SELECTIVE HIPPOCAMPAL-LESIONS IN RATS DISRUPT ACQUISITION AND RETENTION OF A POSITIVE PATTERNING DISCRIMINATION

To determine the contribution of the hippocampus in the processing of a configural positive patterning discrimination (PPD) task, discrimina tion between reinforced presentations of a tone plus light compound st imulus and nonreinforced presentations of each of its components (TL+/ T-,L-) was examined using a conditioned-suppression paradigm. In the f irst experiment, rats demonstrated a rapid acquisition of the PPD with an appropriate discriminative responding. Rats submitted to posttrain ing hippocampal lesions (using multiple injections of ibotenic acid) w ere no longer able to master correctly the previously solved discrimin ation, demonstrating significant differences in their response rates d uring the 2 never-reinforced elemental stimulus presentations. In Expe riment II, lesioned rats were not able to correctly learn the PPD, dem onstrating the same pattern of responding as in Experiment I. These ra ts were also severely disrupted in a radial maze elimination task. Exp eriment ma indicated that, in a simple conditioning task (T+,L+), norm al rats acquired a rapid conditioned suppression for both stimuli, wit h the tone being slightly more susceptible to conditioning than the Li ght stimulus. In Experiment IIIb, conditioning to the compound tone pl us light stimulus led to a clear conditioning to the tone and almost n o conditioning to the light, suggesting an overshadowing from the tone to the light. Similar results were obtained in rats with hippocampal lesions. These results strongly suggest that the disruption showed by rats with hippocampal lesions in the PPD task cannot be due to an alte ration of the relative salience of the stimulus. The inability of rats with hippocampal lesions to solve correctly the PPD is due to difficu lties in eliminating responding to some unimportant events of the situ ation, reflecting a deficit in selective attention processes rather th an in an ability to process configural stimuli. In the discussion, the putative role of the hippocampus in selective attentional processes i s more fully discussed. (C) 1997 Elsevier Science Inc.