NEURAL ORGANIZATION OF COGNITION

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One of the central tenets of modern cognitive neuroscience is that the neural architecture of cognition is organized into anatomically segregated subsystems (modules) that perform different functions. It is usually plausible to assume that the underlying neural segregation was genetically predetermined, but there are cases that this hypothesis cannot easily accommodate. Furthermore, a growing body of evidence in neuroscience has demonstrated that experience can and does lead to what might be called quantitative changes in brain organization (e.g., changes in the size of an existing functional region). It is therefore natural to ask: Can experience lead to qualitative changes in the brain's modular organization (such as the neural segregation of a function that was not previously localized) or are neural modules always hard-wired by genetics? Previous patient work demonstrating selective impairments in experience-dependent functions (e.g., reading and writing) suggests that experience may indeed be able to produce neural segregation, although the interpretation of those findings can be controversial. We propose to develop converging evidence relevant to the issue by using an independent methodology (neuroimaging) that complements lesion studies. Making progress on this issue could have important implications for our understanding of neural organization, for the interpretation of neurological patients, and ultimately for the rehabilitation of patients with brain damage. The method we propose to use is functional magnetic resonance imaging (fMRI). fMRI has a number of features that make it ideal for addressing the issue of experience-dependent neural segregation: (1) its spatial resolution (1-5mm3) is sufficiently fine to detect anatomic segregation of functional modules, (2) significant results can typically be obtained in individuals making it unnecessary to average across subjects (a critical point because the underlying neural organization under study may differ across individuals), and (3) it can be used with human subjects who are neurologically intact. Our plan is to develop converging evidence regarding experience- dependent neural segregation by investigating three sub- questions: (1) Is letter recognition segregated from digit recognition? (2) Is semantic knowledge about numbers segregated from semantic knowledge about other categories? (3) Are different languages segregated in bilinguals? We chose these domains for four reasons. First, in each case the putatively segregated function can be selectively impaired by brain damage (suggesting that it may be segregated). Second, in each case it is problematic to assume that any underlying neural segregation could be hard-wired. Third, the functions can be relatively well matched with control tasks (making it more difficult to attribute positive results to artifacts). And fourth, the functions can be readily studied using fMRI (e.g., they do not require extensive movement). By studying these three domains in parallel we hope to develop converging evidence that will allow us to draw more general conclusions relevant to the issue of experience-dependent neural segregation.