Hemispheric asymmetries in the time course of recognition memory

Effects of sleep insufficiency on spatial working memory in low-pressure and hypoxic environments

BACKGROUND

This study aimed to investigate the effects of sleep insufficiency on spatial working memory in low-pressure and hypoxic environments.

METHODS

We selected 58 insufficient sleepers and 27 normal sleepers among the college students living in high-altitude areas for a long time to receive a spatial 2-back working memory task, while collecting behavioral and electroencephalograph data. We adopted an independent sample t-test and repeated measures analysis of variance to compare the differences in response time and accuracy, P2 and late positive potential components, and theta band energy values in the spatial working memory task between insufficient and normal sleepers.

RESULTS

We found no significant differences in response time and accuracy between the insufficient sleep group and the normal sleep group; however, the P2 peak value and the early theta band energy value were higher in the insufficient sleep group than in the normal sleep group.

CONCLUSIONS

These results suggest that the spatial working memory ability of individuals with sleep insufficiency was weakened under low-pressure and low-oxygen environment.

Connecting and considering: Electrophysiology provides insights into comprehension

The ability to rapidly and systematically access knowledge stored in long-term memory in response to incoming sensory information-that is, to derive meaning from the world-lies at the core of human cognition. Research using methods that can precisely track brain activity over time has begun to reveal the multiple cognitive and neural mechanisms that make this possible. In this article, I delineate how a process of connecting affords an effortless, continuous infusion of meaning into human perception. In a relatively invariant time window, uncovered through studies using the N400 component of the event-related potential, incoming sensory information naturally induces a graded landscape of activation across long-term semantic memory, creating what might be called "proto-concepts". Connecting can be (but is not always) followed by a process of further considering those activations, wherein a set of more attentionally demanding "active comprehension" mechanisms mediate the selection, augmentation, and transformation of the initial semantic representations. The result is a limited set of more stable bindings that can be arranged in time or space, revised as needed, and brought to awareness. With this research, we are coming closer to understanding how the human brain is able to fluidly link sensation to experience, to appreciate language sequences and event structures, and, sometimes, to even predict what might be coming up next.

Is Pupil Activity Associated With the Strength of Memory Signal for Words in a Continuous Recognition Memory Paradigm?

Research on pupillometry provides an increasing evidence for associations between pupil activity and memory processing. The most consistent finding is related to an increase in pupil size for old items compared with novel items, suggesting that pupil activity is associated with the strength of memory signal. However, the time course of these changes is not completely known, specifically, when items are presented in a running recognition task maximizing interference by requiring the recognition of the most recent items from a sequence of old/new items. The sample comprised 42 healthy participants who performed a visual word recognition task under varying conditions of retention interval. Recognition responses were evaluated using behavioral variables for discrimination accuracy, reaction time, and confidence in recognition decisions. Pupil activity was recorded continuously during the entire experiment. The results suggest a decrease in recognition performance with increasing study-test retention interval. Pupil size decreased across retention intervals, while pupil old/new effects were found only for words recognized at the shortest retention interval. Pupillary responses consisted of a pronounced early pupil constriction at retrieval under longer study-test lags corresponding to weaker memory signals. However, the pupil size was also sensitive to the subjective feeling of familiarity as shown by pupil dilation to false alarms (new items judged as old). These results suggest that the pupil size is related not only to the strength of memory signal but also to subjective familiarity decisions in a continuous recognition memory paradigm.

Screen position effects on task performance in a delayed match to sample task

Studies of working memory have used immediate and delayed recall of lists of items. Serial position effects are the phenomena where items at the beginning and end of a studied list are recalled differentially from items in the middle of the list. In matching versions of the task, study items may be presented serially or simultaneously in a grid. After a delay a single probe item is presented for which the participant determines whether or not it was in the study set. The effects of the position of an item when studied on a screen are currently not well understood and are the focus of the current work. Findings from a delayed match to sample task in 49 healthy young adults with 1 to 9 items presented in a 3 by 3 grid, demonstrate that the column of the studied items affect response time and accuracy. The effects of position on accuracy also significantly interact with task demands. The importance of screen position effects is demonstrated with simulations using the mean accuracies split by task demands and screen locations. Simulations demonstrate the possible range of accuracies based on screen effects when the number of trials presented to an individual is less than 20 for each task demand. This has important implications when a small number of trials are administered using randomly generated stimuli which is often the case in neuroimaging studies where tasks are delivered under constrained time limits.

