What can be localized in the brain? Toward a "factor" theory on brain organization of cognition

A theoretical integration attempt among the lesional (neuropsychological), physiological (functional), and psychometric models of cognition is presented in this article. Recent neuroimaging techniques particularly fMRI have shown that there are some brain functions (i.e., simple) that can be localized into single brain areas whereas there are others (i.e., complex) that cannot. Clinical neuropsychology has been able to propose some "cognitive factors" based on empirical observations in patients with brain lesions. Factor analysis in psychometry may provide an additional tool to extract some constitutive elements of psychological functions (factors). "Factors" in factor analysis, however, may have different levels of specificity. Some times they refer to functional systems (complex cognition); in other occasions to elements of cognition ("cognitive factors"). It is emphasized that the very same brain areas (and cognitive factors) may be potentially involved in different types of cognition. It is proposed that complex cognition depends on specific patterns of activation of different brain areas and specific circuitries ("modules"), each one making its own contribution to the whole system (functional system). Impairment in a specific cognitive factor, on the other hand, may result in diverse types of impairments. At the moment, it seems feasible to suppose some cognitive factors responsible for normal neuropsychological performance. Theoretically, the impairment in any of these factors could be responsible for some specific neuropsychological syndromes.