The Psychology and Neuroscience of Forgetting

Sleep leads to changes in the emotional memory trace: evidence from FMRI

After information is encoded into memory, it undergoes an off-line period of consolidation that may occur optimally during sleep. The consolidation process not only solidifies memories but also changes them in useful and adaptive ways. Here, we provide evidence for a shift in the neural structures used to retrieve emotional memories after a night of sleep compared to a day of wakefulness. Although the hippocampus was activated during successful retrieval of negative objects regardless of whether participants slept during a delay, sleep led to a shift from engagement of a diffuse memory retrieval network-including widespread activity in the lateral prefrontal and parietal cortices-to a more refined network of regions-including the amygdala and ventromedial pFC. Effective connectivity analyses revealed stronger connections among limbic regions after sleep versus wake. Although circadian effects may have contributed to these findings, our data strongly suggest that a night of sleep is sufficient to evoke qualitative changes in the emotional memory retrieval network.

Profound retroactive interference in anterograde amnesia: What interferes?

OBJECTIVE

Anterograde amnesia is characterized by a profound inability to retain new information. Recent research suggests that at least some of this severe memory impairment may be the product of retroactive interference. What exactly interferes with memory in amnesic patients, however, remains unknown. The aim of the present study was to examine whether or not postlearning material which is highly dissimilar from the material to be remembered would interfere with amnesic patients' memory.

METHOD

Prose retention was tested in 10 densely amnesic patients and 10 controls following a 10 minute delay period, which was either unfilled (minimal interference) or filled with a tone detection task in which participants were required to listen for piano notes (nonspecific interference).

RESULTS

A significant nonspecific retroactive interference effect was observed in the amnesic patients (p < 0.004): Whereas 7 out of the 10 amnesic patients were able to recall some prose material following the unfilled delay period, only 1 of them was able to recall any material after the tone detection delay.

CONCLUSIONS

The data reveal that some amnesic patients have the capacity to retain new material for much longer than usual but that apparently any new postlearning information profoundly interferes with such retention. This nonspecific retroactive interference effect deviates from the item-specific interference effect that is typically assessed in clinical practice and which is frequently observed in patients with executive impairment. We hypothesize that these interference effects are qualitatively different, occurring during distinct memory processes, namely retrieval (item-specific interference) and consolidation (nonspecific interference).

Dreaming of a learning task is associated with enhanced sleep-dependent memory consolidation

It is now well established that postlearning sleep is beneficial for human memory performance. Meanwhile, human and animal studies have demonstrated that learning-related neural activity is re-expressed during posttraining nonrapid eye movement (NREM) sleep. NREM sleep processes appear to be particularly beneficial for hippocampus-dependent forms of memory. These observations suggest that learning triggers the reactivation and reorganization of memory traces during sleep, a systems-level process that in turn enhances behavioral performance. Here, we hypothesized that dreaming about a learning experience during NREM sleep would be associated with improved performance on a hippocampus-dependent spatial memory task. Subjects were trained on a virtual navigation task and then retested on the same task 5 hr after initial training. Improved performance at retest was strongly associated with task-related dream imagery during an intervening afternoon nap. Task-related thoughts during wakefulness, in contrast, did not predict improved performance. These observations suggest that sleep-dependent memory consolidation in humans is facilitated by the offline reactivation of recently formed memories, and furthermore that dream experiences reflect this memory processing. That similar effects were not observed during wakefulness suggests that these mnemonic processes are specific to the sleep state.

