INTERHIPPOCAMPAL TRANSFER OF PLACE NAVIGATION MONOCULARLY ACQUIRED BYRATS DURING UNILATERAL FUNCTIONAL ABLATION OF THE DORSAL HIPPOCAMPUS AND VISUAL-CORTEX WITH LIDOCAINE

To study the neural mechanisms of interhippocampal transfer of lateral ized place navigation engrams in rats, lidocaine was injected via chro nically implanted cannulae to reversibly inactivate the hippocampal fo rmation and the visual cortex on one side. The eye opposite the blocke d side was occluded. Under these conditions, rats learned the location of an invisible platform in a water maze [mean escape latencies per f our-trial block(t) = 5-6 s at the performance asymptote]. Monocular in tact brain retrieval with the trained eye (t = 7) was better than with the untrained eye (t = 13). However, analysis of each retrieval trial indicated untrained eye performance was only poor on the first trial (t = 30). To test whether trans-commissural read-out alone or write-in (i.e. interhippocampal transfer) of the lateralized engram explains t he above results, rats acquired a new platform location (t = 5). Two g roups were then given a 30-s ''free swim'' (the platform was removed) with intact brain and either the trained or untrained eye occluded. A third group did not have this ''transfer'' trial. Retrieval was tested with the trained hippocampus and visual cortex blocked. With the trai ned eye occluded, retrieval in the rats that had the transfer trial (t = 11) was better than in those that did not (t = 25), but slightly wo rse than in rats tested with the untrained eye, hippocampus and visual cortex blocked (t = 7). Additionally, retrieval was similar, independ ent of whether the trained (t = 12) or untrained (t = 11) eye was open on the transfer swim. The 30-s swim alone did not induce comparable s avings. We conclude that interhippocampal transfer of lateralized plac e learning is easily induced, is equal if the transfer is facultative or imperative, and involves both trans-commissural read-out and write- in processes.