An experimental study on the self-reference effect

Self-reference at encoding increases the probability of recollective experience in recognition memory. Self-reference effect (SRE) in memory is observed when the degree of self-reference of to-be-remembered materials is varied. Usually items encoded in terms of self-reference are associated with significantly higher recall performance relative to items encoded in other ways, including semantic encoding. In our three experiments, four orientating tasks formed four levels in a between-subjects design. The conditions were rating personality traits with reference to self, mother, Lu Xun (a famous Chinese writer) or rating the traits on a negative-positive dimension. In a recognition test, subjects first identified old items and then indicated which of these were accompanied by recollective experience (‘remember’ responses) and which were recognized on some other basis (‘know’ responses). Our results showed that both the self-reference condition and mother condition were associated with the same highest proportion of correct remember responses and the same lowest proportion of correct know responses. In contrast, for Western subjects, the self-reference condition, relative to the mother condition, was associated with a higher proportion of correct remember responses and a lower proportion of correct know responses. We discussed the results with reference to the HERA model (hemispheric encoding/retrieval asymmetry).     

Perirhinal cortex supports encoding and familiarity-based recognition of novel associations

Results from imaging and lesion studies of item recognition memory have suggested that the hippocampus supports memory for the arbitrary associations that form the basis of episodic recollection, whereas the perirhinal cortex (PRc) supports familiarity for individual items. This view has been challenged, however, by findings showing that PRc may contribute to associative recognition, a task thought to measure relational or recollective memory. Here, using functional magnetic resonance imaging, we demonstrate that PRc activity is increased when pairs of items are processed as a single configuration or unit and that this activity predicts subsequent familiarity-based associative memory. These results explain the discrepancy in the literature by showing that novel associations can be encoded in a unitized manner, thereby allowing PRc to support associative recognition based on familiarity.     

The neural substrates of memory suppression: a FMRI exploration of directed forgetting

The directed forgetting paradigm is frequently used to determine the ability to voluntarily suppress information. However, little is known about brain areas associated with information to forget. The present study used functional magnetic resonance imaging to determine brain activity during the encoding and retrieval phases of an item-method directed forgetting recognition task with neutral verbal material in order to apprehend all processing stages that information to forget and to remember undergoes. We hypothesized that regions supporting few selective processes, namely recollection and familiarity memory processes, working memory, inhibitory and selection processes should be differentially activated during the processing of to-be-remembered and to-be-forgotten items. Successful encoding and retrieval of items to remember engaged the entorhinal cortex, the hippocampus, the anterior medial prefrontal cortex, the left inferior parietal cortex, the posterior cingulate cortex and the precuneus; this set of regions is well known to support deep and associative encoding and retrieval processes in episodic memory. For items to forget, encoding was associated with higher activation in the right middle frontal and posterior parietal cortex, regions known to intervene in attentional control. Items to forget but nevertheless correctly recognized at retrieval yielded activation in the dorsomedial thalamus, associated with familiarity-based memory processes and in the posterior intraparietal sulcus and the anterior cingulate cortex, involved in attentional processes.     

Executive control during episodic retrieval: multiple prefrontal processes subserve source memory

During recognition, one may sense items as familiar (item memory) and additionally recollect specific contextual details of the earlier encounters (source memory). Cognitive theory suggests that, unlike item memory, source memory requires controlled cue specification and monitoring processes. Functional imaging suggests that such processes may depend on left prefrontal cortex (PFC). However, the nature and possible anatomical segregation of these processes remains unknown. Using functional magnetic resonance imaging, we isolated distinct response patterns in left PFC during source memory consistent with semantic analysis/cue specification (anterior ventrolateral), recollective monitoring (posterior dorsolateral and frontopolar), and phonological maintenance/rehearsal (posterior ventrolateral). Importantly, cue specification and recollective monitoring responses were not seen during item memory and were unaffected by retrieval success, demonstrating that the mere attempt to recollect episodic detail engages multiple control processes with different left PFC substrates.     

