Deficits in Orienting Gaze Shifts during Muscimol Inactivation of the
Caudal Fastigial Nucleus in the Cat. II. Movement Dynamics and Eye-Head
Coordination.
Laurent Goffart, Denis P‚lisson and Alain Guillaume.
Vision et Motricit‚, INSERM U 94, 16 avenue Doyen L‚pine, 69500 Bron
(France).
APStracts 4:363N, 1997.
ABSTRACT
We have shown in the companion paper that muscimol injection in the caudal
part of the fastigial nucleus (cFN) consistently leads to dysmetria of
visually-triggered gaze shifts that depends on movement direction. Based on
the observations of a constant error and misdirected movements toward the
inactivated side, we have proposed that the cFN contributes to the
specification of the goal of the impending ipsiversive gaze shift. In order to
test this hypothesis and also to better define the nature of the hypometria
that affects contraversive gaze shifts, we report in this paper on various
aspects of movement dynamics and of eye/head coordination patterns.
Unilateral muscimol injection in cFN leads to a slight modification in the
dynamics of both ipsiversive and contraversive gaze shifts (average velocity
decrease = 55 deg/s). This slowing in gaze displacements results from changes
in both eye and head. In some experiments, a larger gaze velocity decrease is
observed for ipsiversive gaze shifts as compared to contraversive ones and
this change is restricted to the deceleration phase. For two particular
experiments testing the effect of visual feedback, we have observed a dramatic
decrease in the velocity of ipsiversive gaze shifts after the animal 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 in 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 ocular and cephalic
components. As a global result, the relative contribution of eye and head to
the amplitude of ipsiversive gaze shifts remains statistically
indistinguishable from that of control responses, and a small (1.6 deg)
increase in the head contribution to contraversive gaze shifts is found. The
delay between eye and head movement onsets is increased by 7.3 +/- 7.4 ms for
contraversive and decreased by 8.3 +/- 10.1 ms for ipsiversive gaze shifts,
corresponding respectively to an increased or decreased lead time of head
movement initiation.
The modest changes in gaze dynamics, the absence of a link between eventual
dynamics changes and dysmetria, and a similar pattern of eye-head coordination
to that of control responses, altogether are compatible with the hypothesis
that the hypermetria of ipsiversive gaze shifts results from an impaired
specification of the metrics of the impending gaze shift. Regarding
contraversive gaze shifts, the weak changes in head contribution do not seem
to reflect a pathological coordination between eye and head but would rather
result from the tonic deviations of gaze and head toward the inactivated side.
Hence, our data suggest that the hypometria of contraversive gaze shifts might
also largely result from an alteration of processes that specify the goal,
rather than the on-going trajectory, of saccadic gaze shifts.
Received 18 February 1997; accepted in final form 8 December 1997.
APS Manuscript Number J145-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997