When Brain Waves Go Traveling

In July last year I asked, Could Traveling Waves Upset Cognitive Neuroscience? This was a post about a paper from David Alexander et al. arguing that neuroscience was overlooking the importance of how neural activity moves or travels through the brain. Now Alexander et al. are back with a new PLoS ONE paper in which they describe traveling waves in human brain activity, as measured with magnetoencephalography (MEG). The authors scanned 20 volunteers during a visual and auditory task. Alexander et al. focussed on "the class of waves which are characterized by a linear trajectory in the Cartesian coordinates of the sensor space." Here's a visualization. On the left we see a classical standing wave, on the right is a traveling wave. On this image, the horizontal axis is time and the vertical axis is space - each row represents one of the MEG scanner's 151 sensors, arranged around the volunteers' head. The color represents the phase of the wave, in this case in the alpha band with a frequency of 9.2 Hz. The diagonal stripe in the right panel shows a wave moving in space. A movie representation of a traveling wave can be seen here.