Cortical organization of action and object naming in Turkish: a transcranial magnetic stimulation study

Abstract

It is controversial whether the linguistic distinction between nouns and verbs is reflected in the cortical organization of the lexicon. Neuropsychological studies of aphasia and neuroimaging studies have associated the left prefrontal cortex, particularly Broca's area, with verbs/actions, and the left posterior temporal cortex, particularly Wernicke's area, with nouns/objects. However, more recent research has revealed that evidence for this distinction is inconsistent. Against this background, the present study employed low-frequency repetitive transcranial magnetic stimulation (rTMS) to investigate the dissociation of action and object naming in Broca's and Wernicke's areas in Turkish. Thirty-six healthy adult participants took part in the study. In two experiments, low-frequency (1 Hz) inhibitory rTMS was administered at 100% of motor threshold for 10 minutes to suppress the activity of the left prefrontal cortex spanning Broca's area or the left posterior temporal cortex spanning Wernicke's area. A picture naming task involving objects and actions was employed before and after the stimulation sessions to examine any pre- to post-stimulation changes in naming latencies. Linear mixed models that included various psycholinguistic covariates including frequency, visual and conceptual complexity, age of acquisition, name agreement and word length were fitted to the data. The findings showed that conceptual complexity, age of acquisition of the target word and name agreement had a significant effect on naming latencies, which was consistent across both experiments. Critically, the findings significantly associated Broca's area, but not Wernicke's area, in the distinction between naming objects and actions. Suppression of Broca's area led to a significant and robust increase in naming latencies (or slowdown) for objects and a marginally significant, but not robust, reduction in naming latencies (or speedup) for actions. The findings suggest that actions and objects in Turkish can be dissociated in Broca's area.