Alexander et al. have analyzed data from magnetoencephalogram (MEG), electroencephalogram (EEG) and electrocorticogram (ECoG), focusing on globally synchronous fields in within-trial evoked brain activity. They quantified several signal components and compared topographies of activation across large-scale cortex. They found the topography of evoked responses was primarily a function of within-trial phase, and within-trial phase topography could be modeled as traveling waves. Traveling waves explained more signal than the trial-averaged phase topography. Here is a edited clip of explanation from Alexander:
The brain can be studied on various scales,..."You have the neurons, the circuits between the neurons, the Brodmann areas – brain areas that correspond to a certain function – and the entire cortex. Traditionally, scientists looked at local activity when studying brain activity, for example, activity in the Brodmann areas. To do this, you take EEG's (electroencephalograms) to measure the brain’s electrical activity while a subject performs a task and then you try to trace that activity back to one or more brain areas."
..."We are examining the activity in the cerebral cortex as a whole. The brain is a non-stop, always-active system. When we perceive something, the information does not end up in a specific part of our brain. Rather, it is added to the brain's existing activity. If we measure the electrochemical activity of the whole cortex, we find wave-like patterns. This shows that brain activity is not local but rather that activity constantly moves from one part of the brain to another. The local activity in the Brodmann areas only appears when you average over many such waves.”
Each activity wave in the cerebral cortex is unique. "When someone repeats the same action, such as drumming their fingers, the motor centre in the brain is stimulated. But with each individual action, you still get a different wave across the cortex as a whole. Perhaps the person was more engaged in the action the first time than he was the second time, or perhaps he had something else on his mind or had a different intention for the action. The direction of the waves is also meaningful. It is already clear, for example, that activity waves related to orienting move differently in children – more prominently from back to front – than in adults. With further research, we hope to unravel what these different wave trajectories mean."
Video: A wave of brain activity measured by the magnetic field it generates externally to the head. The left view of the head is shown on the left side of the image and the right view of the head on the right side of the image. This wave takes about 100 milliseconds to traverse the entire surface of the brain. The travelling wave originates on the lower-left of the head and travels to the lower front-right of the head. Most of the magnetic field shown in this video is generated by brain activity close to the surface of the cortex. The times displayed at the bottom are relative to the subject pressing a button at time zero. The colour scale shows the peak of the wave as hot colours and the trough of the wave as dark colours.