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 l of 0.05% muscimol injection into 26 INC

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... . 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). Abrahams VC, Richmond F, Rose PK. Proceedings: proprioceptive and retinal afferent projections to the superior colliculus of the cat and their connexions to the tectospinal tract. J Physiol 241: 101P-102P, 1974. Anastasopoulos D, Nasios G, Psilas K, Mergner T, Maurer C, Lucking CH. What is straight ahead to a patient with torticollis? Brain 121: 91-101, 1998. Anderson JH. Ocular torsion in the cat after lesions of the interstitial nucleus of Cajal. Ann NY Acad Sci 374: 865-871, 1981. Anderson JH, Precht W, Pappas C. Changes in the vertical vestibulo-ocular reflex due to kainic acid lesions of the interstitial nucleus of Cajal. Neurosci Lett 14: 259 -264, 1979. Averbuch-Heller L, Rottach KG, Zivotofsky AZ, Suarez JI, Pettee AD, Remler BF, Leigh RJ. Torsional eye movements in patients with skew deviation and spasmodic torticollis: responses to static and dynamic head roll. Neurology 48: 506 -514, 1997. Baarsma EA, Collewijn H. Eye movements due to linear accelerations in the rabbit. J Physiol 245: 227-247, 1975. Bertrand C, Molina-Negro P, Martinez SN. Combined stereotactic and peripheral surgical approach for spasmodic torticollis. Appl Neurophysiol 41: 122-133, 1978. Bevington PR. Data Reduction and Error Analysis for the Physical Sciences. New York: McGraw-Hill, 1969. Bhidayasiri R, Plant GT, Leigh RJ. A hypothetical scheme for the brainstem control of vertical gaze. Neurology 54: 1985-1993, 2000. Blazquez PM, Thaera NA, Highstein SM. Distributed vertical integrator function suggested by the response of cells in the posterior part of the SVN.