How emotion strengthens the recollective experience: a time-dependent hippocampal process

Emotion significantly strengthens the subjective recollective experience even when objective accuracy of the memory is not improved. Here, we examine if this modulation is related to the effect of emotion on hippocampal-dependent memory consolidation. Two critical predictions follow from this hypothesis. First, since consolidation is assumed to take time, the enhancement in the recollective experience for emotional compared to neutral memories should become more apparent following a delay. Second, if the emotion advantage is critically dependent on the hippocampus, then the effects should be reduced in amnesic patients with hippocampal damage. To test these predictions we examined the recollective experience for emotional and neutral photos at two retention intervals (Experiment 1), and in amnesics and controls (Experiment 2). Emotional memories were associated with an enhancement in the recollective experience that was greatest after a delay, whereas familiarity was not influenced by emotion. In amnesics with hippocampal damage the emotion effect on recollective experience was reduced. Surprisingly, however, these patients still showed a general memory advantage for emotional compared to neutral items, but this effect was manifest primarily as a facilitation of familiarity. The results support the consolidation hypothesis of recollective experience, but suggest that the effects of emotion on episodic memory are not exclusively hippocampally mediated. Rather, emotion may enhance recognition by facilitating familiarity when recollection is impaired due to hippocampal damage.     

Odor recognition memory: two encoding trials are better than one

The primary purpose of this study was to determine the effect of one versus two encoding trials in the classical yes/no recognition memory paradigm using olfactory stimuli. A group of 24 young adults rated 18 standard microencapsulated odorant targets for familiarity (first encoding block) or pleasantness (second encoding block). Once-encoded targets were in only one block and twice-encoded targets were in both, with items counterbalanced across participants. Participants performed a 20-min nonverbal distractor task followed by a yes/no recognition test incorporating 18 additional odors as foils. Memory performance for twice-encoded targets was superior to that for once-encoded targets. For once-encoded targets, performance did not differ between those rated for familiarity versus those rated for pleasantness. Less pleasant odors produced overall better recognition, with a tendency for less familiar odors to produce overall better recognition. There was a tendency for the second encoding trial to have a larger effect for less pleasant or familiar odors than for more pleasant or familiar odors. The main conclusion is that recognition memory for odors is better for items encoded two times than for items encoded only once. Implications of these findings and suggestions for future research are discussed.     

Are Age Effects in Positivity Influenced by the Valence of Distractors?

An age-related ‘positivity’ effect has been identified, in which older adults show an information-processing bias towards positive emotional items in attention and memory. In the present study, we examined this positivity bias by using a novel paradigm in which emotional and neutral distractors were presented along with emotionally valenced targets. Thirty-five older and 37 younger adults were asked during encoding to attend to emotional targets paired with distractors that were either neutral or opposite in valence to the target. Pupillary responses were recorded during initial encoding as well as a later incidental recognition task. Memory and pupillary responses for negative items were not affected by the valence of distractors, suggesting that positive distractors did not automatically attract older adults’ attention while they were encoding negative targets. Additionally, the pupil dilation to negative items mediated the relation between age and positivity in memory. Overall, memory and pupillary responses provide converging support for a cognitive control account of positivity effects in late adulthood and suggest a link between attentional processes and the memory positivity effect.     

Time and Encoding Effects in the Concealed Knowledge Test

Although the traditional “lie detector” test is used frequently in forensic contexts, it has (like most test of deception) some limitations. The concealed knowledge test (CKT) focuses on participants’ recognition of privileged knowledge rather than lying per-se and has been studied extensively using a variety of measures. A “guilty” suspect’s interaction with and memory of crimescene items may vary. Furthermore, memory for crimescene items may diminish over time. The interaction of encoding quality and test delay on CKT efficiency has been previously implied, but not yet demonstrated. We used a response-time based CKT to detect concealed knowledge from shallow and deep study procedures after 10-min, 24-h, and 1-week delays. Results show that more elaborately encoded information afforded higher detection accuracy than poorly encoded items. Although classification accuracy following deep study was unaffected by delay, detection of poorly elaborated information was initially high, but compromised after 1 week. Thus, choosing optimal test items requires considering both test delay and initial encoding level.     

