No abstract

In-immediate free recall, words recalled successively tend to come from nearby serial positions. M. J. Kahana (1996) documented this effect and showed that this tendency, which the authors refer to as the lag recency effect, is well described by a variant of the search of associative memory (SAM) model (J. G. W. Raaijmakers & R. M. Shiffrin, 1980, 1981). In 2 experiments, participants performed immediate, delayed, and continuous distractor free recall under conditions designed to minimize rehearsal. The lag recency effect, previously observed in immediate free recall, was also observed in delayed and continuous distractor free recall. Although two-store memory models, such as SAM, readily account for the end-of-list recency effect in immediate free recall, and its attenuation in delayed free recall, these models fail to account for the long-term recency effect. By means of analytic simulations, the authors show that both the end of list recency effect and the lag recency effect, across all distractor conditions, can be explained by a single-store model in which context, retrieved with each recalled item, serves as a cue for subsequent recalls.

Electrophysiological and hemodynamic measures of human brain activity have been shown to distinguish between episodes of encoding items that are later recalled versus those that are not recalled (Paller and Wagner, 2002). Using intracranial recordings from 793 widespread cortical and subcortical sites in 10 epileptic patients undergoing invasive monitoring, we compared oscillatory power at frequencies ranging from 2 to 64 Hz as participants studied lists of common nouns. Significant increases in oscillatory power during encoding predicted subsequent recall, with this effect predominantly in the 4-8 Hz (theta) and 28-64 Hz (gamma) frequency bands. Sites exhibiting increased theta activity during successful encoding were clustered in right temporal and frontal cortex, whereas those exhibiting increased gamma activity appeared bilaterally at widespread cortical locations. These findings implicate theta and gamma oscillatory activity, across a widespread network of cortical regions, in the formation of new episodic memories.

We present a new model of free recall based on Howard and Kahana's (2002) temporal context model and Usher and McClelland's (2001) leaky-accumulator decision model. In this model, contextual drift gives rise to both short-term and long-term recency effects, and contextual retrieval gives rise to short-term and long-term contiguity effects, Recall decisions are controlled by a race between competitive leaky-accumulators. The model captures the dynamics of immediate, delayed, and continual distractor free recall, demonstrating that dissociations between short-and long-term recency can naturally arise from a model that uses an internal contextual state as the sole cue for retrieval across time scales.The Law of Recency refers to the observation that memories of recent experiences come to mind more easily than memories from the distant past (T. Brown, 1824;Calkins, 1896). Given the ubiquitous nature of recency across time-scales, memory tasks, and stimulus materials, it is not surprising that it has occupied center stage in theoretical analyses of memory over many decades (Crowder, 1976).Whereas some students of memory have sought a common cause for the varied manifestations of recency (Crowder, 1982;Greene, 1986) others have posited distinct mechanisms for the recency effects observed at short and at long time scales (Atkinson & Shiffrin, 1968). In support of a dual-store explanation of recency, Davelaar, Goshen-Gottstein, Ashkeriazi, Haarmann, and Usher (2005) identified several striking differences between the recency effects observed in immediate free recall and continual distractor free recall. In immediate free recall, participants are asked to recall the list items, in any order, immediately following the last item presentation. In continual distractor free recall, participants are given a demanding distractor task following each list item. After the last period of distraction they are asked to recall the items in any order (see Figure 1 for a graphical description of the free recall tasks). Davelaar et al. (2005) suggest that the existence of dissociations between short-and long-term recency calls into question models that hypothesize a general forgetting process underlying recency phenomena observed at different time scales. One popular class of general forgetting models assumes that a time-varying internal context signal gives rise to recency in both shortterm and long-term recall tasks (see Kahana, Howard, & Polyn, 2008, for a review). Davelaar et al. (2005) instead propose a model of free recall in which an activation-based short-term store (STS) produces recency in immediate free recall, and a time-varying context signal Correspondence concerning this article may be addressed to Per B. Sederberg (persed@princeton.edu). NIH Public Access NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript produces long-term recency via a weight-based long-term store (LTS) in continual distractor free recall.We next review some of the major empirical phenomena observed in free recall and t...

Functional imaging of human cortex implicates a diverse network of brain regions supporting working memory - the capacity to hold and manipulate information for short periods of time. Although we are beginning to map out the brain networks supporting working memory, little is known about its physiological basis. We analyzed intracranial recordings from two epileptic patients as they performed a working memory task. Spectral analyses revealed that, in both patients, gamma (30-60 Hz) oscillations increased approximately linearly with memory load, tracking closely with memory load over the course of the trial. This constitutes the first evidence that gamma oscillations, widely implicated in perceptual processes, support the maintenance of multiple items in working memory.

The hippocampus is thought to contribute to episodic memory in part by binding stimuli to their spatiotemporal context. The present study examined how hippocampal neuronal populations encode spatial and temporal context as rats performed a task in which they were required to remember the order of trial-unique sequences of odors. The results suggest that a gradual change in the pattern of hippocampal activity served as a temporal context for odor sampling events and was important for successful subsequent memory for the order of those odors.

The medial temporal lobe (MTL) has been studied extensively at all levels of analysis, yet its function remains unclear. Theory regarding the cognitive function of the MTL has centered along 3 themes. Different authors have emphasized the role of the MTL in episodic recall, spatial navigation, or relational memory. Starting with the temporal context model (M. W. Howard and M. J. , a distributed memory model that has been applied to benchmark data from episodic recall tasks, the authors propose that the entorhinal cortex supports a gradually changing representation of temporal context and the hippocampus proper enables retrieval of these contextual states. Simulation studies show this hypothesis explains the firing of place cells in the entorhinal cortex and the behavioral effects of hippocampal lesion in relational memory tasks. These results constitute a first step towards a unified computational theory of MTL function that integrates neurophysiological, neuropsychological and cognitive findings.The medial temporal lobe (MTL) is a region that includes the hippocampus proper, the subicular complex and parahippocampal cortical regions, including entorhinal, perirhinal, and parahippocampal/postrhinal cortices. A great deal of data from neuropsychology (e.g. Eichenbaum & Cohen, 2001;Scoville & Milner, 1957;Squire, 1992) and functional imaging (e.g. Fernandez, Effern, Grunwald, et al., 1999;Stern, Corkin, Gonzalez, et al., 1996;Wagner et al., 1998) has converged on the idea that the MTL is important in learning and memory. In order to bridge the gap between cognition and cellular-level physiology, we need a mechanistic, mesoscopic description of MTL computational function. We already have several successful verbally-formulated theories of the cognitive function of the MTL. These are described in turn in the following subsections. This paper will attempt to draw these multiple verbal theories together into a single computational framework that is consistent with known neurophysiological and neuroanatomical data.

The temporal relations among word-list items exert a powerful influence on episodic memory retrieval. Two experiments were conducted with younger and older adults in which the age-related recall deficit was examined by using a decomposition method to the serial position curve, partitioning performance into (a) the probability of first recall, illustrating the recency effect, and (b) the conditional response probability, illustrating the lag recency effect (M. W. Howard & M. J. Kahana, 1999). Although the older adults initiated recall in the same manner in both immediate and delayed free recall, temporal proximity of study items (contiguity) exerted a much weaker influence on recall transitions in older adults. This finding suggests that an associative deficit may be an important contributor to older adults' well-known impairment in free recall.