DIRECTION-DEPENDENT SPECTRAL SENSITIVITY AND INTERAURAL SPECTRAL DIFFERENCE IN A DOLPHIN - EVOKED-POTENTIAL STUDY

Sensitivity and interaural intensity difference (IID) dependence on so und frequency and direction was measured in an Amazon river dolphin In ia geoffrensis by recording the auditory nerve evoked response from th e body surface. The maximal sensitivity in the horizontal plane was fo und when the sound direction was 5-degrees to 10-degrees ipsilateral t o the recorded ear; the direction dependence of sensitivity was more p ronounced at higher frequencies than at lower ones. The IID reached it s peak at small azimuthal angles (7.5-degrees to 15-degrees) and highe r sound frequencies (100 kHz), or at large azimuthal angles (30-degree s to 45-degrees) and lower sound frequencies (20 to 30 kHz). Each soun d direction featured its specific pattern of spectral sensitivity and of interaural spectral difference. The interaural spectral difference fluctuated within a range of more than 20 dB depending on sound direct ion. The data indicate that interaural intensity as well as spectral d ifference may be cues for binaural localization of sound direction by dolphins.