Synchronized brain activity during rehearsal and short-term memory disruption by irrelevant speech is affected by recall mode

Using synchronized brain rhythms to bias memory-guided decisions

Functional interactions between the prefrontal cortex and hippocampus, as revealed by strong oscillatory synchronization in the theta (6-11 Hz) frequency range, correlate with memory-guided decision-making. However, the degree to which this form of long-range synchronization influences memory-guided choice remains unclear. We developed a brain machine interface that initiated task trials based on the magnitude of prefrontal hippocampal theta synchronization, then measured choice outcomes. Trials initiated based on strong prefrontal-hippocampal theta synchrony were more likely to be correct compared to control trials on both working memory-dependent and -independent tasks. Prefrontal-thalamic neural interactions increased with prefrontal-hippocampal synchrony and optogenetic activation of the ventral midline thalamus primarily entrained prefrontal theta rhythms, but dynamically modulated synchrony. Together, our results show that prefrontal-hippocampal theta synchronization leads to a higher probability of a correct choice and strengthens prefrontal-thalamic dialogue. Our findings reveal new insights into the neural circuit dynamics underlying memory-guided choices and highlight a promising technique to potentiate cognitive processes or behavior via brain machine interfacing.

Temporal and Spatial Information Elicit Different Power and Connectivity Profiles during Working Memory Maintenance

Working memory (WM) is the cognitive ability to store and manipulate information necessary for ongoing tasks. Although frontoparietal areas are involved in the retention of visually presented information, oscillatory neural activity differs for temporal and spatial WM processing. In this study, we corroborated previous findings describing the modulation of neural oscillations and expanded our investigation to the network organization underlying the cognitive processing of temporal and spatial information. We utilized MEG recordings during a Sternberg visual WM task. The spectral oscillatory activity in the maintenance phase revealed increased frontal theta (4-8 Hz) and parietal beta (13-30 Hz) in the temporal condition. Source level coherence analysis delineated the prominent role of parietal areas in all frequency bands during the maintenance of temporal information, whereas frontal and central areas showed major contributions in theta and beta ranges during the maintenance of spatial information. Our study revealed distinct spectral profiles of neural oscillations for separate cognitive subdomains of WM processing. The delineation of specific functional networks might have important implications for clinical applications, enabling the development of stimulation protocols targeting cognitive disabilities associated with WM impairments.

Language as the Working Model of Human Mind

The Human Mind, functional aspect of Human Brain, has been envisaged to be working on the tenets of Chaos, a seeming order within a disorder, the premise of Universe. The armamentarium of Human Mind makes use of distributed neuronal networks sub-serving Sensorial Mechanisms, Mirror Neurone System (MNS) and Motor Mechanisms etching a stochastic trajectory on the virtual phase-space of Human Mind, obeying the ethos of Chaos. The informational sensorial mechanisms recruit attentional mechanisms channelising through the window of chaotic neural dynamics onto MNS that providing algorithmic image information flow along virtual phase- space coordinates concluding onto motor mechanisms that generates and mirrors a stimulus- specific and stimulus-adequate response. The singularity of self-iterating fractal architectonics of Event-Related Synchrony (ERS), a Power Spectral Density (PSD) precept of electroencephalographic (EEG) time-series denotes preferential and categorical inhibition gateway and an Event-Related Desynchrony (ERD) represents event related and locked gateway to stimulatory/excitatory neuronal architectonics leading to stimulus-locked and adequate neural response. The contextual inference in relation to stochastic phase-space trajectory of self- iterating fractal of Off-Center α ERS (Central)-On-Surround α ERD-On Surround θ ERS document efficient neural dynamics of working memory., across patterned modulation and flow of the neurally coded information.

Does rehearsal matter? Left anterior temporal alpha and theta band changes correlate with the beneficial effects of rehearsal on working memory

Rehearsal during working memory (WM) maintenance is assumed to facilitate retrieval. Less is known about how rehearsal modulates WM delay activity. In the present study, 44 participants completed a Sternberg Task with either intact novel scenes or phase-scrambled scenes, which had similar color and spatial frequency but lacked semantic content. During the rehearsal condition participants generated a descriptive label during encoding and covertly rehearsed during the delay period. During the suppression condition participants did not generate a label during encoding and suppressed (repeated "the") during the delay period. This was easy in the former (novel scenes) but more difficult in the later condition (phase-scrambled scenes) where scenes lacked semantic content. Behavioral performance and EEG delay activity was analyzed as a function of maintenance strategy. Performance during WM revealed a benefit of rehearsal for phase-scrambled but not intact scenes. Examination of the absolute amplitude revealed three underlying sources of activity for rehearsal, including the left anterior temporal (ATL) and left and midline parietal regions. Increases in alpha and theta activity in ATL were correlated with improvement in performance on WM with rehearsal only when labeling was not automatic (e.g., phase-scrambled scenes), which may reflect differences in labeling and rehearsal (i.e., semantic associations vs. shallow labels). We conclude that rehearsal only benefits memory for visual stimuli that lack semantic information, and that this is correlated with changes in alpha and theta rhythms.

