of 177 neurons was recorded in the prepositus nucleus during either spontaneous eye movement or that induced by natural vestibular and optokinetic stimulation. 2. In 116 cells, eye position and/or eye velocity was precisely and unequivocally encoded whatever the origin of the eye movement. These cells were separated into different populations according to the eye movement variable encoded and the directionality of the neuronal response. The firing rates of the remaining 61 cells were loosely related to eye movements because a variety of discharge patterns were observed during identical eye movements. In the latter case, some other unmeasured variable (e.g., neck or visual) was suggested to be encoded in the firing frequency. 3. Discharge rate changed before the eyes began to move and reached a new steady level during fixation following a saccade into a particular direction of the orbit. The ondirection was horizontal for 59% of the neurons, vertical for 17%, and oblique for 24%. 4. Regardless of their preferred direction, the discharge rate in 19% of the neurons was closely proportional to eye position. The range in sensitivity was from 1.1 to 7.5 spikes X s-1/deg. Weak velocity responses were occasionally observed during the slow phase of vestibular and optokinetic nystagmus including during saccades. This class of neurons exhibited a very regular interspike interval for a given position of fixation. Since mainly eye position was encoded, these cells were called position neurons. 5. Other prepositus neurons showed both position and velocity sensitivity during saccades and fixation. Their firing rate encoded eye position over the same range as above and also coded velocity during the slow phase of vestibular and optokinetic nystagmus. Depending on the weighting between the position and velocity proportionality constants, these neurons were classified into position-velocity (48%) or velocity-position (33%) groups. 6. The distribution of the above responses led to the conclusion that the prepositus nucleus plays a role in vertical and horizontal spatial integration. The predominance of horizontal activity suggested that the nucleus may be a significant site underlying genesis of horizontal eye position.