Frequency resolving power of the dolphin’s hearing measured by rippled noise

Frequency resolving power (FRP) of hearing was studied in bottlenose dolphins in behavioral experiments (go/no-go paradigm) using rippled noise with a phase-reversal test. The rippled noise has a frequency spectrum with periodically alternating peaks and troughs (ripples) which is a convenient spectrum pattern to test frequency resolution. The principle of the phase-reversal test was to find the highest ripple density at which an interchange of peak and trough positions (the phase reversal) in the ripple spectrum is detectable. Measurements were made using narrow-band rippled noise stimuli with center frequencies varying from 2 to 128 kHz. The found FRP was frequency-dependent: it increased from 6.7–7.4 (mean 7.0) at 2 kHz to 33.2– 62.8 (mean 48.3) at 128 kHz. At lower frequencies (2–16 kHz) the found FRP values are close to those of humans. At high frequencies (90–128 kHz) these values are several times higher which indicates very sharp frequency tuning.