Figure 7.

Sensitivity to external perturbations. A, Seizure triggered by local application of 10 mm glutamate. B, During preictal period immediately preceding the next expected seizure, weak external depolarizing electric fields (−4 V/m, 0.75 s) can trigger seizure (C). D, Repeated medium strength (−8 V/m) pulses failed to trigger seizures until later in the interictal period. E, Strong depolarizing field (−40 V/m) reliably triggered seizures early in the interictal period. F, Repeated pulses (as in D) demonstrate the relationship between field strength and shortening of the interictal period. G, Sensitivity of CA1 area to applied fields, of all strengths, as a function of stage during the interictal period (divided into 5 equal segments based on the interictal period measured in the absence of fields). The closer to the predicted seizure onset, the higher the percentage of applied fields of all strengths capable of inducing seizures.