Brainstem Correlates of Comodulation Masking Release for Speech in Normal Hearing Adults

BACKGROUND AND OBJECTIVES: Weak signals embedded in fluctuating masker can be perceived more efficiently than similar signals embedded in unmodulated masker. This release from masking is known as comodulation masking release (CMR). In this paper, we investigate, neural correlates of CMR in the human auditory brainstem. SUBJECTS AND METHODS: A total of 26 normal hearing subjects aged 18-30 years participated in this study. First, the impact of CMR was quantified by a behavioral experiment. After that, the brainstem correlates of CMR was investigated by the auditory brainstem response to complex sounds (cABR) in comodulated (CM) and unmodulated (UM) masking conditions. RESULTS: The auditory brainstem responses are less susceptible to degradation in response to the speech syllable /da/ in the CM noise masker in comparison with the UM noise masker. In the CM noise masker, frequency-following response (FFR) and fundamental frequency (F0) were correlated with better behavioral CMR. Furthermore, the subcortical response timing of subjects with higher CMR was less affected by the CM noise masker, having higher stimulus-to-noise response correlations over the FFR range. CONCLUSIONS: The results of the present study revealed a significant link between brainstem auditory processes and CMR. The findings of the present study show that cABR provides objective information about the neural correlates of CMR for speech stimulus.

[ effects of auditory selective attention on contralateral suppression of stimulus-frequency otoacoustic emissions]

To date, the function of auditory efferent system remains unclear. There is evidence that medial olivocochlear bundle receives input from the cortex. In this study, the effect of auditory selective attention on stimulus-frequency otoacoustic emissions [SFOAE] was analyzed to investigate the modification of peripheral auditory system by auditory cortex activity in frequency specific mode. Thirty-six normal hearing adult subjects with their age ranging from 18 to 30 years [mean age: 21.9 years] participated in this cross-sectional study. Contralateral suppression of stimulus-frequency otoacoustic emissions was recorded in the right ear at 2 KHz. In order to eliminate the auditory attention, subjects were instructed to read a text. Besides, in order to evaluate the effect of auditory attention on contralateral suppression, subjects were instructed to detect target tones in background noise at 1000, 2000, and 4000 KHz. A significant increase at contralateral suppression of stimulus-frequency otoacoustic emissions was observed in auditory selective attention conditions [p