Eye movement control signals carried by cerebellar nucleus neuron population

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The caudal part of the fastigial nucleus, or fastigial oculomotor region (FOR), has been implicated in producing saccades of the correct size. Based on the results of previous studies, it has been hypothesized that burst activity of saccade-related FOR neurons ensures saccade accuracy by reducing the motoneuron burst for ipsiversive saccades that would otherwise overshoot a target (Fuchs et al. 1993 ; Ohtsuka and Noda 1995 ; Robinson 1993). One prediction of the hypothesis is that the horizontal end position error of ipsiversive saccades following FOR inactivation should reflect the burst activity that the FOR cells would normally exhibit. The present study tests this prediction using rhesus monkeys (Macaca mulatta). In agreement with previous reports, unilateral FOR injection resulted in hypermetria of ipsi-, and hypometria of contraversive saccades. Vertical saccades were mildly hypermetric and deviated toward the injection side. For ipsiversive movements, there was a linear relationship between horizontal end position error (overshoot) and horizontal initial retinal error (RE). For vertical saccades, horizontal end position error (deviation) was linearly correlated with vertical initial RE. For unit activity was then analyzed. The number of spikes in the burst (NumSp) for ipsiversive saccades was correlated with horizontal initial RE. For vertical saccades, there was a linear relationship between NumSp and vertical initial RE. The similar dependency of horizontal end position error and fastigial burst activity on RE supports the idea that FOR cells exhibit correction signals that maintain saccade accuracy.