Bidirectional regulation of hippocampal long-term synaptic plasticity and its influence on opposing forms of memory

Reference memory characterizes the long-term storage of information acquired through numerous trials. In contrast, working memory represents the short-term acquisition of trial-unique information. A number of studies in the rodent hippocampus have focused on the contribution of long-term synaptic potentiation (LTP) to long-term reference memory. In contrast, little is known about the synaptic plasticity correlates of hippocampal-based components of working memory. Here, we described a mouse with selective expression of a dominant-negative mutant of the regulatory subunit of protein kinase A (PKA) only in two regions of the hippocampus, the dentate gyrus and area CA1. This mouse showed a deficit in several forms of LTP in both hippocampal subregions and a lowered threshold for the consolidation of long-term synaptic depression (LTD). When trained with one trial per day in a water maze task, mutant mice displayed a deficit in consolidation of long-term memory. In contrast, these mice proved to be more flexible after a transfer test and also showed a delay-dependent increased performance in working memory, when repetitive information (proactive interference) was presented. We suggest that through its bidirectional control over synaptic plasticity PKA can regulate opposing forms of memory. The defect in L-LTP disrupts long-term memory consolidation. The persistence of LTD may allow acquisition of new information by restricting the body of previously stored information and suppressing interference.

Forgetting is regulated through Rac activity in Drosophila

Initially acquired memory dissipates rapidly if not consolidated. Such memory decay is thought to result either from the inherently labile nature of newly acquired memories or from interference by subsequently attained information. Here we report that a small G protein Rac-dependent forgetting mechanism contributes to both passive memory decay and interference-induced forgetting in Drosophila. Inhibition of Rac activity leads to slower decay of early memory, extending it from a few hours to more than one day, and to blockade of interference-induced forgetting. Conversely, elevated Rac activity in mushroom body neurons accelerates memory decay. This forgetting mechanism does not affect memory acquisition and is independent of Rutabaga adenylyl cyclase-mediated memory formation mechanisms. Endogenous Rac activation is evoked on different time scales during gradual memory loss in passive decay and during acute memory removal in reversal learning. We suggest that Rac's role in actin cytoskeleton remodeling may contribute to memory erasure.

Rac in the act of forgetting

Forgetting has been thought to occur as a result of the natural decay of the neuronal changes induced by learning or because of interference from other cognitive functions. In this issue, Shuai et al. (2010) find that the small G protein Rac may function as a switch for remembering versus forgetting.

The memory function of sleep

Sleep has been identified as a state that optimizes the consolidation of newly acquired information in memory, depending on the specific conditions of learning and the timing of sleep. Consolidation during sleep promotes both quantitative and qualitative changes of memory representations. Through specific patterns of neuromodulatory activity and electric field potential oscillations, slow-wave sleep (SWS) and rapid eye movement (REM) sleep support system consolidation and synaptic consolidation, respectively. During SWS, slow oscillations, spindles and ripples - at minimum cholinergic activity - coordinate the re-activation and redistribution of hippocampus-dependent memories to neocortical sites, whereas during REM sleep, local increases in plasticity-related immediate-early gene activity - at high cholinergic and theta activity - might favour the subsequent synaptic consolidation of memories in the cortex.

Delaying interference enhances memory consolidation in amnesic patients

Some patients with amnesia are able to retain new information for much longer than expected when the time that follows new learning is devoid of further stimuli. Animal work shows that the absence or delaying of interference improves long-term memory consolidation. Our study suggests that this is also true for at least some patients with amnesia. Retention of new verbal material was significantly higher in a sample of patients with amnesia (N = 12) when interference occurred at the end of a 9-min delay interval than when it occurred in the middle or at the beginning of the interval. Such findings cannot be accounted for by the mere use of explicit short-term memory rehearsal. Any such rehearsal should have been blocked by the interference, irrespective of interference onset, thus leading to poor retention in all three conditions. The current findings suggest that at least some of the severe forgetting observed in amnesia is the product of a disruption of memory consolidation by immediate postlearning interference.

