Abstract
Elasmobranchs are sensitive to low frequency sound, and electrophysiological studies have demonstrated acoustic responses from the ear. Five primary projections from the ear to the medulla have been found, but individually they could not be identified with either the auditory or the equilibrium modality since they originate in a mixed nerve. Metabolic mapping in the brain of the thornback guitarfish with [14C]2-deoxyglucose autoradiography and acoustic stimulation provided tentative identifications of acoustic centers in cell plate X of the medial octavolateralis nucleus, the anterior octaval nucleus, the nucleus of the lateral lemniscus, and the ventromedial division of the lateral mesencephalic nucleus. Evoked potential recordings confirmed acoustic activity in those sites and additionally in the reticular formation, and the lateral granule cell mass of the auricle. In tests in the mesencephalon the evoked potential disappeared within the range of elasmobranch behavioral thresholds and when the eighth nerves were cut, but was not changed when all 4 lateral line nerves were cut. The identified acoustic centers resemble those found in auditory lemniscal pathways in mammals and other tetrapods, but the most recent ancestor common to elasmobranchs and tetrapods lived 400 million years ago. Therefore, a basic auditory lemniscal pathway may be a longstanding feature of the vertebrate brain.
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