The effects of ablations on primary and secondary epileptic discharges in commissure-sectioned rhesus monkeys

Nine monkeys underwent division of the corpus callosum, anterior commissure, massa intermedia, and posterior commissure. At the same operation, in seven of these animals aluminium hydroxide was applied to the left posterior parietal-prestriate cortex. Monthly EEG recordings were taken from all animals. All seven animals with epileptogenic implants gave evidence of primary abnormal discharges (i.e. from the ipsilateral hemisphere) and secondary (i.e. from the contralateral hemisphere) discharges. The secondary events were of two types: transmitted, i.e. synchronous with primary events; independent, i.e. asynchronous with primary events. After intervals varying from 4 to 22 months all seven monkeys underwent wide ablation of the posterior parietal cortex: on the left in four animals, on the right in three animals. EEG recordings were taken for a further 6 months. All recordings were quantitatively analysed, the number of primary, transmitted secondary and independent secondary events being counted separately. This analysis indicated the following: (1) Primary and transmitted secondary discharges ceased after primary ablations but were unaffected by removal of the secondary cortex. (2) Independent secondary discharges persisted after removal of the primary focus but were abolished by secondary ablations. Possible mechanisms for the development of transmitted and independent secondary discharges are considered.