Theoretical assumptions

Most cognitive neuropsychologists subscribe to the following assumptions (with the exception of the last one):

• The cognitive system exhibits modularity, i.e., there are several relatively independent cognitive processes or modules, each of which functions to some extent in isolation from the rest of the processing system; brain damage typically impairs only some of these modules.

• There is a meaningful relationship between the organisation of the physical brain and that of the mind; this assumption is known as isomorphism.

• Investigation of cognition in brain-damaged patients can tell us much about cognitive processes in normal individuals; this assumption is closely bound up with the other assumptions.

• Most patients can be categorised in terms of syndromes, each of which is based on co-occurring sets of symptoms.

Syndromes

The traditional approach within neuropsychology made much use of syndromes. It was claimed that certain sets of symptoms or impairments are usually found together, and each set of co-occurring symptoms was used to define a separate syndrome (e.g., amnesia; dyslexia). This syndrome-based approach allows us to impose some order on the numerous brain-damaged patients who have been studied by assigning them to a fairly small number of categories. It is also of use in identifying those areas of the brain mainly responsible for cognitive function such as language, because we can search for those parts of the brain damaged in all those patients having a given syndrome.

In spite of its uses, the syndrome-based approach has substantial problems. It exaggerates the similarities among different patients allegedly suffering from the same syndrome. In addition, those symptoms or impairments said to form a syndrome may be found in the same patients solely because the underlying cognitive processes are anatomically adjacent.

There have been attempts to propose more specific syndromes or categories based on our theoretical understanding of cognition. However, the discovery of new patients with unusual patterns of deficits, and the occurrence of theoretical advances, mean that the categorisation system is constantly changing. As Ellis (1987) pointed out, "a syndrome thought at time t to be due to damage to a single unitary module is bound to have fractionated by time t+2 years into a host of awkward subtypes."

How should cognitive neuropsychologists react to these problems? Some cognitive neuro-psychologists (e.g., Parkin, 1996) argue that it makes sense to carry out group studies in which patients with the same syndrome are considered together. He introduced what he called the "significance implies homogeneity [uniformity] rule". According to this rule, "if a group of subjects exhibits significant hetereogeneity [variability] then they will not be capable of generating statistically significant group differences" (Parkin, 1996, p. 16). The potential problem with this rule is that a group of patients can show a significant effect even though a majority of the individual patients fail to show the effect.

Ellis (1987) argued that cognitive neuropsychology should proceed on the basis of intensive single-case studies in which individual patients are studied on a wide range of tasks. An adequate theory of cognition should be as applicable to the individual case as to groups of individuals, and so single-case studies provide a perfectly adequate test of cognitive theories. The great advantage of this approach is that there is no need to make simplifying assumptions about which patients do and do not belong to the same syndrome.

Another argument for single-case studies is that it is often not possible to find a group of patients showing very similar cognitive deficits. As Shallice (1991, p. 432) pointed out, "as finer and finer aspects of the cognitive architecture are investigated in attempts to infer normal function, neuropsychology will be forced to resort more and more to single-case studies."

Ellis (1987) may have overstated the value of single-case studies. If our theoretical understanding of an area is rather limited, it may make sense to adopt the syndrome-based approach until the major theoretical issues have been clarified. Furthermore, many experimental cognitive psychologists disapprove of attaching great theoretical significance to findings from individuals who may not be representative even of braindamaged patients. As Shallice (1991, p. 433) argued:

A selective impairment found in a particular task in some patient could just reflect: the patient's idiosyncratic strategy, the greater difficulty of that task compared with the others, a premorbid lacuna [gap] in that patient, or the way a reorganised system but not the original normal system operates.

A reasonable compromise position is to carry out a number of single-case studies. If a theoretically crucial dissociation is found in a single patient, then there are various ways of interpreting the data. However, if the same dissociation is obtained in a number of individual patients, it is less likely that all the patients had atypical cognitive systems prior to brain damage, or that they have all made use of similar compensatory strategies.

