Lexical access: A perspective from pathology

Cbgnition, 10 (1981) 209-214 @ Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

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Lexical access:A perspectivefrom pathology JOHN C. MARSHALL* FREDA NEWCOMBE The Radcliffe Infirmary

The research program o I* which we (and many other neuropsychologists) have been working for the last decade concerns the nature of the interface between perception and the language faculty. The specific problem that has attracted much of our own attention relates to the ways in which orthographies mediate between vision and lexical retrieval; the prims.ry assay that has been employed is the study of ‘double-dissociations’ (Teuber, 1955) in the residual reading skills of adults who, prior to sustaining brain-injury, were fully literate. The c. igina! goal of Che i3trJgram was to construct a rational classification of the acquired dys!e.:;*.i; (Marshall and Newcombe, 1973). That is, we sought a systematic ar.ai,4s of b:-eakdown that mapped onto a quasi-formal account of the funct@i;til components required in normal reading [Morton, 1979) and that had a fairly direct relationship to some of the strategies encouraged in the acquisition of reading skills (Newcombe and Marshall, in press). The explanatory mode that we adopted was traditional ‘diagram-making”’ (Lichtheim, 1885) Boxes and arrows were employed as a notation for computational centers (or the representations that they assigned) and the information-transmission route; whereby they are interconnected. The behaviour of patients with (often relatively focal) brain-damage was then interpreted in terms of hypothr,!ical ‘lesions’ of the boxes and arrows postulated in the theoretical model Adoption of this research strateg: , analogous to the use of mutations i.n the identification of genes, and hence of control points in a biological system, quickly led to a new classification of the acquired dyslexias that could, at very least, be neatly summarized by relatively simple block-diagrams (Coltheart, Patterson and Marshall, 1980). The primary symptom-complexes of the new taxonomy include deep dyslexia (Marshall and Newcombe, 1966), surface dyslexia (Marshall and Newcombe, 1973), phonological dyslexia Beauvois and D&ouesnC, 1979), direct dyslexia (Schwartz, Saffran, and Marin, 1980), semantic access dyslexia (Warrington and Shallice, 1979). and word-form dysfexia (Wa.rrington and Shallice, 1980). Althougn there are many substantive arguments that still remain to be resolved within the notational conventions of simple informstion-processing models (see Morton, *Requests;or reprints should be addressed to J. C. Marshal& Neuropsychology Unit, Neuroscience Group, The Radcliffe Infirmary, Oxford OX2 6HE, England.

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page 227), it is our current belief that this work is rapidly reaching the stage where the ad hoc proliferation of boxes and arrows will weaken the explanatory power of the approach to the point of vacuity. The reader who is familiar with the history of neurolinguistics will recall that this danger is not without precedent (Moutier, 1908). We accordingly outline in this paper a reconstruction of the original program that will, we hope, provide a framework within which some unifying principles may eventually be found. P;e-theoretically, we assume the notion ‘word’ and ask the question: Given an orthographic string of word-length, how does that string access the semantic, syntactic,. and phonological inforrnation that is arrayed against it in thz competence of a mature reader? Following Marr ( 1980), we further assume that the system so isolated must be understood in terms of four quasiindependent sub-theories. The fnst of these is the theory ot tne orthography itself. Most children learn to read after the core structure of the language, including much of the core vocabulary, has been triggered by auditory/vocal experience; it accordingly makes sense to regard the theory of written language as a statement of the mapping between the form of an orthography and a (set of) level(s) of representation made available by virtue of having acquired the spoken language (Klima, 1972). To a first approximation, extant orthographies can be divided into alphabets, morphophonemic (English) or phonemic-phonetic (SerboCroatian), syllabaries (Japanese Kana), and wordor morpheme-based scripts (Japanese Kanji) according to the smallest units at which correspondences between the written and the spoken language can be expressed. The second sub-theory would specify which of the mappings that the orthography in principle makes available are actually utilized under different conditions, and would provide an account of the algorithms that effect the mappings. Let us illustrate this with a hypothetical example. Consider a shallow orthography such as the international Phonetic Alphabet (narrow transcription). Words written in IPA could be read according to a strategy that segmented the orthographic string into letters and mapped those letters, one at a time from left-to-right, into a phonetic code that was both the representation that triggered articulation (sub.ject to so-articulation constraints) and the access code for retrieval of syntactic and semantic information pertaining to the word. The orthography ullou~ such an algorithm; it does not, however, demand it. Thus it is equally possible in point of logic that an IPA letter-string could be recognized as a stored word, by application, say, of Forstzr’s decision-tree algorithm (Forster, 1978); the word-address computed by the algorithm could then call dire&” the phonological and syntacticosemantic information associated with that a,ddress. In short, the structure of an orthography imposes constraints on possible algorithms (there is no pro-

