Cortical and subcortical generators of normal and abnormal rhythmicity.

The cerebral cortex and thalamus can both generate cyclical oscillations of neuronal activity. Within the thalamus, sleep spindles are generated as a recurrent interaction between thalamocortical and thalamic reticular cells. Abnormally strong activation of the inhibitory thalamic reticular neurons can result in the transformation of this normal rhythm into one that resembles that underlying absence seizures. The cerebral cortex can generate periodic activity at < 1 Hz through recurrent excitation that is controlled by inhibition. Again, loss of inhibitory control allows this normal activity to become epileptiform. Together, the cerebral cortex and thalamus can form cyclical loops of activity that may contribute to some forms of epileptic seizures. It is proposed that hypsarrhythmic activity that is characteristic of children with infantile spasms may be generated through abnormal, locally synchronized bursts of activity within the cerebral cortex.