Graded Effects of Social Conformity on Recognition Memory

Previous studies have shown that the opinion of confederates in a group influences recognition memory, but inconsistent results have been obtained concerning the question of whether recognition of items as old and new are affected similarly, possibly because only one or two confederates are present during the recognition phase. Here, we present data from a study where recognition of novel faces was tested in the presence of four confederates. In a long version of this experiment, recognition of items as old and new was similarly affected by group responses. However, in the short version, recognition of old items depended proportionally on the number of correct group responses, while rejection of new items only decreased significantly when all confederates gave an incorrect response. These findings indicate that differential effects of social conformity on recognition of items as old and new occur in situations with an intermediate level of group pressure.     

Validation of a rodent model of source memory

Source memory represents the origin (source) of information. Recently, we proposed that rats (Rattus norvegicus) remember the source of information. However, an alternative to source memory is the possibility that rats selectively encoded some, but not all, information rather than retrieving an episodic memory. We directly tested this ‘encoding failure’ hypothesis. Here, we show that rats remember the source of information, under conditions that cannot be attributed to encoding failure. Moreover, source memory lasted at least seven days but was no longer present 14 days after studying. Our findings suggest that long-lasting source memory may be modelled in non-humans. Our model should facilitate attempts to elucidate the biological underpinnings of source memory impairments in human memory disorders such as Alzheimer''s disease.     

Neural repetition effects in the medial temporal lobe complex are modulated by previous encoding experience

It remains an intriguing question why the medial temporal lobe (MTL) can display either attenuation or enhancement of neural activity following repetition of previously studied items. To isolate the role of encoding experience itself, we assessed neural repetition effects in the absence of any ongoing task demand or intentional orientation to retrieve. Experiment 1 showed that the hippocampus and surrounding MTL regions displayed neural repetition suppression (RS) upon repetition of past items that were merely attended during an earlier study phase but this was not the case following re-occurrence of items that had been encoded into working memory (WM). In this latter case a trend toward neural repetition enhancement (RE) was observed, though this was highly variable across individuals. Interestingly, participants with a higher degree of neural RE in the MTL complex displayed higher memory sensitivity in a later, surprise recognition test. Experiment 2 showed that massive exposure at encoding effected a change in the neural architecture supporting incidental repetition effects, with regions of the posterior parietal and ventral-frontal cortex in addition to the hippocampus displaying neural RE, while no neural RS was observed. The nature of encoding experience therefore modulates the expression of neural repetition effects in the MTL and the neocortex in the absence of memory goals.     

Eye Tracking,Cortisol, and a Sleep vs. Wake Consolidation Delay: Combining Methods to Uncover an Interactive Effect of Sleep and Cortisol on Memory

Although rises in cortisol can benefit memory consolidation, as can sleep soon after encoding, there is currently a paucity of literature as to how these two factors may interact to influence consolidation. Here we present a protocol to examine the interactive influence of cortisol and sleep on memory consolidation, by combining three methods: eye tracking, salivary cortisol analysis, and behavioral memory testing across sleep and wake delays. To assess resting cortisol levels, participants gave a saliva sample before viewing negative and neutral objects within scenes. To measure overt attention, participants’ eye gaze was tracked during encoding. To manipulate whether sleep occurred during the consolidation window, participants either encoded scenes in the evening, slept overnight, and took a recognition test the next morning, or encoded scenes in the morning and remained awake during a comparably long retention interval. Additional control groups were tested after a 20 min delay in the morning or evening, to control for time-of-day effects. Together, results showed that there is a direct relation between resting cortisol at encoding and subsequent memory, only following a period of sleep. Through eye tracking, it was further determined that for negative stimuli, this beneficial effect of cortisol on subsequent memory may be due to cortisol strengthening the relation between where participants look during encoding and what they are later able to remember. Overall, results obtained by a combination of these methods uncovered an interactive effect of sleep and cortisol on memory consolidation.     

