Oscillatory Neural Activity Research Articles

Phase-Dependent Modulation of Signal Transmission in Cortical Networks through tACS-Induced Neural Oscillations.

Oscillatory neural activity is considered a basis of signal transmission in brain networks. However, the causal role of neural oscillations in regulating cortico-cortical signal transmission has so far not been directly demonstrated. To date, due to methodological limitations, studies on the online modulatory mechanisms of transcranial alternating current stimulation (tACS)-induced neural oscillations are confined to the primary motor cortex. To address the causal role of oscillatory activity in modulating cortico-cortical signal transmission, we have established a new method using concurrent tACS, transcranial magnetic stimulation (TMS) and electroencephalography (EEG). Through tACS, we introduced 6-Hz (theta) oscillatory activity in the human dorsolateral prefrontal cortex (DLPFC). During tACS, we applied single-pulse TMS over the DLPFC at different phases of tACS and assessed propagation of TMS-induced neural activity with EEG. We show that tACS-induced theta oscillations modulate the propagation of TMS-induced activity in a phase-dependent manner and that phase-dependent modulation is not simply explained by the instantaneous amplitude of tACS. The results demonstrate a phase-dependent modulatory mechanism of tACS at a cortical network level, which is consistent with a causal role of neural oscillations in regulating the efficacy of signal transmission in the brain.

Theta Oscillations Related to Orientation Recognition in Unattended Condition: A vMMN Study.

Orientation is one of the important elements of objects that can influence visual processing. In this study, we examined whether changes in orientation could be detected automatically under unattended condition. Visual mismatch negativity (vMMN) was used to analyze this processing. In addition, we investigated the underlying neural oscillatory activity. Non-phase-locked spectral power was used to explore the specific frequency related to unexpected changes in orientation. The experiment consisted of standard (0° arrows) and deviant (90°/270° arrows) stimuli. Compared with standard stimuli, deviant stimuli elicited a larger N170 component (negative wave approximately 170 ms after the stimuli started) and a smaller P2 component (positive wave approximately 200 ms after the stimuli started). Furthermore, vMMN was obtained by subtracting the event-related potential (ERP) waveforms in response to standard stimuli from those elicited in response to deviant stimuli. According to the time–frequency analysis, deviant stimuli elicited enhanced band power compared with standard stimuli in the delta and theta bands. Compared with previous studies, we concluded that theta activity plays an important role in the generation of the vMMN induced by changes in orientation.

Parkinsonism and vigilance: alteration in neural oscillatory activity and phase-amplitude coupling in the basal ganglia and motor cortex.

Oscillatory neural activity in different frequency bands and phase-amplitude coupling (PAC) are hypothesized to be biomarkers of Parkinson's disease (PD) that could explain dysfunction in the motor circuit and be used for closed-loop deep brain stimulation (DBS). How these putative biomarkers change from the normal to the parkinsonian state across nodes in the motor circuit and within the same subject, however, remains unknown. In this study, we characterized how parkinsonism and vigilance altered oscillatory activity and PAC within the primary motor cortex (M1), subthalamic nucleus (STN), and globus pallidus (GP) in two nonhuman primates. Static and dynamic analyses of local field potential (LFP) recordings indicate that 1) after induction of parkinsonism using the neurotoxin MPTP, low-frequency power (8-30 Hz) increased in the STN and GP in both subjects, but increased in M1 in only one subject; 2) high-frequency power (~330 Hz) was present in the STN in both normal subjects but absent in the parkinsonian condition; 3) elevated PAC measurements emerged in the parkinsonian condition in both animals, but in different sites in each animal (M1 in one subject and GPe in the other); and 4) the state of vigilance significantly impacted how oscillatory activity and PAC were expressed in the motor circuit. These results support the hypothesis that changes in low- and high-frequency oscillatory activity and PAC are features of parkinsonian pathophysiology and provide evidence that closed-loop DBS systems based on these biomarkers may require subject-specific configurations as well as adaptation to changes in vigilance.NEW & NOTEWORTHY Chronically implanted electrodes were used to record neural activity across multiple nodes in the basal ganglia-thalamocortical circuit simultaneously in a nonhuman primate model of Parkinson's disease, enabling within-subject comparisons of electrophysiological biomarkers between normal and parkinsonian conditions and different vigilance states. This study improves our understanding of the role of oscillatory activity and phase-amplitude coupling in the pathophysiology of Parkinson's disease and supports the development of more effective DBS therapies based on pathophysiological biomarkers.

