DIRECTION-SPECIFIC DIFFERENCES IN THE MAGNITUDE OF ABDUCENS NERVE RESPONSES DURING OFF-VERTICAL AXIS ROTATION ARE A BASIC PROPERTY OF THE UTRICULO-OCULAR REFLEX IN FROGS

Abducens nerve multiunit responses were recorded in darkness from dece rebrated frogs during steps of angular velocity about an axis tilted w ith respect to the earth vertical (off-vertical axis rotation, OVAR). Thereby, a rotating gravity vector activated utricular hair cells and modulated the abducens nerve discharge sinusoidally as a function of h ead position in space. As expected, a bias velocity response component and nystagmus-related changes in neural activity were absent, since f rogs do not possess a functioning velocity storage mechanism. Response s increased as a function of the tilt angle and of the velocity and di rection of the platform rotation. OVAR in the direction of the recorde d abducens nerve (clockwise for the right and counterclockwise for the left abducens nerve) evoked significantly smaller responses than rota tion in the opposite direction. The possible origin of these direction -specific response properties was further studied after lesioning vari ous structures assumed to modify utriculo-ocular reflexes. Each of the se lesions (ipsilateral hemilabyrinthectomy, cerebellectomy, contralat eral canal nerve sections) had a specific effect on the recorded respo nse properties, but none of them, nor combinations thereof, abolished the direction-specific characteristics of the responses as long as the contralateral utricular nerve branch remained intact. Our results dem onstrate that direction-specificity is a proper ty of the basic utricu lo-ocular reflex that is independent of the velocity storage mechanism in the brainstem, of the intervestibular commissural system, of the i nhibitory control by the cerebellum and of the central convergence of utricular and horizontal canal inputs. A simple, unidirectional intera ction between central utricular neurons with adjacent functional polar ization vectors is suggested as the basic element for the observed dir ection specificity.