Chapter 35 Immediate saccadic substitution for deficits in dynamic vestibular reflexes of frogs with selective peripheral lesions

Abstract

Immediately after the section of one horizontal canal nerve compensatory head movements in the horizontal plane are unidirectional in the dark but symmetrical in amplitude in the light. Responses towards the ‘weak’ (intact) side are saccadic. These head saccades are accompanied by smaller ocular quick phases in the opposite direction. Both head saccades and ocular quick phases are still present even after a bilateral section of the horizontal canal nerves or after removal of both labyrinths. The main sequences of eye-head quick phases and of head saccades are very similar in intact and bilaterally lesioned frogs, indicating that vestibulo-ocular and vestibulocollic reflexes must be suppressed during these fast head movements. The occurrence of these head saccades depends on head-velocity-related proprioceptive signals. This input is facilitated after section of the horizontal canal nerve. Head saccades supplement weak but unaltered optokinetic reflexes at higher frequencies to an extent that compensatory head movements in response to sinusoidal table oscillations are symmetrical in spite of acute unilateral reflex deficits. In some of the chronic hemilabyrinthectomized frogs the vestibulocollic reflexes recovered to an extent that the head slow phase was fast enough to trigger head saccades even in the dark.