The role of offset sensitivity in consonant discrimination in noise
Sound offsets are important cues for recognising, distinguishing and grouping sounds, but the neural mechanisms and perceptual roles of sound-offset sensitivity remain poorly understood. In particular, while it is known that troughs in amplitude modulation are essential to consonant perception, there is a gap in the literature relating physiological studies of sound-offset responses in the auditory brain to the psychophysics of speech perception.
Recent studies in a mouse model of developmental disorder have reported the discovery of an auditory deficit specific to the processing of sound offsets (Anderson & Linden, 2016). This finding raises the possibility that deficits in sound-offset sensitivity might contribute to listening difficulties associated with developmental disorders. Difficulty perceiving speech in noise is the characteristic feature of central auditory processing disorder, and is also associated with other developmental or language disorders.
Here, we used mathematical modelling to investigate how sound-offset sensitivity relates to discrimination of vowel-consonant-vowel (VCV) stimuli in multi-talker babble noise. We used a phenomenological model introduced by Anderson and Linden (2016), based on the assumption that auditory brain activity arises from a sum of inputs from independently weighted onset-sensitive and offset-sensitive channels. By reducing the weighting of the offset-sensitive channel, we simulated reduced offset sensitivity and assessed its influence on the discriminability of model outputs for 48 non-sense VCV speech stimuli in varying levels of multi-talker babble noise (-12, -6, 0, 6, 12 dB SNR). We show that offset salience in noise can be used to categorise phonetic consonants into three groups of high, moderate and low salience, and we identify particular consonants for which discrimination in noise is more strongly or more weakly affected by offset sensitivity. We also report the results of an on-going psychophysical study of offset sensitivity and VCV perception in normal healthy subjects aged 18-60, comparing ratios of sound-onset to sound-offset reaction times with thresholds for gap-in-noise detection and VCV discrimination in noise. Consistent with model predictions, our preliminary results show that differences in consonant discrimination performance for consonants with markedly different offset salience ('w'-'d') varied more strongly with our measure of offset sensitivity than differences in performance for consonants with similar offset salience ('f'-'sh'). We anticipate further testing will reveal greater insight into the role of sound-offset sensitivity in auditory processing.
Anderson, L.A. & Linden, J.F. 2016. Mind the gap: two dissociable mechanisms of temporal processing in the auditory system. Journal of Neuroscience, 36 (6), 1977-95.