Why do absence seizures appear as 3 Hz spike-and-wave over both hemispheres at once? · Grey Matter

The rhythm is generated by a loop between the cortex and the thalamus, which projects to both hemispheres, so the discharge starts everywhere the loop reaches instead of spreading from one spot.

Why do absence seizures appear as 3 Hz spike-and-wave over both hemispheres at once?

The rhythm is generated by a loop between the cortex and the thalamus, which projects to both hemispheres, so the discharge starts everywhere the loop reaches instead of spreading from one spot. Cortex excites the thalamus, the inhibitory neurons of the thalamic reticular nucleus then hyperpolarise the relay cells, and the relay cells answer with a rebound burst carried by T-type calcium channels that drives the cortex again. Each turn of that loop takes about a third of a second, which sets the frequency near 3 per second: the spike is the cortex firing together, the wave is the inhibition that follows.