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cedure: whereby the individual strokes of a Kanji character could be mapped into phonological elements) but it does not uniquely determine a processing algorithm (Marsh&, 1976). Further constraints on the nature of the algorithms that are actually used can be obtained by study of the stimulus-variables that affect either rate of access in normal subjects or yrobcbility of correct access in brain-damaged patients. One such variable is word-frequency, or perhaps the highly-correlated variable of age-of-acquisition. Thus in lexical decision tasks, the speed at which orthographically-permissible tokens can be assigned to the types word or non-word iis, for the word tokens, an inverse function of the frequency with which the tokens appear in a (large) sample of text. One possible algorithm that would capture the constraint can be !informahy) stated as follows: Take the first n elements of a presented letter-string and use them to address directly all the words of the language that begin with those n elements; order by frequency the subset of the vocabulary so selected and evaluate each item in turn against the full input letter-string; accept the first item that matches the fumlinput. An algorithm of (approximately) this nature is presented by Taft (1979). An alternative proposal might involve the Iollowing steps: Assign to each stored word-representation a number that is inversely correlated with the frequency of occurrence of the word in the language;. quantize stimulus-nformation in such a way that numerical weights are associated with ‘fef;tures’ OC the stimulus, where a feature is some visual element or property of the sthmulus array; transmit the quantized and weighted features of the stimulus in parallel to all stored word representations; when a stimulus-feature matches a stored feature of a word add its numerical value to all words containing the stored feature. Continue this process until the sum of such matches reaches the originally-assigned value of a word, An algorithm of (approximately) this nature underlies the models presented by Morton ( 1979). Construction of an appropriate algorithm for word recognition must in turr. be distinguished from an account of the particular mechanisms that realize the algorithm(s). A third sub-theory will specify the nature of the storage devices, adders, multipliers, transducers, feedback loops and so forth that are required to implement the algorithms. Thus one interpretation of Morton’s model, tile interpretation preferred by Morton (1979), req- ‘-es ‘passive’ adders (or multipliers) in order that the ‘level of activation’ of Jred units can be incremented until their ‘threshold’ is reached; it also reqk:ires some forms of transduction into a ‘neutr 11’code in order that contextual (syntactico-semantic) information can summate wl+h stimulus information in a particular modality. The irlodel proposed by Taft (1979~ requires a short-term storage device that can hold stimulus information until the members the

