Specificity of Cognitive Impairment in Neurological Disease: A Methodological Critique of Parkinson’s Disease

Abstract

Multiple cognitive deficits have been recognized in many neurological disorders but the specificity of the findings and the relationship to the underlying neuropathology remain obscure. Definitions of dementia have been proposed based on symptom profiles of the cognitive disorder and qualitative differences have been claimed between dementias of different aetiology. Some conditions have been claimed to show patterns of cognitive deficit that are distinguished from dementia and related to specific neuropathology or psychological processes, e.g. frontal lobe deficits in Parkinson's disease. Sometimes, a relationship has been established between certain cognitive deficits and particular neurochemical deficits which has led to the notion of specific drug treatment, e.g. cholinergic deficits and memory failure in Alzheimer's disease. However, these conclusions are often potentially flawed by methodological inadequacies. This critique presents some methodological issues relevant to the study of brainbehaviour and drug-behaviour relationships in syndromes of multiple cognitive deficit, using Parkinson's disease as the model. The following recommendations are made: rigid diagnostic criteria; representative patient groups; avoidance of arbitrary quantitative criteria to limit definitions of dementia; matching of groups for overall level of cognitive impairment in the search for qualitative cognitive differences related to neuropathology or effects of particular drugs; the use of suitable controls in patient groups, neuropsychological tests and treatment regimes; the use of specific quantitative tests of cognition, affect and motor disability; and longitudinal, compared with cross-sectional, study design.

Classical eyeblink conditioning in Parkinson's disease

“…The first clinical group consisted of 10 medicated patients (PD-MED); the second group comprised nine unmedicated patients (PD-UNM). As the drugs that are used to treat the motor symptoms of PD may have considerable effects on cognitive function (18), medicated and unmedicated patients were studied as separate groups. This procedure should allow some estimation of the contribution of both PD and medication to possible group differences in learning.…”

Frontal Cognitive Function and Memory in Parkinson’s Disease: Toward a Distinction between Prospective and Declarative Memory Impairments?

“…Lange et al, 1993). Yet another variable on which frontal lesion and PD patients will have to be equated before valid comparisons can be made of the quality of their memory problems, is overall severity of cognitive impairment (see Sagar, 1991); .otherwise quantity and quality of impairments are confounded.…”

Space Deficits in Parkinson’s Disease Patients: Quantitative or Qualitative Differences from Normal Controls?

“…These conflicting results can be attributed to various methodological approaches and even to methodological shortcomings, according to Gotham et at. (1988), Sagar (1991) and Taylor et at. (1986).…”