A computational account of dreaming: Learning and memory consolidation

A number of studies have concluded that dreaming is mostly caused by random signals because “dream contents are random impulses”, and argued that dream sleep is unlikely to play an important part in our intellectual capacity. On the other hand, numerous functional studies have suggested that dream sleep does play an important role in our learning and other intellectual functions. Specifically, recent studies have suggested the importance of dream sleep in memory consolidation, following the findings of neural replaying of recent waking patterns in the hippocampus. This study presents a cognitive and computational model of dream process that involves episodic learning and random activation of stored experiences. This model is simulated to perform the functions of learning and memory consolidation, which are two most popular dream functions that have been proposed. The simulations demonstrate that random signals may result in learning and memory consolidation. The characteristics of the model are discussed and found in agreement with many characteristics concluded from various empirical studies.     

Conjunctive representations in learning and memory: principles of cortical and hippocampal function

The authors present a theoretical framework for understanding the roles of the hippocampus and neocortex in learning and memory. This framework incorporates a theme found in many theories of hippocampal function: that the hippocampus is responsible for developing conjunctive representations binding together stimulus elements into a unitary representation that can later be recalled from partial input cues. This idea is contradicted by the fact that hippocampally lesioned rats can learn nonlinear discrimination problems that require conjunctive representations. The authors' framework accommodates this finding by establishing a principled division of labor, where the cortex is responsible for slow learning that integrates over multiple experiences to extract generalities whereas the hippocampus performs rapid learning of the arbitrary contents of individual experiences. This framework suggests that tasks involving rapid, incidental conjunctive learning are better tests of hippocampal function. The authors implement this framework in a computational neural network model and show that it can account for a wide range of data in animal learning.     

Episodic memory and the hippocampus: it's about time

Several recent studies have sought to develop animal models of episodic memory, the capacity to recollect unique personal experiences. However, these studies have not yet provided unambiguous evidence that this capacity is based on recollection of the learning episodes. A recent study that examined memory for the ordering of events within unique experiences, and demonstrated a critical and selective role for the hippocampus, suggests a new and promising model for neurobiological analyses of episodic memory.     

Hippocampal differentiation without recognition: an fMRI analysis of the contextual cueing task

A central role of the hippocampus is to consolidate conscious forms of learning and memory, while performance on implicit tasks appears to depend upon other structures. Recently, considerable debate has emerged about whether hippocampal-dependent tasks necessarily entail task awareness. In the contextual cueing task, repetition facilitation is implicit, but impaired in patients with amnesia. Whether the hippocampus alone or other MTL structures are required is unclear. Event-related functional magnetic resonance imaging revealed hippocampal activity that differentiates novel from repeated arrays. This pattern of results was observed without recognition of the repeating arrays. This finding provides support for the claim that the hippocampus is involved in processes outside the domain of conscious learning and memory.     

Spatial imagery of novel places based on visual scene transformation

The hippocampus is known to maintain memories of object-place associations that can produce a scene expectation at a novel viewpoint. To implement such capabilities, the memorized distances and directions of an object from the viewer at a fixed location should be integrated with the imaginary displacement to the new viewpoint. However, neural dynamics of such scene expectation at the novel viewpoint have not been discussed. In this study, we propose a method of coding novel places based on visual scene transformation as a component of the object-place memory in the hippocampus. In this coding, a novel place is represented by a transformed version of a viewer’s scene with imaginary displacement. When the places of individual objects are stored with the coding in the hippocampus, the object’s displacement at the imaginary viewpoint can be evaluated through the comparison of a transformed viewer’s scene with the stored scene. Results of computer experiments demonstrated that the coding successfully produced scene expectation of a three object arrangement at a novel viewpoint. Such the scene expectation was retained even without similarities between the imaginary scene and the real scene at the location, where the imaginary scenes only functioned as indices to denote the topographical relationship between object locations. The results suggest that the hippocampus uses the place coding based on scene transformation and implements the spatial imagery of object-place associations from the novel viewpoint.     

