Non-auditory, electrophysiological potentials preceding dolphin biosonar click production

Abstract

The auditory brainstem response to a dolphin’s own emitted biosonar click can be measured by averaging epochs of the instantaneous electroencephalogram (EEG) that are time-locked to the emitted click. In this study, averaged EEGs were measured using surface electrodes placed on the head in six different configurations while dolphins performed an echolocation task. Simultaneously, biosonar click emissions were measured using contact hydrophones on the melon and a hydrophone in the farfield. The averaged EEGs revealed an electrophysiological potential (the pre-auditory wave, PAW) that preceded the production of each biosonar click. The largest PAW amplitudes occurred with the non-inverting electrode just right of the midline—the apparent side of biosonar click generation—and posterior of the blowhole. Although the source of the PAW is unknown, the temporal and spatial properties rule out an auditory source. The PAW may be a neural or myogenic potential associated with click production; however, it is not known if muscles within the dolphin nasal system can be actuated at the high rates reported for dolphin click production, or if sufficiently coordinated and fast motor endplates of nasal muscles exist to produce a PAW detectable with surface electrodes.