Computation in the Brain Neuropsychological Inference with an Interactive Brain: A Critique of the “Locality” Assumption Sang Joon Park

Introduction Various parts of the encephalon, through anatomically distinct, are yet so intimately combined and related as to from a complex whole. –Greater or lesser Lesions produce such general perturbation of the functions of the organ. –Highly difficult to trace any connection between the symptoms produced and the lesion as such. Brain damage often rather selective effects on cognitive functioning, impairing some abilities while sparing others. Psychologists interested in describing the “functional architecture” of the mind. –Brain damage is a potentially useful source of constraints on the functional architecture. Focusing on one of the assumptions that frequently underlies the use of neuropsychological data in the development of cognitive theories.

The Locality Assumption Cognitive neuropsychologists generally assume that damage to one component of the functional architecture will have exclusively “local” effects “Modular”, in the sense of being “informationally encapsulated” (Fodor, 1983; see also multiple book review, BBS 8, 1985) One of Fodor’s (1983) other criteria for modulehood, which he suggests will coincide with informationl encapsulation, is that modules make use of dedicated hardware and can therefore be selectively impaired by local brain damage. But ! In contrast, if the different components of the cognitive system were highly interactive, each one depending on input from many or most of the others, then damage to any one component could significantly modify the functioning of the others.

The Locality Assumption (Cont’d) Several Cognitive neuropsychologists have pointed out that informational encapsulation and the locality of the effects of brain damage are assumptions, and expressed varying degrees of confidence in them. Shallice endorses a weaker and more general version of modularity than Fodor’s, according to which components of the functional architecture can be distinguished. –Conceptually in terms of their specialized functions. –Empirically by the relatively selective deficits that ensue upon damage to one of them. Shallice, 1988, Ch. 2 Shallice’s hypothesis is connected to –“isolable subsystems”, Posner (1978) –distinguishing modular systems with some mutual dependence among modules from fully interactive systems, Tulving (1983)

The Locality Assumption (Cont’d) “Transparency assumption”, Caramazza (1984; 1986) –Closely related to the locality assumption but weaker. –Transparency requires only that the behavior of the damaged system be understandable in therms of the functional architecture of the normal system. However, assuming that the relevant ability has been experimentally isolated and that the deficit is truly selective, the locality assumption allows us to delineate and characterize the components of the functional architecture in a direct, almost algorithm way.

The Locality Assumption is Ubiquitous in Cognitive Neuropsychology With the domain of reading, phonological dyslexics show a selective deficit in tasks that require grapheme-to-phonological translation. Similarly, in surface dyslexia a selective deficit in reading irregular words Coltheart, 1985 These lexical items are represented by a separate system In the domain of vision, some right hemisphere-damaged patients shows an apparently selective impairment in the recognition of objects viewed from unusual perspectives. Warrington, 1985 Therefore, assuming that the effects of the lesion on the lesion on the functioning of the system are local to the lesioned component.

Two Empirical Issues about the Locality Assumption We do not yet know whether the brain is such a separated functional system. Is it, locality assumption, really indispensable to cognitive neuropsychology? The most fruitful approach to answering these two questions would there involve an analysis of the body of cognitive neuropsychological research, or at least an extensive sample of it.

An Architecture for Interactive Processing Alternative assumption The parallel distributed processing (PDP) framework will be used as a source of principled constraints on the ways in which the remaining parts of the system behave after local damage. –Distributed representation of knowledge –Graded nature of information processing –Interactivity.

Reinterpreting Dissociations Without the Locality Assumption: Three Case Studiees The Functional Architecture of Semantic Memory : Category-Specific? The Functional Architecture of Visual Attention : A “Disengage” Module? The Functional Architecture of Visual Face Recognition : Separate Components for Visual Processing and Awareness?

Reinterpreting Dissociations Without the Locality Assumption: Three Case Studiees The Functional Architecture of Semantic Memory : Category-Specific?

