Acoustically evoked potentials in midbrain auditory structures in sea lions (Pinnipedia)

Summary Twelve sea lions (Zalophus californianus) and one harbor seal (Phoca vitulina) were examined by recording evoked potentials in response to sound from the inferior colliculus and adjacent structures, under barbiturate or after implanting and coming out of anesthesia. Results were similar in air and under water.The averaged response evoked by a sharply rising tone consists of early, brief peaks and later, slow waves (Fig. 1). The latency of the earliest deflection is 3.5 to 4.8 ms from the moment of arrival of a sound pip at the ear. The potential increases in size with sound intensity approximately as a power function, over a dynamic range of 60–70 db (Fig. 2). Masking is qualitatively similar to that in common laboratory species.The properties of the midbrain response are strikingly different from those in porpoises, reported elsewhere. The pinniped is not so specialized for extremely short duration, fast rise time, sounds or for rapid recovery or ultrasonic frequencies (Figs. 3, 4, 7, 8). Evoked potentials fail to show response above 30–35 kHz at 100 db SPL; best frequency is about 4–6 kHz (Figs. 5, 6). Threshold by this method is about 20 db SPL in air. Frequency modulated tones are markedly more effective in some loci but less so than in porpoises under water. The receptive field is essentially total and directionality weak, in contrast with porpoises.Physiological results cannot settle the question whether echolocation is employed but they can indicate lack of high specialization for the types of sounds bats and porpoises use.This study was aided by the Office of Naval Research, the Air Force Office of Scientific Research, the National Science Foundation, and the National Institute of Neurological Diseases and Stroke through grants to T. H. Bullock, and by the Naval Undersea Research and Development Center through S. H. Ridgway.The assistance of R. F. Green in developing anatomical landmarks and that of R. H. Hamstra, Jr. in electronic problems was especially valuable. The staffs of both laboratories were the essence of cooperation.