Representations of conceptual information during automatic and active semantic access

When we read a word or see an object, conceptual meaning is automatically accessed. However, previous research investigating non-perceptual sensitivity to semantic class has employed active tasks. In this fMRI study, we tested whether conceptual representations in regions constituting the semantic network are invoked during passive semantic access and whether these representations are modulated by the need to access deeper knowledge. Seventeen healthy subjects performed a semantically active typicality judgment task and a semantically passive phonetic decision task, in both the written and the spoken input-modalities. Stimuli consisted of one hundred forty-four concepts drawn from six semantic categories. Multivariate Pattern Analysis (MVPA) revealed that the left posterior middle temporal gyrus (pMTG), posterior ventral temporal cortex (pVTC) and pars triangularis of the left inferior frontal gyrus (IFG) showed a stronger sensitivity to semantic category when active rather than passive semantic access is required. Using a cross-task training/testing classifier, we determined that conceptual representations were not only active in these regions during passive semantic access but that the neural representation of these categories was common to both active and passive access. Collectively, these results show that while representations in the pMTG, pVTC and IFG are strongly modulated by active conceptual access, consistent representational patterns are present during active and passive conceptual access in these same regions.