Visual Cues Signaling Object Grasp Reduce Interference in Motor Learning

Recent motor learning studies show that human subjects and nonhuman primates form neural representations of novel mechanical environments and associated forces. Whereas proficient adaptation is seen for a single force field, when faced with multiple novel force environments, movement performance and in particular the ability to switch between different force environments declines. It is difficult to reconcile these findings with the notion that primates can proficiently switch between multiple motor skills. Conceivably, particular kinds of sensory, cognitive, or perceptual contextual cues are required. This study examined the effect of visual feedback on motor learning, in particular, cues that simulated interaction with a virtual object. A robot arm was used to deliver novel patterns of forces (force fields) to the limb during reaching movements. We tested the possibility that subjects transition more easily between novel forces and their sudden absence when they are accompanied by visual cues that relate to object grasp. We used a virtual display system to present subjects with different kinds of visual feedback during reaching, including illusory feedback, indicating grasp of a virtual object during reaching in the force field, and object release in the absence of forces. Throughout the experiment, subjects in fact maintained grasp of the robot. We found that, indeed, the most effective visual cues were those associating the force field with grasp of the virtual object and the absence of the force field with release of the object. Our findings show more broadly that specific visual cues can protect motor skills from interference.

Sleep and rest facilitate implicit memory in a visual search task

Several forms of learning have been demonstrated to show improvements with sleep. Based on rodent models, it has been suggested that replay of waking events in the hippocampus during sleep may underlie memory consolidation in humans. However, behavioral data for the role of sleep in human hippocampal-related memory have been inconsistent. To further investigate the role of sleep in hippocampal-mediated learning, we tested subjects in two sessions of a contextual cueing paradigm, a form of hippocampus-dependent implicit learning, separated by intervals of sleep, active wake, or carefully controlled quiet rest. Participants completed a visual search task, and unbeknownst to them, some search displays were occasionally repeated in the experiment. Contextual cueing was revealed by faster search speed on repeated trials (Old) than unrepeated ones (New), even though subjects were unaware of the trial repetition. Notably, performance in a second testing session was equivalent for participants who underwent quiet resting, daytime sleep, or nocturnal sleep between the two sessions. These four groups showed equivalent transfer of learning from Session 1. Notably, learning of New configurations in Session 2 was absent in the active wake group, but was equally strong among the other three groups. These results indicate that this form of hippocampal learning is independent of sleep, and vulnerable to proactive interference during active wake. They prompt a reevaluation of the hippocampal replay hypothesis as a general model of sleep-dependent learning.

Predicting not to predict too much: how the cellular machinery of memory anticipates the uncertain future

Although the faculty of memory holds information about the past, it is mostly about the present and the future, because it permits adaptive responses to ongoing events as well as to events yet to come. Since many elements in the future are uncertain, the plasticity machinery that encodes memories in the brain has to operate under the assumption that stored information is likely to require fast and recurrent updating. This assumption is reflected at multiple levels of the brain, including the synaptic and the cellular level. Recent findings cast new light on how combinations of plasticity and metaplasticity mechanisms could permit the brain to balance over time between stability and plasticity of the information stored.

Adaptation and learning: characteristic time scales of performance dynamics

A multiple time scales landscape model is presented that reveals structures of performance dynamics that were not resolved in the traditional power law analysis of motor learning. It shows the co-existence of separate processes during and between practice sessions that evolve in two independent dimensions characterized by time scales that differ by about an order of magnitude. Performance along the slow persistent dimension of learning improves often as much and sometimes more during rest (memory consolidation and/or insight generation processes) than during a practice session itself. In contrast, the process characterized by the fast, transient dimension of adaptation reverses direction between practice sessions, thereby significantly degrading performance at the beginning of the next practice session (warm-up decrement). The theoretical model fits qualitatively and quantitatively the data from Snoddy's [Snoddy, G. S. (1926). Learning and stability. Journal of Applied Psychology, 10, 1-36] classic learning study of mirror tracing and other averaged and individual data sets, and provides a new account of the processes of change in adaptation and learning.