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candidate recognition list can be fed back for evaluation; the evaluation process itself presumably requires some kind of ‘comparator’, the nature of which must be specified. The algorithm for direct conversion between a letter-string and its phonological representation (illustrated in simplified form in the IPA example and believed to be implicated in the performance of patients with ‘surface dyslexia’) requires some kind of scanning device for segmenting the input string and fee&:; it(: elements sequentially to a ‘look-up’ table for the assignment of grapheme-phoneme correspondences. The mechanisms that realize an algorithm are the first theoretical entities for which it makes sense to raise the issue of neuronal representation. Finally then, a fourth sub-theory will provide an account of the neuronal hardware that instantiates the mechanisms. The relevant hardware is deduced, in the first place, from clinico-pathological correlation. Thus necropsy studies show that relatively ‘pure’ disorders ijf written language processing are associated with pathology centered on the junction of the occipital, parietal, and temporal lobes of the dominant (.typically left) hemisphere. Functional (or at least symptomatic) considerations thus suggest that ‘modular’ areas should be found in this region. If we defme areas anatomically, that , :$ in terms of a well defmed specialized cytoarchitecture and well defined ingut and output connections it does indeed seem that distirlct areas can be isolated within the broad region of pathology that can be demonstrated by histology at necropsy (Braak, 1980; Galaburda and Sanides, 1980). In some ins:ances , classical clinico-pathological studies have provided quite tight correlations between sym]ptom-eomplexzs of acquired dyslexia and focal damage. Thus dyslexia without dysgraihia is frequently seen with combined lesion of left calcarine cortex and the splenium of the corpus callosum (Geschwind, 1962), and with ‘subangular’ lesions located deep in the white : natter below the angular gyrus (Greenblatt, 1976); dysphasia with dysgraphia is often seen with pathology centered on the angular gyrus itself (Geschwind, 1962). One must note, however, that lesion sites only make ‘functional sense if the detailed character of the patient’s impaired and preserved skills can be interpreted in terms of lccalized mechanidns and their interaction. Thus in the taxonomy proposed in Newcombe and Marshall (19s 1), dyslexia with-and-without dysgraphia are no longer unitary syndromes but rnusf rather be fractio.nated into a variety of distinct conditions. Our general line of argument, then, follows Marr (1980) in claiming that the capacities of’ any complex biological system should be studied at all four levels of interpretation. We also assume that ‘bridge-laws’ must be formulated to link the levels. We accordingly see no virtue to the suggestion that theories formulated at one level should be impervious to criticism derived from data

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obtained at another level. What one should seek to maximize is the power of the theory as a whole; if this involves reinterpreting or overthrowing a claim about algorithms or mechanisms on the basis of what is known about basic component and circuit analysis for neuronal elements, then so be it. Some of the serial algorithms that have been proposed (admittedly somewhat tonguein-cheek) for word-recognition (Goldiamond and Hawkins, 1958) can be ruled out on the basis of the fact that speed of nervous conduction precludes their operation in real-time. Conversely, the requirement that a plausible mechanism (and algorithm) be ;;*vailablemight give one pause before accepting a particular anatomical interpretation of lesion data. Many ‘disconnectionist’ analyses that demand callosal crossing thus fail to specify the nature of the encoding and decoding machinery that would be needed to effect the requisite transfer of information. And, more seriously, one might expect that white matter lesions (within or between hemispheres) may well affect the computational capacities of their destinations over and above the role that long fibre tracts play in information-transmission. We will conclude with. a brief summary of why we believe that a partitioning into four levels of theoretical interpretation is a prerequisite of further advances in the neurolinguistics of reading (and neuropsychology more generally). A concern with the nature of orthcgraphies should draw attention to the necessity of studying a much wider variety of structural types than is currently under consideration (but see Sasanuma, 1980); we want to distinguish between ‘core’ reading skills that are needed to acquire and fluently manipulate any evolved orthography and a ‘marked’ periphery of skills that are only applicable to subsets of orthographies. An inn&e bias to ‘search’ the orthography of one’s native language for word-, syllable-, phonemic- and phonetic-level structure in that order would lead to fewer blind alleys than the reverse order of search. Most current work on disorders of reading i:; best regarded as pertaining to the algorithmic sub-thecry. Explicitly distinguishing stlch a level serves to remind m that formal algorithms must be constructed that are in principle capable of performing the operations that ale tacitly assumed in our block diagrams. The level of mechanism; the internal structure of computational centres and codes, should provide an alternative locus fog the interpretation of the rapidly increasing number of doubledissociations that are being uncovered in detailed singlecase studies (Shallice, 1979). To invoke a new box or arrow for each stimulusdimension and each input or output modality that can be selectively impaired just cannot be the way forward. And finally, we hope that good guesses at the level of mechanism will help us to interpret functionally the extremely exciting architectonic studies of language-committed cortex that have been appearing over the last decade.

