The human brain embodies a variety of interacting brain structures implicated in attention. These brain structures can be treated as computational objects and their interaction defined as message passing. Causal models based on functional neuroanatomy can be then implemented in computer simulations to reflect the dynamical interaction of brain structures. When this is done, the activity changes of these simulated brain structures are observed to mimic the analog changes in real brains. A working simulation is presented of changes in the pattern of brain structure activity during attention.