The inferior frontal gyrus and phonological processing: an investigation using rTMS

Repetitive transcranial magnetic stimulation (rTMS) offers a powerful new technique for investigating the distinct contributions of the cortical language areas. We have used this method to examine the role of the left inferior frontal gyrus (IFG) in phonological processing and verbal working memory. Functional neuroimaging studies have implicated the posterior part of the left IFG in both phonological decision making and subvocal rehearsal mechanisms, but imaging is a correlational method and it is therefore necessary to determine whether this region is essential for such processes. In this paper we present the results of two experiments in which rTMS was applied over the frontal operculum while subjects performed a delayed phonological matching task. We compared the effects of disrupting this area either during the delay (memory) phase or at the response (decision) phase of the task. Delivered at a time when subjects were required to remember the sound of a visually presented word, rTMS impaired the accuracy with which they subsequently performed the task. However, when delivered later in the trial, as the subjects compared the remembered word with a given pseudoword, rTMS did not impair accuracy. Performance by the same subjects on a control task that required the processing of nonverbal visual stimuli was unaffected by the rTMS. Similarly, performance on both tasks was unaffected by rTMS delivered over a more anterior site (pars triangularis). We conclude that the opercular region of the IFG is necessary for the normal operation of phonologically based working memory mechanisms. Furthermore, this study shows that rTMS can shed further light on the precise role of cortical language areas in humans.