Contrasting hippocampal and perirhinalcortex function using immediate early gene imaging

The perirhinal cortex and hippocampus have close anatomical links, and it might, therefore, be predicted that they have close, interlinked roles in memory. Lesion studies have, however, often failed to support this prediction, providing dissociations and double dissociations between the two regions on tests of object recognition and spatial memory. In a series of rat studies we have compared these two regions using the expression of the immediate early gene c-fosas a marker of neuronal activity. This gene imaging approach makes it possible to assess the relative involve-ment of different brain regions and avoids many of the limitations of the lesion approach. A very consistent pattern of results was found as the various hippocampal subfields but not the peri-rhinal cortex show increased c-fosactivity following tests of spatial learning. In contrast, the perirhinal cortex but none of the hippocampal subfields show increased c-fosactivity when presented with novel rather than familiar visual objects. When novel scenes are created by the spatial rearrangement of familiar objects it is the hippocampus and not the perirhinal cortex that shows c-foschanges. This double dissociation for gene expression accords with that found from lesion studies and highlights the different contributions of the perirhinal cortex and hippocampus to memory.     

Case-Based Planning: A Framework for Planning from Experience

This article presents a view of planning as a task supported by a dynamic memory. This view attempts to ingegrate models of memory, learning, and planning into a single system that learns about planning by creating new plans and analyzing how they interact with the world. We call this view of planning case-based planning. A case-based planner makes use of its own past experience in developing new plans. It relies on its memory of observed effects, rather than a set of causal rules, to create and modify new plans. Memories of past successes are accessed and modified to create new plans. Memories of past failures are used to warn the planner of impending problems, and memories of past repairs are called upon to tell the planner how to deal with them. This view of planning from experience supports and is supported by a learning system that incorporates new experiences into the planner's episodic memory. This learning algorithm gains from the planner's failures as well as its successes. Successful plans are stored in memory, indexed by the goals they satisfy and the problems they avoid. Failures are also stored and indexed by the features in the world that predict them. By storing failures as well as successes, the planner is able to anticipate and avoid future plan failures. Case-based planning is aimed at improving planning behavior in three areas: failure avoidance, plan repair, and plan reuse. It also attempts gains over current learning systems, in that the learning is driven by the functional needs of a planner.     

Memory for context becomes less specific with time

Context memories initially require the hippocampus, but over time become independent of this structure. This shift reflects a consolidation process whereby memories are gradually stored in distributed regions of the cortex. The function of this process is thought to be the extraction of statistical regularities and general knowledge from specific experiences. The current study examined this idea in mice by measuring the specificity of context memories during consolidation. In the first experiment, separate groups of animals were trained with a single shock and tested in the training context or a novel environment 1, 14, 28, or 36 d later. We found a systematic increase in generalization over this period. Initially, mice froze more in the training context, but fear of the novel environment grew over time until animals eventually froze an equivalent amount in both contexts. The second experiment demonstrated that the increase in generalization was due to a loss of detailed information about the context and not fear incubation. In this experiment, mice were exposed to the context and then trained with an immediate shock 1 or 36 d later. Animals trained 1 d after exposure acquired robust context fear that did not generalize across environments. In contrast, mice trained 36 d after exposure froze an equivalent amount in the training context and the novel environment. The same profile was observed in H-ras mutants that exhibit enhanced hippocampal plasticity and learning. These results suggest that context memories are specific early after training when they require the hippocampus, and become more general as they are permanently stored in the cortex.     

Amygdala and "emotional" modulation of the relative use of multiple memory systems

The basolateral amygdala modulates the cognitive and habit memory processes mediated by the hippocampus and caudate nucleus, respectively. The present experiments used a plus-maze task that can be acquired using either hippocampus-dependent "place" learning or caudate-dependent "response" learning to examine whether peripheral or intra-basolateral amygdala injection of anxiogenic drugs would bias rats towards the use of a particular memory system. In Experiment 1, adult male Long-Evans rats were trained to swim from the same start point to an escape platform located in a consistent goal arm, and received pre-training peripheral injections of the alpha(2)-adrenoceptor antagonists yohimbine (2.5 or 5.0 mg/kg), RS 79948-197 (0.05, 0.1, or 0.2 mg/kg), or vehicle. On a drug-free probe trial from a novel start point administered 24h following acquisition, vehicle treated rats predominantly displayed hippocampus-dependent place learning, whereas rats previously treated with yohimbine (2.5, 5.0 mg/kg) or RS 79948-197 (0.1 mg/kg) predominantly displayed caudate-dependent response learning. In Experiment 2, rats receiving pre-training intra-basolateral amygdala infusions of RS 79948-197 (0.1 microg/0.5 microl) also predominantly displayed response learning on a drug-free probe trial. The findings indicate (1) peripheral injections of anxiogenic drugs can influence the relative use of multiple memory systems in a manner that favors caudate-dependent habit learning over hippocampus-dependent cognitive learning, and (2) intra-basolateral amygdala infusion of anxiogenic drugs is sufficient to produce this modulatory influence of emotional state on the use of multiple memory systems.     

