Varying effect of noise on sound onset and acoustic change evoked auditory cortical N1 responses evoked by a vowel-vowel stimulus

Introduction: According to previous studies noise causes prolonged latencies and decreased amplitudes in acoustic change evoked cortical responses. Particularly for a consonant-vowel stimulus, speech shaped noise leads to more pronounced changes on onset evoked response than acoustic change evoked response. Reasoning that this may be related to the spectral characteristics of the stimuli and the noise, in the current study a vowel-vowel stimulus (/ui/) was presented in white noise during cortical response recordings. The hypothesis is that the effect of noise will be higher on acoustic change N1 compared to onset N1 due to the masking effects on formant transitions. Methods: Onset and acoustic change evoked auditory cortical N1-P2 responses were obtained from 21 young adults with normal hearing while presenting 1000 ms /ui/ stimuli in quiet and in white noise at +10 dB and 0 dB signal-to-noise ratio (SNR). Results: In the quiet and +10 dB SNR conditions, the N1-P2 responses to both onset and change were present. In the +10 dB SNR condition acoustic change N1-P2 peak-to-peak amplitudes were reduced and N1 latencies were prolonged compared to the quiet condition. Whereas there was not a significant change in onset N1 latencies and N1-P2 peak-to-peak amplitudes in the +10 dB SNR condition. In the 0 dB SNR condition change responses were not observed but onset N1-P2 peak-to-peak amplitudes were significantly lower, and onset N1 latencies were significantly higher compared to the quiet and the 10 dB SNR conditions. Onset and change responses were also compared with each other in each condition. N1 latencies and N1-P2 peak to peak amplitudes of onset and acoustic change were not significantly different in the quiet condition. Whereas at 10 dB SNR, acoustic change N1 latencies were higher and N1-P2 amplitudes were lower than onset latencies and amplitudes. To summarize, presentation of white noise at 10 dB SNR resulted in the reduction of acoustic change evoked N1-P2 peak-to-peak amplitudes and the prolongation of N1 latencies compared to quiet. Same effect on onsets were only observed at 0 dB SNR, where acoustic change N1 was not observed. In the quiet condition, latencies and amplitudes of onsets and changes were not different. Whereas at 10 dB SNR, acoustic change N1 latencies were higher, amplitudes were lower than onset N1. Discussion/conclusions: The effect of noise was found to be higher on acoustic change evoked N1 response compared to onset N1. This may be related to the spectral characteristics of the utilized noise and the stimuli, possible differences in acoustic features of sound onsets and acoustic changes, or to the possible differences in the mechanisms for detecting acoustic changes and sound onsets. In order to investigate the possible reasons for more pronounced effect of noise on acoustic changes, future work with different vowel-vowel transitions in different noise types is suggested.