AGRAMMATISM in APHASICS and NORMALS Language Deficits
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Agrammatism in Aphasics and Normals 1 Running head: AGRAMMATISM IN APHASICS AND NORMALS Language Deficits, Localization, and Grammar: Evidence for a Distributive Model of Language Breakdown in Aphasics and Normals Frederic Dick and Elizabeth Bates Jennifer Aydelott Utman Center for Research in Language and Department of Center for Research in Language, University of Cognitive Science, California, San Diego, and University of California, San Diego Department of Experimental Psychology, Oxford University Beverly Wulfeck Nina Dronkers Center for Research in Language and VA Northern California Health Care System Department of Language and Communicative Disorders, San Diego State University Morton Ann Gernsbacher Department of Psychology and The Waisman Center, University of Wisconsin-Madison This is a pre-print of an article in press in Psychological Review Corresponding Author: Frederic Dick Center for Research in Language Department of Cognitive Science UCSD 9500 Gilman Drive, Mail Code 0526 La Jolla, CA 92037-0526 [email protected] Agrammatism in Aphasics and Normals 2 Abstract discoveries launched a century of debate (still unresolved) revolving around the nature of these and Selective deficits in aphasics' grammatical other contrasting forms of aphasia, and their neural production and comprehension are often cited as correlates. Although any dichotomy is an evidence that syntactic processing is modular and oversimplification when it is applied to questions of localizable in discrete areas of the brain (e.g., this magnitude, the poles of this debate have been Grodzinsky, 2000). In this paper, we review a large defined (and can still be defined) in terms of the body of experimental evidence suggesting that theorist’s stand on three related issues: localization, morphosyntactic deficits can be observed in a number transparency of mapping, and domain specificity. of aphasic and neurologically intact populations. We present new data showing that receptive Localizationists argue in favor of the idea that agrammatism is found not only over a range of language (and/or specific subcomponents of aphasic groups, but is also observed in neurologically language) is represented and processed in one or intact normals processing under stressful conditions. more bounded regions of the brain. This belief, which We suggest that these data are most compatible with a has continuous and explicit roots in Franz Gall’s domain-general account of language, one that doctrine of phrenology (Fodor, 1983; Gall, 1810), is emphasizes the interaction of linguistic distributions usually accompanied by two corollaries: (a) there is a with the properties of an associative processor transparent mapping between specific functions (i.e., working under normal or suboptimal conditions. specific behaviors, experiences, and/or domains of knowledge) and the neural regions that mediate those functions, and (b) these neural regions are dedicated exclusively to the functional domains that they serve Introduction (e.g., an area that mediates grammar is not involved in other forms of sequenced perceptual or motor The primary purpose of this paper is to provide behavior). If one accepts these assumptions, then it is empirical arguments in support of a new view of meaningful to describe a given stretch of tissue as a language deficits and their neural correlates, “language zone” or even a “grammar zone”. particularly in the realm of syntax. "Selective" syntactic deficits are often cited as evidence that the The alternative to localization is sometimes cast human brain contains a bounded and well-defined in negative terms, i.e., as “nonlocalization.” Early faculty or module dedicated exclusively to the nonlocalizationists used terms like “holism” representation and/or processing of syntax (Caplan & (Goldstein, 1948), “equipotentiality”, and “mass Waters, 1999; Grodzinsky, 1995a, 1995b, 2000; action” (Lashley, 1950), to describe their alternative Pinker, 1994). Here, we present a wide range of view, emphasizing a lack of specificity in cortical experimental evidence (both new and old) showing organization for language. These amorphous terms that such deficits are more accurately—and have not outlived their authors, and with good reason: parsimoniously—characterized as interactions It is now indisputable that the brain is a highly between specific linguistic environments and a differentiated organ at birth, with substantial division domain-general (as opposed to language-specific) of labor from one region to another (Clancy, processor. Darlington, & Finlay, 2000; Elman, Bates, Johnson, Karmiloff-Smith, Parisi, & Plunkett 1996). The link between brain injury and aphasia has been known for at least 3000 years (O'Neill, 1980). The modern alternative to classic localization For more than 100 years, we have also known that embraces this central tenet, but casts it in a different aphasia is more likely following injuries to the left form: complex functions like language emerge from side of the brain, and that different kinds of aphasia the conjoint activity of many brain regions which may can result depending on the nature of the injury and be spatially discontinuous and widely distributed. For its locus within the left hemisphere (Goodglass, present purposes, we will use the term “distributivity” 1993). Paul Broca was the first to associate deficits in to describe this alternative to the localizationist view. language production with damage to an anterior Distributivity almost always coexists with the region of the left hemisphere, now known as “Broca’s rejection of transparent mapping and domain area”. Carl Wernicke is credited with the subsequent specificity: A given region may be relevant for discovery of a contrasting form of aphasia, language, participate in language, and even be characterized by a comprehension deficit in the essential for language, but its relationship to language presence of fluent speech, usually associated with is not transparent, nor is it dedicated exclusively to damage to a posterior region of the left hemisphere the processing of language or any of its that is now called “Wernicke’s area”. These subcomponents. Instead, the regions involved in Agrammatism in Aphasics and Normals 3 language processing are also involved in the were explained along sensorimotor lines, rooted in mediation of processes that language shares with rudimentary principles of neuroanatomy. For other domains, including specific forms of memory, example, the symptoms associated with damage to attention, perception and motor planning. From this Broca’s area were referred to collectively as motor point of view, it is no more appropriate to refer to a aphasia: slow and effortful speech, with a reduction in participating region as a “language zone” or a grammatical complexity, despite the apparent “grammar zone” than it would be to refer to the elbow preservation of speech comprehension at a clinical as a “tennis organ”. level. This definition made sense in view of the fact that Broca’s area lies in frontal cortex, anterior to Our goal in this paper is to provide evidence in what we now refer to as the “motor strip”. favor of a distributive approach to grammar, and against the claim that deficits in the processing of Conversely, the symptoms associated with grammar necessarily derive from damage to a damage to Wernicke’s area were defined collectively localized, bounded, and self-contained module or as sensory aphasia: moderate to severe problems in organ dedicated exclusively to this aspect of speech comprehension in the presence of fluent but language. We will show that (a) deficits in empty speech and moderate to severe word-finding grammatical processing (both receptive and problems. This characterization also made sense to expressive) are not restricted to any single type of early neurologists, because Wernicke’s area lies in aphasia, and hence are not associated with damage to posterior cortex, at the interface between auditory specific regions; and (b) the specific profile of cortex and the various sensory association areas that deficits referred to as “receptive agrammatism” can were presumed to mediate or contain word meaning. be reproduced by testing college students under Isolated problems with repetition were further adverse processing conditions (e.g., temporal and/or ascribed to fibers that link Broca’s and Wernicke’s spectral degradation of the acoustic signal), area; other syndromes involving the selective sparing suggesting that these grammatical deficits may have or impairment of reading or writing were proposed, causes that lie outside of the linguistic domain. We with speculations about the fibers that connect visual will review recent evidence on both these points, and cortex with the classical language areas (for an will also provide two new sets of data in their influential and highly critical historical review, see support. The first involves a large group of aphasic Head, 1926). patients, who vary in their symptom profiles as well as the nature and location of their lesions. The This classification of aphasia types in terms of second involves a large sample of college students sensory and motor “centers” still appears in many who are tested on the same stimuli under one of nine medical texts, perhaps because of its appealing different stress conditions. In both these data sets, we neuroanatomical simplicity. However, as Sigmund target what has been called the “core data” of Freud pointed out in his remarkably prescient book agrammatism (Hickok & Avrutin,