Evidence from electromagnetic recordings in humans (Arnal et al., 2011 and Iversen et al., 2009) suggest that top-down prediction and bottom-up prediction error may use different frequency bands; where descending predictions are transmitted in the beta range (blue arrows in the figure) and ascending prediction errors are conveyed predominantly in the gamma band (red arrows in the figure).
See (Bastos et al., 2012) for a fuller discussion.