Language, Cognition and Neuroscience, 2015 Vol. 30, No. 4, 357–366, http://dx.doi.org/10.1080/23273798.2014.980750 Towards a computational(ist) neurobiology of language: correlational, integrated and explanatory neurolinguistics David Embicka* and David Poeppelb,c aDepartment of Linguistics, University of Pennsylvania, 619 Williams Hall, Philadelphia, PA 19104-6305, USA; bDepartment of Psychology, New York University, 6 Washington Place, New York, NY 10003, USA; cMax-Planck-Institute (MPIEA), Grüneburgweg 14, 60322 Frankfurt, Germany We outline what an integrated approach to language research that connects experimental, theoretical and neurobiological (NB) domains of inquiry would look like and ask to what extent unification is possible across the domains. At the centre of the programme is the idea that computational/representational (CR) theories of language must be used to investigate its NB foundations. We consider different ways in which CR and NB might be connected. These are (1) a correlational way, in which NB computation is correlated with the CR theory; (2) an integrated way, in which NB data provide crucial evidence for choosing among CR theories; and (3) an explanatory way, in which properties of NB explain why a CR theory is the way it is. We examine various questions concerning the prospects for explanatory connections in particular, including to what extent it makes sense to say that NB could be specialised for particular computations. Keywords: computation; linguistic theory; unification; explanation; reductionism; levels 1. The most general question confronting current research most research in cognitive neuroscience of language, in cognitive neuroscience is how increasingly detailed where it is typically assumed that neural data have neuroanatomical and imaging work can be employed in ‘epistemological priority’ over data from linguistics or the development of theories in which there are explanat- psycholinguistics. We argue instead that computational- ory connections between cognition and neurobiology representational (CR) theories that are the foundation of (NB). Recent technological advances constitute great language research invite not reduction to the biological improvements over earlier ways of looking at brain infrastructure that is described by the neurosciences, but function. At the same time, though, it is not clear that rather conceptual change and ultimately unification – these technological successes have been matched by the subsequent to the identification of linking hypotheses conceptual advances that are required for the development between these domains of study. In short, we support a of truly integrated or unified theories that link cognition more ‘muscular’ linguistics and cognitive science. and NB. Thus, although cognitive theories and NB 2.Weassumethatlanguage comprises a set of repre- theories are advancing in their own terms, there are few sentations (e.g. ‘morpheme’)andcomputations(e.g. (if any) substantive linking hypotheses connecting these ‘concatenation’)whoseformalpropertiesaretheobject domains. of research in (psycho)linguistic theory as currently We take the questions about how to unify cognitive and practiced. We assume, moreover, a view of language NB lines of inquiry to be completely general to the that takes it to be part of the natural sciences, in which Downloaded by [University of Pennsylvania] at 11:20 21 July 2015 cognitive neurosciences. Under ideal circumstances – i.e. questions of biology, in general, and neurobiology, in with sufficient time and resources – it would be worth particular, play a prominent role – in principle, if not in looking in detail for linking hypotheses throughout the practice.3 entire ‘human cognome’ (the comprehensive ‘parts list’ of Our discussion is directed at a specific question that is the human mind), in order to determine what progress presumably the centre of the ‘cognitive neuroscience of towards unification has been made after 25 years of language’ (and which, in terms of our first section, could intensive neuroimaging.1 In this paper, we frame some of be generalised to cognitive neuroscience as a whole): Can the foundational issues confronting unification with refer- discoveries about the structure and functional organiza- ence to our narrow focus, language.2 It is fair to say that tion of the brain explain properties of the computations the perspective we develop here differs markedly from and representations that constitute language? That is, *Corresponding author. Email: [email protected] Our goal in this paper is to outline a framework for understanding connections between computational–representational and neurobiological theories of language. For a variety of reasons (both of principle and of practice) we have been unable to discuss in-depth a wide range of actual examples from current language work. Our hope is that we will be able to present a number of such case studies in a companion piece to this article. © 2014 Taylor & Francis 358 D. Embick and D. Poeppel under what conditions can it be said that there is an the papers collected in Chomsky (2000)). Here we explanatory relationship between some aspect of neuro- sometimes use the term computations as a cover for both biology and some aspect of linguistic computation? computations and representations. Our goal here, as in the case of earlier forays into The second line of research, implementational in these themes (Poeppel, 2012;Poeppel&Embick,2005), character, comes from NB: the study of the structure and is to introduce a particular set of questions into discus- function of the brain. Under this general heading we sion of language and the brain and to outline a research include any approach that makes claims about how brain programme that attempts to address them. We do this structures and different forms of brain activity underpin with particular emphasis on some possible forms that perception and cognition. In practice, theories on the NB answers to these questions might take, in the hope that side differ greatly along various dimensions; e.g. in terms this will provide a concrete foundation for further of the size of the objects they concentrate on (small = development. channels, neurons; large = populations, areas, etc.), or in 3. Since our goal is to address the potential for unifying terms of the methodologies that they employ to study the two lines of inquiry, from the cognitive and NB domains, brain (different types of invasive and non-invasive some initial assumptions concerning the nature of each imaging, with different dependent variables). For present need to be clarified. It is convenient to develop some of purposes, we look beyond these differences and treat all brain-centred theories of cognition, ranging from biophys- our main themes with reference to Marr’s(1982) distinc- tions between the computational, algorithmic and imple- ical models to systems – and cognitive – neuroscience, mentational levels of analysis: under the heading NB. When we speak of potentially unifying CR and NB COMPUTATIONAL THEORY: What is the goal of the computa- theories, we do so with a very specific understanding of tion, why is it appropriate, and what is the logic of the the overall goal of research in cognitive neuroscience: the strategy by which it can be carried out? goal is to understand why language has the cognitive – REPRESENTATION AND ALGORITHM: How can this computa- that is to say, computational and representational – tional theory be carried out? In particular, what is the properties that it has. This stance might look like we are representation for the input and output, and what is the privileging CR, since clearly a CR type of theory looks algorithm for the transformation? like it is better adapted to explaining cognition than an NB HARDWARE IMPLEMENTATION: How can the representation theory. This is not our intention, though; it is not our and algorithm be realized physically? intention to simply assert that CR theories of language, which are by definition interested in cognition, are prima Current research on language in the cognitive domain facie better at explaining the cognitive properties of concentrates on the first two levels; over the last 60 years language than NB theories. This point is obvious, but or so of research, much progress has been made, not very instructive. Our goal is to highlight the idea that especially considering the immense complexity of lan- some crucial explanatory force in understanding language guage and its connections with other systems. The theories will come from understanding neurobiology. Thus, we do that have been developed make claims both about how not see any point to attempting to achieve reduction of CR language is structured as a formal system, a grammar, and to NB, or vice versa (whatever that would mean). Rather, there are also theories of how grammars are used by Downloaded by [University of Pennsylvania] at 11:20 21 July 2015 our goal is to outline a framework for unified inquiry into speakers in real time in production and in comprehension. language, in which CR and NB are investigated together We use (psycho)linguistic theory as a cover term for these and on equal epistemic footing. lines of research on language, with no distinction being made at present between ‘theoretical linguistics’ and 4. There are two major problems that can be identified ‘psycholinguistics’ (the typical categories for the former when we try to link CR and NB theories (Poeppel & and latter types of theories mentioned above). When