Induced cortical oscillations in turtle cortex are coherent at the mesoscale of population activity, but not at the microscale of the membrane potential of neurons.

Bursts of oscillatory neural activity have been hypothesized to be a core mechanism by which remote brain regions can communicate. Such a hypothesis raises the question to what extent oscillations are coherent across spatially distant neural populations. To address this question, we obtained local field potential (LFP) and membrane potential recordings from the visual cortex of turtle in response to visual stimulation of the retina. The time-frequency analysis of these recordings revealed pronounced bursts of oscillatory neural activity and a large trial-to-trial variability in the spectral and temporal properties of the observed oscillations. First, local bursts of oscillations varied from trial to trial in both burst duration and peak frequency. Second, oscillations of a given recording site were not autocoherent; i.e., the phase did not progress linearly in time. Third, LFP oscillations at spatially separate locations within the visual cortex were more phase coherent in the presence of visual stimulation than during ongoing activity. In contrast, the membrane potential oscillations from pairs of simultaneously recorded pyramidal neurons showed smaller phase coherence, which did not change when switching from black screen to visual stimulation. In conclusion, neuronal oscillations at distant locations in visual cortex are coherent at the mesoscale of population activity, but coherence is largely absent at the microscale of the membrane potential of neurons.NEW & NOTEWORTHY Coherent oscillatory neural activity has long been hypothesized as a potential mechanism for communication across locations in the brain. In this study we confirm the existence of coherent oscillations at the mesoscale of integrated cortical population activity. However, at the microscopic level of neurons, we find no evidence for coherence among oscillatory membrane potential fluctuations. These results raise questions about the applicability of the communication through coherence hypothesis to the level of the membrane potential.

Vibroacoustic Stimulation and Brain Oscillation: From Basic Research to Clinical Application

Abstract: This paper addresses the importance of steady state brain oscillation for brain connectivity and cognition. Given that a healthy brain maintains particular levels of oscillatory activity, it argues that disturbances or dysrhythmias of this oscillatory activity can be implicated in common health conditions including Alzheimer’s disease, Parkinson’s Disease, pain, and depression. Literature is reviewed that shows that electric stimulation of the brain can contribute to regulation of neural oscillatory activity and the alleviation of related health conditions. It is then argued that specific frequencies of sound in their vibratory nature can serve as a means to brain stimulation through auditory and vibrotactile means and as such can contribute to regulation of oscillatory activity. The frequencies employed and found effective in electric stimulation are reviewed with the intent of guiding the selection of sound frequencies for vibroacoustic stimulation in the treatment of AD, PD, Pain, and depression.

Anterior Thalamic High Frequency Band Activity Is Coupled with Theta Oscillations at Rest.

Cross-frequency coupling (CFC) between slow and fast brain rhythms, in the form of phase–amplitude coupling (PAC), is proposed to enable the coordination of neural oscillatory activity required for cognitive processing. PAC has been identified in the neocortex and mesial temporal regions, varying according to the cognitive task being performed and also at rest. PAC has also been observed in the anterior thalamic nucleus (ATN) during memory processing. The thalamus is active during the resting state and has been proposed to be involved in switching between task-free cognitive states such as rest, in which attention is internally-focused, and externally-focused cognitive states, in which an individual engages with environmental stimuli. It is unknown whether PAC is an ongoing phenomenon during the resting state in the ATN, which is modulated during different cognitive states, or whether it only arises during the performance of specific tasks. We analyzed electrophysiological recordings of ATN activity during rest from seven patients who received thalamic electrodes implanted for treatment of pharmacoresistant focal epilepsy. PAC was identified between theta (4–6 Hz) phase and high frequency band (80–150 Hz) amplitude during rest in all seven patients, which diminished during engagement in tasks involving an external focus of attention. The findings are consistent with the proposal that theta–gamma coupling in the ATN is an ongoing phenomenon, which is modulated by task performance.

