A number of conflicting claims have been advanced regarding the role of the left inferior frontal gyrus, inferior parietal lobe and posterior middle temporal gyrus in action recognition, driven in part by an ongoing debate about the capacities of putative mirror systems that match observed and planned actions. We report data from 43 left hemisphere stroke patients in two action recognition tasks in which they heard and saw an action word ('hammering') and selected from two videoclips the one corresponding to the word. In the spatial recognition task, foils contained errors of body posture or movement amplitude/timing. In the semantic recognition task, foils were semantically related (sawing). Participants also performed a comprehension control task requiring matching of the same verbs to objects (hammer). Using regression analyses controlling for both the comprehension control task and lesion volume, we demonstrated that performance in the semantic gesture recognition task was predicted by per cent damage to the posterior temporal lobe, whereas the spatial gesture recognition task was predicted by per cent damage to the inferior parietal lobule. A whole-brain voxel-based lesion symptom-mapping analysis suggested that the semantic and spatial gesture recognition tasks were associated with lesioned voxels in the posterior middle temporal gyrus and inferior parietal lobule, respectively. The posterior middle temporal gyrus appears to serve as a central node in the association of actions and meanings. The inferior parietal lobule, held to be a homologue of the monkey parietal mirror neuron system, is critical for encoding object-related postures and movements, a relatively circumscribed aspect of gesture recognition. The inferior frontal gyrus, on the other hand, was not predictive of performance in any task, suggesting that previous claims regarding its role in action recognition may require refinement.

Scientific interest in the relationship between cognition and action has increased markedly in the past several years, fueled by the discovery of mirror neurons in monkey prefrontal and parietal cortex and by the emergence of a movement in cognitive psychology, termed the embodied cognition framework, which emphasizes the role of simulation in cognitive representations. Guided by a functional neuroanatomic model called the Two Action Systems account, which posits numerous points of differentiation between structure- and function-based actions, we focus on two of the major issues under recent scrutiny: the relationship between representations for action production and recognition, and the role of action in object representations. We suggest that mirror neurons in humans are not critical for full action understanding, and that only function-based (and not structure-based) action is a component of embodied object concepts.

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The current research aimed at specifying the activation time course of different types of semantic information during object conceptual processing and the effect of context on this time course. We distinguished between thematic and functional knowledge and the specificity of functional similarity. Two experiments were conducted with healthy older adults using eye tracking in a word-to-picture matching task. The time course of gaze fixations was used to assess activation of distractor objects during the identification of manipulable artifact targets (e.g., broom). Distractors were (a) thematically related (e.g., dustpan), (b) related by a specific function (e.g., vacuum cleaner), or (c) related by a general function (e.g., sponge). Growth curve analyses were used to assess competition effects when target words were presented in isolation (Experiment 1) and embedded in contextual sentences of different generality levels (Experiment 2). In the absence of context, there was earlier and shorter lasting activation of thematically related as compared to functionally related objects. The time course difference was more pronounced for general functions than specific functions. When contexts were provided, functional similarities that were congruent with context generality level increased in salience with earlier activation of those objects. Context had little impact on thematic activation time course. These data demonstrate that processing a single manipulable artifact concept implicitly activates thematic and functional knowledge with different time courses and that context speeds activation of context-congruent functional similarity.

Previous studies suggest that action representations are activated during object processing, even when task-irrelevant. In addition, there is evidence that lexical-semantic context may affect such activation during object processing. Finally, prior work from our laboratory and others indicates that function-based (“use”) and structure-based (“move”) action subtypes may differ in their activation characteristics. Most studies assessing such effects, however, have required manual object-relevant motor responses, thereby plausibly influencing the activation of action representations. The present work utilizes eyetracking and a Visual World Paradigm task without object-relevant actions to assess the time course of activation of action representations, as well as their responsiveness to lexical-semantic context. In two experiments, participants heard a target word and selected its referent from an array of four objects. Gaze fixations on non-target objects signal activation of features shared between targets and non-targets. The experiments assessed activation of structure-based (Experiment 1) or function-based (Experiment 2) distractors, using neutral sentences (“S/he saw the …”) or sentences with a relevant action verb (Experiment 1: “S/he picked up the……”; Experiment 2: “S/he used the….”). We observed task-irrelevant activations of action information in both experiments. In neutral contexts, structure-based activation was relatively faster-rising but more transient than function-based activation. Additionally, action verb contexts reliably modified patterns of activation in both Experiments. These data provide fine-grained information about the dynamics of activation of function-based and structure-based actions in neutral and action-relevant contexts, in support of the “Two Action System” model of object and action processing (e.g., Buxbaum & Kalénine, 2010).

Converging evidence supports the existence of functionally and neuroanatomically distinct taxonomic (similarity-based; e.g., hammer-screwdriver) and thematic (event-based; e.g., hammer-nail) semantic systems. Processing of thematic relations between objects has been shown to selectively recruit the left posterior temporoparietal cortex. Similar posterior regions have been also been shown to be critical for knowledge of relationships between actions and manipulable human-made objects (artifacts). Based on the hypothesis that thematic relationships for artifacts are based, at least in part, on action relationships, we assessed the prediction that the same regions of the left posterior temporoparietal cortex would be critical for conceptual processing of artifact-related actions and thematic relations for artifacts. To test this hypothesis, we evaluated processing of taxonomic and thematic relations for artifact and natural objects as well as artifact action knowledge (gesture recognition) abilities in a large sample of 48 stroke patients with a range of lesion foci in the left hemisphere. Like control participants, patients identified thematic relations faster than taxonomic relations for artifacts, whereas they identified taxonomic relations faster than thematic relations for natural objects. Moreover, response times for identifying thematic relations for artifacts selectively predicted performance in gesture recognition. Whole brain Voxel Based Lesion-Symptom Mapping (VLSM) analyses and Region of Interest (ROI) regression analyses further demonstrated that lesions to the left posterior temporal cortex, overlapping with LTO and visual motion area hMT+, were associated both with relatively slower response times in identifying thematic relations for artifacts and poorer artifact action knowledge in patients. These findings provide novel insights into the functional role of left posterior temporal cortex in thematic knowledge, and suggest that the close association between thematic relations for artifacts and action representations may reflect their common dependence on visual motion and manipulation information.

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Substantial evidence suggests that conceptual processing of manipulable objects is associated with potentiation of action. Such data have been viewed as evidence that objects are recognized via access to action features. Many objects, however, are associated with multiple actions. For example, a kitchen timer may be clenched with a power grip to move it, but pinched with a precision grip to use it. The present study tested the hypothesis that action evocation during conceptual object processing is responsive to the visual scene in which objects are presented. Twenty-five healthy adults were asked to categorize object pictures presented in different naturalistic visual contexts that evoke either move- or use-related actions. Categorization judgments (natural vs. artifact) were performed by executing a move- or use-related action (clench vs. pinch) on a response device, and response times were assessed as a function of contextual congruence. Although the actions performed were irrelevant to the categorization judgment, responses were significantly faster when actions were compatible with the visual context. This compatibility effect was largely driven by faster pinch responses when objects were presented in use- compared to move-compatible contexts. The present study is the first to highlight the influence of visual scene on stimulus-response compatibility effects during semantic object processing. These data support the hypothesis that action evocation during conceptual object processing is biased toward context-relevant actions.