The effects of an acute psychosocial stressor on episodic memory

Although stressors are believed to impair memory, experimental studies with humans have provided inconsistent support for this conclusion. The current study was designed to examine the effect of an acute psychosocial stressor, and subsequent reactivity, on episodic memory. One hundred participants completed a list-recall task before and after random assignment into a stressor or nonstressor condition. Participants assigned to the stressor condition exhibited both impaired delayed and immediate recall, and also exhibited increasesin the commission of intrusions and perseverations. The experience of off-task thoughts and intentional suppression of such thoughts, were associated with greater impairment of immediate recall. Changes in state anxiety, negative mood, and heart rate were unrelated to changes in memory. These data indicate that exposure to a stressor impaired the recall of previously learned information, and compromised the recall of newly acquired information. Furthermore, cognitive interference is an important factor regarding stress-related impairments of episodic memory. memory.

REM, not incubation, improves creativity by priming associative networks

The hypothesized role of rapid eye movement (REM) sleep, which is rich in dreams, in the formation of new associations, has remained anecdotal. We examined the role of REM on creative problem solving, with the Remote Associates Test (RAT). Using a nap paradigm, we manipulated various conditions of prior exposure to elements of a creative problem. Compared with quiet rest and non-REM sleep, REM enhanced the formation of associative networks and the integration of unassociated information. Furthermore, these REM sleep benefits were not the result of an improved memory for the primed items. This study shows that compared with quiet rest and non-REM sleep, REM enhances the integration of unassociated information for creative problem solving, a process, we hypothesize, that is facilitated by cholinergic and noradrenergic neuromodulation during REM sleep.

Does sleep really influence face recognition memory?

Mounting evidence implicates sleep in the consolidation of various kinds of memories. We investigated the effect of sleep on memory for face identity, a declarative form of memory that is indispensable for nearly all social interaction. In the acquisition phase, observers viewed faces that they were required to remember over a variable retention period (0-36 hours). In the test phase, observers viewed intermixed old and new faces and judged seeing each before. Participants were classified according to acquisition and test times into seven groups. Memory strength (d') and response bias (c) were evaluated. Substantial time spent awake (12 hours or more) during the retention period impaired face recognition memory evaluated at test, whereas sleep per se during the retention period did little to enhance the memory. Wakefulness during retention also led to a tightening of the decision criterion. Our findings suggest that sleep passively and transiently shelters face recognition memory from waking interference (exposure) but does not actively aid in its long-term consolidation.

The role of sleep in false memory formation

Memories are not stored as exact copies of our experiences. As a result, remembering is subject not only to memory failure, but to inaccuracies and distortions as well. Although such distortions are often retained or even enhanced over time, sleep's contribution to the development of false memories is unknown. Here, we report that a night of sleep increases both veridical and false recall in the Deese-Roediger-McDermott (DRM) paradigm, compared to an equivalent period of daytime wakefulness. But while veridical memory deteriorates across both wake and sleep, false memories are preferentially preserved by sleep, actually showing a non-significant improvement. The same selectivity of false over veridical memories was observed in a follow-up nap study. Unlike previous studies implicating deep, slow-wave sleep (SWS) in declarative memory consolidation, here veridical recall correlated with decreased SWS, a finding that was observed in both the overnight and nap studies. These findings lead to two counterintuitive conclusions - that under certain circumstances sleep can promote false memories over veridical ones, and SWS can be associated with impairment rather than facilitation of declarative memory consolidation. While these effects produce memories that are less accurate after sleep, these memories may, in the end, be more useful.

