ORIENTING GAZE SHIFTS DURING MUSCIMOL INACTIVATION OF CAUDAL FASTIGIAL NUCLEUS IN THE CAT - II - DYNAMICS AND EYE-HEAD COUPLING

We have shown in the companion paper that muscimol injection in the ca udal part of the fastigial nucleus (cFN) consistently leads to dysmetr ia of visually triggered gaze shifts that depends on movement directio n. Based on the observations of a constant error and misdirected movem ents toward the inactivated side, we have proposed that the cFN contri butes to the specification of the goal of the impending ipsiversive ga ze shift. To test this hypothesis and also to better define the nature of the hypometria that affects contraversive gaze shifts, we report i n this paper on various aspects of movement dynamics and of eye/head c oordination patterns. Unilateral muscimol injection in cFN leads to a slight modification in the dynamics of both ipsiversive and contravers ive gaze shifts (average velocity decrease = 55 degrees/s). This slowi ng in gaze displacements results from changes in both eye and head. In some experiments, a larger gaze velocity decrease is observed for ips iversive gaze shifts as compared with contraversive ones, and this cha nge is restricted to the deceleration phase. For two particular experi ments testing the effect of visual feedback, we have observed a dramat ic decrease in the velocity of ipsiversive gaze shifts after the anima l had received visual information about its inaccurate gaze responses; but virtually no change in hypermetria was noted. These observations suggest that there is no obvious causal relationship between changes i n dynamics and in accuracy of gaze shifts after muscimol injection in the cFN. Eye and head both contribute to the dysmetria of gaze. Indeed , muscimol injection leads to parallel changes in amplitude of both oc ular and cephalic components. As a global result, the relative contrib ution of eye and head to the amplitude of ipsiversive gaze shifts rema ins statistically indistinguishable from that of control responses, an d a small (1.6 degrees) increase in the head contribution to contraver sive gaze shifts is found. The delay between eye and head movement ons ets is increased by 7.3 +/- 7.4 ms for contraversive and decreased by 8.3 +/- 10.1 ms for ipsiversive gaze shifts, corresponding respectivel y to an increased or decreased lead time of head movement initiation. The modest changes in gaze dynamics, the absence of a link between eve ntual dynamics changes and dysmetria, and a similar pattern of eye-hea d coordination to that of control responses, altogether are compatible with the hypothesis that the hypermetria of ipsiversive gaze shifts r esults from an impaired specification of the metrics of the impending gaze shift. Regarding contraversive gaze shifts, the weak changes in h ead contribution do not seem to reflect a pathological coordination be tween eye and head but would rather result from the tonic deviations o f gaze and head toward the inactivated side. Hence, our data suggest t hat the hypometria of contraversive gaze shifts also might result larg ely from an alteration of processes that specify the goal rather than the on-going trajectory, of saccadic gaze shifts.