Dynamic changes in the medial temporal lobe during incidental learning of object-location associations

No Object-Location Memory Improvement through Focal Transcranial Direct Current Stimulation over the Right Temporoparietal Cortex

Remembering objects and their associated location (object-location memory; OLM), is a fundamental cognitive function, mediated by cortical and subcortical brain regions. Previously, the combination of OLM training and transcranial direct current stimulation (tDCS) suggested beneficial effects, but the evidence remains heterogeneous. Here, we applied focal tDCS over the right temporoparietal cortex in 52 participants during a two-day OLM training, with anodal tDCS (2 mA, 20 min) or sham (40 s) on the first day. The focal stimulation did not enhance OLM performance on either training day (stimulation effect: -0.09, 95%CI: [-0.19; 0.02], p = 0.08). Higher electric field magnitudes in the target region were not associated with individual performance benefits. Participants with content-related learning strategies showed slightly superior performance compared to participants with position-related strategies. Additionally, training gains were associated with individual verbal learning skills. Consequently, the lack of behavioral benefits through focal tDCS might be due to the involvement of different cognitive processes and brain regions, reflected by participant's learning strategies. Future studies should evaluate whether other brain regions or memory-relevant networks may be involved in the modulation of object-location associations, investigating other target regions, and further exploring individualized stimulation parameters.

Prior Reward Conditioning Dampens Hippocampal and Striatal Responses during an Associative Memory Task

Offering reward during encoding typically leads to better memory [Adcock, R. A., Thangavel, A., Whitfield-Gabrieli, S.,Knutson, B., & Gabrieli, J. D. E. Reward-motivated learning: Mesolimbic activation precedes memory formation. Neuron, 50, 507-517, 2006]. Whether such memory benefit persists when tested in a different task context remains, however, largely understudied [Wimmer, G. E., & Buechel, C. Reactivation of reward-related patterns from single past episodes supports memory-based decision making. Journal of Neuroscience, 36, 2868-2880, 2016]. Here, we ask whether reward at encoding leads to a generalized advantage across learning episodes, a question of high importance for any everyday life applications, from education to patient rehabilitation. Although we confirmed that offering monetary reward increased responses in the ventral striatum and pleasantness judgments for pictures used as stimuli, this immediate beneficial effect of reward did not carry over to a subsequent and different picture-location association memory task during which no reward was delivered. If anything, a trend for impaired memory accuracy was observed for the initially high-rewarded pictures as compared to low-rewarded ones. In line with this trend in behavioral performance, fMRI activity in reward (i.e., ventral striatum) and in memory (i.e., hippocampus) circuits was reduced during the encoding of new associations using previously highly rewarded pictures (compared to low-reward pictures). These neural effects extended to new pictures from same, previously highly rewarded semantic category. Twenty-four hours later, delayed recall of associations involving originally highly rewarded items was accompanied by decreased functional connectivity between the hippocampus and two brain regions implicated in value-based learning, the ventral striatum and the ventromedial PFC. We conclude that acquired reward value elicits a downward value-adjustment signal in the human reward circuit when reactivated in a novel nonrewarded context, with a parallel disengagement of memory-reward (hippocampal-striatal) networks, likely to undermine new associative learning. Although reward is known to promote learning, here we show how it may subsequently hinder hippocampal and striatal responses during new associative memory formation.

Memory-guided attention: bilateral hippocampal volume positively predicts implicit contextual learning

Several studies have begun to demonstrate that contextual memories constitute an important mechanism to guide our attention. Although there is general consensus that the hippocampus is involved in the encoding of contextual memories, it is controversial whether this structure can support implicit forms of contextual memory. Here, we combine automated segmentation of structural MRI with neurobehavioral assessment of implicit contextual memory-guided attention to test the hypothesis that hippocampal volume would predict the magnitude of implicit contextual learning. Forty healthy subjects underwent 3T magnetic resonance imaging brain scanning with subsequent automatic measurement of the total brain and hippocampal (right and left) volumes. Implicit learning of contextual information was measured using the contextual cueing task. We found that both left and right hippocampal volumes positively predicted the magnitude of implicit contextual learning. Larger hippocampal volume was associated with superior implicit contextual memory performance. This study provides compelling evidence that implicit contextual memory-guided attention is hippocampus-dependent.

Fast track to the neocortex: A memory engram in the posterior parietal cortex

Models of systems memory consolidation postulate a fast-learning hippocampal store and a slowly developing, stable neocortical store. Accordingly, early neocortical contributions to memory are deemed to reflect a hippocampus-driven online reinstatement of encoding activity. In contrast, we found that learning rapidly engenders an enduring memory engram in the human posterior parietal cortex. We assessed microstructural plasticity via diffusion-weighted magnetic resonance imaging as well as functional brain activity in an object–location learning task. We detected neocortical plasticity as early as 1 hour after learning and found that it was learning specific, enabled correct recall, and overlapped with memory-related functional activity. These microstructural changes persisted over 12 hours. Our results suggest that new traces can be rapidly encoded into the parietal cortex, challenging views of a slow-learning neocortex.

