How neurons make meaning: brain mechanisms for embodied and abstract-symbolic semantics
Authors: Friedemann Pulvermüller
Journal: Trends in Cognitive Sciences (2013)

•Semantic processing requires distinct neural mechanisms with different brain bases.•These include referential, combinatorial, emotional-affective and abstract-symbolic semantics.•All semantic processes are grounded; only some are embodied in action and perception.•Neural processes of disembodiment are explained by neurobiological principles. How brain structures and neuronal circuits mechanistically underpin symbolic meaning has recently been elucidated by neuroimaging, neuropsychological, and neurocomputational research. Modality-specific 'embodied' mechanisms anchored in sensorimotor systems appear to be relevant, as are 'disembodied' mechanisms in multimodal areas. In this paper, four semantic mechanisms are proposed and spelt out at the level of neuronal circuits: referential semantics, which establishes links between symbols and the objects and actions they are used to speak about; combinatorial semantics, which enables the learning of symbolic meaning from context; emotional-affective semantics, which establishes links between signs and internal states of the body; and abstraction mechanisms for generalizing over a range of instances of semantic meaning. Referential, combinatorial, emotional-affective, and abstract semantics are complementary mechanisms, each necessary for processing meaning in mind and brain. How brain structures and neuronal circuits mechanistically underpin symbolic meaning has recently been elucidated by neuroimaging, neuropsychological, and neurocomputational research. Modality-specific 'embodied' mechanisms anchored in sensorimotor systems appear to be relevant, as are 'disembodied' mechanisms in multimodal areas. In this paper, four semantic mechanisms are proposed and spelt out at the level of neuronal circuits: referential semantics, which establishes links between symbols and the objects and actions they are used to speak about; combinatorial semantics, which enables the learning of symbolic meaning from context; emotional-affective semantics, which establishes links between signs and internal states of the body; and abstraction mechanisms for generalizing over a range of instances of semantic meaning. Referential, combinatorial, emotional-affective, and abstract semantics are complementary mechanisms, each necessary for processing meaning in mind and brain. What is the brain basis of meaning or semantics? Cognitive psychologists have proposed that semantic and conceptual processes can be attributed to a dedicated 'symbolic semantic system' specialized for handling information about meaning and concepts related to signs [1Collins A.M. Loftus E.F. 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Action-based language: a theory of language acquisition, comprehension, and production.Cortex. 2012; 48: 905-922Abstract Full Text Full Text PDF PubMed Scopus (27) Google Scholar]. Because words are used to speak about different types of objects and actions, this approach implies specific semantic mechanisms that depend on word category (e.g., object vs action words, animal vs tool names) [15Shallice T. From Neuropsychology to Mental Structure. Cambridge University Press, 1988Crossref Google Scholar, 16Pulvermüller F. Words in the brain's language.Behav. Brain Sci. 1999; 22: 253-336Crossref PubMed Scopus (461) Google Scholar]. Support for different category-specific semantic brain mechanisms comes from patients with focal brain lesions and category-specific semantic deficits [17Warrington E.K. Shallice T. Category specific semantic impairments.Brain. 1984; 107: 829-854Crossref PubMed Google Scholar, 18Gainotti G. 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Neuropsychol. 2010; 27: 428-461Crossref PubMed Scopus (13) Google Scholar]. To explain semantic processing and representation in the human mind and brain, the new data and positions that have emerged in this 'embodiment debate' need to be integrated with each other. In this review, four different semantic mechanisms are identified as essential and spelt out neuromechanistically in an attempt to develop an integrative neurosemantic proposal that covers both embodied and abstract-symbolic processes in both modality-specific and multimodal areas of the brain. First, referential semantic mechanisms for linking symbols to objects and actions are contrasted with mechanisms for combining symbols with each other. Then symbol grounding in emotion and the wide applicability spectrum of some terms are discussed in an attempt to account for aspects of abstract concepts. A final section focuses on the interplay between 'embodied' and 'disembodied' semantic systems in abstract sentence processing. Semantic processing may occur in an integration center or 'semantic hub' that joins together the various aspects of a word's meaning [3Patterson K. et al.Where do you know what you know? The representation of semantic knowledge in the human brain.Nat. Rev. Neurosci. 