Language-specificity in early cortical responses to speech sounds

Cutler, A., Baldacchino, J., Wagner, A., & Peter, V. (2016). Language-specificity in early cortical responses to speech sounds. Poster presented at the Eighth Annual Meeting of the Society for the Neurobiology of Language (SNL 2016), London, UK.
The continuity of speech articulation ensures that in all languages, spoken sounds influence one another. Thus there are potentially cues to a sound’s identity in the realisation of surrounding sounds. Listeners make use of such coarticulatory cues – but not always. It has long been known (Harris, Lang. Sp., 1958) that English-speakers use this coarticulation to identify [f] but not [s]. The reason is that place of articulation cues can distinguish [f] from its very close perceptual competitor [θ] (deaf/death), while [s] has no such perceptual competitor and hence less need of such disambiguation. In languages with [f] but no [θ] (e.g., Dutch, Polish), listeners do not use coarticulation to identify [f], whereas listeners do use coarticulation to identify [s] where [s] has close competitors (Polish; Wagner et al., JASA, 2006). The patterning of coarticulation cue use is thus language-specific. In those studies, listeners’ use of coarticulatory cues was revealed by comparing responses to the same sounds in matching versus mismatching phonetic context (e.g., in afa, asa either as originally recorded, or with the consonants cross-spliced); sensitivity to this difference signals attention to coarticulation. We used this same method to assess whether language-specificity could be observed in the early cortical responses to speech, by measuring auditory evoked potentials in response to change in an ongoing sound (Acoustic Change Complex [ACC]; Martin & Boothroyd, JASA, 2000). 18 undergraduate native speakers of Australian English (11 females) heard, while watching silent video, 900 bisyllables (150 repetitions each of afa and asa in original, identity-spliced and cross-spliced realisation, where identity-spliced afa has initial [a] from another utterance of afa, cross-spliced afa has [a] from asa). If the ACC exhibits the language-specific differential response to [f] versus [s], we predict a significant difference across stimulus types (cross-spliced versus the other two stimulus types) for afa but not for asa. Listeners’ EEG was recorded (BioSemi, 64 channels), filtered between 0.1-30 Hz, divided into epochs from -100 to +1000 ms from token onset, and the epochs averaged separately for each bisyllable and stimulus type. The ACC amplitude was calculated from the grand averaged waveform across listeners as the difference in amplitude between the N1 and P2 peaks at the Fz electrode site; these differences were analysed in Bonferroni-corrected planned comparisons across the three stimulus types (unspliced, identity-spliced, cross-spliced) for each of afa and asa. For asa, the planned comparisons showed no differences at all between stimulus types. For afa, in contrast, the comparison between unspliced and cross-spliced stimulus types revealed that cross-spliced tokens generated a significantly smaller ACC: F(1,17)=5.98, p<.05. The amplitudes from the unspliced and identity-spliced afa stimuli however did not significantly differ. These findings indicate that English-speaking listeners’ coarticulation usage patterns – sensitivity to cues in a preceding vowel in the case of [f], insensitivity in the case of [s] – can be detected in the ACC, suggesting that native language experience tailors even the initial cortical responses to speech sounds.
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