Autism, Attention, and Alpha Oscillations: An Electrophysiological Study of Attentional Capture

BackgroundAutism spectrum disorder (ASD) is associated with deficits in adaptively orienting attention to behaviorally relevant information. Neural oscillatory activity plays a key role in brain function and provides a high-resolution temporal marker of attention dynamics. Alpha-band (8–12 Hz) activity is associated with both selecting task-relevant stimuli and filtering task-irrelevant information. MethodsThe present study used electroencephalography to examine alpha-band oscillatory activity associated with attentional capture in 19 children with ASD and 21 age- and IQ-matched typically developing children. Participants completed a rapid serial visual presentation paradigm designed to investigate responses to behaviorally relevant targets and contingent attention capture by task-irrelevant distractors, which either did or did not share a behaviorally relevant feature. Participants also completed 6 minutes of eyes-open resting electroencephalography. ResultsIn contrast to their typically developing peers, children with ASD did not evidence posterior alpha desynchronization to behaviorally relevant targets. Additionally, reduced target-related desynchronization and poorer target detection were associated with increased ASD symptomatology. Typically developing children also showed behavioral and electrophysiological evidence of contingent attention capture, whereas children with ASD showed no behavioral facilitation or alpha desynchronization to distractors that shared a task-relevant feature. Lastly, children with ASD had significantly decreased resting alpha power, and for all participants increased resting alpha levels were associated with greater task-related alpha desynchronization. ConclusionsThese results suggest that in ASD underresponsivity and impairments in orienting to salient events within their environment are reflected by atypical electroencephalography oscillatory neurodynamics, which may signify atypical arousal levels and/or an excitatory/inhibitory imbalance.

Frequency-specific Transcranial Modulation of Oscillatory Dynamics in Visuospatial Attention Networks

The ability to deploy neural resources to discrete stimulus features or groupings of features within the visual space is important to normative cognitive function. Population-level neural oscillatory activity across multiple frequencies and spatial locations has been linked to attentional deployment, but the inherent temporal dynamics of these neural oscillations have stymied efforts to image these responses in humans. Further, the impact of modulating neural activity in these networks on attentional performance remains unknown. Herein, we apply 2.0 mA transcranial direct current stimulation (tDCS) for 20 minutes over key nodes of the visual attention network (i.e., the visual and prefrontal cortices) using two opposite polarity current configurations, as well as a sham condition. Participants then performed an offline visuospatial discrimination task during concurrent recording with magnetoencephalography (MEG). Recordings were decomposed into time-frequency space and significant changes from baseline were identified and imaged. Time-series were then extracted from peak voxels for subsequent analysis, including functional connectivity analyses. Stimulation significantly altered behavioral performance in the attention task and basal activity in a polarity specific way, such that cathodal and anodal stimulation each affected the dynamics of attention circuits at distinct frequencies. Interestingly, the magnitudes of oscillatory neural responses at these frequencies were not significantly different between stimulation conditions, meaning that the tDCS-induced shifts in basal activity (i.e., baseline) carried through to the absolute amplitude of neural responses during attentional deployment. These findings shed light on the multi-spectral effects of tDCS on human neural circuits, particularly within and between prototypical attention network hubs.

Internal mechanisms underlying anticipatory language processing: Evidence from event-related-potentials and neural oscillations.

Although numerous studies have demonstrated that the language processing system can predict upcoming content during comprehension, there is still no clear picture of the anticipatory stage of predictive processing. This electroencephalograph study examined the cognitive and neural oscillatory mechanisms underlying anticipatory processing during language comprehension, and the consequences of this prediction for bottom-up processing of predicted/unpredicted content. Participants read Mandarin Chinese sentences that were either strongly or weakly constraining and that contained critical nouns that were congruent or incongruent with the sentence contexts. We examined the effects of semantic predictability on anticipatory processing prior to the onset of the critical nouns and on integration of the critical nouns. The results revealed that, at the integration stage, the strong-constraint condition (compared to the weak-constraint condition) elicited a reduced N400 and reduced theta activity (4-7Hz) for the congruent nouns, but induced beta (13-18Hz) and theta (4-7Hz) power decreases for the incongruent nouns, indicating benefits of confirmed predictions and potential costs of disconfirmed predictions. More importantly, at the anticipatory stage, the strongly constraining context elicited an enhanced sustained anterior negativity and beta power decrease (19-25Hz), which indicates that strong prediction places a higher processing load on the anticipatory stage of processing. The differences (in the ease of processing and the underlying neural oscillatory activities) between anticipatory and integration stages of lexical processing were discussed with regard to predictive processing models.

Altered sensorimotor cortical oscillations in individuals with multiple sclerosis suggests a faulty internal model.