2007; 8: 976-987Crossref PubMed Scopus (435) Google Scholar], for example, in the case of the word 'fish', about shape, color, smell and taste. Although a specialized area is in fact not necessary for integration of semantic information – because the binding of multimodal semantic features into one coherent representation may rely on long-range cortico–cortical connections [16Pulvermüller F. Words in the brain's language.Behav. Brain Sci. 1999; 22: 253-336Crossref PubMed Scopus (461) Google Scholar] – it is possible that such a single hub exists. However, neuroimaging and neuropsychological studies have revealed several cortical regions that may support general meaning processes (Figure 1, top):(i)Inferior frontal cortex (iFC): the anterior part of Broca's area and adjacent tissue in left iFC (Brodmann areas, BA, 44, 45, and 47) are active in semantic processing and functional changes in iFC impact on them [4Bookheimer S. Functional MRI of language: new approaches to understanding the cortical organization of semantic processing.Annu. Rev. Neurosci. 2002; 25: 151-188Crossref PubMed Scopus (633) Google Scholar, 22Devlin J.T. et al.Semantic processing in the left inferior prefrontal cortex: a combined functional magnetic resonance imaging and transcranial magnetic stimulation study.J. Cogn. Neurosci. 2003; 15: 71-84Crossref PubMed Scopus (213) Google Scholar].(ii)Superior temporal cortex (sTC): Wernicke's area, the classic posterior language area in and adjacent to the superior temporal gyrus and sulcus [23Lichtheim L. 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Recent work shows the best correlation between semantic deficit and reductions in metabolic brain activation in anterior fusiform gyrus rather than TP [30Mion M. et al.What the left and right anterior fusiform gyri tell us about semantic memory.Brain. 2010; 133: 3256-3268Crossref PubMed Scopus (73) Google Scholar]; data from stroke patients indicate similar roles of left aTC and other semantic areas in causing semantic errors [31Tsapkini K. et al.The function of the left anterior temporal pole: evidence from acute stroke and infarct volume.Brain. 2011; 134: 3094-3105Crossref PubMed Scopus (13) Google Scholar]. Although each candidate semantic hub is, by itself, consistent with the idea of an amodal semantic system [1Collins A.M. Loftus E.F. A spreading activation theory of semantic processing.Psychol. Rev. 1975; 82: 407-428Crossref Scopus (1870) Google Scholar, 2Ellis A.W. Young A.W. Human Cognitive Neuropsychology. Lawrence Erlbaum Associates, 1988Google Scholar], all the results taken together suggest that a wide variety of areas are important for general meaning processing, and this undermines the idea of a unique local center for semantic integration. Semantic integration mechanisms draw on higher association areas of the neocortex, that is, the multimodal convergence zones in prefrontal, posterior parietal, and anterior, inferior, and posterior temporal cortex, and possibly even anterior and posterior cingulate and other corticolimbic structures [6Binder J.R. Desai R.H. The neurobiology of semantic memory.Trends Cogn. Sci. 2011; 15: 527-536Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar], where sensory, motor, and affective emotional information converges. These areas are known to function as convergence zones of information from several modalities, rather than as functionally isolated modules of amodal information processing [32Damasio H. et al.A neural basis for lexical retrieval.Nature. 1996; 380: 499-505Crossref PubMed Scopus (819) Google Scholar]. On closer examination, all of the candidate semantic hubs appear to contribute to semantics differentially. Typical lesions in each of the five potential hubs do not affect all words alike; instead, words from one semantic category may suffer more than words from other categories. Concordant with these observations, neuroimaging studies show that semantic areas are activated to different degrees when different semantic types are being processed:(i)Left iFC and bilateral frontocentral motor systems become most strongly active in the processing of action-related words and phrases, and lesions in these areas, as seen with stroke and dementia, lead to relatively pronounced impairments in processing of action-related words and concepts compared to matched control items (Box 1) [12Pulvermüller F. Fadiga L. Active perception: sensorimotor circuits as a cortical basis for language.Nat. Rev. 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However, recent evidence showed that frontoparietal areas in the dorsal stream (red, purple, and green areas) are also active in semantic processing and that lesions here lead to severe linguistic–conceptual impairments, especially in the processing of action-related meaning [12Pulvermüller F. Fadiga L. Active perception: sensorimotor circuits as a cortical basis for language.Nat. Rev. Neurosci. 