Intrinsic brain activity sets the stage for expression of motivated behavior.

Research in many species has provided increasingly detailed information on relevant, primarily subcortical brain systems supporting the expression of basic appetites and drives. While basic appetites and drives are essential for adaptation and survival in any environment, they are naturally constrained by an organism's inherent biology and modulated as circumstances dictate. The brain mechanisms which serve to constrain and modulate them, however, remain much less well understood. We suggest that the manner in which such constraint and potential modulation is achieved likely involves processes that emerge from the coordinated behavior of multiple brain systems, and functional brain imaging techniques such as PET and fMRI are beginning to help us understand aspects of such coordination. In this review we argue that, in pursuit of this understanding, we must focus not only on changes evoked in brain systems during various behaviors, but also on the ongoing and very costly intrinsic activity within these systems, for the latter may be at least as important as the evoked activity in terms of brain function in general and the constraint and modulation of basic appetites and drives in particular. Distinguishing intrinsic from evoked activity in the context of functional brain imaging experiments is challenging, however. Here we review some evolving strategies for doing so. (c) 2005 Wiley-Liss, Inc.