A single bout of torpor in mice protects memory processes

Memory consolidation is the process by which new and labile information is stabilized as long-term memory. Consolidation of spatial memories is thought to involve the transfer of information from the hippocampus to cortical regions. While the hypometabolic and hypothermic state of torpor dramatically changes hippocampal connectivity, little work has considered the functional consequences of these changes. The present study examines the role of a single bout of shallow torpor in the process of memory consolidation in mice. Adult female C57Bl/6NHSD mice were trained on the Morris Water Maze (MWM) task. Immediately following acquisition, the mice were exposed to one of four experimental manipulations for 24 h: fasted at an ambient temperature of 19 degrees C, fasted at 29 degrees C, allowed free access to food at 19 degrees C, or allowed free access to food at 29 degrees C. Mice fasted at 19 degrees C entered a bout of torpor as assessed by core body temperature while none of the mice in the other conditions did so. Spatial biases were then assessed with a probe trial in the MWM. During the probe trial, mice that had entered torpor and mice that were fed at 29 degrees C spent twice as much time in the prior target platform location than mice that were fed at 19 degrees C and those that were fasted at 29 degrees C. These findings demonstrate that, while food restriction or cool ambient temperature independently disrupt memory processes, together they cause physiological changes including the induction of a state of torpor that result in functional preservation of the memory process.

No temporal decay in verbal short-term memory

Many models of short-term memory (STM) ascribe an important role to temporal decay and forgetting because of the passage of time alone. We argue against decay as the primary form of forgetting from STM, and suggest that new experimental methodologies and recent models provide new perspectives on the old issue of the causes of forgetting. We show that several classic sources of evidence for time-based forgetting can be re-interpreted in terms of an interference-based view, and that new experiments provide compelling evidence against decay. We conclude that progress requires moving beyond demonstrations of qualitative effects and focusing instead on testing quantitative predictions of models.

Posterior parietal cortex and episodic encoding: insights from fMRI subsequent memory effects and dual-attention theory

The formation of episodic memories--memories for life events--is affected by attention during event processing. A leading neurobiological model of attention posits two separate yet interacting systems that depend on distinct regions in lateral posterior parietal cortex (PPC). From this dual-attention perspective, dorsal PPC is thought to support the goal-directed allocation of attention, whereas ventral PPC is thought to support reflexive orienting to information that automatically captures attention. To advance understanding of how parietal mechanisms may impact event encoding, we review functional MRI studies that document the relationship between lateral PPC activation during encoding and subsequent memory performance (e.g., later remembering or forgetting). This review reveals that (a) encoding-related activity is frequently observed in human lateral PPC, (b) increased activation in dorsal PPC is associated with later memory success, and (c) increased activation in ventral PPC predominantly correlates with later memory failure. From a dual-attention perspective, these findings suggest that allocating goal-directed attention during event processing increases the probability that the event will be remembered later, whereas the capture of reflexive attention during event processing may have negative consequences for event encoding. The prevalence of encoding-related activation in parietal cortex suggests that neurobiological models of episodic memory should consider how parietal-mediated attentional mechanisms regulate encoding.

Sleep preferentially enhances memory for emotional components of scenes

Central aspects of emotional experiences are often well remembered at the expense of background details. Previous studies of such memory trade-offs have focused on memory after brief delays, but little is known about how these components of emotional memories change over time. We investigated the evolution of memory for negative scenes across 30 min, 12 daytime hours spent awake, and 12 nighttime hours including sleep. After 30 min, negative objects were well remembered at the expense of information about their backgrounds. Time spent awake led to forgetting of the entire negative scene, with memories of objects and their backgrounds decaying at similar rates. Sleep, in contrast, led to a preservation of memories of negative objects, but not their backgrounds, a result suggesting that the two components undergo differential processing during sleep. Memory for a negative scene develops differentially across time delays containing sleep and wake, with sleep selectively consolidating those aspects of memory that are of greatest value to the organism.

Motor learning and consolidation: the case of visuomotor rotation

Adaptation to visuomotor rotation is a particular form of motor learning distinct from force-field adaptation, sequence learning, and skill learning. Nevertheless, study of adaptation to visuomotor rotation has yielded a number of findings and principles that are likely of general importance to procedural learning and memory. First, rotation learning is implicit and appears to proceed through reduction in a visual prediction error generated by a forward model, such implicit adaptation occurs even when it is in conflict with an explicit task goal. Second, rotation learning is subject to different forms of interference: retrograde, anterograde through aftereffects, and contextual blocking of retrieval. Third, opposite rotations can be recalled within a short time interval without interference if implicit contextual cues (effector change) rather than explicit cues (color change) are used. Fourth, rotation learning consolidates both over time and with increased initial training (saturation learning).

