RELATIONAL OBJECT ASSOCIATION LEARNING IN RATS WITH HIPPOCAMPAL-LESIONS

The contribution of the hippocampus to relational association learning was examined using a paired associate task in the rat. Using a go/no-go paradigm, rats were trained to respond quickly on positive pair presentations, and to withhold responding on negative pair presentations. Using four constant three-dimensional objects (A, B, C and D), pairing of either AB or CD was assigned as positive pairs, whereas the other possible object combinations (AC, DB, AD, and CB) were never associated with reinforcement. A further type of negative pair trial was also studied which involved object-pairs composed of one of the four constant objects and another object, which varied from trial to trial, and which was selected from a set of 120 junk objects. Electrolytic lesions of the hippocampus were performed on the rats which had previously acquired the paired associate task. Results indicated that hippocampal lesions did not significantly impair the ability of the animals to solve the paired associate task. To study further the exact nature of the learned associations, a transfer test which involved a reversal of the spatial position within each of the six positive as well as negative pair-types was conducted. Results of the transfer test demonstrated that hippocampal-lesioned as well as control rats performed normally by distinguishing the positive from negative pairs regardless of the relative location of stimuli within the object pairs. These data suggest that the hippocampus might not be critical for relational object associations. Further studies involving additional transfer tests or different types of stimuli, especially spatial, are required to ascertain whether or not the hippocampus mediates all types of relational associations in rats.