Dynamic properties of eye position coded neurons in the alert monkey during saccades

Single units in the regions of the III, IV and VI nuclei were recorded together with EOG's for horizontal and vertical eye positions in alert macaques. The sequential analysis for several dynamic parameters of the activity patterns in correlation to the saccade velocity for saccades in the on-direction leads to the results that: 1. eye position coded neurons can clearly be separated into two main classes [early peak (EP) and late peak (LP] by means of their activity patterns during saccades in the on-direction; 2. the maximum impulse rate of EP neurons shows a better correlation with saccade velocity than the difference between maximum and initial impulse rate while the opposite is valid for LP neurons. EP neurons are likely to be motoneurons which initiate saccadic eye movements whereas LP neurons are too slow for this task because they reach their maximum impulse rate after half the saccadic time. The dynamic properties of LP neurons have several features similar to those of primary stretch receptors during ramp-like stretches. The possible influence of fusimotor activity on the oculomotor system is discussed. The fact that the relationship between dynamic index and saccade velocity shows subgroups of data supports the assumption that the state of alertness changes instantaneously in untrained monkeys.