Three-dimensional eye-head coordination after injection of muscimol into the interstitial nucleus of Cajal (INC).

The interstitial nucleus of Cajal (INC) is thought to be the "neural integrator" for torsional/vertical eye position and head posture. Here, we investigated the coordination of eye and head movements after reversible INC inactivation. Three-dimensional (3-D) eye-head movements were recorded in three head-unrestrained monkeys using search coils. INC sites were identified by unit recording/electrical stimulation and then reversibly inactivated by 0.3 mul of 0.05% muscimol injection into 26 INC sites. After muscimol injection, the eye and head 1) began to drift (an inability to maintain stable fixation) torsionally: clockwise (CW)/counterclockwise (CCW) after left/right INC inactivation respectively. 2) The eye and head tilted torsionally CW/CCW after left/right INC inactivation, respectively. Horizontal gaze/head drifts were inconsistently present and did not result in considerable position offsets. Vertical eye drift was dependent on both vertical eye position and the magnitude of the previous vertical saccade, as in head-fixed condition. This correlation was smaller for gaze and head drift, suggesting that the gaze and head deficits could not be explained by a first-order integrator model. Ocular counterroll (OC) was completely disrupted. The gain of torsional vestibuloocular reflex (VOR) during spontaneous eye and head movements was reduced by 22% in both CW/CCW directions after either left or right INC inactivation. Our results suggest a complex interdependence of eye and head deficits after INC inactivation during fixation, gaze shifts, and VOR. Some of our results resemble the symptoms of spasmodic torticollis (ST).