A number of laboratories have been suggesting that top-down vs bottom-up attention signals may be transmitted across the cortex via neural synchronization at beta vs gamma frequencies, respectively (Buschman and Miller, 2007; Bosman et al, 2012; Gregoriou et al, 2009, see review by Wang 2010).  Chanes et al (2013) tested this by entraining the human frontal cortex at those frequencies.  This produced the predicted top-down vs bottom-up effects on behavior: Beta modulated (top-down) response criterion whereas gamma modulated (bottom-up) perceptual sensitivity.  This supports observations that different frequencies of neural synchrony support feedback vs feedforward cortical processing.  It also shows how neural synchrony supports multiplexing of function: Activity from the same neurons has different functional outcomes depending on their rhythmic dynamics.
Chanes et al (2013)

Buschman, T.J. and Miller, E.K. (2007) Top-down versus bottom-up control of attention in the prefrontal and posterior parietal cortices. Science. 315: 1860-1862  The Scientist’s “Hot Paper” for October 2009. View PDF »

G.G. Gregoriou, S.J. Gotts, H. Zhou, R. Desimone (2009) High-frequency, long-range coupling between prefrontal and visual cortex during attention Science, 324 (2009), pp. 1207–1210.

C.A. Bosman, J.-M. Schoffelen, N. Brunet, R. Oostenveld, A.M. Bastos, T. Womelsdorf, B. Rubehn, T. Stieglitz, P. De Weerd, P. Fries (2012) Attentional stimulus selection through selective synchronization between monkey visual areas. Neuron, 75 (2012), pp. 875–888

X.-J. Wang (2010) Neurophysiological and computational principles of cortical rhythms in cognition.  Physiol. Rev., 90 (2010), pp. 1195–1268