Imaging brain activity using voltage sensitive dyes has revealed propagating waves of neuronal activity in the brain. It’s been hypothesized that propagating waves of activity may contribute to cerebral cortical signal processing by determining when and where the cortex is depolarized in relation to a sensory or motor event.

Spiral waves are a particular kind of propagating wave that rotate around a center point. Spiral waves have been observed in turtle visual cortex and in brain slices but it wasn’t known if they may be found in the intact mammalian brain. Today’s paper “Spiral Wave Dynamics in Neocortex” (published December 9, 2010 in Neuron) reports on an investigation into the existence of spiral waves in intact rat brains.

The research team placed rats into two states and then looked for spiral waves emerging from the cerebral cortex. One state was induced by applying carbachol and bicuculline to the brain. Local excitatory connections in the cerebral cortex were greatly enhanced in this condition and oscillations at around 10 Hertz were induced. The other state was achieved with low levels of pentobarbital anesthesia. In this state, the cortex alternated between theta (around 6 Hz) and delta (1-4 Hz) rhythms that resembled the rapid eye movement (REM) state in rodent natural sleep.

Spiral waves were observed under both conditions. Furthermore, the emergence of spiral waves appeared to have a large impact on the oscillation frequency, spatial coherence, and amplitude of cortical activity.