Research: Evoked Potential Audiometry (EPA)
Currently, very little is known about the hearing sensitivity of most marine mammals, yet the impact of sound on marine mammals in the wild is of increasing concern. Efforts to develop safe exposure guidelines to protect dolphins and whales from harmful exposure to underwater sounds are hampered by the limited information regarding the hearing abilities of most species.
Evoked potential audiometry is a non-invasive means of testing an animal's hearing. In the past, hearing in dolphins has been tested by behavioral methods. Animals were trained to perform a particular behavior (e.g. whistle or paddle push) in response to the presence of a sound, if they heard it. They were trained to perform an alternative behavior (e.g. remain silent or press an alternative paddle) if no sound was heard or no sound was present. The process of training the behaviors necessary for testing, and performance of the test itself, typically takes many months to complete. In contrast, EPA can be used to test the entire frequency range of hearing in a dolphin within a single test session and without the need to train behaviors specific to the process.
Through EPA, the dolphin's hearing is tested by measuring the electrical activity generated by the brain in response to a sound. The sounds are computer generated and played for the dolphin through speakers, called "jawphones", placed against the dolphin's lower jaw. The dolphin's response is measured by computer analysis of electrical activity in the dolphin's brain which is received through a transducer on the dolphin's head.
Information obtained from this procedure will improve our understanding of how the age and sex of the animal, as well as its noise exposure history, affect hearing. With an understanding of how hearing sensitivity varies within a population, and with information on the hearing sensitivity of species that were previously untested, regulatory agencies will be better empowered to mitigate potential impacts to marine mammals in the wild that result from anthropogenic sound exposure.
The use of EPA to test the hearing of dolphins in facilities like the Dolphin Conservation Center not only contributes to a more broad understanding of the dolphins, but enhances their care through improved knowledge of individual limitations in hearing. The sense of hearing is the most important of the senses to dolphins because of their use of sound to navigate, communicate and hunt. Identification of hearing problems allows behavioral issues and variations in animal responsiveness to training and human interaction to be gauged against their ability to hear. This knowledge may then be used to modify animal care and husbandry procedures and potentially increase the overall quality and efficiency of care.