Characterizing synchrony patterns across cognitive task stages of associative recognition memory

Numerous studies seek to understand the role of oscillatory synchronization in cognition. This problem is particularly challenging in the context of complex cognitive behavior, which consists of a sequence of processing steps with uncertain duration. In this study, we analyzed oscillatory connectivity measures in time windows that previous computational models had associated with a specific sequence of processing steps in an associative memory recognition task (visual encoding, familiarity, memory retrieval, decision making, and motor response). The timing of these processing steps was estimated on a single‐trial basis with a novel hidden semi‐Markov model multivariate pattern analysis (HSMM‐MVPA) method. We show that different processing stages are associated with specific patterns of oscillatory connectivity. Visual encoding is characterized by a dense network connecting frontal, posterior, and temporal areas as well as frontal and occipital phase locking in the 4–9 Hz theta band. Familiarity is associated with frontal phase locking in the 9–14 Hz alpha band. Decision making is associated with frontal and temporo‐central interhemispheric connections in the alpha band. During decision making, a second network in the theta band that connects left‐temporal, central, and occipital areas bears similarity to the neural signature for preparing a motor response. A similar theta band network is also present during the motor response, with additionally alpha band connectivity between right‐temporal and posterior areas. This demonstrates that the processing stages discovered with the HSMM‐MVPA method are indeed linked to distinct synchronization patterns, leading to a closer understanding of the functional role of oscillations in cognition.

Decreased global field synchronization of multichannel frontal EEG measurements in obsessive-compulsive disorders

Global field synchronization (GFS) quantifies the synchronization level of brain oscillations. The GFS method has been introduced to measure functional synchronization of EEG data in the frequency domain. GFS also detects phase interactions between EEG signals acquired from all of the electrodes. If a considerable amount of local brain neurons has the same phase, these neurons appear to interact with each other. EEG data were received from 17 obsessive-compulsive disorder (OCD) patients and 17 healthy controls (HC). OCD effects on local and large-scale brain circuits were studied. Analysis of the GFS results showed significantly decreased values in the delta and full frequency bands. This research suggests that OCD causes synchronization disconnection in both the frontal and large-scale regions. This may be related to motivational, emotional and cognitive dysfunctions.

Ventral Midline Thalamus Is Critical for Hippocampal-Prefrontal Synchrony and Spatial Working Memory

Maintaining behaviorally relevant information in spatial working memory (SWM) requires functional synchrony between the dorsal hippocampus and medial prefrontal cortex (mPFC). However, the mechanism that regulates synchrony between these structures remains unknown. Here, we used a unique dual-task approach to compare hippocampal-prefrontal synchrony while rats switched between an SWM-dependent task and an SWM-independent task within a single behavioral session. We show that task-specific representations in mPFC neuronal populations are accompanied by SWM-specific oscillatory synchrony and directionality between the dorsal hippocampus and mPFC. We then demonstrate that transient inactivation of the reuniens and rhomboid (Re/Rh) nuclei of the ventral midline thalamus abolished only the SWM-specific activity patterns that were seen during dual-task sessions within the hippocampal-prefrontal circuit. These findings demonstrate that Re/Rh facilitate bidirectional communication between the dorsal hippocampus and mPFC during SWM, providing evidence for a causal role of Re/Rh in regulating hippocampal-prefrontal synchrony and SWM-directed behavior.

SIGNIFICANCE STATEMENT

Hippocampal-prefrontal synchrony has long been thought to be critical for spatial working memory (SWM) and the ventral midline thalamic reuniens and rhomboid nuclei (Re/Rh) have long been considered a potential site for synchronizing the hippocampus and medial prefrontal cortex. However, the hypothesis that Re/Rh are critical for hippocampal-prefrontal synchrony and SWM has not been tested. We first used a dual-task approach to identify SWM-specific patterns of hippocampal-prefrontal synchrony. We then demonstrated that Re/Rh inactivation concurrently disrupted SWM-specific behavior and the SWM-specific patterns of hippocampal-prefrontal synchrony seen during dual-task performance. These results provide the first direct evidence that Re/Rh contribute to SWM by modulating hippocampal-prefrontal synchrony.

