Friday, 7 October 2011

neuropsychology, the assumptions in

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Neuropsychology essays.


Cognitive Neuropsychology (CN) studies and uses the relationship between normal and abnormal cognitive functioning to attain the following goals.

1. Using models of normal functioning to understanding impaired functioning.

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. Using knowledge of impaired and preserved cognitive abilities of brain-damaged patients to improve models of normal functioning.

In order to address the underlying assumptions in CN there is a need to explain why the assumptions are necessary. Despite many years of research in both clinical and cognitive neuropsychology there is no definitive unambiguous model of actual (normal) cognitive processes. Assumptions are therefore required to isolate functions, record and measure them and then to assess the impacts of impairment to those processes. The assumptions are therefore an aid to the process of applying normal and impaired systems to refine the normal models.

There are several key assumptions, which underpin the study of CN. These include modularity, isomorphism, universality, subtractivity, and transparency. The relevance and implications of these assumptions remains an important topic of debate. This essay discusses these assumptions, and their implications for the interpretation of neuropsychological data, counter-arguments against their validity and concludes with how these issues help further the understanding of CN as a science and enrichment of the general debates of brain function. All sciences are based upon inferences and develop over time so the assumptions and implications arising from the CN field are shown to be an important contributor to this body of work.

The first and possibly the most important assumption in cognitive neuropsychology is that there is a functional organisation of the brain i.e. neuroanatomy affects cognitive process. This is developed by the theory of neurological specificity (isomorphism) which states that different mental functions occupy different brain regions. This is concurs with clinical psychology, which espouses theories on the specific cognitive processes and the areas of the brain responsible for them. Evidence confirming this has been compiled through a number of studies, in particular the abnormalities in brain regions of murderers, whereby specific regions were found through imaging to differ when compared to a set of control (Raine et al, 17).

Developing on this the assumption of modularity in CN states that effects of lesions or damage are localised in the brain leaving other parts of the brain undamaged. Evidence for this is found in neurophysiology where responses are selective and specific. Modularity also assumes that each module processes only one particular type of input. This is further enhanced with the theorem of double dissociation (Patterson & Marcel, 177) which hypothesises that there is an independent existence of various modules comprising a system i.e. concrete and abstract words (truth and table) are processed in different parts of the brain. What has yet to be fully determined is whether the modular processing system works in a serial or parallel nature.

Attempts have been made to prove modularity through the application of PET and fMRI studies. Assuming we knew the actual components of cognition, and the modularity assumption were true, then neuroimaging studies could reliably localize cognitive components in brain regions. However, considering the complexity of matching step of process with region through a series of snapshots may not be as reliable as cognitive neuropsychologists would like it to be. Changes in methodology between studies are likely to result in diverging rather than converging evidence, recent efforts to localize the phonology component offer a good test case (Paap, 00).

A component of this is Fractionation, which assumes there is a certain degree of functional specialisation of the brain and that any damage results in selective impairment of specific cognitive functions. However, argument arises whether or not neurons are clustered or distributed (are they all together in the small place or spread throughout different regions of the brain), shared versus dedicated (do they share functions or are they dedicated to a single function), and large versus small number of neurons (are there lots of neurons performing the same task or a small number performing the same task). Each of these impacts on the degree of brain damage caused by a lesion to a particular region of the brain, and whether the results of comparisons of differing size lesions are truly comparable. (You should then lead on to put something in here about single cases v multiple cases for studies).

CN also assumes that there is a relationship between pathological conditions and the normal system. Neuropsychologists assume that that impaired performance has the same relation to a model of the damaged cognitive system as that of normal performance to the normal cognitive system. This step involves the attempt to explicate the possible relationship between damaged and normal cognitive systems using the assumption of subtractivity. However, there is no unambiguous proof of the functioning of the ‘normal’ system, which leads to an inability to compare a normal system to an impaired system leading to a fundamental flaw in the assumption.

