Using fMRI during a lexical decision task, we investigated the neural correlates of semantic priming under masked and unmasked prime presentation conditions in a repeated measurement design of the same group of 24 participants (14 women). The task was to discriminate between pseudowords and words. Masked and unmasked prime words differed in their degree of semantic relatedness with target stimuli. Neural correlates of priming were defined as significantly different neural activations upon semantically unrelated minus related trials. Left fusiform gyrus, left posterior inferior frontal gyrus, and bilateral pre-SMA showed priming effects independent of the masking condition. By contrast, bilateral superior temporal gyri, superior parietal lobules, and the SMA proper demonstrated greater neural priming in the unmasked compared with the masked condition. The inverted contrast (masked priming minus unmasked priming) did not show significant differences even at lowered thresholds of significance. The conjoint effects of priming in the left fusiform gyrus suggest its involvement as a direct consequence of the neural organization of semantic memory. Activity in brain regions showing significantly more neural priming in the unmasked condition possibly reflected participants' evaluation of the prime–target relationship, presumably in the context of semantic matching. The present results therefore indicate that masked and unmasked semantic priming partially depend on dissociable mechanisms at the neural and most likely also at the functional level.

Traditionally, concepts are assumed to be situational invariant mental knowledge entities (conceptual stability), which are represented in a unitary brain system distinct from sensory and motor areas (amodality). However, accumulating evidence suggests that concepts are embodied in perception and action in that their conceptual features are stored within modality-specific semantic maps in the sensory and motor cortex. Nonetheless, the first traditional assumption of conceptual stability largely remains unquestioned. Here, we tested the notion of flexible concepts using functional magnetic resonance imaging and event-related potentials (ERPs) during the verification of two attribute types (visual, action-related) for words denoting artifactual and natural objects. Functional imaging predominantly revealed crossover interactions between category and attribute type in visual, motor, and motion-related brain areas, indicating that access to conceptual knowledge is strongly modulated by attribute type: Activity in these areas was highest when nondominant conceptual attributes had to be verified. ERPs indicated that these category-attribute interactions emerged as early as 116 msec after stimulus onset, suggesting that they reflect rapid access to conceptual features rather than postconceptual processing. Our results suggest that concepts are situational-dependent mental entities. They are composed of semantic features which are flexibly recruited from distributed, yet localized, semantic maps in modality-specific brain regions depending on contextual constraints.