One of the surprising consequences following the explosion of functional brain imaging research over the last 10 years, is the rediscovery of individual differences. Early Positron Emission Tomography (PET) studies focused on cerebral blood flow or glucose metabolic rate comparisons between groups, say schizophrenics and normal controls. Most early studies measured the brain during a resting state, often eyes closed and lying quietly. As many psychologists discovered the PET technique, studies began to focus on comparisons between periods of different task activations in normal controls. Only a few early studies correlated actual task performance with regional cerebral function. These studies deliberately selected tasks that showed individual differences in performance rather than tasks where performance was uniform from subject to subject. Moreover, finding negative as well as positive correlations highlighted that regional brain deactivation may be as important for cognition as activation. Currently, many functional brain imaging studies examine correlations between task performance and regional brain function, even in cognitive psychology where individual differences historically have not been emphasized.