Abstract
Neuroimaging and neuropsychological experiments suggest that modality-preferential cortices, including motor- and somatosensory areas, contribute to the semantic processing of action related concrete words. Still, a possible role of sensorimotor areas in processing abstract meaning remains under debate. Recent fMRI studies indicate an involvement of the left sensorimotor cortex in the processing of abstract-emotional words (e.g., “love”) which resembles activation patterns seen for action words. But are the activated areas indeed necessary for processing action-related and abstract words? The current study now investigates word processing in two patients suffering from focal brain lesion in the left frontocentral motor system. A speeded Lexical Decision Task on meticulously matched word groups showed that the recognition of nouns from different semantic categories – related to food, animals, tools, and abstract-emotional concepts – was differentially affected. Whereas patient HS with a lesion in dorsolateral central sensorimotor systems next to the hand area showed a category-specific deficit in recognizing tool words, patient CA suffering from lesion centered in the left supplementary motor area was primarily impaired in abstract-emotional word processing. These results point to a causal role of the motor cortex in the semantic processing of both action-related object concepts and abstract-emotional concepts and therefore suggest that the motor areas previously found active in action-related and abstract word processing can serve a meaning-specific necessary role in word recognition. The category-specific nature of the observed dissociations is difficult to reconcile with the idea that sensorimotor systems are somehow peripheral or ‘epiphenomenal’ to meaning and concept processing. Rather, our results are consistent with the claim that cognition is grounded in action and perception and based on distributed action perception circuits reaching into modality-preferential cortex.
Highlights
A fundamental theoretical debate about the nature of meaning and concepts dominates the cognitive and brain sciences
Some recent attempts to amalgamate both positions into one integrative proposal either maintain that semantic processing is processed by an amodal system, whereas modality-preferential cortices, such as the sensorimotor areas, play an optional, merely “coloring” role (Mahon and Caramazza, 2008; Caramazza et al, 2014), or they postulate semantic integration in a ‘semantic hub’ and allow for additional modality-specific semantic centers across the cortex (Patterson et al, 2007; for review, see Binder and Desai, 2011; Kiefer and Pulvermüller, 2012)
In patient CA, who suffered from a focal lesion of the left supplementary motor area (SMA), this impairment was most pronounced for abstract-emotional nouns, whereas patient HS, who suffered from a mild paresis of the right extremities and focal lesion just inferior to the typical hand representation in the primary motor cortex, showed a category-specific deficit in recognizing tool-related nouns
Summary
A fundamental theoretical debate about the nature of meaning and concepts dominates the cognitive and brain sciences. Classic cognitive psychologists propose that semantic and conceptual processes are carried by a dedicated symbolic semantic system functionally detached from sensory and motor modules and specialized for handling information about meaning and concepts related to signs (e.g., Ellis and Young, 1988). Sometimes referred to by the terms ‘embodiment’ and ‘semantic grounding’, states that meaning is intrinsically related to (or grounded in) action and perception information and processed in the brain by distributed action perception circuits that reach into motor and sensory brain areas (Barsalou, 1999, 2008; Pulvermüller, 2005; Glenberg and Gallese, 2012). It is not clear whether this type of ‘integrative’ position allows for the explanation of category-specific deficits arising from a focal lesion in one modality-preferential cortical system
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