Controlling for individual differences in fMRI brain activation to tones, syllables, and words.

Previous neuroimaging studies have consistently reported bilateral activation to speech stimuli in the superior temporal gyrus (STG) and have identified an anteroventral stream of speech processing along the superior temporal sulcus (STS). However, little attention has been devoted to the possible confound of individual differences in hemispheric dominance for speech. The present study was designed to test for speech-selective activation while controlling for inter-individual variance in auditory laterality, by using only subjects with at least 10% right ear advantage (REA) on the dichotic listening test. Eighteen right-handed, healthy male volunteers (median age 26) participated in the study. The stimuli were words, syllables, and sine wave tones (220-2600 Hz), presented in a block design. Comparing words > tones and syllables > tones yielded activation in the left posterior MTG and the lateral STG (upper bank of STS). In the right temporal lobe, the activation was located in the MTG/STS (lower bank). Comparing left and right temporal lobe cluster sizes from the words > tones and syllables > tones contrasts on single-subject level demonstrated a statistically significant left lateralization for speech sound processing in the STS/MTG area. The asymmetry analyses suggest that dichotic listening may be a suitable method for selecting a homogenous group of subjects with respect to left hemisphere language dominance.