Modularity

The whole enterprise of cognitive neuropsychology is based on the assumption that there are numerous modules or cognitive processors in the brain. These modules function relatively independently, so that damage to one module does not directly affect other modules. Modules are anatomically distinct, so that brain damage will often affect some modules while leaving others intact. Cognitive neuropsychology may help the discovery of these major building blocks of cognition. A double dissociation indicates that two tasks make use of different modules or cognitive processors, and so a series of double dissociations can be

Syndrome-based approach vs. single-case studies syndrome-based approach Single-case studies

Advantages Advantages

Provides a means of imposing order and categorising Avoids oversimplifying assumptions, No need to patients. Allows identification of cognitive functions find groups of patients with very similar cognitive of brain areas. Useful while major theoretical issues deficits. remain to be clarified.

Disadvantages Disadvantages

Oversimplification based on theoretical Evidence lacks generalisability and can even be assumptions. Exaggeration of similarities among misleading.

patients._

used to provide a sketch-map of our modular cognitive system.

The notion of modularity was emphasised by Fodor (1983), who identified the following distinguishing features of modules:

• Informational encapsulation: each module functions independently from the functioning of other modules.

• Domain specificity: each module can process only one kind of input (e.g., words; faces).

• Mandatory or compulsory operation: the functioning of a module is not under any form of voluntary control.

• Innateness: modules are inborn.

Fodor's ideas have been influential. However, many psychologists have criticised mandatory operation and innateness as criteria for modularity. Some modules may operate automatically, but there is little evidence to suggest that they all do. It is implausible to assume the innateness of modules underlying skills such as reading and writing, as these are skills that the human race has developed only comparatively recently.

From the perspective of cognitive neuropsychologists, these criticisms do not pose any special problems. If the assumptions of information encapsulation and domain specificity remain tenable, then data from brain-damaged patients can continue to be used in the hunt for cognitive modules. This would still be the case even if it turned out that several modules or cognitive processors were neither mandatory nor innate.

It is not only cognitive neuropsychologists who subscribe to the notion of modularity. Most experimental cognitive psychologists, cognitive scientists, and cognitive neuroscientists also believe in modularity. The four groups differ mainly in terms of their preferred methods for showing modularity.

Isomorphism

Cognitive neuropsychologists assume there is a meaningful relationship between the way in which the brain is organised at a physical level and the way in which the mind and its cognitive modules are organised. This assumption has been called isomorphism, meaning that two things (e.g., brain and mind) have the same shape or form. Thus, it is expected that each module will have a different physical location within the brain. If this expectation is disconfirmed, then cognitive neuropsychology and cognitive neuroscience will become more complex enterprises.

An assumption that is related to isomorphism is that there is localisation of function, meaning that any specific function or process occurs in a given location within the brain (Figure 1.7). The notion of localisation of function seems to be in conflict with the connectionist account, according to which a process (e.g., activation of a concept) can be distributed over a wide area of the brain. There is as yet no definitive evidence to support one view over the other.

Evaluation

Are the various theoretical assumptions underlying cognitive neuropsychology correct? It is hard to tell. Modules do not actually "exist", but are convenient theoretical devices used to clarify our understanding. Therefore, the issue of whether the theoretical assumptions are valuable or not is probably best resolved by considering the extent to which cognitive neuropsychology is successful in increasing our knowledge of cognition. In other words, the proof of the pudding is in the eating. Farah (1994) argued that the evidence does not support what she termed the locality assumption, according to which damage to one module has only "local" effects. According to Farah (1994, p. 101), "The conclusion that the locality assumption may be false is a disheartening one. It undercuts much of the special appeal of neuropsychological architecture."

PET scans can be used to show localisation of function within the brain. This three-dimensional PET scan shows the metabolic activity within the brain during a hand exercise. The exercise involved moving the fingers of the right hand. The front of the brain is at the left. The most active area appears white; this is the motor cortex in the cerebral cortex where movement is coordinated. Photo credit: Montreal Neurological Institute/McGill University/CNRI/Science Photo Library.

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