Hippocampectomy and the attenuation of blocking

Abstract

Rats with dorsal and ventral hippocampal lesions and sham-operated controls were tested in two experiments using Kamin's [(1968). In M. R. Jones (Ed.), “Miami Symposium on the Prediction of Behavior.” Miami: University of Miami Press; (1969). In B. Campbell and R. Church (Eds.), “Punishment and Aversive Behavior.” New York: Appleton-Century-Crofts] two-stage blocking paradigm. In Experiment 1 hippocampal and sham rats conditioned to a compound conditioned stimulus (CS) (tone + light) suppressed on nonreinforced test trials to either element. In contrast, hippocampal and sham rats trained to tone or to light prior to conditioning to the compound CS performed differentially when tested to the redundant cue: Conditioning was blocked for shams, while blocking was severely attenuated with hippocampal rats. In Experiment 2 two parameters known to affect learning rate were manipulated to determine whether a general or specific deficit was responsible for the unblocking in hippocampal rats. Neither manipulation produced blocking in hippocampal rats suggesting that the attenuation of blocking observed in hippocampal rats was due to an impairment in attention rather than to a general learning deficit. The results indicate hippocampectomy produces an inability to exclude irrelevant or redundant information from the stimulus flux.