Phonetic perception and the temporal cortex.

Abstract

Recent functional neuroimaging studies have emphasized the role of the different areas within the left superior temporal sulcus (STS) for the perception of various speech stimuli. We report here the results of three independent studies additionally demonstrating hemodynamic responses in the vicinity of the planum temporale (PT). In these studies we used consonant-vowel (CV) syllables, tones, white noise, and vowels as acoustic stimuli in the context of whole-head functional magnetic resonance imaging, applying a long TR to attenuate possible masking effects by the scanner noise. To summarize, we obtained the following results for the contrasts comparing hemodynamic responses obtained during the perception of CV syllables compared to tones or white noise: (i) stronger activation in the vicinity of the left PT with two distinct foci of activation, one in a lateral position and the other more medial in the vicinity of Heschl's sulcus; (ii) stronger activation in the vicinity of the right PT; and (iii) stronger bilateral activation within the mid-STS. Further contrasts revealed the following findings: (iv) stronger bilateral activation to CV syllables than to vowels in the medial PT, (v) stronger left-sided activation to CV syllables than to vowels in the mid-STS, and (vi) stronger activation to CV syllables with voiceless initial consonants than to CV syllables with voiced initial consonants in the left medial PT. The results are compatible with the hypothesis that the STS contains neurons specialized for speech perception. However, these results also emphasize the role of the PT in the analysis of phonetic features, namely the voice-onset-time. Yet this does not mean that the PT is solely specialized for phonetic analysis. We hypothesize rather that the PT contains neurons specialized for the analysis of rapidly changing cues as was suggested by P. Tallal et al. (1993, Ann. N. Y. Acad. Sci. 682: 27-47).