Multiple sclerosis (MS) is a demyelinating disease that results in a broad array of symptoms, including impaired motor performance. How such demyelination of fibers affects the inherent neurophysiological activity in motor circuits, however, remains largely unknown. Potentially, the movement errors associated with MS may be due to imperfections in the internal model used to make predictions of the motor output that will meet the task demands. Prior magnetoencephalographic (MEG) and electroencephalographic brain imaging experiments have established that the beta (15-30 Hz) oscillatory activity in the sensorimotor cortices is related to the control of movement. Specifically, it has been suggested that the strength of the post-movement beta rebound may indicate the certainty of the internal model. In this study, we used MEG to evaluate the neural oscillatory activity in the sensorimotor cortices of individuals with MS and healthy individuals during a goal-directed isometric knee force task. Our results showed no difference between the individuals with MS and healthy individuals in the beta activity during the planning and execution stages of movement. However, we did find that individuals with MS exhibited a weaker post-movement beta rebound in the pre/postcentral gyri relative to healthy controls. Additionally, we found that the behavioral performance of individuals with MS was aberrant, and related to the strength of the post-movement beta rebound. These results suggest that the internal model may be faulty in individuals with MS. Hum Brain Mapp 38:4009-4018, 2017. © 2017 Wiley Periodicals, Inc.

Neural indices of phonemic discrimination and sentence-level speech intelligibility in quiet and noise: A P3 study

This study examined how speech babble noise differentially affected the auditory P3 responses and the associated neural oscillatory activities for consonant and vowel discrimination in relation to segmental- and sentence-level speech perception in noise. The data were collected from 16 normal-hearing participants in a double-oddball paradigm that contained a consonant (/ba/ to /da/) and vowel (/ba/ to /bu/) change in quiet and noise (speech-babble background at a −3 dB signal-to-noise ratio) conditions. Time-frequency analysis was applied to obtain inter-trial phase coherence (ITPC) and event-related spectral perturbation (ERSP) measures in delta, theta, and alpha frequency bands for the P3 response. Behavioral measures included percent correct phoneme detection and reaction time as well as percent correct IEEE sentence recognition in quiet and in noise. Linear mixed-effects models were applied to determine possible brain-behavior correlates. A significant noise-induced reduction in P3 amplitude was found, accompanied by significantly longer P3 latency and decreases in ITPC across all frequency bands of interest. There was a differential effect of noise on consonant discrimination and vowel discrimination in both ERP and behavioral measures, such that noise impacted the detection of the consonant change more than the vowel change. The P3 amplitude and some of the ITPC and ERSP measures were significant predictors of speech perception at segmental- and sentence-levels across listening conditions and stimuli. These data demonstrate that the P3 response with its associated cortical oscillations represents a potential neurophysiological marker for speech perception in noise.

Neural oscillatory activities of the motor thalamus in dystonia (P3.023)

Neural oscillatory activities of the motor thalamus in dystonia (P3.023)

Nonsinusoidal Beta Oscillations Reflect Cortical Pathophysiology in Parkinson's Disease.

Oscillations in neural activity play a critical role in neural computation and communication. There is intriguing new evidence that the nonsinusoidal features of the oscillatory waveforms may inform underlying physiological and pathophysiological characteristics. Time-domain waveform analysis approaches stand in contrast to traditional Fourier-based methods, which alter or destroy subtle waveform features. Recently, it has been shown that the waveform features of oscillatory beta (13-30 Hz) events, a prominent motor cortical oscillation, may reflect near-synchronous excitatory synaptic inputs onto cortical pyramidal neurons. Here we analyze data from invasive human primary motor cortex (M1) recordings from patients with Parkinson's disease (PD) implanted with a deep brain stimulator (DBS) to test the hypothesis that the beta waveform becomes less sharp with DBS, suggesting that M1 input synchrony may be decreased. We find that, in PD, M1 beta oscillations have sharp, asymmetric, nonsinusoidal features, specifically asymmetries in the ratio between the sharpness of the beta peaks compared with the troughs. This waveform feature is nearly perfectly correlated with beta-high gamma phase-amplitude coupling (r = 0.94), a neural index previously shown to track PD-related motor deficit. Our results suggest that the pathophysiological beta generator is altered by DBS, smoothing out the beta waveform. This has implications not only for the interpretation of the physiological mechanism by which DBS reduces PD-related motor symptoms, but more broadly for our analytic toolkit in general. That is, the often-overlooked time-domain features of oscillatory waveforms may carry critical physiological information about neural processes and dynamics.SIGNIFICANCE STATEMENT To better understand the neural basis of cognition and disease, we need to understand how groups of neurons interact to communicate with one another. For example, there is evidence that parkinsonian bradykinesia and rigidity may arise from an oversynchronization of afferents to the motor cortex, and that these symptoms are treatable using deep brain stimulation. Here we show that the waveform shape of beta (13-30 Hz) oscillations, which may reflect input synchrony onto the cortex, is altered by deep brain stimulation. This suggests that mechanistic inferences regarding physiological and pathophysiological neural communication may be made from the temporal dynamics of oscillatory waveform shape.