Contrasting hippocampal and perirhinal cortex function using immediate early gene imaging.

The perirhinal cortex and hippocampus have close anatomical links, and it might, therefore, be predicted that they have close, interlinked roles in memory. Lesion studies have, however, often failed to support this prediction, providing dissociations and double dissociations between the two regions on tests of object recognition and spatial memory. In a series of rat studies we have compared these two regions using the expression of the immediate early gene c-fos as a marker of neuronal activity. This gene imaging approach makes it possible to assess the relative involvement of different brain regions and avoids many of the limitations of the lesion approach. A very consistent pattern of results was found as the various hippocampal subfields but not the perirhinal cortex show increased c-fos activity following tests of spatial learning. In contrast, the perirhinal cortex but none of the hippocampal subfields show increased c-fos activity when presented with novel rather than familiar visual objects. When novel scenes are created by the spatial rearrangement of familiar objects it is the hippocampus and not the perirhinal cortex that shows c-fos changes. This double dissociation for gene expression accords with that found from lesion studies and highlights the different contributions of the perirhinal cortex and hippocampus to memory.     

新颖语义联结在顿悟促进记忆效果中的作用

采用成语谜题选择任务, 通过学习-测验范式探究顿悟促进记忆的认知神经机制。实验1采用行为实验, 验证成语谜题选择范式在探究顿悟促进记忆中的有效性, 结果显示, 相比于寻常联结条件, 新颖联结条件下被试在学习阶段具有更高的顿悟感得分, 在测试阶段具有更高的正确率, 范式的有效性得以验证。实验2采用fMRI技术探究顿悟促进记忆的关键脑区。结果显示, 相比于失败记忆新颖联结条件, 成功记忆新颖联结条件更强地激活了顿悟过程相关脑区, 包括海马、杏仁核、额中回、颞上回和颞中回。这说明在学习阶段的顿悟问题解决过程中, 对信息的深加工与积极情绪促进了随后的记忆; 对其进一步分析发现, 相比于寻常联结记忆, 新颖联结对记忆的促进效应主要与右侧海马激活有关, 它可能反映了在顿悟问题解决中新颖联结形成过程建立了情节记忆以及新颖且有价值的语义联结。研究结果表明新颖语义联结形成在顿悟促进记忆中发挥了重要作用。     

Learning of novel semantic relationships via sudden comprehension is associated with a hippocampus-independent network

Sudden comprehension—or insight—during problem-solving can enhance learning, but the underlying neural processes are largely unknown. We investigated neural correlates of learning from sudden comprehension using functional magnetic resonance imaging and a verbal problem-solving task. Solutions and “solutions” to solvable and unsolvable verbal problems, respectively, were presented to induce sudden comprehension or continued incomprehension. We found activations of the hippocampus, medial prefrontal cortex (mPFC), amygdala, and striatum during sudden comprehension. Notably, however, mPFC and temporo-parietal neocortical structures rather than the hippocampus were associated with later learning of suddenly comprehended solutions. Moreover, difficult compared to easy sudden comprehension elicited midbrain activations and was associated with successful learning, pointing to learning via intrinsic reward. Sudden comprehension of novel semantic associations may constitute a special case of long-term memory formation primarily mediated by the mPFC, expanding our knowledge of its role in prior-knowledge-dependent memory.     

