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Abstract
<p class="first" id="P1">Speech sounds are encoded by distributed patterns of activity
in bilateral superior
temporal cortex. However, it is unclear whether speech sounds are topographically
represented in cortex, or which acoustic or phonetic dimensions might be spatially
mapped. Here, using functional MRI, we investigated the potential spatial representation
of vowels, which are largely distinguished from one another by the frequencies of
their first and second formants, i.e. peaks in their frequency spectra. This allowed
us to generate clear hypotheses about the representation of specific vowels in tonotopic
regions of auditory cortex. We scanned participants as they listened to multiple natural
tokens of the vowels [ɑ] and [i], which we selected because their first and second
formants overlap minimally. Formant-based regions of interest were defined for each
vowel based on spectral analysis of the vowel stimuli and independently acquired tonotopic
maps for each participant. We found that perception of [ɑ] and [i] yielded differential
activation of tonotopic regions corresponding to formants of [ɑ] and [i], such that
each vowel was associated with increased signal in tonotopic regions corresponding
to its own formants. This pattern was observed in Heschl’s gyrus and the superior
temporal gyrus, in both hemispheres, and for both the first and second formants. Using
linear discriminant analysis of mean signal change in formant-based regions of interest,
the identity of untrained vowels was predicted with ~73% accuracy. Our findings show
that cortical encoding of vowels is scaffolded on tonotopy, a fundamental organizing
principle of auditory cortex that is not language-specific.
</p>