This article in last week's Science seems interesting:
Top-Down Versus Bottom-Up Control of Attention in the Prefrontal and Posterior Parietal Cortices
Timothy J. Buschman and Earl K. Miller
Attention can be focused volitionally by "top-down" signals derived from task demands and automatically by "bottom-up" signals from salient stimuli. The frontal and parietal cortices are involved, but their neural activity has not been directly compared. Therefore, we recorded from them simultaneously in monkeys. Prefrontal neurons reflected the target location first during top-down attention, whereas parietal neurons signaled it earlier during bottom-up attention. Synchrony between frontal and parietal areas was stronger in lower frequencies during top-down attention and in higher frequencies during bottom-up attention. This result indicates that top-down and bottom-up signals arise from the frontal and sensory cortex, respectively, and different modes of attention may emphasize synchrony at different frequencies.
At the end, they speculate that this difference in frequency has to do with the greater transmissibility of low frequency signals across different areas.
Lower-frequency bands are more robust to spike timing delays and thus may be better suited for longer-range coupling between multiple, distant areas (28-30). The increase in low-frequency synchrony during search could reflect a "broadcast" of top-down signals on a larger anatomical scale.
That's a simple information theoretic argument; the idea being that a low-rate signal is more resistant to degradation by noise across brain areas.
Buschman TJ, Miller EK. 2007. Top-down versus bottom-up control of attention in the prefrontal and posterior parietal cortices. Science 315:1860-1862. doi:10.1126/science.1138071