FUNCTIONAL-HETEROGENEITY OF LEFT INFERIOR FRONTAL-CORTEX AS REVEALED BY FMRI

USING functional magnetic resonance imaging (fMRI), Rie mapped brain a ctivity in six normal volunteers during two silent verbal fluency task s, one with a phonemic (letter) cue and one with a semantic (category) cue. In comparison with resting state, both tasks activated the anter ior triangular portion of the left inferior frontal gyrus (IFG or F3, for third frontal gyrus) and the left thalamus. There were also areas activated in one task but not in the other: the posterior opercular po rtion of the left IFG for phonemic fluency, and the left retrosplenial region for semantic fluency. Our findings concur with normal psychoph ysical data and neuropsychological observations to suggest the recruit ment of two overlapping but dissociable systems for the two tasks, and demonstrate functional heterogeneity within the left IFG (Broca's are a), where the opercular portion is responsible for obtaining access to words through a phonemic/articulatory route.