Neural mechanisms underlying the reward-related enhancement of motivation when remembering episodic memories with high difficulty

The motivation to receive rewards enhances episodic memories, and the motivation is modulated by task difficulty. In episodic retrieval, however, functional neuroimaging evidence regarding the motivation that mediates interactions between reward and task difficulty is scarce. The present fMRI study investigated this issue. During encoding performed without fMRI, participants encoded Japanese words using either deep or shallow strategies, which led to variation in difficulty level during subsequent retrieval. During retrieval with fMRI, participants recognized the target words in either high or low monetary reward conditions. In the behavioral results, a reward-related enhancement of memory was found only when the memory retrieval was difficult, and the rewarding effect on subjective motivation was greater in the retrieval of memories with high difficulty than those with low difficulty. The fMRI data showed that reward-related increases in the activation of the substantia nigra/ventral tegmental area (SN/VTA), medial temporal lobe (MTL), dorsomedial prefrontal cortex (dmPFC), and dorsolateral prefrontal cortex (dlPFC) were greater during the retrieval of memories with high difficulty than those with low difficulty. Furthermore, reward-related enhancement of functional connectivity between the SN/VTA and MTL and between the SN/VTA and dmPFC during the retrieval of memories with high difficulty was significantly correlated with reward-related increases of retrieval accuracy and subjective motivation. The reward-related enhancement of episodic retrieval and retrieval-related motivation could be most effective when the level of retrieval difficulty is optimized. Such reward-related enhancement of memory and motivation could be modulated by a network including the reward-related SN/VTA, motivation-related dmPFC, and memory-related MTL. Hum Brain Mapp 38:3428-3443, 2017. © 2017 Wiley Periodicals, Inc.

Working memory contributes to the encoding of object location associations: Support for a 3-part model of object location memory

A recent model by Postma and colleagues posits that the encoding of object location associations (OLAs) requires the coordination of several cognitive processes mediated by ventral (object perception) and dorsal (spatial perception) visual pathways as well as the hippocampus (feature binding) [1]. Within this model, frontoparietal network recruitment is believed to contribute to both the spatial processing and working memory task demands. The current study used functional magnetic resonance imaging (fMRI) to test each step of this model in 15 participants who encoded OLAs and performed standard n-back tasks. As expected, object processing resulted in activation of the ventral visual stream. Object in location processing resulted in activation of both the ventral and dorsal visual streams as well as a lateral frontoparietal network. This condition was also the only one to result in medial temporal lobe activation, supporting its role in associative learning. A conjunction analysis revealed areas of shared activation between the working memory and object in location phase within the lateral frontoparietal network, anterior insula, and basal ganglia; consistent with prior working memory literature. Overall, findings support Postma and colleague's model and provide clear evidence for the role of working memory during OLA encoding.

Functional correlates of likelihood and prior representations in a virtual distance task

Spatial navigation is an imperative cognitive function, in which individuals must interact with their environment in order to accurately reach a destination. Previous research has demonstrated that, when traveling a predetermined distance, humans must balance between noise in the measurement process and the prior history of traveled distances. This tradeoff has recently been formally described using Bayesian estimation; however, the neural correlates of Bayesian estimation during distance reproduction have yet to be investigated. Here, human subjects performed a virtual reality distance reproduction task during functional Magnetic Resonance Imaging (fMRI), in which they were required to reproduce various traveled distances in the absence of overt navigational cues. As previously demonstrated, subjects exhibited a central tendency effect, wherein reproduced distances gravitated to the mean of the stimulus set. fMRI activity during this task revealed distance-sensitive activity in a network of regions, including prefrontal and hippocampal regions. Using a computational index of central tendency, we found that activity in the retrosplenial cortex, a region highly implicated in spatial navigation, negatively covaried between subjects with the degree of central tendency observed; conversely, we found that activity in the anterior hippocampus/amygdala complex was positively correlated with the central tendency effect of gravitating to the average reproduced distance. These findings suggest dissociable roles for the retrosplenial cortex and hippocampal complex during distance reproduction, with both regions coordinating with the prefrontal cortex the influence of prior history of the environment with present experience. Hum Brain Mapp 37:3172-3187, 2016. © 2016 Wiley Periodicals, Inc.

