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.[Supplemental material is available for this article.]The dorsal hippocampus-HP (the CA fields, dentate gyrus, and subicular complex) is generally crucial for mammalian memory, but its exact role in memory formation is still hotly debated around a conspicuous number of rival theories. Although, in the early 1980s, Olton and colleagues (Wible et al. 1992;Wan et al. 1994) showed that HP lesions produced an impairment on both spatial and nonspatial memory independently on the retention time interval, converging evidence presented a dichotomous view between spatial and item memory (Brown and Aggleton 2001), and between short-term (STM) and long-term (LTM) memory (Shrager et al. 2008), regarding the role of the HP. Brown and Aggleton (2001) proposed that the HP is essential for associational, multi-item, re-collective aspects of recognition memory, but has little or no involvement for single item recognition, which would be mediated by the perirhinal cortex. Squire and colleagues (Broadbent et al. 2004), in contrast, suggested that the HP is selectively involved in LTM, and not in STM or working memory (WM), independently of the nature of the information.Studies in rodents unequivocally demonstrated that HP lesions impair memory consolidation of newly acquired spatial information, and quite consistently affect spatial STM, thus suggesting that the HP is involved in spatial memory
