Supernormal excitability of fibers of the dorsal hippocampal commissure.

The excitability modifications induced by a conditioning stimulus were studied in the three groups of fibers coursing in the dorsal hippocampal commissure of the guinea pig. The response of all fibers was potentiated after a conditioning stimulus greater than and equal to the test stimulus. The potentiation lasted 120 to 600 ms, 180 to 1000 ms, and 120 to 700 ms, respectively, in the three fiber groups. The data indicate that the fibers of the dorsal hippocampal commissure exhibit a prolonged supernormal period during which threshold is reduced for both previously discharged and previously inactive fibers.