PET was used to investigate the neural correlates of action knowledge in object representations, particularly the left lateralized network of activations previously implicated in the processing of tools and their associated actions: ventral premotor cortex (VPMCx), posterior middle temporal gyrus (PMTG), and intraparietal sulcus (IPS). Judgments were made about the actions and functions associated with manipulable man-made objects (e.g., hammer); this enabled us to measure activations in response to both explicit and implicit retrieval of knowledge about actions associated with manipulable tools. Function judgments were also made about nonmanipulable artifacts (e.g., traffic light) providing a direct comparison for manipulable objects. Although neither the left VPMCx nor the left PMTG were selective for tool stimuli (nonmanipulable objects also activated these areas relative to a visual control condition), both regions responded more strongly to manipulable objects, suggesting a role for these cortical areas in the processing of knowledge associated with tools. Furthermore, these activations were insensitive to retrieval task, suggesting that visually presented tools automatically recruit both left VPMCx and left PMTG in response to action features that are inherent in tool representations. In contrast, the IPS showed clear selectivity for explicit retrieval of action information about manipulable objects. No regions of cortex were more activated by function relative to action judgments about artifacts. These results are consistent with the brain's preferential responsiveness to how we interact with objects, rather than what they are used for.

Event-related potentials (ERPs) were used to investigate whether processing differences between nouns and verbs can be accounted for by the differential salience of visual-perceptual and motor attributes in their semantic specifications. Three subclasses of nouns and verbs were selected, which differed in their semantic attribute composition (abstract, high visual, high visual and motor). Single visual word presentation with a recognition memory task was used. While multiple robust and parallel ERP effects were observed for both grammatical class and attribute type, there were no interactions between these. This pattern of effects provides support for lexical—semantic knowledge being organized in a manner that takes account both of category-based (grammatical class) and attribute-based distinctions.

The processing nature of N400, an event-related brain potential (ERP) component associated with semantic processing, was investigated in a paradigm combining a semantic priming lexical decision task and color-cued selective attention. Semantic priming effects on ERPs and reaction time (RT) were examined when targets and preceding semantically related primes were either both attended or both unattended, and when only either the prime or target was the focus of attention. Priming effects were determined by comparing semantically primed target ERPs (and RTs when appropriate) to their prime (in those conditions where the prime and target had the same attentional status) and/or to an unprimed control target matched to the attentional status of the primed target. Control stimuli were examined for ERP effects of color-cued selective attention unconfounded by priming factors. Experiment 1 required overt responses to words and nonwords in a binary choice task, while Experiment 2 required response only to nonwords. RTs in Experiment 1 indicated facilitatory priming effects to all semantically primed attended targets. In Experiments 1 and 2 ERPs to primes and controls were consistently more negative than the primed target trace in the N400 latency range in conditions with attended primes, suggesting priming effects on N400 are contingent on attentional processing of the prime. Removal of the attention manipulation (Experiment 3) resulted in an N400 component with a well-defined peak not evident in the first two experiments, indicating modulation of N400 by overlapping effects of attention.