In-air and underwater auditory evoked cortical responses in the dolphin

Abstract

Auditory signal processing in dolphins is of special interest because these animals echolocate and possess high upper-cutoffs of hearing (∼140 kHz). The few previous measurements of auditory cortical responses (ACRs) in dolphins have been performed in air or with a restrained dolphin half submerged in water. Our goal was to record ACRs from an unrestrained dolphin fully submerged in water utilizing far-field sound stimulation. We performed a series of experiments in air and under water manipulating stimulus presentation rate and level. An increasing stimulus rate from 2 to 4 Hz showed only a nominal decrease in ACR potential; a greater decrease in ACR amplitude occurred with an 8-Hz rate. Decreasing stimulus sound pressure level (SPL) decreased the ACR in a manner consistent with auditory brainstem responses to similar SPLs. Taken together, these findings support the idea that valid cortical potentials can be obtained in air and underwater, though differences in ACR amplitude and polarity require further investigation. Brainstem responses are consistent between both environments; however, the cortical response is attenuated underwater, and a polarity reversal along the anterior-posterior axis observed in air is not present underwater.