We did not find many significant or large group effects in terms of behavioural measures. This may at first seem disappointing, given the behavioural deficits that emerge in studies of patients with more advanced disease. However, the lack of major effects in terms of behavioural data provides more relevance to the significant differences between patients and controls in the functional imaging: functional MRI may be more sensitive to the factors that modify the function of neural systems than the cognitive performance that depend on those systems at least at early stages of the disease; and the specificity of region by group inter-actions also raises the possibility that at early stages of the disease, compensatory mechanisms can allow for a normalperformance. It also reduces the ambiguity in interpreting func-tional MRI data that otherwise arises if there are marked behavioural differences such that activation differences could be thecause or consequence of altered behaviour (Price and Friston, 1999; Poldrack, 2007).
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