Evidence for Selective Impairments in Knowledge of Living and Nonliving Things Materials –4 patients –Had recovered from herpes encephalitis and had sustained bilateral temporal lobe damage Methods –Each 2 of patients were studied in detail and showed a selective impairment for living things across a range of tasks, both visual and verbal Results Warrington and Shallice (1984)

Evidence for Selective Impairments in Knowledge of Living and Nonliving Things (Cont’d)

Results –Although these patients are not entirely normal in their knowlegde of the relative spared category, –they are markedly worse at recognizing, defining, or answering questions about items from the impaired category.

Interpretation of “Living Things” and “Nonliving Things” Deficits Relative to the Functional Architecture of Semantic Memory Category-specific functional architecture for semantic memory

Interpretation of “Living Things” and “Nonliving Things” Deficits Relative to the Functional Architecture of Semantic Memory (Cont’d) Modality-specific functional architecture for semantic memory However, it is not able to account for the impaired ability of these patients to retrieve non-visual information about living things.

Accounting for Category-Specific Impairments with an Interactive Modality-Specific Architecture Picture-to-name & name-to-picture Functional semantics is damaged Accuracy with Functional semantic memory information

Reinterpreting Dissociations Without the Locality Assumption: Three Case Studiees The Functional Architecture of Visual Attention : A “Disengage” Module?

Evidence for the Disengage Deficit Materials –Parietally damaged patients are tested. Methods

Evidence for the Disengage Deficit Results –They perform roughly normally on validly cued trials when the target appears on the side of space ipsilateral to their lesions –However, their reaction times are greatly slowed to invalidly cued contralesional targets. –This means as if once attention has been engaged on the ipsilesional side it cannot be disengaged to be moved to a target occuring on the contralesional side.

Evidence for the Disengage Deficit Interpretation of the Disengage Deficit Relative to the Functional Architecture of Visual Attention Posner et al. (1984)

The Functional Architecture of Visual Attention : A “Disengage” Module? (Cont’d) Accounting for Category-Specific Impairments with an Interactive Modality-Specific Architecture Cohen et al. (1994)

The Functional Architecture of Visual Attention : A “Disengage” Module? (Cont’d) Posner et al. (1994)

The Functional Architecture of Visual Attention : A “Disengage” Module? (Cont’d) Relevance of the Locality Assumption for Architecture of Visual Attention –After damage to the attention units on one side of the model, the non-damaged attention units on the other side function differently. –This is because of the reduced ability of the attention units on the damaged side to recapture activation from the intact side. –The ability of this model to account for the disengage deficit depends critically upon this non-local aspect of its response to damage.

Reinterpreting Dissociations Without the Locality Assumption: Three Case Studiees The Functional Architecture of Visual Face Recognition : Separate Components for Visual Processing and Awareness?

Evidence for Dissociated Recognition and Awareness of Recognition

Evidence for Dissociated Recognition and Awareness of Recognition (Cont’d) Haan et al. (1992) The prosopagnosic patient who was tested in this task showed the same pattern of results, implying that he was unconsciously recognizing the faces fully.

The Functional Architecture of Visual Face Recognition : Separate Components for Visual Processing and Awareness? Interpretation of Covert Recognition relative to the Functional Architecture of Visual Recognition and Conscious Awareness

The Functional Architecture of Visual Face Recognition : Separate Components for Visual Processing and Awareness? Accounting for Dissociated Covert and Overt Recognition with an Interactive Architecture At levels of damage corresponding to removal of 62.5% and 75% of the face units in a given layer.

The Functional Architecture of Visual Face Recognition : Separate Components for Visual Processing and Awareness? In contrast, the damaged network showed faster learning of correct face name associations.

The Functional Architecture of Visual Face Recognition : Separate Components for Visual Processing and Awareness? When the network was presented with name patterns and the time it too to classify them according to occupation.

General Discussion 1.Evaluating the Truth and Methodological Necessity of the Locality Assumption 2.Possible Objections 1.PDP and Box-and-Arrow : Apples and Oranges? 2.The Locality Assumption can be Saved with more Fine-Grained Empirical Analysis of the Deficit 3.PDP Could be False 3.General Implications of Denying the Locality Assumption 1.Modularity 2.Top-Down Versus Bottom-Up Research Strategies 3.Implications for Cognitive Neuropsychology

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