CONTRIBUTION OF THE ROSTRAL FASTIGIAL NUCLEUS TO THE CONTROL OF ORIENTING GAZE SHIFTS IN THE HEAD-UNRESTRAINED CAT

The implication of the caudal part of the fastigial nucleus (cFN) in t he control of saccadic shifts of the visual axis is now well establish ed. In contrast a possible involvement of the rostral part of the fast igial nucleus (rFN) remains unknown. In the current study we investiga ted in the head-unrestrained cat the contribution of the rFN to the co ntrol of visually triggered saccadic gaze shifts by measuring the defi cits after unilateral muscimol injection in the rFN. A typical gaze dy smetria was observed: gaze saccades directed toward the inactivated si de were hypermetric, whereas those with an opposite direction were hyp ometric. For both movement directions, gaze dysmetria was proportional to target retinal eccentricity and could be described as a modified g ain in the translation of visual signals into eye and head motor comma nds. Correction saccades were triggered when the target remained visib le and reduced the gaze fixation error to 2.7 +/- 1.3 degrees (mean +/ - SD) on average. The hypermetria of ipsiversive gaze shifts resulted predominantly from a hypermetric response of the eyes, whereas the hyp ometria of contraversive gaze shifts resulted from hypometric response s of both eye and head. However, even in this latter case, the eye sac cade was more affected than the motion of the head. As a consequence, for both directions of gaze shift the relative contributions of the ey e and head to the overall gaze displacement were altered by muscimol i njection. This was revealed by a decreased contribution of the head fo r ipsiversive gaze shifts and an increased head contribution for contr aversive movements. These modifications were associated with slight ch anges in the delay between eye and head movement onsets. Inactivation of the rFN also affected the initiation of eye and head movements. ind eed, the latency of ipsiversive gaze and head movements decreased to 8 8 and 92% of normal, respectively, whereas the latency of contraversiv e ones increased to 149 and 145%. The deficits induced by rFN inactiva tion were then compared with those obtained after muscimol injection i n the cFN of the same animals. Several deficits differed according to the site of injection within the fastigial nucleus (tonic orbital eye rotation, hypermetria of ipsiversive gaze shifts and fixation offset, relationship between dysmetria and latency of contraversive gaze shift s, postural deficit). Tn conclusion, the present study demonstrates th at the rFN is involved in the initiation and the control of combined e ye-head gaze shifts. In addition our findings support a functional dis tinction between the rFN and cFN for the control of orienting gaze shi fts. This distinction is discussed with respect to the segregated fast igiofugal projections arising from the rFN and cFN.