Classical theories of semantic memory assume that concepts are represented in a unitary amodal memory system. In challenging this classical view, pure or hybrid modality-specific theories propose that conceptual representations are grounded in the sensory–motor brain areas, which typically process sensory and action-related information. Although neuroimaging studies provided evidence for a functional–anatomical link between conceptual processing of sensory or action-related features and the sensory–motor brain systems, it has been argued that aspects of such sensory–motor activation may not directly reflect conceptual processing but rather strategic imagery or postconceptual elaboration. In the present ERP study, we investigated masked effects of acoustic and action-related conceptual features to probe unconscious automatic conceptual processing in isolation. Subliminal feature-specific ERP effects at frontocentral electrodes were observed, which differed with regard to polarity, topography, and underlying brain electrical sources in congruency with earlier findings under conscious viewing conditions. These findings suggest that conceptual acoustic and action representations can also be unconsciously accessed, thereby excluding any postconceptual strategic processes. This study therefore further substantiates a grounding of conceptual and semantic processing in action and perception.

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.

Perception and action are classically thought to be supported by functionally and neuroanatomically distinct mechanisms. However, recent behavioral studies using an action priming paradigm challenged this view and showed that action representations can facilitate object recognition. This study determined whether action representations influence object recognition during early visual processing stages, that is, within the first 150 msec. To this end, the time course of brain activation underlying such action priming effects was examined by recording ERPs. Subjects were sequentially presented with two manipulable objects (e.g., tools), which had to be named. In the congruent condition, both objects afforded similar actions, whereas dissimilar actions were afforded in the incongruent condition. In order to test the influence of the prime modality on action priming, the first object (prime) was presented either as picture or as word. We found an ERP effect of action priming over the central scalp as early as 100 msec after target onset for pictorial, but not for verbal primes. A later action priming effect on the N400 ERP component known to index semantic integration processes was obtained for both picture and word primes. The early effect was generated in a fronto-parietal motor network, whereas the late effect reflected activity in anterior temporal areas. The present results indicate that action priming influences object recognition through both fast and slow pathways: Action priming affects rapid visuomotor processes only when elicited by pictorial prime stimuli. However, it also modulates comparably slow conceptual integration processes independent of the prime modality.

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.

Concepts are composed of features related to different sensory and motor modalities such as vision, sound, and action. It is a matter of controversy whether conceptual features are represented in sensory-motor areas reflecting the specific learning experience during acquisition. In order to address this issue, we assessed the plasticity of conceptual representations by training human participants with novel objects under different training conditions. These objects were assigned to categories such that for one class of categories, the overall shape was diagnostic for category membership, whereas for the other class, a detail feature affording a particular action was diagnostic. During training, participants were asked to either make an action pantomime toward the detail feature of the novel object or point to it. In a categorization task at test, we assessed the neural correlates of the acquired conceptual representations by measuring electrical brain activity. Here, we show that the same object is differentially processed depending on the sensory-motor interactions during knowledge acquisition. Only in the pantomime group did we find early activation in frontal motor regions and later activation in occipito-parietal visual-motor regions. In the pointing training group, these effects were absent. These results show that action information contributes to conceptual processing depending on the specific learning experience. In line with modality-specific theories of conceptual memory, our study suggests that conceptual representations are established by the learning-based formation of cell assemblies in sensory-motor areas.

Automatic processes are usually thought to occur independently of any cognitive resources. This traditional view has been recently challenged by showing that temporal attention to a target stimulus is a prerequisite for “automatic” response priming. The event-related potential (ERP) study reported here extends this research by pursuing a somewhat different approach. In two experiments, it was investigated whether masked semantic priming effects can be modulated by temporal attention to the prime using a cueing procedure. We hypothesized that masked priming is amplified when attention is directed to the stimulus stream in the time window of masked prime presentation, even in the absence of any prime awareness. ERPs were recorded while subjects performed a primed lexical decision task. Target words were preceded by semantically related or unrelated masked prime words, which were not consciously identified. A cue stimulus prompted subjects to direct their attention to the stimulus stream either shortly before the masked prime (short cue interval) or a long time interval before. Priming affected the amplitude of the N400 ERP component, an electrophysiological index of semantic processing. Unrelated prime–target pairs elicited a larger N400 than related pairs (N400 priming effect). Most importantly, this masked N400 priming effect was strongest when the cue interval and the stimulus onset asynchronies were short. The present results show that temporal attention to the prime is a prerequisite for obtaining masked N400 priming effects. They also demonstrate that unconscious automatic processes are susceptible to attentional modulation.

In the present study, the significance of category-related brain activations as an index of semantic memory structure was assessed within a repetition-priming paradigm during a lexical decision task. The interpretation of category-related effects has been debated since previous studies observed category-related brain activity mainly in tasks requiring explicit semantic categorization. Furthermore, categories were frequently associated with behavioral performance differences, which could have influenced the pattern of brain activation. Event-related potentials (ERPs) to words denoting objects from artifactual (e.g., tools) and natural categories (e.g., animals) were recorded while subjects were presented with words and pseudowords as distracters, which were repeatedly presented. Category-related ERP differences emerged in the time window of the N400, an electrophysiological index of semantic processing, over occipito-parietal and fronto-central regions as well as in the following window of a late positive potential. Repetition priming modulated these category-related ERP effects whereas behavioral repetition priming (faster reactions to repeated words) was comparable for both categories. Differences in ERP repetition effects were specifically due to diminished category-related activity at repeated presentation. The present results show that category-related brain activation is not confined to tasks requiring explicit semantic categorization. Most importantly, the study demonstrates that category-related brain activation can be specifically modulated by repetition priming in the absence of corresponding behavioral performance differences. These findings therefore substantiate the significance of category-related brain activations as reflections of semantic memory structure and support the notion of multiple cortical semantic systems.