Role of the dorsal hippocampus in object memory load.

The dorsal hippocampus is crucial for mammalian spatial memory, but its exact role in item memory is still hotly debated. Recent evidence in humans suggested that the hippocampus might be selectively involved in item short-term memory to deal with an increasing memory load. In this study, we sought to test this hypothesis. To this aim we developed a novel behavioral procedure to study object memory load in mice by progressively increasing the stimulus set size in the spontaneous object recognition task. Using this procedure, we demonstrated that naive mice have a memory span, which is the number of elements they can remember for a short-time interval, of about six objects. Then, we showed that excitotoxic selective lesions of the dorsal hippocampus did not impair novel object discrimination in the condition of low memory load. In contrast, the same lesion impaired novel object discrimination in the high memory load condition, and reduced the object memory span to four objects. These results have important heuristic and clinical implications because they open new perspective toward the understanding of the role of the hippocampus in item memory and in memory span deficits occurring in human pathologies, such as Alzheimer's disease and schizophrenia.