“Disorganized in time”: Impact of bottom-up and top-down negative emotion generation on memory formation among healthy and traumatized adolescents

“Travelling in time,” a central feature of episodic memory is severely affected among individuals with Post Traumatic Stress Disorder (PTSD) with two opposite effects: vivid traumatic memories are unorganized in temporality (bottom-up processes), non-traumatic personal memories tend to lack spatio-temporal details and false recognitions occur more frequently that in the general population (top-down processes). To test the effect of these two types of processes (i.e. bottom-up and top-down) on emotional memory, we conducted two studies in healthy and traumatized adolescents, a period of life in which vulnerability to emotion is particularly high. Using negative and neutral images selected from the international affective picture system (IAPS), stimuli were divided into perceptual images (emotion generated by perceptual details) and conceptual images (emotion generated by the general meaning of the material). Both categories of stimuli were then used, along with neutral pictures, in a memory task with two phases (encoding and recognition). In both populations, we reported a differential effect of the emotional material on encoding and recognition. Negative perceptual scenes induced an attentional capture effect during encoding and enhanced the recollective distinctiveness. Conversely, the encoding of conceptual scenes was similar to neutral ones, but the conceptual relatedness induced false memories at retrieval. However, among individuals with PTSD, two subgroups of patients were identified. The first subgroup processed the scenes faster than controls, except for the perceptual scenes, and obtained similar performances to controls in the recognition task. The second subgroup group desmonstrated an attentional deficit in the encoding task with no benefit from the distinctiveness associated with negative perceptual scenes on memory performances. These findings provide a new perspective on how negative emotional information may have opposite influences on memory in normal and traumatized individuals. It also gives clues to understand how intrusive memories and overgeneralization takes place in PTSD.     

Recognition memory and the human hippocampus

The capacity for declarative memory depends on the hippocampal region and adjacent cortex within the medial temporal lobe. One of the most widely studied examples of declarative memory is the capacity to recognize recently encountered material as familiar, but uncertainty remains about whether intact recognition memory depends on the hippocampal region itself and, if so, what the nature of the hippocampal contribution might be. Seven patients with bilateral damage thought to be limited primarily to the hippocampal region were impaired on three standard tests of recognition memory. In addition, the patients were impaired to a similar extent at Remembering and Knowing, measures of the two processes thought to support recognition performance: the ability to remember the learning episode (episodic recollection) and the capacity for judging items as familiar (familiarity).     

Choline: Selective enhancement of serial learning and encoding of low imagery words in man

The effect of a single oral dose of choline chloride (10g) on two kinds of memory measures (serial learning and a selective reminding task) was studied in 10 normal volunteers. Choline decreased significantly the number of trials required to master a serial learning word test. On the selective reminding task only the recall of poorly imageable words (which are ordinarily difficult to remember) was enhanced by choline. Words which were highly imageable (and ordinarily easy to remember) were unaffected. The degree of enhancement with choline correlated inversely with performance of subjects on placebo. Our data indicate that brain acetylcholine may play a facilitatory role in encoding and storage of specific information items in man.     

Auditory short-term memory behaves like visual short-term memory

Are the information processing steps that support short-term sensory memory common to all the senses? Systematic, psychophysical comparison requires identical experimental paradigms and comparable stimuli, which can be challenging to obtain across modalities. Participants performed a recognition memory task with auditory and visual stimuli that were comparable in complexity and in their neural representations at early stages of cortical processing. The visual stimuli were static and moving Gaussian-windowed, oriented, sinusoidal gratings (Gabor patches); the auditory stimuli were broadband sounds whose frequency content varied sinusoidally over time (moving ripples). Parallel effects on recognition memory were seen for number of items to be remembered, retention interval, and serial position. Further, regardless of modality, predicting an item's recognizability requires taking account of (1) the probe's similarity to the remembered list items (summed similarity), and (2) the similarity between the items in memory (inter-item homogeneity). A model incorporating both these factors gives a good fit to recognition memory data for auditory as well as visual stimuli. In addition, we present the first demonstration of the orthogonality of summed similarity and inter-item homogeneity effects. These data imply that auditory and visual representations undergo very similar transformations while they are encoded and retrieved from memory.     

Sleep loss produces false memories

People sometimes claim with high confidence to remember events that in fact never happened, typically due to strong semantic associations with actually encoded events. Sleep is known to provide optimal neurobiological conditions for consolidation of memories for long-term storage, whereas sleep deprivation acutely impairs retrieval of stored memories. Here, focusing on the role of sleep-related memory processes, we tested whether false memories can be created (a) as enduring memory representations due to a consolidation-associated reorganization of new memory representations during post-learning sleep and/or (b) as an acute retrieval-related phenomenon induced by sleep deprivation at memory testing. According to the Deese, Roediger, McDermott (DRM) false memory paradigm, subjects learned lists of semantically associated words (e.g., "night", "dark", "coal",...), lacking the strongest common associate or theme word (here: "black"). Subjects either slept or stayed awake immediately after learning, and they were either sleep deprived or not at recognition testing 9, 33, or 44 hours after learning. Sleep deprivation at retrieval, but not sleep following learning, critically enhanced false memories of theme words. This effect was abolished by caffeine administration prior to retrieval, indicating that adenosinergic mechanisms can contribute to the generation of false memories associated with sleep loss.     