Utilizing the extent of theta-gamma synchronization to estimate visuospatial memory ability

The ability of human memory declines due to normal aging and cognitive diseases, which means that everyone will eventually be affected by this problem. Fortunately, memory ability can be improved by training, and early detection and treatment can even actively prevent serious memory loss. Based on this principle, we aimed to identify a method for estimating the memory ability in order to slow the progression of memory loss. Numerous studies have found that coupling between the theta and gamma bands (also referred to as theta-gamma synchronization) reflect memory processes, with this being more pronounced when maintaining working memory. This study measured the extent of theta-gamma synchronization, and used a wavelet transform to observe the activity in the theta and gamma bands during a visuospatial memory task. The findings showed that there was a pronounced change during the encoding and retrieval phases of a working-memory task, and a significant correlation between the rate of correct responses and the synchronization index in the parietal brain area. We propose that the extent of theta-gamma synchronization can be used to estimate the working-memory ability. The further application of theta-gamma synchronization in both clinical and home situations may be expected in the future.

For whom the bell tolls: periodic reactivation of sensory cortex in the gamma band as a substrate of visual working memory maintenance

Working memory (WM) is central to human cognition as it allows information to be kept online over brief periods of time and facilitates its usage in cognitive operations (Luck and Vogel, 2013). How this information maintenance actually is implemented is still a matter of debate. Several independent theories of WM, derived, respectively, from behavioral studies and neural considerations, advance the idea that items in WM decay over time and must be periodically reactivated. In this proposal, we show how recent data from intracranial EEG and attention research naturally leads to a simple model of such reactivation in the case of sensory memories. Specifically, in our model the amplitude of high-frequency activity (>50 Hz, in the gamma-band) underlies the representation of items in high-level visual areas. This activity decreases to noise-levels within 500 ms, unless it is reactivated. We propose that top-down attention, which targets multiple sensory items in a cyclical or rhythmic fashion at around 6-10 Hz, reactivates these decaying gamma-band representations. Therefore, working memory capacity is essentially the number of representations that can simultaneously be kept active by a rhythmically sampling attentional spotlight given the known decay rate. Since attention samples at 6-10 Hz, the predicted WM capacity is 3-5 items, in agreement with empirical findings.

Frontal midline theta connectivity is related to efficiency of WM maintenance and is affected by aging

Representations in working memory (WM) are temporary, but can be refreshed for longer periods of time through maintenance mechanisms, thereby establishing their availability for subsequent memory tests. Frontal brain regions supporting WM maintenance operations undergo anatomical and functional changes with advancing age, leading to age related decline of memory functions. The present study focused on age-related functional connectivity changes of the frontal midline (FM) cortex in the theta band (4-8 Hz), related to WM maintenance. In the visual delayed-match-to-sample WM task young (18-26 years, N=20) and elderly (60-71 years N=16) adults had to memorize sample stimuli consisting of 3 or 5 items while 33 channel EEG recording was performed. The phase lag index was used to quantify connectivity strength between cortical regions. The low and high memory demanding WM maintenance periods were classified based on whether they were successfully maintained (remembered) or unsuccessfully maintained (unrecognized later). In the elderly reduced connectivity strength of FM brain region and decreased performance were observed. The connectivity strength between FM and posterior sensory cortices was shown to be sensitive to both increased memory demands and memory performance regardless of age. The coupling of frontal regions (midline and lateral) and FM-temporal cortices characterized successfully maintained trials and declined with advancing age. The findings provide evidence that a FM neural circuit of theta oscillations that serves a possible basis of active maintenance process is especially vulnerable to aging.

Fronto-parietal and fronto-temporal theta phase synchronization for visual and auditory-verbal working memory

In humans, theta phase (4-8 Hz) synchronization observed on electroencephalography (EEG) plays an important role in the manipulation of mental representations during working memory (WM) tasks; fronto-temporal synchronization is involved in auditory-verbal WM tasks and fronto-parietal synchronization is involved in visual WM tasks. However, whether or not theta phase synchronization is able to select the to-be-manipulated modalities is uncertain. To address the issue, we recorded EEG data from subjects who were performing auditory-verbal and visual WM tasks; we compared the theta synchronizations when subjects performed either auditory-verbal or visual manipulations in separate WM tasks, or performed both two manipulations in the same WM task. The auditory-verbal WM task required subjects to calculate numbers presented by an auditory-verbal stimulus, whereas the visual WM task required subjects to move a spatial location in a mental representation in response to a visual stimulus. The dual WM task required subjects to manipulate auditory-verbal, visual, or both auditory-verbal and visual representations while maintaining auditory-verbal and visual representations. Our time-frequency EEG analyses revealed significant fronto-temporal theta phase synchronization during auditory-verbal manipulation in both auditory-verbal and auditory-verbal/visual WM tasks, but not during visual manipulation tasks. Similarly, we observed significant fronto-parietal theta phase synchronization during visual manipulation tasks, but not during auditory-verbal manipulation tasks. Moreover, we observed significant synchronization in both the fronto-temporal and fronto-parietal theta signals during simultaneous auditory-verbal/visual manipulations. These findings suggest that theta synchronization seems to flexibly connect the brain areas that manipulate WM.

