The paper considers the question of why subjects are poor at performing two tasks simultaneously if both require attention. It is shown using positron emission tomography (PET) that during new learning of a motor sequence task the prefrontal and anterior cingulate cortex are extensively activated, but that they are no longer activated when a motor sequence has been practiced for an hour until it is automatic. It is also shown that early in motor learning there is interference if subjects are required to generate verbs at the same time, but that the interference is much less if they are required to do this late in motor learning. The prefrontal and anterior cingulate cortex are activated during verb generation. It is therefore suggested that the interference occurs centrally, and that it occurs in either prefrontal or anterior cingulate cortex.