Groups of neurons firing in unison across the cortex synchronise at many different time scales. The most notable phase time scales are the gamma band (30-40 hertz) and the beta band (15-25 hertz). There is much debate in the community as to the role of phase synchronisation. Many think they are a mere epiphenomenon whilst others believe they play a vital role such as binding disparate parts of brain activation together (e.g. a population encoding for red and a population encoding for circle synchronize to encode for a red circle). Opinions are very polarized and I will not enter that debate right now.
What I would like to mention is Pascal Fries talk at the Brain Connectivity Workshop today. In his work studying phase synchrony in monkey’s brains he states that topologically higher areas in the visual hierarchy exhibit attentive influence on lower areas and in doing so they also manipulate synchrony. In his recent research he uses Granger causality, an analysis technique that can give you a metric for how much one part of a system at a particular time affects another at a later time. His results show that top down processes in the visual cortex have a causal synchronizing effect on lower areas but not the other way round. The implication of this is that higher level areas may facilitate the binding with or between lower areas through attentive modulation. For more details read here.