The nature of remote memory impairment in patients with medial temporal lobe damage is the subject of some debate. While some investigators have found that retrograde amnesia in such patients is temporally graded, with relative sparing of remote memories (Squire and Alvarez, 1995), others contend that impairment is of very long duration and that remote memories are not necessarily spared (Sanders and Warrington, 1971;Nadel and Moscovitch, 1997). In this study, remote memory was assessed in 25 patients with unilateral temporal lobe epilepsy and 22 non-neurologically impaired controls using the Autobiographical Memory Interview (Kopelman et al., 1989). Results indicate that patients have impaired personal episodic memory but intact personal semantic memory. The impairment extends even to the most remote time periods in early childhood, long before seizure onset in many patients. As well, patients awaiting temporal lobectomy for control of seizures perform as poorly as those who have already undergone resective surgery. These results support the hypothesis that temporal lobe damage or dysfunction, caused by recurrent seizures or surgical excision, results in extensive retrograde amnesia for personal episodic memories. Interestingly, patients with radiological evidence of hippocampal sclerosis were not significantly more impaired than those without obvious sclerosis. These results indicate that even minimal damage to medial temporal lobes results in significant impairment to autobiographical episodic memory. These findings are more compatible with a memory loss or retrieval deficit rather than a consolidation account of remote memory impairment. Key words: remote memory; episodic memory; epilepsy; temporal gradient; multiple trace theory; consolidation; autobiographical memoryThe study of remote memory is key to understanding the role of the hippocampal complex in memory consolidation. The development of a standardized test, the Autobiographical Memory Interview (AMI), by Kopelman et al. (1989Kopelman et al. ( , 1999 has made it easier to assess personal remote memory reliably in humans. The AMI measures personal semantic and episodic memories from different time periods-childhood, early adulthood, and the recent past-permitting examination of the possible differential effects of brain damage on these two types of autobiographical memory.In keeping with earlier observations (Ribot, 1882;Burnham, 1903;Scoville and Milner, 1957) Squire (1992) and Squire and Alvarez (1995) noted a temporal gradient to retrograde amnesia (RA) associated with medial temporal damage with the most remote memories differentially spared. He concluded that the hippocampus is involved in the establishment of new memories and their temporary storage but that over time these memories become independent of the hippocampus through consolidation. Once consolidation is complete, the memory trace resides elsewhere, rendering the hippocampus unnecessary for retrieval. According to consolidation theory, patients with temporal lobe epilepsy (TLE) and d...

People with whom one is personally acquainted tend to elicit richer and more vivid memories than people with whom one does not have a personal connection. Recent findings from neurons in the human medial temporal lobe ( epilepsy ͉ hippocampus ͉ memory ͉ familiarity ͉ single-unit recordings H umans are self-absorbed by nature. One virtually infallible method of enhancing memory is simply to relate the to-be-remembered information to one's self. The self-reference effect (1) is a well-documented encoding enhancement: people are more likely to remember items that are personally relevant, than items that have undergone some other deep or semantically elaborative encoding processes (2). One mechanism by which the self-referent effect might operate is via the incidental recollection of a rich network of information related to past experiences with that particular item. In addition, incidental recollection of related autobiographical associations can lead to performance advantages such as enhanced memory and speeded responding (3). This incidental recollection has been shown to occur in the context of identifying famous people (3), and has also been shown to involve the hippocampus (4, 5). Furthermore, famous faces and names have long been used to query the integrity of recent and remote semantic memory in patients with temporal lobe epilepsy (TLE) (3, 6-11). Finally, autobiographical memory retrieval consistently engages the medial temporal lobe (12), suggesting that cells in this region may respond differentially to faces that elicit autobiographical memory retrieval.Recently, recordings from the human medial temporal lobe (MTL) have shown that individual neurons can be highly selective in terms of the stimuli to which they respond (13). Out of a set of about 100 visual images across several categories, such as faces, animals, and landmarks, some cells showed robust responses to only a handful of pictures pertaining to a single conceptual category, such a particular famous person. This selectivity provides an important clue as to the mechanism by which the brain represents information currently in awareness (14). In animals, selective sparse coding has also been observed in recordings from the MTL (15, 16). In fact, the idea that the MTL represents information using a sparse code is one of the basic tenets of most contemporary memory models (17)(18)(19). Given that the MTL assigns a relatively small number of neurons to a specific stimulus (20), and that the number of stimuli in the environment is very large, does a feature such as personal relevance, which may be related to the incidental recollection of autobiographically significant information (3), make a stimulus more likely to elicit a selective excitatory response from a cell? Here, we address the question of whether individual neurons in the MTL show a preference for personally relevant pictures.In this study, patients with intracranial electrodes implanted for clinical reasons were shown photographs of varying personal relevance: previously unknown faces and...

