It has been proposed that a core network of brain regions, including the hippocampus, supports both past remembering and future imagining. We investigated the importance of the hippocampus for these functions. Five patients with bilateral hippocampal damage and one patient with large medial temporal lobe lesions were tested for their ability to recount autobiographical episodes from the remote past, the recent past, and to imagine plausible episodes in the near future. The patients with hippocampal damage had intact remote autobiographical memory, modestly impaired recent memory, and an intact ability to imagine the future. The patient with large medial temporal lobe lesions had intact remote memory, markedly impaired recent memory, and also had an intact ability to imagine the future. The findings suggest that the capacity for imagining the future, like the capacity for remembering the remote past, is independent of the hippocampus.episodic memory | semantic memory | medial temporal lobe | remote memory | amnesia B ilateral damage to medial temporal lobe structures impairs the formation of new memories and also impairs recall of facts, events, and autobiographical experiences that were acquired during the years before the damage occurred (1, 2). This finding suggests that common mechanisms may underlie the ability to form new memories and the ability to recollect recent memories. There has also been interest in the possible link between remembering past experiences and imagining plausible episodes in the future (3). It was noted, for example, that the memory-impaired patient KC was impaired at generating autobiographical details about his past and also could not imagine future autobiographical episodes (4, 5). A link between past remembering and future imagining has received additional support from other patient studies. Thus, the densely amnesic patient DB had difficulty imagining future episodes (6). Similarly, four of five memory-impaired patients with lesions involving the hippocampus were reported to have difficulty constructing future autobiographical scenarios (7). Moreover, elderly individuals who provided fewer specific details about the recent past also provided fewer specific details about the future (8). Last, patients with mild Alzheimer's disease were impaired at providing autobiographical details about both past and future events (9).Consistent with these observations, neuroimaging studies have described substantial overlap between the brain regions activated when volunteers retrieve past memories and when they imagine future experiences (e.g., refs. 10-14). Schacter et al. (14) suggested that a core network of brain regions supports past remembering and future imagining. The key components of this network are proposed to be the medial prefrontal cortex, posterior regions in medial and lateral parietal cortex, lateral temporal cortex, and the medial temporal lobe including hippocampus (14,15).Within this network, the importance of the hippocampus and related medial temporal lobe structures for f...

Habit memory is thought to involve slowly acquired associations between stimuli and responses and to depend on the basal ganglia. Habit memory has been well studied in experimental animals but is poorly understood in humans because of their strong tendency to acquire information as conscious (declarative) knowledge. Here we show that humans have a robust capacity for gradual trial-and-error learning that operates outside awareness for what is learned and independently of the medial temporal lobe. We tested two patients with large medial temporal lobe lesions and no capacity for declarative memory. Both patients gradually acquired a standard eight-pair object discrimination task over many weeks but at the start of each session could not describe the task, the instructions or the objects. The acquired knowledge was rigidly organized, and performance collapsed when the task format was altered.

The hippocampus is important for autobiographical memory, but its role is unclear. In the study, patients with hippocampal damage and controls were taken on a 25-min walk on the University of California, San Diego, campus during which 11 planned events occurred. Memory was tested directly after the walk. In addition, a second group of controls took the same walk and were tested after 1 mo. Patients with hippocampal damage remembered fewer details than controls tested directly after the walk but remembered a similar number of details as controls tested after 1 mo. Notably, the details that were reported by patients had the characteristics of episodic recollection and included references to particular places and events. Patients exhibited no special difficulty remembering spatial details in comparison with nonspatial details. Last, whereas both control groups tended to recall the events of the walk in chronological order, the order in which patients recalled the events was unrelated to the order in which they occurred. The findings illuminate the role of the hippocampus in autobiographical memory and in the spatial and nonspatial aspects of episodic recollection.hippocampus | prospective memory | autobiographical memory A utobiographical memory represents the experiences of our lives and provides our sense of self. We can reexperience events from the past, and we can imagine events in the future. Without this faculty, our conscious life would be a series of unconnected moments.The severely amnesic patient K.C. cannot remember a single personal event from his life and cannot describe what he did yesterday or what he might do tomorrow (1). K.C.'s amnesia was caused by a closed-head injury, which damaged the hippocampus and adjacent cortex, as well as regions of the frontal and parietal lobes (2). Although the extent of K.C.'s lesions makes it difficult to relate his impairment to anatomy, other work has studied autobiographical memory in patients with more circumscribed damage.A number of studies have identified the hippocampus as an important structure for autobiographical memory (3-5), but its role remains unclear. Some findings emphasize its function in forming new memories about both events and facts (episodic and semantic memory) (6). Other studies suggest that the hippocampus is particularly important for the episodic content of autobiographical memory (e.g., time, place, and perceptual information) and that, as a result, patients with hippocampal damage must rely on semantic memory alone (7). Still other work suggests that the hippocampus is especially important for spatial cognition and that impaired autobiographical memory after hippocampal damage is due to a difficulty in constructing spatially coherent scenes (8).Here, we describe a different approach to the study of autobiographical memory and hippocampal function. Patients with hippocampal damage and healthy volunteers were taken individually on a 25-min walk on the University of California, San Diego, campus during which 11 planned events occurred (Fig...

We evaluated recent proposals that the hippocampus supports certain kinds of visual discrimination performance, for example, when spatial processing is required and the stimuli have a high degree of feature overlap. Patients with circumscribed hippocampal lesions tried to discriminate between images of similar faces or images of similar scenes. In one condition, elements of the stimulus display repeated from trial to trial, and in another condition every trial was unique. In the repeated condition for both faces and scenes, controls gradually improved their performance across testing. In the trial-unique condition, no improvement occurred. The patients were impaired for both faces and scenes in the repeated condition where controls could benefit from learning. However, the patients were fully intact in the trial-unique condition. The results suggest that previous reports of impaired discrimination performance after medial temporal lobe damage may reflect impaired learning rather than impaired visual perception. The findings support the fundamental idea that memory is a distinct cerebral function separable from other perceptual and cognitive abilities.