Auditory lexical decision, categorical perception, and FM direction discrimination differentially engage left and right auditory cortex

Recent neuroimaging and neuropsychological data suggest that speech perception is supported in bilaterally auditory areas. We evaluate this issue building on well-known behavioral effects. While undergoing positron emission tomography (PET), subjects performed standard auditory tasks: direction discrimination of frequency-modulated (FM) tones, categorical perception (CP) of consonant-vowel (CV) syllables, and word/non-word judgments (lexical decision, LD). Compared to rest, the three conditions led to bilateral activation of the auditory cortices. However, lateralization patterns differed as a function of stimulus type: the LD task generated stronger responses in the left, the FM task a stronger response in the right hemisphere. Contrasts between either words or syllables versus FM were associated with significantly greater activity bilaterally in superior temporal gyrus (STG) ventro-lateral to Heschl's gyrus. These activations extended into the superior temporal sulcus (STS) and the middle temporal gyrus (MTG) and were greater in the left. The same areas were more active in the LD than the CP task. In contrast, the FM task was associated with significantly greater activity in the right lateral-posterior STG and lateral MTG. The findings argue for a view in which speech perception is mediated bilaterally in the auditory cortices and that the well-documented lateralization is likely associated with processes subsequent to the auditory analysis of speech.