The Brain's Concepts: the Role of the Sensory-Motor
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COGNITIVE NEUROPSYCHOLOGY, 2005, 21 (0), XXX–XXX THE BRAIN’S CONCEPTS: THE ROLE OF THE SENSORY-MOTOR SYSTEM IN CONCEPTUAL KNOWLEDGE Vittorio Gallese Università di Parma, Italy George Lakoff University of California, Berkeley, USA Concepts are the elementary units of reason and linguistic meaning. They are conventional and rela- tively stable. As such, they must somehow be the result of neural activity in the brain. The questions are: Where? and How? A common philosophical position is that all concepts—even concepts about action and perception—are symbolic and abstract, and therefore must be implemented outside the brain’s sensory-motor system. We will argue against this position using (1) neuroscientific evidence; (2) results from neural computation; and (3) results about the nature of concepts from cognitive lin- guistics. We will propose that the sensory-motor system has the right kind of structure to charac- terise both sensory-motor and more abstract concepts. Central to this picture are the neural theory of language and the theory of cogs, according to which, brain structures in the sensory-motor regions are exploited to characterise the so-called “abstract” concepts that constitute the meanings of gram- matical constructions and general inference patterns. INTRODUCTION First-generation cognitive science was strongly influenced by the analytic tradition of philosophy Concepts are the elementary units of reason and of language, from which it inherited the propen- linguistic meaning. They are conventional and rel- sity to analyse concepts on the basis of formal atively stable. As such, they must somehow be the abstract models, totally unrelated to the life of the result of neural activity in the brain. The questions body, and of the brain regions governing the are: Where? and How? body’s functioning in the world. Correspondence should be addressed to Vittorio Gallese, Dipartimento di Neuroscienze, Sezione di Fisiologia, Università di Parma, Via Volturno 39, 43100 Parma, Italy/(E-mail: [email protected]). For many of the ideas put forward in the present paper the authors are indebted to, and would like to thank, the following members of the NTL Project at the International Computer Science Institute of the University of California at Berkeley: Jerome Feldman, Srini Narayanan, Lokendra Shastri, Eve Sweetser, Nancy Chang, Shweta Narayan, Ellen Dodge, Keith Sanders, and Ben Bergen. Many thanks also to Giacomo Rizzolatti and Maria Alessandra Umiltà, for their most valuable comments on an earlier version of this paper. Vittorio Gallese, who was George Miller Visiting Professor at the University of California at Berkeley, would like to thank the Miller Institute for awarding him the fellowship that made this work possible. He was also supported by the EU Mirrorbot Project, and by the Eurocores Project “The Origin of Man, Language, and Languages.” © 2005 Psychology Press Ltd 1 http://www.tandf.co.uk/journals/pp/02643294.html DOI:10.1080/02643290442000310 pCGN-SIOBA9.qxd GALLESE AND LAKOFF Concepts, from this perspective, were con- neural substrate. One can reason about grasping ceived of as abstract, amodal, and arbitrary, repre- without grasping; yet one may still use the same Q1 sented in some “language of thought” (Fodor, neural substrate in the sensory-motor system. 1975, 1987), made up of symbols and having the Indeed, that is just what we shall argue. In doing properties of productivity and compositionality, so, we will extend what we know about doing and Q1 among others. In Fodor’s theory (see Fodor, 1975), imagining sharing a common substrate via the fol- the purported amodal nature of concepts draws a lowing hypothesis: The same neural substrate used in sharp dividing line between the modular input/ imagining is used in understanding. output brain structures and a generalised cognitive Consider a simple sentence, like “Harry picked system (unanalysed at the level of the brain), whose up the glass.” If you can’t imagine picking up a functioning rules are totally independent from glass or seeing someone picking up a glass, then those governing the input/output modules. The you can’t understand that sentence. Our hypothe- propositional picture of the mind conveyed by early sis develops this fact one step further. It says that cognitivism is that of a functional system whose understanding is imagination, and that what you processes can be described in terms of manip- understand of a sentence in a context is the meaning of ulations of abstract symbols according to a set of that sentence in that context. Q1 formal syntactic rules (see Fodor, 1981; Pylyshyn, Our proposal is not an internalist theory of 1984). Knowledge is therefore represented in meaning. The reason is that imagination, like per- amodal symbolic form. Meaning is referential, in ceiving and doing, is embodied, that is, structured that it derives from a posited correspondence by our constant encounter and interaction with the between the system of abstract symbols and their world via our bodies and brains. The result is