‘‘Internal spectra’’ in the dolphin’s auditory system obtained by a minimal-masking technique

Abstract

To understand signal processing in the auditory system, it is of importance to know the ‘‘internal spectrum’’ representations of test stimuli, i.e., profiles of excitation along the frequency-representation axis. Such profiles may be obtained using a ‘‘minimal-masking’’ technique: An excitation profile is reflected by a masking curve (masker level dependence on frequency) when a masking criterion is not the threshold response to the probe stimulus but the threshold masking effect—a very small standard decrease of the probe response. The dolphin’s auditory system provides good opportunities for such measurements because of the large amplitude and high consistency of the auditory brain-stem-evoked potentials (ABR). Minimal-masking curves were obtained in bottlenosed dolphins for various probe stimuli: narrow-band, rectangular, and rippled spectrum stimuli. At low stimulus levels, the profiles reproduced the stimulus spectrum well, except for a little wider bandwidth; this widening corresponded to the bandwidth of peripheral auditory filters. Stimuli with rectangular spectra evoked excitation profiles with enhanced edges, thus indicating enhancement of spectral contrast by lateral effects. Stimuli with rippled spectra evoked profiles reproducing ripples; however, edge ripples were also markedly enhanced. At high stimulus levels, all stimulus types evoked excitation profiles markedly expanded to high frequencies, far beyond the stimulus bandwidth.