Bilateral communication between vestibular labyrinths in pigeons.

Extracellular action potentials from single horizontal semicircular canal primary afferent fibers were recorded in paralysed decerebrate pigeons during pulse mechanical stimulation of the contralateral horizontal semicircular canal. Clear responses to the contralateral membranous duct displacement stimuli were observed in 51% of the tested 158 horizontal semicircular canal afferents. Generally, three different types of responses were obtained in the primary afferent fibers including excitation, inhibition, and a few complex type neural activity profiles. Inhibitory responses were of larger amplitude and had longer time constants than did excitatory responses. The few complex type responses observed were characterized by an initial excitatory discharge followed by a longer duration decrease in the fiber's firing rate. The sensitivity to stimulation and type of response obtained for each afferent was significantly correlated with the fiber's coefficient of variation value. Regular firing afferents were less sensitive and exhibited primarily excitatory responses (71%) to contralateral canal stimulation. Irregular firing afferents were more sensitive and exhibited mostly inhibitory responses (84%). The present results demonstrate that a communication network for information exchange between the bilateral labyrinths exists in pigeons. The observed responses in primary afferent fibers to contralateral horizontal semicircular canal stimulation are proposed to be mediated by the vestibular efferent system, which could provide an anatomical pathway for information exchange from vestibular receptors on opposite sides of the head.