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Refen:nces &auvois, M. F. and D&ouesni, J. (1979) Phonological alexia: Three dissociations. J Neur@l.,!Veuroaurg., psych., 82, 1115-1124. Braak, H. (1980) Addtectonics of the Human Telencephulic Correx. Berlin, Springer. Coftheart, M., Patterson, K. E.. and Marshall, J. C. (eds.) (1980) Deep Dyslexia. London, Routledge and Kegan Paul. Fonder, K. I. (1978) Acceskg the mental lexicon. In E. Walker (ed.), Explorations jn the Biology of L.anguuge. Montgomery, Vermont, Bradford Books. Galaburda, A. M. and Sanides, F. (1980) Cytoarchitectonic organization of the human auditory cortex. J. compar. Neural... 190. 597-610. Geschwind, N. (1962) The anatomy of acquired disorders of reading. In J. Money (ed.), Reading Disorders. &&more, Johns Hopkins. Goldtimond, I. and Hawkins, W. F. (1958) Vexiervsrsuch: The logarithmic relationship between wordfrequency and recognition obtained in the absence of stimulus words. J. exper. Psychol.. 56. 457-463. Greenblatt, S. II. (1976) Subangular alexia without agraphia or hemianopsia. Br. Long., 3, 229-245. Klima, E, S. (1972) How alphabets might reflect language. In J. F. Kavanagh and I. G. Mattingly (eds.), Lungrrage by Ear and by Eye. Cambridge, Mass., MIT Press. Lichtheim, I,. (1885) On ap’hasia.Bruin, 7,433-484. Marr, D. (1’980) Visual information processing: The structure and creation of visual representations. PhZ Trans. R. Sot. London B, 290. 193-218. NlarshaIl, 5. C. (1976b Neuropsychological aspects of orthographic representation. In R. J. Wales and E. Walker (e&s.), New Approaches to Lunguage Mechanisms. Amsterdam, North-Holland PubIishing Co. .MarshaU,J. 1C.and Newcombe, F. (1966) Syntactic and semantic errors in paralexia. Neuropsychol., 4, 169-176. ‘Marshall,J. C. and Newcombe, F. (1973)Pattems of paralexia: A psycholinguistic appr0ach.X Psycho&g. Res. 2, 175-199. Morton, J. (1979) Word recognition. In J. Morton and J. C. Marsha8 (eds.), l?sycho,liflgustics Series, Vol. Z!.London, Elek. Moutier, F. (1908) L’aghosie de Broca. Paris, SteinheiI. Newcombe, F. and *Marshall,J. C. (In press) On psycholinguistic classifications of the acquired dyslexias,. Bull. Orton Sot. Sasanuma, 8. (1980) Acquired dyslexia in Japanese: Cliiical features and underlying mechanisms. In M. GDltheart, K. Patterson, and I. C. Marshall (eds.), Deep Dyslexia. London, Routledge and Kegan Paul. Schwartz, M. F., Saffran, E. M., and Marin, 0. S. M. (1980) Fractionating the reading process in dementia: evidence for wordkpecific print-tosound associations. In M. Coltheart, K. Patterson, and J. C. Marshall (eds.), Deep Dyslexia. London, Routledge and Kegan Paul. ShaiIice, T. (1979): Case study approach in neuropsychological research. J. clin. Neuropsychol.. I, 183-211. Taft, M. (1979) Lexical access via an orthographic code: The basic orthographic syllable structure (BOBS).J. verb. &urn. verb. Behav.. 18, 21-39. Teuber, N. L. (1955) Physiological psychology. An. Rev. Aychol., 9, 267-296. wass@ton, E. K. and Shallice, T. (1979) Semantic access dyslexia. Bruin, 202,43-63. Wanington, E. K. and Shallice, T. (1980) Word-fonm dyslexia. B&n, 203, 99-l 12.