No evidence for dissociation of Turkish nouns and verbs in Broca's and Wernicke's areas: A transcranial magnetic stimulation study

Abstract

It is not clear whether the grammatical distinction between nouns and verbs serves as an organizational principle for representation of the lexicon in the brain, or whether semantic differences between the two categories such as imageability account for any cortical segregation between them. In this study, we used repetitive transcranial magnetic stimulation (rTMS) and lexical decision tasks to test whether Broca's area would be associated with verbs and Wernicke's area with nouns, and whether imageability and lexical status (real words versus pseudowords) would modulate representation of nouns and verbs in Broca's area and Wernicke's area. We assumed that if nouns and verbs are dissociated in these regions then their suppression would lead to a selective slowdown in lexical decision times for one or the other word category, which may be modulated by imageability and lexical status. On two different days, Broca's area and Wernicke's area were suppressed using low-frequency rTMS, and lexical decision times on Turkish nouns and verbs were collected before and immediately after the stimulation sessions. Using linear mixed-effects models with item- and trial-level predictors and covariates (imageability, lemma frequency, length in letters and presentation order), we failed to find any evidence for dissociation of nouns and verbs in Broca's area and Wernicke's area, or for an effect of imageability and lexical status on such purported dissociation. The analyses revealed a significant interaction between stimulation session and lexical status (real words versus pseudowords) in Broca's area, but not in Wernicke's area, implicating Broca's area with real words more than pseudowords. In addition, several behavioral effects were observed including the word frequency effect (faster RTs for frequent than infrequent words), word superiority effect (faster RTs for real words than pseudowords) and word category effect (faster RTs for nouns than verbs). In conclusion, our findings on Turkish nouns and verbs do not provide any evidence that grammatical category is a lexical organizational principle in Broca's or Wernicke's areas.