Hippocampectomy disrupts the topography of the rat eyeblink response during acquisition and extinction of classical conditioning.

The effect of hippocampal lesions on acquisition and extinction of eyeblink conditioning in rats was examined with the procedure described by Schmajuk and Christiansen (Physiol. Behav., 48 (1990) 755-758). Male Long-Evans rats received either sham, cortical control or hippocampal lesions. Animals were trained in a delay conditioning paradigm with a 500-ms, 98-dB, white noise conditioned stimulus; a 150-ms, 5-psi air puff unconditioned stimulus; a 350-ms interstimulus interval; and a 60-s average intertrial interval. Each animal received 50 trials daily until it reached a criterion of 80% conditioned responses (CR) on a given day. After acquisition criterion was reached, extinction began the next day and continued until each animal achieved a criterion of one CR in a block of 10 consecutive trials. Although hippocampal lesions affected neither acquisition nor extinction rates, hippocampal lesioned animals showed significantly shorter CR onset latency during acquisition and extinction, and larger CR peak amplitude during acquisition. The results are discussed in terms of hippocampal modulation of cerebellar learning.