Learning of a sequential motor skill comprises explicit and implicit components that consolidate differently

The ability to perform accurate sequential movements is essential to normal motor function. Learning a sequential motor behavior is comprised of two basic components: explicit identification of the order in which the sequence elements should be performed and implicit acquisition of spatial accuracy for each element. Here we investigated the time course of learning of these components for a first sequence (SEQA) and their susceptibility to interference from learning a second sequence (SEQB). We assessed explicit learning with a discrete index, the number of correct anticipatory movements, and implicit learning with a continuous variable, spatial error, which decreased during learning without subject awareness. Spatial accuracy to individual sequence elements reached asymptotic levels only when the whole sequence order was known. Interference with recall of the order of SEQA persisted even when SEQB was learned 24 h after SEQA. However, there was resistance to interference by SEQB with increased initial training with SEQA. For implicit learning of spatial accuracy, SEQB interfered at 5 min but not 24 h after SEQA. As in the case of sequence order, prolonged initial training with SEQA induced resistance to interference by SEQB. We conclude that explicit sequence learning is more susceptible to anterograde interference and implicit sequence learning is more susceptible to retrograde interference. However, both become resistant to interference with saturation training. We propose that an essential feature of motor skill learning is the process by which discrete explicit task elements are combined with continuous implicit features of movement to form flawless sequential actions.

Tripartite mechanism of extinction suggested by dopamine neuron activity and temporal difference model

Extinction of behavior enables adaptation to a changing world and is crucial for recovery from disorders such as phobias and drug addiction. However, the brain mechanisms underlying behavioral extinction remain poorly understood. Midbrain dopamine (DA) neurons appear to play a central role in most acquisition processes of appetitive conditioning. Here, we show that the responses of putative DA neurons to conditioned reward predicting cues also dynamically encode two classical features of extinction: decrement in amplitude of previously learned excitatory responses and rebound of responding on subsequent retesting (spontaneous recovery). Crucially, this encoding involves development of inhibitory responses in the DA neurons, reflecting new, extinction-specific learning in the brain. We explored the implications of this finding by adding such inhibitory inputs to a standard temporal difference model of DA cell activity. We found that combining extinction-triggered plasticity of these inputs with a time-dependent spontaneous decay of weights, equivalent to a forgetting process as described in classical behavioral extinction literature, enabled the model to simulate several classical features of extinction. A key requirement to achieving spontaneous recovery was differential rates of spontaneous decay for weights representing original conditioning and for subsequent extinction learning. A testable prediction of the model is thus that differential decay properties exist within the wider circuits regulating DA cell activity. These findings are consistent with the hypothesis that extinction processes at both cellular and behavioral levels involve a dynamic interaction between new (inhibitory) learning, forgetting, and unlearning.

Reorganization of associative memory in humans with long-standing hippocampal damage

Conflicting theories have been advanced to explain why hippocampal lesions affect distinct memory domains and spare others. Recent findings in monkeys suggest that lesion-induced plasticity may contribute to the seeming preservation of some of these domains. We tested this hypothesis by investigating visuo-spatial associative memory in two patient groups with similar surgical lesions to the right medial temporal lobe, but different preoperative disease courses (benign brain tumours, mean: 1.8 +/- 0.6 years, n = 5, age: 28.2 +/- 4.0 years; hippocampal sclerosis, mean: 16.8 +/- 1.9 years, n = 9, age: 38.9 +/- 4.1 years). Compared to controls (n = 14), tumour patients showed a significant delay-dependent deficit in memory of colour-location associations. No such deficit was observed in hippocampal sclerosis patients, which appeared to benefit from a compensatory mechanism that was inefficient in tumour patients. These results indicate that long-standing hippocampal damage can yield significant functional reorganization of the neural substrate underlying memory in the human brain. We suppose that this process accounts for some of the discrepancies between results from previous lesion studies of the human medial temporal lobe.