The experience of force: the role of haptic experience of forces in visual perception of object motion and interactions, mental simulation, and motion-related judgments

Forces are experienced in actions on objects. The mechanoreceptor system is stimulated by proximal forces in interactions with objects, and experiences of force occur in a context of information yielded by other sensory modalities, principally vision. These experiences are registered and stored as episodic traces in the brain. These stored representations are involved in generating visual impressions of forces and causality in object motion and interactions. Kinematic information provided by vision is matched to kinematic features of stored representations, and the information about forces and causality in those representations then forms part of the perceptual interpretation. I apply this account to the perception of interactions between objects and to motions of objects that do not have perceived external causes, in which motion tends to be perceptually interpreted as biological or internally caused. I also apply it to internal simulations of events involving mental imagery, such as mental rotation, trajectory extrapolation and judgment, visual memory for the location of moving objects, and the learning of perceptual judgments and motor skills. Simulations support more accurate judgments when they represent the underlying dynamics of the event simulated. Mechanoreception gives us whatever limited ability we have to perceive interactions and object motions in terms of forces and resistances; it supports our practical interventions on objects by enabling us to generate simulations that are guided by inferences about forces and resistances, and it helps us learn novel, visually based judgments about object behavior.     

Growth points in research on memory and hippocampus.

We present an overview of two of our on-going projects relating processes in the hippocampus to memory. We are trying to understand why retrograde amnesia occurs after damage to the hippocampus. Our experiments establish the generality of several new retrograde amnesia phenomena that are at odds with the consensus view of the role of the hippocampus in memory. We show in many memory tasks that complete damage to the hippocampus produces retrograde amnesia that is equivalent for recent and remote memories. Retrograde amnesia affects a much wider range of memory tasks than anterograde amnesia. Normal hippocampal processes can interfere with retention of a long-term memory stored outside the hippocampus. We conclude that the hippocampus competes with nonhippocampal systems during memory encoding and retrieval. Finally, we outline a project to understand and manipulate adult hippocampal neurogenesis in order to repair damaged hippocampal circuitry to recover lost cognitive functions.     

Amygdala and “emotional” modulation of the relative use of multiple memory systems

The basolateral amygdala modulates the cognitive and habit memory processes mediated by the hippocampus and caudate nucleus, respectively. The present experiments used a plus-maze task that can be acquired using either hippocampus-dependent “place” learning or caudate-dependent “response” learning to examine whether peripheral or intra-basolateral amygdala injection of anxiogenic drugs would bias rats towards the use of a particular memory system. In Experiment 1, adult male Long–Evans rats were trained to swim from the same start point to an escape platform located in a consistent goal arm, and received pre-training peripheral injections of the α₂-adrenoceptor antagonists yohimbine (2.5 or 5.0 mg/kg), RS 79948-197 (0.05, 0.1, or 0.2 mg/kg), or vehicle. On a drug-free probe trial from a novel start point administered 24 h following acquisition, vehicle treated rats predominantly displayed hippocampus-dependent place learning, whereas rats previously treated with yohimbine (2.5, 5.0 mg/kg) or RS 79948-197 (0.1 mg/kg) predominantly displayed caudate-dependent response learning. In Experiment 2, rats receiving pre-training intra-basolateral amygdala infusions of RS 79948-197 (0.1 μg/0.5 μl) also predominantly displayed response learning on a drug-free probe trial. The findings indicate (1) peripheral injections of anxiogenic drugs can influence the relative use of multiple memory systems in a manner that favors caudate-dependent habit learning over hippocampus-dependent cognitive learning, and (2) intra-basolateral amygdala infusion of anxiogenic drugs is sufficient to produce this modulatory influence of emotional state on the use of multiple memory systems.     

Excitotoxic lesions restricted to the dorsal CA1 field of the hippocampus impair spatial memory and extinction learning in C57BL/6 mice

Recent studies in patients with hippocampal lesions have indicated that the degree of memory impairment is proportional to the extent of damage within the hippocampus. Particularly, patients with damage restricted to the CA1 field demonstrate moderate to severe anterograde amnesia with only slight retrograde amnesia. Comparable results are also seen in other species such as non-human primates and rats; however, the effect of selective damage to CA1 has not yet been characterized in mice. In the present study, we investigated the effects of excitotoxic (NMDA) lesions of dorsal CA1 on several aspects of learning and memory performance in mice. Our data indicate that dorsal CA1 lesioned mice are hyperactive upon exposure to a novel environment, have spatial working memory impairments in the Y-maze spontaneous alternation task, and display deficits in an 8-arm spatial discrimination learning task. Lesioned mice are able to acquire an operant lever-press task but demonstrate extinction learning deficits in this appetitive operant paradigm. Taken together, our results indicate that lesions to dorsal CA1 in mice induce selective learning and memory performance deficits similar to those observed in other species, and extend previous findings indicating that this region of the hippocampus is critically involved in the processing of spatial information and/or the processing of inhibitory responses.     

