COMMENTARY The Units Of Thought

That associative processing provides the vehicle of thought is a long-standing idea. We describe here observations from cognitive neuroimaging that elucidate the neural processing that medi- ates this element. This account further allows a more specific ascription of a cognitive function to the brain's ''default'' activity in mindwander- ing. We extend this account to argue that one primary outcome of asso- ciative processing is the generation of predictions, which approximate the immediately relevant future and thus facilitate perception, action, and the progression of thought. V C 2007 Wiley-Liss, Inc.