There's a constant level of electrical activity in the brain, higher or lower, depending on whether the brain is at work or resting. The hallmark of a seizure is an abnormally excitatory electrical discharge within a cluster of neurons, which spreads over the brain as the inappropriate firing of one cell kindles the firing of cells it communicates with.

If the snowball effect goes unchecked,
you have a full, grand mal seizure; if it's stopped, you experience a partial seizure, perhaps a sudden jerking of a limb, a momentary blackout, or an inexplicable sensation of smell, depending upon the location of the seizure in the brain. Epileptics will often experience an aura of "deja vu" prior to blacking out when seizures begin in the temporal lobe, where memory is located.

My seizures always begin in the occipital lobe, where visual phenomena are controlled. Photosensitive is the term. An EEG revealed a marked susceptiblity to colors in the red/orange end of the spectrum, and at frequencies between ten and thirteen pulses per second, the alpha frequencies. I know a seizure is impending when my vision becomes a wild array of violently shifting images, liquid patterns, and exploding lights, and I feel a giddiness down to the pit of my stomach.