The EPL is engaged in several lines of investigation into how the brain perceives speech. We are interested in how the brain represents speech sounds, how it uses those representations to make predictions, and whether those representations affect our ability to learn new sounds.
Underspecification
Underspecification
Rotated Speech
The lab has most recently engaged in research probing whether the Mismatch Negativity (MMN) is truly comparing linguistic representations, or if there may be domain general explanations for asymmetries found in the MMN (see Crosslinguistic Studies!). To test this, we are using spectrally rotated speech to generate waveforms that do not sound like speech, but retain many features in the signal. Preliminary results are showing an opposite MMN asymmetry to rotated speech!
Crosslinguistic Studies
In a series of studies, the lab has collaborated with researchers at Michigan State University and Waseda University (Tokyo) and used MMN asymmetries to probe phoneme representations for underspecification. In these two studies we show that the phonological grammar of speakers of two different languages are reflected in opposite MMN asymmetries to the same stimuli, which is predicted by the two languages having opposite abstract underspecification patterns:
Hestvik, A., & Durvasula, K. (2016). Neurobiological evidence for voicing underspecification in English. Brain and Language. https://doi.org/10.1016/j.bandl.2015.10.007
Hestvik, A., Shinohara, Y., Durvasula, K., Verdonschot, R. G., & Sakai, H. (2020). Abstractness of human speech sound representations. Brain Research, 146664. https://doi.org/10.1016/j.brainres.2020.146664
Ad Hoc Phonetic Representations
Ad Hoc Phonetic Representations
There is abundant evidence from phonological rule application that phonemes are a fundamental organizational unit of speech sounds, but there is less direct evidence about when and how phonemes are deployed in the course of speech perception. Initial coding of speech sounds has been found to be detailed and phonetic, but some evidence has been found that the auditory system opts for abstract phoneme representations when making predictions over sets of varying speech sounds.
Using ERPs, we are probing the types of representations and predictions generated within auditory sensory memory in response to streams of varying input sounds. Does the brain use pre-defined, abstract phoneme representations to make predictions, or will it generate novel, ad hoc representations?
We use a passive MMN paradigm to measure the brain’s response to sounds that deviate from the set of varying input sounds.
The amplitude of the MMN response has previously been found to correlate with the magnitude of the difference between standards and deviants. We used two sets of varying standards – one in a higher range of voice onset time (VOT) and one in a lower range (while still occupying the same category – voiceless). If the brain uses phoneme representations to make predictions over sets of varying speech sounds, we predict no amplitude difference between these conditions. If the brain generates ad hoc representations, the two sets of standards should be treated differently, leading to a difference in MMN amplitude.
We have observed no difference between these conditions in several experiments, suggesting that the brain is lazily using pre-defined phoneme representations under these conditions, rather than making more accurate predictions using ad hoc phonetic representations.
However, we also find that the brain can discriminate standard from deviant when all tokens are in the same (voiceless) category, suggesting that while a pre-defined category representation is being used, it may still contain phonetic information!
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