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Comparative Study
, 25 (35), 8010-6

Dissociating Linguistic Processes in the Left Inferior Frontal Cortex With Transcranial Magnetic Stimulation

Affiliations
Comparative Study

Dissociating Linguistic Processes in the Left Inferior Frontal Cortex With Transcranial Magnetic Stimulation

Patricia M Gough et al. J Neurosci.

Abstract

Is the left inferior frontal cortex (LIFC) a single functional region, or can it be subdivided into distinct areas that contribute differently to word processing? Here we used transcranial magnetic stimulation (TMS) to investigate anterior and posterior LIFC when the meaning and sound of words were being processed. Relative to no stimulation, TMS of the anterior LIFC selectively increased response latencies when participants focused on the meaning of simultaneously presented words (i.e., synonym judgments) but not when they focused on the sound pattern of the words (i.e., homophone judgments). In contrast, the opposite dissociation was observed in the posterior LIFC, where stimulation selectively interfered with the phonological but not the semantic task. This double dissociation shows functionally distinct subdivisions of the LIFC that can be understood in terms of separable corticocortical connections linking the anterior LIFC to temporal pole regions associated with semantic memory and the posterior LIFC to temporoparietal regions involved in auditory speech processing.

Figures

Figure 1.
Figure 1.
Results of functional localization. The bar plots illustrate group mean RTs for the category judgment task used to localize the anterior stimulation site (top left) and the rhyme judgment tasks used to localize the posterior stimulation site (top right). In both regions, stimulation (dark gray) significantly increased RTs relative to trials without TMS (light gray), but more importantly, a consistent increase in RTs was observed in both locations for 9 of 11 individuals (dark lines). Two subjects (light lines) were excluded from the main experiment for not showing a consistent TMS-induced RT increase in the posterior localization task. Bottom left, Average scalp measurements relative to the canther–tragus (CT) line for the anterior (A) and posterior (P) sites. Bottom right, The same sites on a parasagittal slice through the mean structural image of the four participants with MRI scans. Anterior locations for each individual are marked with crosses; posterior locations are marked with circles. Scale bar, 10 mm.
Figure 2.
Figure 2.
Effects of anterior and posterior stimulation on performance during synonym judgments (left), homophone judgments (middle), and visual matching (right). A, Accuracy measures across tasks and conditions. B, Normalized TMS effects expressed as a change relative to the non-TMS baseline trials. C, Group mean reaction times per condition. Trials without TMS are shown in light gray; those with TMS are shown in dark gray. Error bars indicate SEM adjusted to reflect the between-condition variance used in repeated-measure designs (Loftus and Masson, 1994).

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