Z. vergl. Physiologic 74, 372-387 (1971) 9 by Springer-Verlag 1971 Acoustically Evoked Potentials in Midbrain Auditory Structures in Sea Lions (Pinnipedia) THEODORE H. BULLOCK, SAM I-I. RIDGWAY and NOBUO SUGA* Department of Neuroseiences, School of Medicine, Neurobiology Unit, Scripps Institution of Oceanography, Universitiy of California, San Diego and Naval Undersea Research and Development Center, San Diego, California Received June 22, 1971 Summary. Twelve sea lions (Zalophus cali/ornianus) 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 e~rliest 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 pimliped 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 SFL in air. Frequency modulated tones are markedly more effective in some loci but less so than in porpoises under water. The receptive field is essen- tially total and directionality weak, in contrast with porpoises. Physiological results cannot settle the question whether echolocation is em- ployed but they can indicate lack of high specialization for the types of sounds bats and porpoises use. Introduction Besides the interest in comparing pinnipeds, which belong to the order Pinnipedia, closely related to the Carnivora, with porpoises of the order Cetacea (Bullock et al., 1968; Bullock and Ridgway, 1971a, b), there are special reasons for examing the central and in particular the midbrain auditory responses in these animals. Although echolocation is a well accepted phenomenon in porpoises, controversy exists in recent * Present address: Department of Biology, Washington University, St. Louis, Missouri.