Learning and retention by monkeys with epileptogenic implants in anteromedial temporal cortex

Alumina cream was implanted bilaterally in anteromedial temporal cortex of monkeys. After the onset of epileptic discharges eight implanted and eight normal monkeys were trained on tasks of visual and somesthetic discriminations and delayed alternation. All monkeys were tested for retention on the three tasks at 3-week intervals, for a total experimental period of 52 weeks. The normal monkeys received epileptogenic implants after their first retention tests. Serial scalp EEGs were recorded and several monkeys were implanted with cortical recording electrodes. On acquisition training the epileptic group was significantly retarded in comparison with the normals, but also exhibited significantly greater variabilities in individual learning rates. On the retention tests the majority of epileptic monkeys showed little or no deficits. In these monkeys the epileptic discharges remained restricted to the implanted areas. The retention deficits in several monkeys could be related to propagations of epileptiform patterns and secondary foci. These results are consistent with earlier findings with focal epileptogenic implants in different cortical areas of deficient acquisition rates on tasks specific to the focal areas, but unimpaired retention performance. The finding can be explained in relation to labile and consolidated mnemonic processes.