LEARNING IMPAIRMENT INDUCED BY LESION OF THE CA1 FIELD OF THE PRIMATEHIPPOCAMPUS - ATTEMPTS TO AMELIORATE THE IMPAIRMENT BY TRANSPLANTATION OF FETAL CA1 TISSUE

Monkeys with bilateral excitotoxic lesion of the CA1 field of the hipp ocampus were severely impaired at learning visuospatial conditional ta sks. This was not a general spatial impairment, because the animals we re not impaired on serial spatial reversal, which requires response fl exibility in the spatial domain; they were not impaired at learning to choose the position fur thest away from a single stimulus, which requ ires analysis of spatial layout of the test area, and they were not im paired at discriminating between two patterns that differed only in or ientation, CA1-lesioned monkeys were impaired at learning a visuospati al conditional task when trials of the two component types ''if AA go left'' and ''if BE go right'' were presented according to either a pse udorandom or alternating schedule; but they were not impaired if one c omponent type of trial was presented until three consecutive correct r esponses were made, followed by the other type of trial, to three cons ecutive correct responses. In all cases testing continued until a crit erion of 27 of 30 consecutive correct responses across both types of t rial was achieved. Although this suggests that CA1-lesioned animals ar e particularly prone to interference effects, they had no difficulty i n learning ten concurrent visual discriminations presented against eit her a uniform background or with each discrimination presented against its own distinctive background, a condition that might reduce interfe rence in unoperated monkeys. Interference following hippocampal damage might occur at a deeper level than stimulus identification such that ani mals with hippocampal damage may be able to learn about many aspec ts of different stimuli in parallel but may be unable to learn about m ultiple related aspects of the same subject matter. Monkeys with graft s of fetal CA1 tissue in the lesioned CA1 field showed significant imp rovement relative to CA1-lesioned animals on those tasks on which CA1- lesioned animals were impaired, although they remained impaired relati ve to control animals. This suggests that the grafts had produced some improvement in performance. Grafted monkeys did not differ from unope rated control monkeys or from CA1-lesioned monkeys on those tasks that were not sensitive to CAI damage. This demonstrates that the grafts d id not have an additional deleterious effect on cognitive performance.