The effects of lesions to the rat hippocampus or rhinal cortex on olfactory and spatial memory: retrograde and anterograde findings

The role of the hippocampal system in retrograde and anterograde amnesia was investigated by using a novel olfactory-guided paradigm and a traditional test of spatial learning. In the retrograde study, rats were trained on a sequence of two-choice olfactory discriminations in the weeks prior to receiving neurotoxic lesions of the hippocampus or aspiration lesions of the perirhinal-entorhinal cortex. Memory tests for preoperatively learned discriminations revealed no statistical impairment for subjects with damage to the hippocampus on a problem learned remote in time from surgery (i.e., 4 weeks +) or on the two recently learned discriminations (i.e., 1–3 weeks prior to surgery). The performance of subjects with perirhinal-entorhinal damage provided an important comparison for subjects with specific hippocampal lesions. Despite showing intact memory for the remotely learned problem, perirhinalentorhinal damage resulted in numerically (although not significantly) weaker performance on postoperative tests of retention for the discriminations learned in the 3 weeks prior to surgery. In the anterograde portion of the study, long-term memory for newly acquired discriminations was spared in subjects with damage to the hippocampus, whereas subjects in the perirhinal-entorhinal lesion group again showed the weakest memory performance on these tests of 5-day retention. Postoperative water maze learning was uniformly impaired in subjects with damage to the hippocampus and perirhinalentorhinal cortex, thus confirming the effect of these lesions and supporting the involvement of these brain areas in spatial processes. These findings further dissociate the specific involvement of the hippocampus in tasks of a spatial-relational nature versus nonrelational tasks, such as discrimination learning and recognition memory (e.g., Duva et al., 1997; Eichenbaum, 1997; Eichenbaum, Schoenbaum, Young, & Bunsey, 1996). Moreover, the results suggest that damage to the hippocampus itself does not contribute to retrograde or anterograde memory impairments for all types of information, whereas the data suggest a more important role for the perirhinal-entorhinal cortex in recognition memory, irrespective of modality.     

Hippocampal inactivation enhances taste learning

Learning tasks are typically thought to be either hippocampal-dependent (impaired by hippocampal lesions) or hippocampal-independent (indifferent to hippocampal lesions). Here, we show that conditioned taste aversion (CTA) learning fits into neither of these categories. Rats were trained to avoid two taste stimuli, one novel and one familiar. Muscimol infused through surgically implanted intracranial cannulae temporarily inactivated the dorsal hippocampus during familiarization, subsequent CTA training, or both. As shown previously, hippocampal inactivation during familiarization enhanced the effect of that familiarization on learning (i.e., hippocampal inactivation enhanced latent inhibition of CTA); more novel and surprising, however, was the finding that hippocampal inactivation during training sessions strongly enhanced CTA learning itself. These phenomena were not caused by specific aspects of our infusion technique--muscimol infusions into the hippocampus during familiarization sessions did not cause CTAs, muscimol infusions into gustatory cortex caused the expected attenuation of CTA, and hippocampal inactivation caused the expected attenuation of spatial learning. Thus, we suggest that hippocampal memory processes interfere with the specific learning mechanisms underlying CTA, and more generally that multiple memory systems do not operate independently.     

不同动机和材料因素对学习判断影响的实验研究

本研究主要考察不同动机和材料因素对学习判断的影响。采用标准R-J-R实验程序,运用有意义与无意义词对对120名被试进行实验。结果发现：（1）成就目标、记忆自我效能、材料因素对JOL有显著影响,材料因素对JOL影响最大,记忆自我效能次之,成就目标最小;（2）成就目标与记忆自我效能存在交互作用对JOL有显著影响;（3）成就目标以记忆自我效能为中介影响JOL;（4）JOL准确性与回忆成绩存在显著正相关。