Acoustic evoked activity in the brain in sharks
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Averaged evoked potentials (AEP's) were recorded in the medulla, cerebellum, mesencephalon and telencephalon of several species of carcharhinid and triakid sharks, in the anesthetized animal with microelectrodes in the exposed brain and in the unanesthetized animal with implanted electrodes.
A preparation is described for recording from implanted electrodes with the unanesthetized shark suspended in the water by rubber bands, subject to air- or water-borne acoustic stimuli, or electric fields or photic stimuli.
AEP's were found in each of the levels named above, to acoustic as well as to electric and photic stimuli. The responsive loci are discrete and small. The loci of best response are distinct for each of these three modalities. Anatomical localizations are given to within about one tenth of a neuromere but rarely to the microscopic level.
The form, latencies and recovery times of AEP's are given for the several levels and modalities. No interaction occurred between modalities at least with brief stimuli.
The best acoustic stimulus for AEP amplitude is a “click” with a resonance of a few hundred Hz. The best tone stimulus is a rapidly rising burst of about 300 Hz. This value may be a function of size of animal, species, and electrode position. The lowest sound pressure threshold observed was −8 dB re 1 μbar near the shark's head (=66 dB SPL), to a click delivered to the water surface. We do not know the velocity-wave amplitude, although it is believed to be the more relevant quantity.
Acoustic AEP's were markedly suppressed by background white noise or tones — best at about 100 Hz.
When sound was delivered very locally the largest AEP occurred if the sound source was directly over the parietal fossa in the dorsal midline of the head. When sound was delivered at a distance, from a larger speaker, experimental occlusion of the parietal fossa usually suppressed the acoustic AEP. We interpret this to support the view that the fossa is an important portal for sound.
In two experiments bilateral section of the VIIIth nerve twig to the macula neglecta, together with some incidental damage to the sacculus greatly reduced the acoustic AEP. This supports the view that the macula neglecta is an important concentration of acoustic receptors but does not definitely confirm that proposal. The evidence argues against any substantial role of the lateral line in these species in response to acoustic stimuli at low amplitudes.
KeywordsAcoustic Stimulus Acoustic Receptor Background White Noise Photic Stimulus Exposed Brain
Averaged evoked potential
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