Hyojin Kim1, Viktorija Ratkute1, Bastian Epp1
Hearing Systems Section, Department of Health Technology, Technical University of Denmark, Lyngby, Denmark

Comodulated masking noise and binaural cues can facilitate detecting a target sound from noise. These cues induce a decrease in detection thresholds, quantified as comodulation masking release (CMR) and binaural masking level difference (BMLD), respectively. Previous studies showed that CMR is affected by a preceding masker, possibly due to the adaptation of the auditory system to features of the preceding masker. However, its relevance to speech perception is unclear. Here, we investigated the effect of the duration of preceding maskers on CMR and BMLD, and its ecological validity using sounds with speech-like spectro-temporal dynamics.
We hypothesized that the adaptation results from top-down processing, and both CMR and BMLD will be affected with increased preceding maskers’ length. We measured CMR and BMLD when the length of preceding maskers varied from 0 (no preceding masker) to 500 ms. We used four different maskers: the reference condition with uncorrelated masker (RR), and three maskers consist of a comodulated masker preceded by three different maskers: uncorrelated masker (RC), comodulated masker (CC), and the masker with comodulated flanking-band (FC). For BMLD, we used interaural phase difference (IPD) of pi. Results showed that CMR was more affected with longer preceding masker only for FC condition while the preceding masker did not affect BMLD. This indicates that grouping of frequencies in preceding masker has influence on following frequency grouping by comodulation.
We further evaluated the ecological validity of such grouping effect with stimuli reflecting formant changes in speech. We set three masker bands at formant frequencies F1, F2, and F3 based on CV combination: /GU/, /FU/, and /PU/. We found that the CMR was little (~2 dB) while BMLD was comparable to previous findings (~9 dB). In conclude, we suggest that there are factors that play a role in frequency grouping in speech other than comodulation.