an corresponding extensions, the objects and events interactionist theory of meaning. in the world. Thus, following the line of arguments Accordingly, we will argue that a key aspect of of early cognitivism, concepts are symbolic repre- human cognition is neural exploitation—the adap- sentations by nature, and as thinking, they can be tation of sensory-motor brain mechanisms to reduced to symbolic (not neural) computation. serve new roles in reason and language, while We will propose a radically different view. We retaining their original functions as well. We will will argue that conceptual knowledge is embodied, discuss two cases: conceptual metaphor and cogs. that is, it is mapped within our sensory-motor As we shall see, circuitry across brain regions system. We will argue that the sensory-motor links modalities, infusing each with properties of system not only provides structure to conceptual others. The sensory-motor system of the brain is content, but also characterises the semantic content thus “multimodal” rather than modular. Accordingly, of concepts in terms of the way that we function language is inherently multimodal in this sense, with our bodies in the world. Before delving deeply that is, it uses many modalities linked together— into the argument, we should discuss a major find- sight, hearing, touch, motor actions, and so on. ing in neuroscience that we will be assuming Language exploits the pre-existing multimodal throughout: Imagining and doing use a shared neural character of the sensory-motor system. If this is substrate. true, it follows that there is no single “module” for When one imagines seeing something, some of language—and that human language makes use of the same part of the brain is used as when one mechanisms also present in nonhuman primates. actually sees. When we imagine moving, some of According to our proposal, the concept grasp, the same part of the brain is used as when we actu- from which we will start, gets its meaning via our ally move. Note that these facts undermine the ability to imagine, perform, and perceive grasping. traditional rationale given above. We can imagine Our ability to imagine grasping makes use of the grasping an object without actually grasping it. same neural substrate as performing and perceiv- From this, it does not follow that actual grasping ing grasping.According to our proposal, imagining and imaginary grasping do not use a common is a form of simulation—a mental simulation of 2 COGNITIVE NEUROPSYCHOLOGY, 2005, 21 (0) THE BRAIN’S CONCEPTS: THE ROLE action or perception, using many of the same sensory-functional theory, correlated structure neurons as actually acting or perceiving (Gallese, theory, and domain-specific theory (see Martin & 2003a). Caramazza, 2003; Simmons & Barsalou, 2003). Q2 Before developing our arguments, we would These theories differ along many dimensions, the like to conclude this introductory part by framing principal one being the extent to which conceptual our proposal within the extant and copious litera- knowledge is structured—and henceforth selectively ture on the neural underpinnings of conceptual affected by localised brain damage, by property or by knowledge. Any serious attempt to provide a neuro- category. scientific account of conceptual content as nested Similarly, it has recently been argued (Gainotti, in the activity of the brain faces the challenge of 2004) that the overall picture provided by func- explaining how the localised patterns of activation tional brain imaging studies is by no means con- of different neural cortical networks can enable the sistent and clear-cut (see also Gallese, 2003c). It Q3 capacity to distinguish, recognise, categorise, and should be added that even if one could provide ultimately conceptualise objects, events, and the unambiguous evidence for the neural correlates of state of affairs in the world. specific object concepts, the general principle Two main approaches have so far challenged defining the topology of such neural representa- the view held by first-generation cognitive science tion would need to be convincingly demonstrated. on the nature of conceptual knowledge: clinical Unfortunately, as argued by Malach et al. (2002), neuropsychology and cognitive neuroscience. Clin- to date no such convincing solution has been ical neuropsychology has established a relationship proposed. between given patterns of localised brain damage Our present proposal does not aim to settle the and corresponding deficits in conceptual knowledge. clinical issues, and so we will not deal with clinical Cognitive neuroscience has more recently tried to cases. Our goal is to follow an alternative route, establish, mainly by means of brain imaging exper- that is, to provide a testable embodied theory of iments, which brain regions are activated by differ- concepts, based on the results of research in neuro- ent conceptual categories. science, neural computation, and cognitive linguis- During the last two decades, an impressive tics, capable of reconciling both concrete and amount of clinical data has accumulated, describ- abstract concepts within a unified framework.