Head-unrestrained gaze shifts after muscimol injection in the caudal fastigial nucleus of the monkey.

The effects of unilateral cFN inactivation on horizontal and vertical gaze shifts generated from a central target toward peripheral ones were tested in two head unrestrained monkeys. After muscimol injection, the eye component was hypermetric during ipsilesional gaze shifts, hypometric during contralesional ones and deviated toward the injected side during vertical gaze shifts. The ipsilesional gaze hypermetria increased with target eccentricity until approximately 24 degrees after which it diminished and became smaller than the hypermetria of the eye component. Contrary to eye saccades, the amplitude and peak velocity of which were enhanced, the amplitude and peak velocity of head movements were reduced during ipsilesional gaze shifts. These changes in head movement were not correlated with those affecting the eye saccades. Head movements were also delayed relative to the onset of eye saccades. The alterations in head movement and the faster eye saccades likely explained the reduced head contribution to the amplitude of ipsilesional gaze shifts. The contralesional gaze hypometria increased with target eccentricity and was associated with uncorrelated reductions in eye and head peak velocities. When compared with control movements of similar amplitude, contralesional eye saccades had lower peak velocity and longer duration. This slowing likely accounted for the increase in head contribution to the amplitude of contralesional gaze shifts. These data suggest different pathways for the fastigial control of eye and head components during gaze shifts. Saccade dysmetria was not compensated by appropriate changes in head contribution, raising the issue of the feedback control of movement accuracy during combined eye-head gaze shifts.