Human hippocampal processing of environmental novelty during spatial navigation

The detection and processing of novel information encountered as we explore our environment is crucial for learning and adaptive behavior. The human hippocampus has been strongly implicated in laboratory tests of novelty detection and episodic memory, but has been less well studied during more ethological tasks such as spatial navigation, typically used in animals. We examined fMRI BOLD activity as a function of environmental and object novelty as humans performed an object-location virtual navigation task. We found greater BOLD response to novel relative to familiar environments in the hippocampus and adjacent parahippocampal gyrus. Object novelty was associated with increased activity in the posterior parahippocampal/fusiform gyrus and anterior hippocampus extending into the amygdala and superior temporal sulcus. Importantly, whilst mid-posterior hippocampus was more sensitive to environmental novelty than object novelty, the anterior hippocampus responded similarly to both forms of novelty. Amygdala activity showed an increase for novel objects that decreased linearly over the learning phase. By investigating how participants learn and use different forms of information during spatial navigation, we found that medial temporal lobe (MTL) activity reflects both the novelty of the environment and of the objects located within it. This novelty processing is likely supported by distinct, but partially overlapping, sets of regions within the MTL.

Memory consolidation by replay of stimulus-specific neural activity

Memory consolidation transforms initially labile memory traces into more stable representations. One putative mechanism for consolidation is the reactivation of memory traces after their initial encoding during subsequent sleep or waking state. However, it is still unknown whether consolidation of individual memory contents relies on reactivation of stimulus-specific neural representations in humans. Investigating stimulus-specific representations in humans is particularly difficult, but potentially feasible using multivariate pattern classification analysis (MVPA). Here, we show in healthy human participants that stimulus-specific activation patterns can indeed be identified with MVPA, that these patterns reoccur spontaneously during postlearning resting periods and sleep, and that the frequency of reactivation predicts subsequent memory for individual items. We conducted a paired-associate learning task with items and spatial positions and extracted stimulus-specific activity patterns by MVPA in a simultaneous electroencephalography and functional magnetic resonance imaging (fMRI) study. As a first step, we investigated the amount of fMRI volumes during rest that resembled either one of the items shown before or one of the items shown as a control after the resting period. Reactivations during both awake resting state and sleep predicted subsequent memory. These data are first evidence that spontaneous reactivation of stimulus-specific activity patterns during resting state can be investigated using MVPA. They show that reactivation occurs in humans and is behaviorally relevant for stabilizing memory traces against interference. They move beyond previous studies because replay was investigated on the level of individual stimuli and because reactivations were not evoked by sensory cues but occurred spontaneously.

Segregating cognitive functions within hippocampal formation: a quantitative meta-analysis on spatial navigation and episodic memory

The most important cognitive domains where hippocampal formation is crucially involved are navigation and memory. Some evidence suggests that different hippocampal subregions mediate these domains. However, a quantitative meta-analysis on neuroimaging studies of spatial navigation versus memory is lacking. By means of activation likelihood estimation (ALE), we investigate concurrence of brain regions activated during spatial navigation encoding and retrieval as well as during episodic memory encoding and retrieval tasks in humans. During encoding in spatial navigation, activity was located in more posterior regions of the hippocampal formation, whereas episodic memory encoding was located in more anterior regions. Retrieval in spatial navigation was more strongly lateralized to the right compared to episodic memory retrieval. Within studies on spatial navigation retrieval, immediate recall was located more posterior and delayed recall more anterior. Overlap between concurrence of activation in spatial navigation and episodic memory was rather limited in comparison to uniquely involved regions. This argues in favor of two distinct networks, one for spatial navigation the other for episodic memory within the hippocampal formation.

What makes a pattern? Matching decoding methods to data in multivariate pattern analysis

Research in neuroscience faces the challenge of integrating information across different spatial scales of brain function. A promising technique for harnessing information at a range of spatial scales is multivariate pattern analysis (MVPA) of functional magnetic resonance imaging (fMRI) data. While the prevalence of MVPA has increased dramatically in recent years, its typical implementations for classification of mental states utilize only a subset of the information encoded in local fMRI signals. We review published studies employing multivariate pattern classification since the technique’s introduction, which reveal an extensive focus on the improved detection power that linear classifiers provide over traditional analysis techniques. We demonstrate using simulations and a searchlight approach, however, that non-linear classifiers are capable of extracting distinct information about interactions within a local region. We conclude that for spatially localized analyses, such as searchlight and region of interest, multiple classification approaches should be compared in order to match fMRI analyses to the properties of local circuits.

Procedural learning and associative memory mechanisms contribute to contextual cueing: Evidence from fMRI and eye-tracking

Using a combination of eye tracking and fMRI in a contextual cueing task, we explored the mechanisms underlying the facilitation of visual search for repeated spatial configurations. When configurations of distractors were repeated, greater activation in the right hippocampus corresponded to greater reductions in the number of saccades to locate the target. A psychophysiological interactions analysis for repeated configurations revealed that a strong functional connectivity between this area in the right hippocampus and the left superior parietal lobule early in learning was significantly reduced toward the end of the task. Practice related changes (which we call "procedural learning") in activation in temporo-occipital and parietal brain regions depended on whether or not spatial context was repeated. We conclude that context repetition facilitates visual search through chunk formation that reduces the number of effective distractors that have to be processed during the search. Context repetition influences procedural learning in a way that allows for continuous and effective chunk updating.