Currently, the best understood of the primary generalized seizures is the Absence Seizure(also called Petit Mal seizure).
So we will focus on it. In contrast to secondary generalized seizures, where synchronicity begins in a specific foci in the brain within an aggregate of neurons and then spreads to the entire brain; the primary generalized seizures arises from central brain regions like Thalamus and then spreads rapidly to both hemishpheres.
To understand the pathophysiology of absence seizures, we first have to be acquainted with the physiology of slow-wave(Stage 3) sleep; since they both have similar EEG reading patterns; i.e., the 3-per-second spike-and-wave activity.
In the awake state, the thalamocortical circuits are in "transmission" mode, whereby incoming sensory informations are faithfully transmitted to the cerebral cortex. Whereas in slow-wave sleep, these circuits are in"burst" mode, because of the bursting activity of a unqiue, dendritic T-type Calcium channel in the thalamus which alters the incoming sensory signals in such a way that the output signals to the cortex have an oscillatory firing rate; but no sensory information is transmitted to the cortex. Something similar happens in Absence Seizure.
In absence seizure, there is abnormal, abrupt activation of this T-type calcium channel in the awake state. This has been postulated to be due to hyperpolarizaion of relay cells in thalamus which in turn is due to increased GABAergic input from the reticular nuclei.
Hence, drugs that block T-type calcium channels (Ethosuximide, Valproate, Lamotrigine, Clonazepam) are used in the treatment of these seizures while Barbiturates which augment the GABAergic activity in the reticulothalamic relay circuits exacerbate the condition.