Reactivation and Consolidation of Memory During Sleep

Recent research has shown compellingly that sleep supports the consolidation of declarative memories for events and facts. During consolidation, memories are stabilized against future interference and undergo qualitative changes with regard to their “explicitness” and underlying neural representation. In this article, we argue that declarative memory consolidation during sleep is based on covert reactivations of newly encoded memory traces in the hippocampus. During slow-wave sleep (SWS), the prominent slow oscillations act to synchronize the repeated reactivation of the newly encoded representations in hippocampal networks with the generation of spindle activity in the thalamus, supporting changes in neocortical networks that contribute to long-term memory storage. In this view, sleep plays an active role in the consolidation of memories, in which the neuronal reactivation of newly acquired memories is critical for the redistribution and integration of these memories into the network of pre-existing long-term memories.

Distinct haptic cues do not reduce interference when learning to reach in multiple force fields

Background

Previous studies of learning to adapt reaching movements in the presence of novel forces show that learning multiple force fields is prone to interference. Recently it has been suggested that force field learning may reflect learning to manipulate a novel object. Within this theoretical framework, interference in force field learning may be the result of static tactile or haptic cues associated with grasp, which fail to indicate changing dynamic conditions. The idea that different haptic cues (e.g. those associated with different grasped objects) signal motor requirements and promote the learning and retention of multiple motor skills has previously been unexplored in the context of force field learning.

Methodology/Principle Findings

The present study tested the possibility that interference can be reduced when two different force fields are associated with differently shaped objects grasped in the hand. Human subjects were instructed to guide a cursor to targets while grasping a robotic manipulandum, which applied two opposing velocity-dependent curl fields to the hand. For one group of subjects the manipulandum was fitted with two different handles, one for each force field. No attenuation in interference was observed in these subjects relative to controls who used the same handle for both force fields.

Conclusions/Significance

These results suggest that in the context of the present learning paradigm, haptic cues on their own are not sufficient to reduce interference and promote learning multiple force fields.

The mind and brain of short-term memory

The past 10 years have brought near-revolutionary changes in psychological theories about short-term memory, with similarly great advances in the neurosciences. Here, we critically examine the major psychological theories (the "mind") of short-term memory and how they relate to evidence about underlying brain mechanisms. We focus on three features that must be addressed by any satisfactory theory of short-term memory. First, we examine the evidence for the architecture of short-term memory, with special attention to questions of capacity and how--or whether--short-term memory can be separated from long-term memory. Second, we ask how the components of that architecture enact processes of encoding, maintenance, and retrieval. Third, we describe the debate over the reason about forgetting from short-term memory, whether interference or decay is the cause. We close with a conceptual model tracing the representation of a single item through a short-term memory task, describing the biological mechanisms that might support psychological processes on a moment-by-moment basis as an item is encoded, maintained over a delay with some forgetting, and ultimately retrieved.

Long-term memory for 400 pictures on a common theme

Long-term memory for large numbers of color photographs with a common motif--doors--was studied using pictures with two levels of informative cues: original photographs, and edited pictures in which extraneous information on details such as vegetation, paint scratches, signs, and lamp posts was removed. In the study phase, subjects viewed 400 pictures and were subsequently tested for memory on two-alternative forced-choice discriminations between studied and distracter pictures from the same picture category, at retention intervals between 0.5 h and 9 days. When tested with the nonedited original photographs immediate memory performance was close to 85% correct; when pictorial details were removed memory performance dropped by 20%. The decay functions were shallow with parallel paths for the categories of pictures. It is concluded that specific details of visual scenes contributed to long-term memory of those scenes.

