Kometer et al show that psilocybin decreased spontaneous alpha oscillations which precluded the usual decrease in alpha when a visual stimulus is presented.  Psilocybin may result in a brain state in which normal stimulus-driven cortical excitation is overwhelmed by spontaneous neuronal excitation resulting in altered perception and hallucinations.

We recently found evidence that alpha oscillations are useful for clearing out unwanted thoughts (neural ensembles) that could interfere with the current cognitive demands:

• Buschman, T.J., Denovellis, E.L., Diogo, C., Bullock, D. and Miller, E.K. (2012) Synchronous oscillatory neural ensembles for rules in the prefrontal cortex. Neuron, 76: 838-846.  View PDF