Evaluating the affective and cognitive effects of a drug: theoretical and methodological considerations.

Current research leads to new drugs with new properties which are clinically screened toward a sum of traditional validation methods and clinical expression of biological models. Two complementary but different theoretical procedures underlie clinical studies. Drugs can be evaluated in "categorial" or "dimensional" perspective. In the first case, their effects are studied on a nosographic entity (for example meeting DSM III criteria for dementia or depression) by global scales whatever clinical heterogeneity or biological specific properties of the drug. In the second case, drugs effects are assessed on a single clinical dimension constituting a continuous parameter from normal to pathological state. Result of these different levels of organisation cannot be compared or correlate. Irrespect of this principle led, in the past, to confusion and lack of interpretable results. A coherent approach may clarify all the process of development of new drugs.