Long-term exposure to high altitude attenuates verbal and spatial working memory: Evidence from an event-related potential study

INTRODUCTION

This study aimed to determine the neurocognitive basis underlying the effects of long-term high-altitude (HA) exposure on working memory (WM).

METHODS

Using event-related potentials (ERPs), we compared the performance of an HA group (individuals who had lived at HA for 3 years but were born and raised at low altitude [LA]) to that of an LA group (individuals who had only lived at LA) on verbal and spatial n-back tasks (i.e., 1- and 2-back memory load).

RESULTS

Response accuracy of the HA group was significantly decreased in comparison to the LA group in both the verbal and spatial 2-back tasks. The P2 amplitude was larger in the HA than in the LA group in the spatial, but not the verbal 2-back task. A smaller late-positive potential (LPP) amplitude was found in the HA group in both the verbal and spatial 2-back tasks.

CONCLUSIONS

These results suggest that HA impairs the matching (P2) process in spatial WM tasks and the maintenance (LPP) process in both verbal and spatial WM tasks, indicating that HA had a different effect on verbal and spatial 2-back task performance.

One is all you need: intrahemispheric processing benefits nonverbal visual recognition

Several attempts have been made to understand when and how the two hemispheres of the brain work together to encode and retrieve information during memory tasks, but it remains unclear whether they are equally capable of encoding and retrieval, particularly when the stimuli do not evoke a leftward processing asymmetry. Using a divided visual field paradigm, we presented nonverbal visual stimuli to one visual field/hemisphere at encoding, and at retrieval presented the stimuli either to the same or opposite visual field/hemisphere. Recognition responses were faster and more accurate when the stimuli were initially presented at encoding and retrieval to the same hemisphere (Experiment 1), even when delay intervals between study and test were short (Experiment 2). Taken together, these findings suggest that recognition decisions for stimuli initially presented to a single hemisphere occur more quickly at shorter lags, perhaps due to a stronger memory representation in the original hemisphere of input compared to the indirectly activated hemisphere. Our results are significant because they demonstrate that each hemisphere of the brain can function to encode and retrieve memory representations equally well, as long as the stimuli contain no linguistic information.

Exploring laterality and memory effects in the haptic discrimination of verbal and non-verbal shapes

The brain's left hemisphere often displays advantages in processing verbal information, while the right hemisphere favours processing non-verbal information. In the haptic domain due to contra-lateral innervations, this functional lateralization is reflected in a hand advantage during certain functions. Findings regarding the hand-hemisphere advantage for haptic information remain contradictory, however. This study addressed these laterality effects and their interaction with memory retention times in the haptic modality. Participants performed haptic discrimination of letters, geometric shapes and nonsense shapes at memory retention times of 5, 15 and 30 s with the left and right hand separately, and we measured the discriminability index d'. The d' values were significantly higher for letters and geometric shapes than for nonsense shapes. This might result from dual coding (naming + spatial) or/and from a low stimulus complexity. There was no stimulus-specific laterality effect. However, we found a time-dependent laterality effect, which revealed that the performance of the left hand-right hemisphere was sustained up to 15 s, while the performance of the right-hand-left hemisphere decreased progressively throughout all retention times. This suggests that haptic memory traces are more robust to decay when they are processed by the left hand-right hemisphere.

Hemispheric processing of memory is affected by sleep

Sleep is known to affect learning and memory, but the extent to which it influences behavioural processing in the left and right hemispheres of the brain is as yet unknown. We tested two hypotheses about lateralised effects of sleep on recognition memory for words: whether sleep reactivated recent experiences of words promoting access to the long-term store in the left hemisphere (LH), and whether sleep enhanced spreading activation differentially in semantic networks in the hemispheres. In Experiment 1, participants viewed lists of semantically related words, then slept or stayed awake for 12h before being tested on seen, unseen but related, or unrelated words presented to the left or the right hemisphere. Sleep was found to promote word recognition in the LH, and to spread activation equally within semantic networks in both hemispheres. Experiment 2 ensured that the results were not due to time of day effects influencing cognitive performance.

The effect of cognitive load on hemispheric asymmetries in true and false memory

Studies examining hemispheric asymmetries in false memory have shown that the right hemisphere (RH) is more susceptible to false memories compared to the left hemisphere (LH). Theories suggest that hemispheric asymmetries in true and false memory may be due to differences in representational coding and the use of top-down mechanisms in each hemisphere. In the current study, the Deese-Roediger-McDermott false memory paradigm was used in conjunction with divided visual field presentation to examine the role of top-down mechanisms in hemispheric asymmetries of true and false memory. In Experiment 1, participants studied lists of related words while completing secondary cognitive load tasks. In Experiment 2, the secondary tasks were administered during memory retrieval instead of memory encoding. Results revealed that cognitive loads imposed during the study phase influenced veridical memory in the LH more than the RH, but cognitive loads imposed during retrieval did not influence veridical memory in either hemisphere. Surprisingly, false memory rates were not influenced by cognitive loads and were higher in the LH. These data provide evidence that, at least for veridical memory, top-down control mechanisms are used more readily for the encoding of information into memory in the LH compared to the RH.

