Processing of sub-syllabic speech units in the posterior temporal lobe: an fMRI study

The objective of this study was to investigate phonological processing in the brain by using sub-syllabic speech units with rapidly changing frequency spectra. We used isolated stop consonants extracted from natural speech consonant-vowel (CV) syllables, which were digitized and presented through headphones in a functional magnetic resonance imaging (fMRI) paradigm. The stop consonants were contrasted with CV syllables. In order to control for general auditory activation, we used duration- and intensity-matched noise as a third stimulus category. The subjects were seventeen right-handed, healthy male volunteers. BOLD activation responses were acquired on a 1.5-T MR scanner. The auditory stimuli were presented through MR compatible headphones, using an fMRI paradigm with clustered volume acquisition and 12 s repetition time. The consonant vs. noise comparison resulted in unilateral left lateralized activation in the posterior part of the middle temporal gyrus and superior temporal sulcus (MTG/STS). The CV syllable vs. noise comparison resulted in bilateral activation in the same regions, with a leftward asymmetry. The reversed comparisons, i.e., noise vs. speech stimuli, resulted in right hemisphere activation in the supramarginal and superior temporal gyrus, as well as right prefrontal activation. Since the consonant stimuli are unlikely to have activated a semantic-lexical processing system, it seems reasonable to assume that the MTG/STS activation represents phonetic/phonological processing. This may involve the processing of both spectral and temporal features considered important for phonetic encoding.