Spatio-temporal cortical dynamics of phonemic and semantic fluency.

Hemodynamic brain imaging and lesion studies have suggested differential involvement of expressive language-related cortical regions based on the phonemic versus semantic characteristics of verbal cues. The aims of this study were: 1) to elucidate the relative timing of the activity of inferior frontal and anterior insular versus motor and supplementary motor cortex during a fluency task and 2) to assess potential differences in the location or timing of activity in anterior and posterior language areas based on letter versus category cues. Using magnetic source imaging (MSI), we found significantly earlier onset latencies and a greater number of activity sources in motor and supplementary motor compared with inferior frontal and anterior insular regions. We also observed greater left versus right hemispheric asymmetry of activation for letter compared with category cues. This study provides new insights into cortico-cortical interactions during expressive language tasks.