Memory consolidation and accelerated forgetting in children with idiopathic generalized epilepsy

Whether children with idiopathic generalized epilepsy exhibit accelerated forgetting of verbal and nonverbal information in comparison to healthy controls matched for age and IQ was explored. Twenty-one children with IGE were compared with 21 healthy controls on measures of verbal and visuospatial memory at delays of 30 minutes and 1 week by use of a minimum-learning criterion controlled for initial learning. For the auditory-verbal memory test, group performance was comparable at 30 minutes, but children with IGE recalled significantly less than controls at 1 week. When the number of learning trials to criterion was controlled, the main effects of group and delay became nonsignificant. No group differences were found with respect to recognition performance. Comparisons for the visuospatial task were nonsignificant. Overall, poor initial learning efficiency led to retrieval difficulties, specifically at the longer delay, and was more common in the IGE group. These results, although preliminary, have implications for education planning in childhood IGE.

Nap and melatonin-induced changes in hippocampal activation and their role in verbal memory consolidation

Overnight sleep contributes to memory consolidation; even a short nap improves memory performance. Such improvement has been linked to hippocampal activity during sleep. Melatonin has been shown to affect the human hippocampus and to induce 'sleep like' changes in brain activation. We therefore conducted and compared two functional magnetic resonance imaging studies: the first study assessed the effect of a 2-hr mid-day nap versus an equal amount of wakefulness on a verbal memory task (unrelated word pair association); the second assessed the effect of melatonin versus placebo (both conditions without nap) on a similar task. We report that following a nap relative to wakefulness, successful retrieval-related activation in the parahippocampus is decreased. A smaller decrease is seen in wakefulness with melatonin but not placebo. In parallel, an improvement in verbal memory recall was found after a nap compared with wakefulness but not with melatonin during wakefulness compared with placebo. Our findings demonstrate effects of melatonin that resemble those of sleep on verbal memory processing in the hippocampus thus suggesting that melatonin, like sleep, can initiate offline plastic changes underlying memory consolidation; they also suggest that concomitant rest without interferences is necessary for enhanced performance.

Retrograde facilitation under midazolam: the role of general and specific interference

In a double-blind, placebo-controlled experiment that used midazolam, a benzodiazepine that creates temporary amnesia, we compared acquisition and retention of paired associates of different types. Some word pairs were studied before the injection of saline or midazolam, and two lists of word pairs were studied after the injection. Critical comparisons involved retention of pairs that were practiced on all three lists, pairs studied on only one list, and pairs that involved recombining cue and response terms from one list to the next, as a function of drug condition. Previous research with benzodiazepines had found retrograde facilitation for material acquired prior to injection, compared with the control condition. One explanation for this facilitation is that the anterograde amnesia produced by the benzodiazepine frees up the hippocampus to better consolidate previously learned material (Wixted, 2004, 2005). We accounted for a rich data set using a simple computational model that incorporated interference effects (cue overload) at retrieval for both general (experimental context) interference and specific (stimulus term) interference without the need to postulate a role for consolidation. The computational model as an Excel spreadsheet may be downloaded from www.psychonomic.org/archive.

Forgetting due to retroactive interference: a fusion of Müller and Pilzecker's (1900) early insights into everyday forgetting and recent research on anterograde amnesia

Ebbinghaus' seminal work suggested that forgetting occurred as a function of time. However, it raised a number of fundamental theoretical issues that still have not been resolved in the literature. Müller and Pilzecker (1900) addressed some of these issues in a remarkable manner but their observations have been mostly ignored in recent years. Müller and Pilzecker (1900) showed that the materials and the task that intervene between presentation and recall may interfere with the to-be-remembered items, and they named this phenomenon "retroactive interference" (RI). They further asked whether there is a type of RI that is based only on distraction, and not on the similarity between the memoranda and the interfering stimuli. Their findings, and our follow up research in healthy volunteers and amnesiacs, confirm that forgetting can be induced by any subsequent mentally effortful interpolated task, irrespective of its content; the interpolated "interfering" material does not have to be similar to the to-be-remembered stimuli.