Influence of background noise on auditory evoked responses to rippled-spectrum signals

The resolution of spectral patterns in adaptation background noise was investigated in a beluga whale, Delphinapterus leucas, using the evoked-potential technique. The resolution of spectral patterns was investigated using rippled-spectrum test stimuli of various levels and ripple densities and recording the rhythmic evoked responses (the rate following response, RFR) to ripple phase reversals. In baseline (no adaptation background noise) experiments, the highest RFR magnitude was observed at signal sound pressure levels (SPLs) of 100-110 dB re 1 μPa; at SPLs both below the optimum (down to 80 dB re 1 μPa) and above the optimum (up to 140 dB re 1 μPa), the RFR magnitude decreased. For high signal levels (above 110 dB re 1 μPa), low-level adaptation background noise (from -10 to -20 dB re signal level) increased RFR magnitude compared to baseline. This effect is considered to be a result of the optimization of the sensation level of the high-SPL signals due to decreasing hearing sensitivity caused by the adaptation background noise.