TMS studies of preictal cortical excitability change

The transition between the interictal and ictal states may be characterised in terms of the dynamics of a complex system. Seizures may emerge because of a change in system parameters, but these parameters may be invisible to passive observation. Therefore, a number of investigators have developed methods to probe the system using stimulation; these probing stimuli may reveal important hidden parameters. Here we describe studies from two sets of investigators working independently, which have shown that motor responses to transcranial magnetic brain stimulation (TMS) differ between the interictal state remote from any seizure, and a period of hours immediately prior to a seizure. We place these studies in the context of the known physiology of motor responses to TMS and discuss how actively probing the state of brain excitability may open new windows on its dynamics.