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The difficulty of reasoning tasks depends on their relational complexity, which increases with the number of relations that must be considered simultaneously to make an inference, and on the number of irrelevant items that must be inhibited. The authors examined the ability of younger and older adults to integrate multiple relations and inhibit irrelevant stimuli. Young adults performed well at all but the highest level of relational complexity, whereas older adults performed poorly even at a medium level of relational complexity, especially when irrelevant information was presented. Simulations based on a neurocomputational model of analogical reasoning, Learning and Inference with Schemas and Analogies (LISA), suggest that the observed decline in reasoning performance may be explained by a decline in attention and inhibitory functions in older adults.Human reasoning depends in part on the ability to integrate multiple relations and inhibit irrelevant information. For example, if Bill is taller than Carl and Abe is taller than Bill, one must integrate the two "taller than" relations to make the inference that Abe is taller than Carl. A relational analysis of reasoning provides a framework that makes it possible to define levels of complexity for particular reasoning tasks. According to Halford (1998;Halford & Wilson, 1980;Halford, Wilson, & Phillips, 1998), the processing load for any step in a task is determined by the number of dimensions, or relations, that must be processed simultaneously to make the decisions required for that step. Dimensions are viewed as analogous to degrees of freedom, or the number of independent sources of variation. At the first level of complexity (Level 1), the reasoner needs to consider only one relation to solve the task correctly.1 At Level 2, the reasoner must integrate two relations, and so on. For example, it is necessary to integrate two relations to correctly solve the transitive inference problem described previously.More generally, we define relational complexity as the number of relations that a reasoner must simultaneously "hold in mind" to generate the solution. This framework makes it possible to study the processing demands of complex reasoning and to better characterize changes in reasoning ability that occur during normal aging. Although it is known that reasoning ability declines with age (for reviews, see Salthouse, 1992aSalthouse, , 2005, the underlying mechanisms of this decline are not well established. In the current study, we investigate the effects of age on the ability to solve a complex analogical reasoning problem and consider possible algorithmic reasons for the observed decline with age.The ability to hold in mind multiple relations relies on working memory capacity. (See Morrison, 2005, for a review of the role of working memory in reasoning.) Miller (1956) originally postulated that the processing capacity of what is now called working memory is seven plus or minus two "chunks," which are independent units of information. This estimate has since bee...

Different structures within the medial-temporal lobe likely make distinct contributions to declarative memory. In particular, several current psychological and computational models of memory predict that the hippocampus and parahippocampal regions play different roles in the formation and retrieval of declarative memories [e.g., Norman, K. A., & O'Reilly, R. C. Modeling hippocampal and neocortical contributions to recognition memory: A complementary-learning systems approach. Psychological Review, 110, 611-646, 2003]. Here, we examined the neuronal firing patterns in these two regions during recognition memory. Recording directly from neurons in humans, we find that cells in both regions respond to novel stimuli with an increase in firing (excitation). However, already on the second presentation of a stimulus, neurons in these regions show very different firing patterns. In the parahippocampal region there is dramatic decrease in the number of cells responding to the stimuli, whereas in the hippocampus there is recruitment of a large subset of neurons showing inhibitory (decrease from baseline firing) responses. These results suggest that inhibition is a mechanism used by cells in the human hippocampus to support sparse coding in mnemonic processing. The findings also provide further evidence for the division of labor in the medial-temporal lobe with respect to declarative memory processes.