Posterior middle temporal gyrus is involved in verbal and non-verbal semantic cognition: Evidence from rTMS

Background: Left posterior middle temporal gyrus (pMTG) is reliably activated in functional neuroimaging studies of semantic processing and is frequently damaged in patients with comprehension impairments following stroke (e.g., Wernicke's aphasia). Its precise function remains elusive, however. Some researchers take the view that pMTG is a multimodal semantic area, involved in verbal and non-verbal semantic cognition. Others ascribe a lexical-semantic function to the region, positing that it is involved in mapping between phonology and conceptual knowledge. Aims: We investigated whether pMTG was involved in non-verbal as well as verbal semantic cognition by using rTMS to induce temporary, focal “virtual lesions” to this region in healthy participants. Methods & Procedures: Participants completed picture and word versions of a semantic association test before and after receiving 10 minutes of 1-Hz offline rTMS to left pMTG. They also completed a difficulty-matched visual decision task on scrambled pictures. An occipital lobe control site was stimulated in a separate session. Outcomes & Results: TMS slowed responses to word and picture versions of the test to an equal degree. There was no slowing on a non-semantic visual-matching task, or following TMS to the control site. Conclusions: These results indicate that pMTG is involved in both verbal and non-verbal semantic cognition. This region could be key to understanding the multimodal semantic deficits often observed following stroke.