ABSTRACT
Experiments examined differential coding of acoustic particle motion axis in the auditory midbrain of goldfish. Animals were exposed to vibratory stimuli varying in axis orientation as action potentials were recorded from single units in the central neuropil of nucleus centralis in the torus semicircularis. Response magnitudes as a function of stimulation axis were visualized in three dimensional plots called directional response profiles. These are generally comparable to directional responses observed among primary saccular afferents in having substantially vertical orientations. Distortions in shape from the peripheral patterns indicate neural information processing. A three-dimensional model was used to evaluate the hypothesis that responses in the auditory midbrain reflect the convergence of excitatory and inhibitory primary afferent-like responses. Model afferent inputs were generated and combined arithmetically. This analysis gives insight into the mechanisms of information processing that appear to occur in brainstem nuclei. The lack of diversity in best axis directions suggests that this mechanism alone cannot account for directional hearing abilities in this species. The roles that this directional representation and processing may play in directional hearing and sound source localization are not yet clear. Implications of these data on current models of fish directional hearing are discussed.