The effect of sound level and signal-to-noise ratio on adaptation to noise in spectral and temporal modulation detection.

Adaptation to noise is the improvement in auditory function as the signal of interest is delayed in the noise. Previous research has shown that adaptation to noise occurs in the detection of temporal and spectral modulations when these modulations occur separately but not when they occur simultaneously. The aim of the present study was to investigate if adaptation to noise in spectral and temporal modulation detection depends on the signal level or on the signal-to-noise ratio (SNR). The signal was a 200-ms spectrally (2 cycles/oct) or temporally (10 Hz) modulated ripple noise embedded in white noise with noise-signal onset delays of 50 ms (early condition) and 800 ms (late condition). Adaptation was calculated as the improvement in the modulation detection threshold in the late condition with respect to the early condition. To explore if adaptation depends on the sound level (Experiment 1), adaptation was measured for ripple noise levels of 45, 60 and 75 dB SPL and a fixed SNR of 0 dB. Statistically significant adaptation occurred in spectral modulation detection at all three levels. Adaptation in temporal modulation detection was statistically significant at 60 and 75 dB SPL, but not at 45 dB SPL. To explore if adaptation to noise depends on the SNR (Experiment 2), adaptation was measured for five SNRs (-10, -5, 0, 5 and 10 dB) while fixing the level of the white noise at 60 dB SPL. Adaptation in spectral modulation detection was statistically significant for all SNRs except +5 dB. Adaptation in temporal modulation detection was statistically significant only at -10 and 0 dB SNR. Findings are discussed with regard to potential mechanisms involved in adaptation to noise.