Vowel sound extraction in anterior superior temporal cortex

Abstract

We investigated the functional neuroanatomy of vowel processing. We compared attentive auditory perception of natural German vowels to perception of nonspeech band-passed noise stimuli using functional magnetic resonance imaging (fMRI). More specifically, the mapping in auditory cortex of first and second formants was considered, which spectrally characterize vowels and are linked closely to phonological features. Multiple exemplars of natural German vowels were presented in sequences alternating either mainly along the first formant (e.g., [u]-[o], [i]-[e]) or along the second formant (e.g., [u]-[i], [o]-[e]). In fixed-effects and random-effects analyses, vowel sequences elicited more activation than did nonspeech noise in the anterior superior temporal cortex (aST) bilaterally. Partial segregation of different vowel categories was observed within the activated regions, suggestive of a speech sound mapping across the cortical surface. Our results add to the growing evidence that speech sounds, as one of the behaviorally most relevant classes of auditory objects, are analyzed and categorized in aST. These findings also support the notion of an auditory "what" stream, with highly object-specialized areas anterior to primary auditory cortex.

Figure 1

Upper panel, right: F₁,F₂‐vowel space of the vowel stimuli used. Note the spectral properties of the vowel categories and their correspondence to the articulatory and phonological features “place of articulation” and “tongue height” (left). Lower panel: Illustration of the experimental setup with corresponding oscillograms and spectrograms, exemplified on the [e]‐[o] condition.

Figure 2

a: Overview of overactivations compared to band‐passed noise (BPN; t values) seen in the fixed‐effects analysis (P < 10⁻⁶, cluster extent k > 50). Left: Activations of all vowel conditions greater than BPN. Smaller panels on the right: activations of single vowel conditions greater than BPN. b: Comparison of Front > BPN (blue colors) and Back > BPN (red colors) seen in the random‐effects analysis (P < 10⁻³, k > 3). Although activation overlaps, a change in center of gravity is seen in both anterior superior temporal gyrus (aSTG) and inferior frontal gyrus (IFG).

Figure 3

Upper panel: Axial slices of the standard statistical parametric mapping (SPM) brain template (Montreal Neurological Institute coordinates, x = 60, z = −4) with activation in the random‐effects analysis overlaid (P < 10⁻³). Lower panels: For the corresponding locations along the supratemporal plane (anterior–posterior dimension; bold line indicating different y‐coordinates), bar plots indicate percent signal change in all five conditions of the random‐effects model. The left‐most graph displays a preference of the most posterior voxels for the Front vowel categories [i] and [e] and shows no signal change that is significantly different from zero for Back vowels [u] and [o]. The reverse is seen in the two right‐most graphs (most anterior voxels). The two intermediate vowel conditions High ([u]‐[i]) and Mid ([o]‐[e]) show no significant signal change compared to the global mean; the reference condition of band‐passed noise (BPN) bursts displays a relative suppression.

Figure 4

Direct statistical comparison of Back vowel to Front vowel activation. Given are t‐values (P < 10⁻¹¹) for the clusters in the fixed‐effects (FFX) model that survived small‐volume correction for the “all‐vowels > band‐passed noise” comparison derived from the random‐effects model (shown in the right‐hemispheric brain activation; right panel). The two clusters differ largely along the posterior–anterior axis (Montreal Neurological Institute coordinates).

Figure 5

Single‐subject activations to Back vowels [u,o] (red), Front vowels [i,e] (blue), as well as areas of overlap (shown in purple), plotted onto individual T₁ structural images (neurological convention, left being left). All activations are confined to regions of interest in temporal (Brodmann area [BA] 21, 22, 38, 41, and 42) and frontal lobe (BA 44 and 45), compared against band‐passed noise (BPN) and corrected to have an overall false discovery rate P < 0.05 (cluster extent k > 15). Note the gradual shift due to vowel features (from blue to red) in anterior superior temporal cortex (aST).