Hemispheric differences in orthographic and semantic processing as revealed by event-related potentials

Differences in how the right and left hemispheres (RH, LH) apprehend visual words were examined using event-related potentials (ERPs) in a repetition paradigm with visual half-field (VF) presentation. In both hemispheres (RH/LVF, LH/RVF), initial presentation of items elicited similar and typical effects of orthographic neighborhood size, with larger N400s for orthographically regular items (words and pseudowords) than for irregular items (acronyms and meaningless illegal strings). However, hemispheric differences emerged on repetition effects. When items were repeated in the LH/RVF, orthographically regular items, relative to irregular items, elicited larger repetition effects on both the N250, a component reflecting processing at the level of visual form (orthography), and on the N400, which has been linked to semantic access. In contrast, in the RH/LVF, repetition effects were biased toward irregular items on the N250 and were similar in size across item types for the N400. The results suggest that processing in the LH is more strongly affected by wordform regularity than in the RH, either due to enhanced processing of familiar orthographic patterns or due to the fact that regular forms can be more readily mapped onto phonology.

Two sides of meaning: the scalp-recorded n400 reflects distinct contributions from the cerebral hemispheres

The N400, a component of the event-related potential (ERP) associated with the processing of meaning, is sensitive to a wide array of lexico-semantic, sentence-level, and discourse-level manipulations across modalities. In sentence contexts, N400 amplitude varies inversely and nearly linearly with the predictability of a word in its context. However, recent theories and empirical evidence from studies employing the visual half-field technique (to selectively bias processing to one cerebral hemisphere) suggest that the two hemispheres use sentence context information in different ways. Thus, each hemisphere may not respond to manipulations of contextual predictability in an equivalent manner. This possibility was investigated by recording ERPs while presenting [in the left and right visual fields (VFs)] sentence-final words that varied over the full range of sentence-level predictability. RVF/left hemisphere items were facilitated (as evidenced by reduced N400 amplitudes) over a broader range of predictability compared with LVF/right hemisphere items, although both strongly predictable and completely unexpected items evoked similar responses in each VF/hemisphere. Further, the pattern of N400 amplitudes over the full range of predictability significantly differed from a linear response function for both VFs/hemispheres. This suggests that the N400 response recorded with standard central field presentation comprises different contributions from both cerebral hemispheres, neither of which on its own is sensitive to contextual predictability in an evenly graded manner. These data challenge the notion of a singular or unitary mode of comprehension and instead support the view that the left and right hemispheres instantiate unique, complementary language comprehension architectures in parallel.

Lateral head turning affects temporal memory

Spatial attention is a key factor in the exploration and processing of the surrounding environment, and plays a role in linking magnitudes such as space, time, and numbers. The present work evaluates whether shifting the coordinates of spatial attention through rotational head movements may affect the ability to estimate the duration of different time intervals. A computer-based implicit timing task was employed, in which participants were asked to concentrate and report verbally on colour changes of sequential stimuli displayed on a computer screen; subsequently, they were required to reproduce the temporal duration (ranging between 5 and 80 sec.) of the perceived stimuli using the computer keyboard. There was statistically significant overestimation of the 80-sec. intervals exclusively on the rightward rotation head posture, whereas head posture did not affect timing performances on shorter intervals. These findings support the hypothesis that the coordinates of spatial attention influence the ability to process time, consistent with the existence of common cortical metrics of space and time in healthy humans.

When side matters: hemispheric processing and the visual specificity of emotional memories

Previous studies have shown that the right hemisphere processes the visual details of objects and the emotionality of information. These two roles of the right hemisphere have not been examined concurrently. In the present study, the authors examined whether right hemisphere processing would lead to particularly good memory for the visual details of emotional stimuli. Participants viewed positive, negative, and neutral objects, displayed to the left or right of a fixation cross. Later, participants performed a recognition task in which they evaluated whether items were "same" (same visual details), "similar" (same verbal label, different visual details), or "new" (unrelated) in comparison with the studied objects. Participants remembered the visual details of negative items well, and this advantage in memory specificity was particularly pronounced when the items had been presented directly to the right hemisphere (i.e., to the left of the fixation cross). These results suggest that there is an episodic memory benefit conveyed when negative items are presented directly to the right hemisphere, likely because of the specialization of the right hemisphere for processing both visual detail and negatively valenced emotional information.

