Modality-independent involvement of the left BA 44 during lexical decision making

During a lexical decision task, lexical decision making can be distinguished from lexical retrieval. Lexical decision making is independent of stimulus modality and not reflected in the decision times, whereas the opposite holds for lexical retrieval. In neuroimaging studies investigating lexical decision tasks with either visual or auditory stimuli these two processes have so far been confused. Therefore, it is not clear whether the activation of Broca's region, consisting of the left Brodmann's area (BA) 44 and BA 45, reported in such studies really reflects lexical decision making. The present event-related fMRI study investigated the role of Broca's region in lexical decision by analyzing brain activation that is independent of stimulus modality or decision times. Twenty-two healthy participants performed lexical decisions on visual and auditory real words and pseudo-words. The left BA 44 was conjointly activated during visual and auditory lexical decisions as compared to rest indicating the modality-independent involvement of BA 44 in lexical decision tasks in general. To identify brain activation related to lexical decision making rather than lexical retrieval the decision times were entered as covariates into the fMRI analysis. In this analysis, the left BA 44 was activated for visual and for auditory lexical decision making. These results indicate that the left BA 44 as a distinct sub-part of Broca's region plays an important role in lexical decision making independently of stimulus modality and decision times.