Effective visual working memory capacity: an emergent effect from the neural dynamics in an attractor network

The study of working memory capacity is of outmost importance in cognitive psychology as working memory is at the basis of general cognitive function. Although the working memory capacity limit has been thoroughly studied, its origin still remains a matter of strong debate. Only recently has the role of visual saliency in modulating working memory storage capacity been assessed experimentally and proved to provide valuable insights into working memory function. In the computational arena, attractor networks have successfully accounted for psychophysical and neurophysiological data in numerous working memory tasks given their ability to produce a sustained elevated firing rate during a delay period. Here we investigate the mechanisms underlying working memory capacity by means of a biophysically-realistic attractor network with spiking neurons while accounting for two recent experimental observations: 1) the presence of a visually salient item reduces the number of items that can be held in working memory, and 2) visually salient items are commonly kept in memory at the cost of not keeping as many non-salient items.OUR MODEL SUGGESTS THAT WORKING MEMORY CAPACITY IS DETERMINED BY TWO FUNDAMENTAL PROCESSES: encoding of visual items into working memory and maintenance of the encoded items upon their removal from the visual display. While maintenance critically depends on the constraints that lateral inhibition imposes to the mnemonic activity, encoding is limited by the ability of the stimulated neural assemblies to reach a sufficiently high level of excitation, a process governed by the dynamics of competition and cooperation among neuronal pools. Encoding is therefore contingent upon the visual working memory task and has led us to introduce the concept of effective working memory capacity (eWMC) in contrast to the maximal upper capacity limit only reached under ideal conditions.     

Electrophysiological evidence for immature processing capacity and filtering in visuospatial working memory in adolescents

The present study investigated the development of visuospatial working memory (VSWM) capacity and the efficiency of filtering in VSWM in adolescence. To this end, a group of IQ-matched adults and adolescents performed a VSWM change detection task with manipulations of WM-load and distraction, while performance and electrophysiological contralateral delay activity (CDA) were measured. The CDA is a lateralized ERP marker of the number of targets and distracters that are selectively encoded/maintained in WM from one hemifield of the memory display. Significantly lower VSWM-capacity (Cowan's K) was found in adolescents than adults, and adolescents' WM performance (in terms of accuracy and speed) also suffered more from the presence of distracters. Distracter-related CDA responses were partly indicative of higher distracter encoding/maintenance in WM in adolescents and were positively correlated with performance measures of distracter interference. This correlation suggests that the higher interference of distracters on WM performance in adolescents was caused by an inability to block distracters from processing and maintenance in WM. The lower visuospatial WM-capacity (K) in adolescents in the high load (3 items) condition was accompanied by a trend (p<.10) towards higher CDA amplitudes in adolescents than adults, whereas CDA amplitudes in the low load (1 item) condition were comparable between adolescents and adults. These findings point to immaturity of frontal-parietal WM-attention networks that support visuospatial WM processing in adolescence.     

Human hippocampal neurons predict how well word pairs will be remembered

What is the neuronal basis for whether an experience is recalled or forgotten? In contrast to recognition, recall is difficult to study in nonhuman primates and rarely is accessible at the single neuron level in humans. We recorded 128 medial temporal lobe (MTL) neurons in patients implanted with intracranial microelectrodes while they encoded and recalled word paired associates. Neurons in the amygdala, entorhinal cortex, and hippocampus showed altered activity during encoding (9%), recall (22%), and both task phases (23%). The responses of hippocampal neurons during encoding predicted whether or not subjects later remembered the pairs successfully. Entorhinal cortex neuronal activity during retrieval was correlated with recall success. These data provide support at the single neuron level for MTL contributions to encoding and retrieval, while also suggesting there may be differences in the level of contribution of MTL regions to these memory processes.