Ontogeny of delay versus trace eyeblink conditioning in the rat.

The ontogeny of delay versus trace eyeblink conditioning was examined in 19-, 23-, and 30-day-old rat pups. Pairings of a tone conditioned stimulus (CS) and periocular shock unconditioned stimulus (US; 100-ms) were presented in one of three conditioning paradigms: standard delay [380-ms CS, 280-ms interstimulus interval (ISI)], trace (380-ms CS, 500-ms trace interval), or long-delay (980-ms CS, 880-ms ISI). The results of two experiments indicated that standard delay conditioning emerged between 19 and 23 days of age whereas trace and long-delay eyeblink conditioning emerged more slowly from postnatal Days 19 to 30. Because the acquisition profile for long-delay paralleled that of trace and not standard delay, it appears that the relative deficits in the emergence of trace eyeblink conditioning during development reflect difficulty in forming associations over long ISIs rather than the short-term memory demands of the trace conditioning paradigm.