Mechanisms underlying seizure generation are traditionally thought to act o
ver seconds to minutes before clinical seizure onset. We analyzed continuou
s 3- to 14-day intracranial EEG recordings from five patients with mesial t
emporal lobe epilepsy obtained during evaluation for epilepsy surgery. We f
ound localized quantitative EEG changes identifying prolonged bursts of com
plex epileptiform discharges that became more prevalent 7 hr before seizure
s and highly localized subclinical seizure-like activity that became more f
requent 2 hr prior to seizure onset. Accumulated energy increased in the 50
min before seizure onset, compared to baseline. These observations, from a
small number of patients, suggest that epileptic seizures may begin as a c
ascade of electrophysiological events that evolve over hours and that quant
itative measures of preseizure electrical activity could possibly be used t
o predict seizures far in advance of clinical onset.