Treating working memory deficits in schizophrenia: a review of the neurobiology

Cognitive deficits are a core feature of schizophrenia. Among these deficits, working memory impairment is considered a central cognitive impairment in schizophrenia. The prefrontal cortex, a region critical for working memory performance, has been demonstrated as a critical liability region in schizophrenia. As yet, there are no standardized treatment options for working memory deficits in schizophrenia. In this review, we summarize the neuronal basis for working memory impairment in schizophrenia, including dysfunction in prefrontal signaling pathways (e.g., γ-aminobutyric acid transmission) and neural network synchrony (e.g., gamma/theta oscillations). We discuss therapeutic strategies for working memory dysfunction such as pharmacological agents, cognitive remediation therapy, and repetitive transcranial magnetic stimulation. Despite the drawbacks of current approaches, the advances in neurobiological and translational treatment strategies suggest that clinical application of these methods will occur in the near future.

The role of phase synchronization in memory processes

In recent years, studies ranging from single-unit recordings in animals to electroencephalography and magnetoencephalography studies in humans have demonstrated the pivotal role of phase synchronization in memory processes. Phase synchronization - here referring to the synchronization of oscillatory phases between different brain regions - supports both working memory and long-term memory and acts by facilitating neural communication and by promoting neural plasticity. There is evidence that processes underlying working and long-term memory might interact in the medial temporal lobe. We propose that this is accomplished by neural operations involving phase-phase and phase-amplitude synchronization. A deeper understanding of how phase synchronization supports the flexibility of and interaction between memory systems may yield new insights into the functions of phase synchronization in general.

ERP correlates of the irrelevant sound effect

The irrelevant sound effect refers to a decrement in serial-recall performance when auditory distractors are played during encoding or retention of the to-be-remembered items. We examined the event-related brain potentials (ERPs) that were elicited in response to the auditory distractors during encoding and retention of visually presented target sequences. Changing-state distractor sequences that consisted of several different distractor items interfered more with serial recall than steady-state sequences that consisted of repetitions of a single distractor item. The ERP responses that were elicited in response to the distractors comprised the exogenous N1 component and were further characterized by a subsequent positive wave, and a late negativity. The changing-state effect was associated with an increased N1 and a P3a. The results support the attention-capture account of the irrelevant sound effect.

Oscillatory mechanisms of process binding in memory

A central topic in cognitive neuroscience is the question, which processes underlie large scale communication within and between different neural networks. The basic assumption is that oscillatory phase synchronization plays an important role for process binding--the transient linking of different cognitive processes--which may be considered a special type of large scale communication. We investigate this question for memory processes on the basis of different types of oscillatory synchronization mechanisms. The reviewed findings suggest that theta and alpha phase coupling (and phase reorganization) reflect control processes in two large memory systems, a working memory and a complex knowledge system that comprises semantic long-term memory. It is suggested that alpha phase synchronization may be interpreted in terms of processes that coordinate top-down control (a process guided by expectancy to focus on relevant search areas) and access to memory traces (a process leading to the activation of a memory trace). An analogous interpretation is suggested for theta oscillations and the controlled access to episodic memories.

N1 and the mismatch negativity are spatiotemporally distinct ERP components: Disruption of immediate memory by auditory distraction can be related to N1

Event-related potentials (ERPs) were recorded for ignored tones presented during the retention interval of a delayed serial recall task. The mismatch negativity (MMN) and N1 ERP components were measured to discern spatiotemporal and functional properties of their generation. A nine-token sequence with nine different tone pitches was more disruptive than an oddball (two-token) sequence, yet this oddball sequence was no more disruptive than a single repeating tone (one-token). Tones of the nine-token sequence elicited augmented N1 amplitudes compared to identical tones delivered in the one-token sequence, yet deviants elicited an additional component (MMN) with distinct temporal properties and topography. These results suggested that MMN and N1 are separate, functionally distinct components. Implications are discussed for the N1 hypothesis and the changing-state hypothesis of the disruption of serial recall performance by auditory distraction.