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Patient Classification in Neuropsychological Research

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BRAIN AND COGNITION 10, 256-295 (1989) Patient Classification in Neuropsychological Research ALFONSO CARAMAZZA AND WILLIAM BADECKER Cognitive Neuropsychology Laboratory, Cognitive Science Center, The Johns Hopkins University In a number of papers we have been concerned with the type of inferences that are legitimate in “experiments of nature” where the experimenter does not and cannot control the modifications to the cognitive system that are introduced by brain damage. We have argued that in such cases very restrictive conditions must he met in order to be able to draw valid inferences about the structure of normal cognitive mechanisms. Two consequences of these conditions are (I) patient classification into syndrome types (e.g., phonological dysgraphia. agram- matism, and so forth) can play no useful role in research concerned with issues about the structure of normal cognitive functioning or its dissolution under con- ditions of brain damage; and (2) only single-patient studies allow valid inferences about the structure of cognitive mechanisms from the analysis of impaired per- formance. Zurif, Gardner, and Brownell (1989. Brrri/z and Cognition, 10, 237- 255) have taken exception to our conclusions and propose to show the limitations of our arguments. In this paper we respond to their criticisms. Q 1989 Academic Press, Inc. Cognitive neuropsychological research is motivated by the assumption that the analysis of impaired performance in brain-damaged patients pro- vides a window into the structure of normal cognitive systems-that is, by the assumption that the results generated by this research activity will provide insights into the structure of cognitive mechanisms-and that it will ultimately provide important constraints for theories of the neural basis for cognitive functioning. Along with many others, we share in this conviction. However, it must be emphasized that it is not immediately obvious that a research enter- prise based on the analysis of pathological states will in fact generate During the writing of this paper the authors were supported in part by NIH Grants NS23836 and NS22201 and by a grant from the Seaver Institute. We gratefully acknowledge their support. We would also like to thank Argye Hillis, Brenda Rapp and Kathleen Straub for their helpful comments on an earlier draft of this paper. Address correspondence and reprint requests to Alfonso Caramazza, Cognitive Neuropsychology Laboratory, Cognitive Science Center, The Johns Hopkins University, Baltimore, MD 21218. 256 0278-2626189 $3.00 Copyright 0 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.NOTES AND DISCUSSION 2.57 the promised, nontrivial empirical constraints for theories of normal cog- nitive processing. Whether this research enterprise will live up to its promise is not an issue that can be decided merely by logic and reflection. It is strictly an empirical matter: We will have to see how useful neuropsy- chological data turn out to be in constraining detailed claims about the nature of cognitive mechanisms. But, while we cannot decide whether, in the long run, cognitive neuropsychology will be scientifically produc- tive, we can evaluate the logic of its practice. In other words, we can evaluate whether, independently of its ultimate usefulness, the practice of cognitive neuropsychology is methodologically coherent. For, unless we can show that the inferences about normal cognitive mechanisms that may be drawn from the analysis of cognitive deficits are, at least on the basis of our current understanding, free of basic fallacies, then there can be no reason to expect that the enterprise could be scientifically productive in the first place. If there are reasons for believing that certain of its practices cannot lead to valid inferences, then we would be well advised to exclude them from the armory of the cognitive neuro- psychologist. In a number of papers we have tried to deal with the issue of the logic of inferences in cognitive neuropsychology. Specifically, we have been concerned with the type of inferences that are legitimate in “experiments of nature” where the experimenter does not and cannot control the modijications to the cognitive system that are introduced by brain dam- age. The results of these analyses have been quite controversial and have even engendered hostility. We have argued that there are very restrictive conditions that must be met in order to be able to draw valid inferences about the structure of normal cognitive mechanisms from the analysis of impaired performance consequent to brain damage. Two prac- tical consequences of these conditions are that (1) patient classification into syndrome types (e.g., Wernicke’s aphasia, phonological dysgraphia, semantic access dyslexia, optic aphasia, agrammatism, and so forth) plays no useful role (and is harmful) in research concerned with issues about the structure of normal cognitive functioning or its dissolution under conditions of brain damage; and (2) only single-patient studies allow valid inferences about the structure of cognitive mechanisms from the analysis of impaired performance.’ The detailed arguments on which these conclusions depend have been presented in various publications ’ Note that these are the consequences of having to satisfy a set of conditions which minimally ensure that valid inferences for normal cognition could be drawn from patho- logical states. These practical consequences have gotten all the attention at the expense of the really interesting issues about the assumptions we must make to be able to draw any inferences at all about normal cognition from pathological states. Of special interest is the assumption of transparency (Caramazza, 1984)-the assumption on which the whole enterprise is based but which has not received serious scrutiny.258


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