Time-dependent hemispheric shift of the cortical control of volitional swallowing

An important part of the cortical processing of swallowing takes place in the sensorimotor cortex, predominantly in the left hemisphere. However, until now, only deglutition related brain activation with low time resolution exceeding a time interval of 1 s has been reported. In this study, we have examined the chronological sequence of cortical swallowing processing in humans by means of high temporal resolution magnetoencephalography (MEG). The cortical MEG activity was recorded during self-paced volitional swallowing in 10 healthy subjects. Data were analyzed using synthetic aperture magnetometry and the group analysis was performed using a permutation test. Swallowing-related muscle activity was recorded by electromyography. Within the time interval of 1 s of the most pronounced muscular swallowing execution, the MEG analysis revealed neural activation in the primary sensorimotor cortex. During the first 600 ms, only left hemispheric activation was found, bihemispheric activation during the next 200 ms and a right hemispheric activation during the last 200 ms. Thus, our results demonstrate a time-dependent shift of neural activation from left to right sensorimotor cortex during deglutition with left hemispheric dominance in the early stage of volitional swallowing and right hemispheric dominance during its later part.

Left and right memory revisited: electrophysiological investigations of hemispheric asymmetries at retrieval

Hemispheric differences in the use of memory retrieval cues were examined in a continuous recognition design, using visual half-field presentation to bias the processing of test words. A speeded recognition task revealed general accuracy and response time advantages for items whose test presentation was biased to the left hemisphere. A second experiment recorded event-related brain potentials in the same design and replicated these behavioral effects, but found no electrophysiological support for the hypothesis that test words biased to the left hemisphere elicit superior recognition. Instead, successful retrieval was accompanied by memory components of identical strength regardless of test field. That robust visual field effects in response accuracy and speed were not mimicked in memory components that generally do correlate with such behavioral differences suggests that patterns in overt responses may be dominated by the left hemisphere's superior ability to apprehend words. Differences between the data pattern observed in the present study with lateralized retrieval and that in a prior study with lateralized encoding [Evans, K. M., & Federmeier, K. D. (2007). The memory that's right and the memory that's left: Event-related potentials reveal hemispheric asymmetries in the encoding and retention of verbal information. Neuropsychologia 45(8), 1777-1790.] support the notion that hemispheric processing is highly integrated in the intact brain, and highlight the need to treat lateralization at different stages as distinct.

Thinking ahead: the role and roots of prediction in language comprehension

Reviewed are studies using event-related potentials to examine when and how sentence context information is used during language comprehension. Results suggest that, when it can, the brain uses context to predict features of likely upcoming items. However, although prediction seems important for comprehension, it also appears susceptible to age-related deterioration and can be associated with processing costs. The brain may address this trade-off by employing multiple processing strategies, distributed across the two cerebral hemispheres. In particular, left hemisphere language processing seems to be oriented toward prediction and the use of top-down cues, whereas right hemisphere comprehension is more bottom-up, biased toward the veridical maintenance of information. Such asymmetries may arise, in turn, because language comprehension mechanisms are integrated with language production mechanisms only in the left hemisphere (the PARLO framework).

The memory that's right and the memory that's left: event-related potentials reveal hemispheric asymmetries in the encoding and retention of verbal information

We examined the nature and timecourse of hemispheric asymmetries in verbal memory by recording event-related potentials (ERPs) in a continuous recognition task. Participants made overt recognition judgments to test words presented in central vision that were either novel (new words) or had been previously presented in the left or right visual field (old words). An ERP memory effect linked to explicit retrieval revealed no asymmetries for words repeated at short and medium retention intervals, but at longer repetition lags (20-50 intervening words) this 'old/new effect' was more pronounced for words whose study presentation had been biased to the right hemisphere (RH). Additionally, a repetition effect linked to more implicit recognition processes (P2 amplitude changes) was observed at all lags for words preferentially encoded by the RH but was not observed for left hemisphere (LH)-encoded words. These results are consistent with theories that the RH encodes verbal stimuli more veridically whereas the LH encodes in a more abstract manner. The current findings provide a critical link between prior work on memory asymmetries, which has emphasized general LH advantages for verbal material, and on language comprehension, which has pointed to an important role for the RH in language processes that require the retention and integration of verbal information over long time spans.