2010; 11: 351-360Crossref PubMed Scopus (128) Google Scholar, 20Kiefer M. Pulvermüller F. Conceptual representations in mind and brain: theoretical developments, current evidence and future directions.Cortex. 2012; 48: 805-825Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar, 33Kemmerer D. et al.Behavioural patterns and lesion sites associated with impaired processing of lexical and conceptual knowledge of action.Cortex. 2012; 48: 826-848Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 35Bak T.H. Chandran S. What wires together dies together: verbs, actions and neurodegeneration in motor neuron disease.Cortex. 2012; 48: 936-944Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar]. For example, motor neuron disease, a degenerative brain disease that affects frontoparietal cortex including precentral motor systems, is characterized by motor deficits and concordant conceptual–semantic deficits in the processing of action information [35Bak T.H. Chandran S. What wires together dies together: verbs, actions and neurodegeneration in motor neuron disease.Cortex. 2012; 48: 936-944Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar]. In functional imaging studies, semantic somatotopy was found in precentral motor and premotor cortex, showing that words (both verbs and nouns), phrases, and sentences semantically related to different parts of the body (e.g., 'talk', 'fork', and 'walk') activate ventral face/mouth, lateral arm/hand, and dorsal leg/foot motor regions, respectively [80Hauk O. et al.Somatotopic representation of action words in the motor and premotor cortex.Neuron. 2004; 41: 301-307Abstract Full Text Full Text PDF PubMed Scopus (519) Google Scholar]. Semantic–somatotopic activation has been confirmed by a range of fMRI studies (Figure I; [81Carota F. et al.Body-part-specific representations of semantic noun categories.J. Cogn. 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Moving the hands and feet specifically impairs working memory for arm- and leg-related action words.Cortex. 2013; 49: 222-231Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar] – on the processing of action-related language.There are two main reasons why dorsal-stream action semantics is important for cognitive theory. First, claims about semantic category specificity and its relationship to motor and sensory brain systems had remained suggestive because most category-specific deficits were seen for large lesions [17Warrington E.K. Shallice T. Category specific semantic impairments.Brain. 1984; 107: 829-854Crossref PubMed Google Scholar] and category-specific functional activation was typically seen adjacent to, but not in, sensory regions [43Martin A. The representation of object concepts in the brain.Annu. Rev. Psychol. 2007; 58: 25-45Crossref PubMed Scopus (464) Google Scholar]. 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Category specific semantic impairments.Brain. 1984; 107: 829-854Crossref PubMed Google Scholar, 32Damasio H. et al.A neural basis for lexical retrieval.Nature. 1996; 380: 499-505Crossref PubMed Scopus (819) Google Scholar] and in SD due to TP lesion [44Pulvermüller F. et al.The word processing deficit in semantic dementia: all categories are equal but some categories are more equal than others.J. Cogn. Neurosci. 2010; 22: 2027-2041Crossref PubMed Scopus (25) Google Scholar, 45Gainotti G. The format of conceptual representations disrupted in semantic dementia: a position paper.Cortex. 2012; 48: 521-529Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar]. Some brain language models attribute semantics to areas in the temporo-occipital ventral stream of processing of visual object-related information (yellow/orange areas in top middle and right panel of Figure 1). However, recent evidence showed that frontoparietal areas in the dorsal stream (red, purple, and green areas) are also active in semantic processing and that lesions here lead to severe linguistic–conceptual impairments, especially in the processing of action-related meaning [12Pulvermüller F. Fadiga L. Active perception: sensorimotor circuits as a cortical basis for language.Nat. Rev. Neurosci. 2010; 11: 351-360Crossref PubMed Scopus (128) Google Scholar, 20Kiefer M. Pulvermüller F. Conceptual representations in mind and brain: theoretical developments, current evidence and future directions.Cortex. 2012; 48: 805-825Abstract Full Text Full Text PDF PubMed Scopus (45) Google Scholar, 33Kemmerer D. et al.Behavioural patterns and lesion sites associated with impaired processing of lexical and conceptual knowledge of action.Cortex. 2012; 48: 826-848Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar, 35Bak T.H. Chandran S. What wires togeth