All of these assumptions do not imply that the relationship between impaired performance and the normal cognitive system is simple or direct; cognitive neuropsychologists do however require that it be transparent under analysis (the transparency assumption, Caramazza, 186). The use therefore is strictly to infer the structure of normal cognition, with the goal of these inferences in practice leading to significant insight into the nature of normal cognitive processing. This should 1/ allow one to explain a wide range of facts deemed relevant to the domain of inquiry, / generate predictions and receives empirical confirmation with this.

However, Transparency theory in CN assumes that the damage results in the local modification of a cognitive process and the brain does not reorganise or remodify the process after the damage occurs. The plasticity of the brain (especially in brain damaged children) is now recognised throughout the neuroscience’s (Kandel, 000) and may disprove the transparency theory in time to come.

An illustration of the productive use of impaired performance in brain-damaged subjects is the case of NG (Carmazza and Hillis, 10). The patient in question suffered a left-hemisphere stroke that resulted difficulties reproducing details on the right part of objects when drawing. The areas affected by the stroke were a large infraction to the left parietal white matter and a smaller area in the left anterior basal ganglia. In relation to symptoms, no reduction in her visual field was present no were signs of aphasia or dysarthia present.

Conclusion made about normal word recognition on the basis on NG’s performance were extensive but the question remains is why we should accept the results reported for NG as relevant for distinguishing among levels of representation in normal reading. On the other hand there is no for excluding the performance of this subject from the range of facts that may be relevant for the purpose of deciding among competing accounts of the process of word recognition. Therefore it would be imprudent to prejudge the potential contribution of these and similar results as not useful for theories of normal cognition.

The link between observation and theory in all empirical sciences is mediated by complex background assumptions that motivate the relevance of particular types of observations for constraining theory (Gooding et al., 18).

A continuation of this is the assumption of universality in CN, it must be considered whether the cognitive structure of a single case can be assumed be applied universally. Shallice (17) lists recommendations to give ensure validity in reporting single case studies. These are the importance of replicating the study to ensure the results are consistent and theoretical conclusions should be supported by data from more than one test. However, when examining a single-case individual differences in structure and strategy for processing are considered, furthermore it is rare that prior testing of the cognitive structure prior to damage was conducted therefore inference of the structure is dubious. However, studying a single case is important regardless of these issues because not many damaged cases present themselves and the opportunity to study the affects of damage is still relevant to furthering the understanding to the brain.

Various studies have been undertaken to credit and discredit the assumptions used in CN. The most influential of recent times was Kosslyn and Van Kleek’s critique of CN which concluded that the study of brain-damaged subjects for the purpose of constraining theories of normal cognitive processing is doomed to failure. Kosslyn and Van Kleek argue that the analysis of performance of brain-damaged patients is not particularly useful in its own right. The central tenet for this statement is the lack of guidance cognitive neuropsychologist take from the anatomical or neurophysical considerations. An example of this is the conclusions drawn from the case of NG, they are not, according to Kosslyn and Van Kleek, valid since theoretical considerations on which they are based are not ‘cast in light of facts about the brain. However, KVK’s criticism of CN is very general and it spans both the assumptions that underlie the enterprise as well as its practice.

The issues relating to the assumptions which underlie CN are not married to this particular branch of science. In all sciences, changes in methodology can create diverging conclusions. Inference is key to all sciences because there is the likelihood of exceptions, extraneous variables unaccounted for such as individual differences. The issues raised with each assumption do not discredit this as a science but rather assist in creating new avenues for exploration and further understanding of the area as a whole.

The assumption that there is an orderly relationship between the type of stimulus and cell response. That is, that there is a standardised reaction to stimuli. Similarly as above, the full range of cell responses are not definitively known, in particular whether there is an orderly or random reaction to stimuli. Patients may respond and process in different ways to reach the same end result, ‘trained brain strategies’ (Errington-Power, 00).

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