Frequency-dependent oscillatory neural profiles during imitation

Imitation is a complex process that includes higher-order cognitive and motor function. This process requires an observation-execution matching system that transforms an observed action into an identical movement. Although the low-gamma band is thought to reflect higher cognitive processes, no studies have focused on it. Here, we used magnetoencephalography (MEG) to examine the neural oscillatory changes including the low-gamma band during imitation. Twelve healthy, right-handed participants performed a finger task consisting of four conditions (imitation, execution, observation, and rest). During the imitation and execution conditions, significant event-related desynchronizations (ERDs) were observed at the left frontal, central, and parietal MEG sensors in the alpha, beta, and low-gamma bands. Functional connectivity analysis at the sensor level revealed an imitation-related connectivity between a group of frontal sensors and a group of parietal sensors in the low-gamma band. Furthermore, source reconstruction with synthetic aperture magnetometry showed significant ERDs in the low-gamma band in the left sensorimotor area and the middle frontal gyrus (MFG) during the imitation condition when compared with the other three conditions. Our results suggest that the oscillatory neural activities of the low-gamma band at the sensorimotor area and MFG play an important role in the observation-execution matching system related to imitation.

Inhibitory interneuron circuits at cortical and spinal levels are associated with individual differences in corticomuscular coherence during isometric voluntary contraction

Corticomuscular coherence (CMC) is an oscillatory synchronization of 15–35 Hz (β-band) between electroencephalogram (EEG) of the sensorimotor cortex and electromyogram of contracting muscles. Although we reported that the magnitude of CMC varies among individuals, the physiological mechanisms underlying this variation are still unclear. Here, we aimed to investigate the associations between CMC and intracortical inhibition (ICI) in the primary motor cortex (M1)/recurrent inhibition (RI) in the spinal cord, which probably affect oscillatory neural activities. Firstly, we quantified ICI from changes in motor-evoked potentials induced by paired-pulse transcranial magnetic stimulation in M1 during tonic isometric voluntary contraction of the first dorsal interosseous. ICI showed a significant, negative correlation with the strength of EEG β-oscillation, but not with the magnitude of CMC across individuals. Next, we quantified RI from changes in H-reflexes induced by paired-pulse electrical nerve stimulation to the posterior tibial nerve during isometric contraction of the soleus muscle. We observed a significant, positive correlation between RI and peak CMC across individuals. These results suggest that the local inhibitory interneuron networks in cortical and spinal levels are associated with the oscillatory activity in corticospinal loop.

SA74. Visual Cortex Gamma-Band Power is Reduced in Individuals at Ultra-High Risk for Schizophrenia: An MEG Study

Abstract Background: Emerging evidence suggests that schizophrenia (ScZ) is associated with aberrant neural oscillatory activity, in particular at γ-band frequencies (>30 Hz). Our group has reported several studies showing impaired generation of high-gamma band oscillatory activity (~50–120 Hz) in chronic ScZ patients as well as in unmedicated patients with first-episode psychosis (FEP). Currently, however, it is unclear whether gamma-band activity is already impaired in individuals at Ultra High Risk (UHR) for schizophrenia. The current MEG study investigates this question, using a visual task that is known to induce strong high-γ band activity over visual cortical areas. Methods: MEG data were recorded, while 50 UHR and 30 healthy control (CON) participants performed an inward moving grating task. Participants fixated on sine-wave circular gratings and detected (by button-press) acceleration of inward movement, occurring at an unpredictable moment during trial presentation. MEG analysis focused on frequency domain reconstructed MEG data collected from central nodes placed in 80 (sub)cortical regions defined within the AAL-atlas, using LCMV beamformer source-analysis algorithms to optimally suppress both sensor-level noise sources and contributions from neighboring brain areas. Results: Independent of group (i.e., across all participants), 14 AAL-atlas-derived regions in the visual cortex showed significantly increased sustained gamma-band (53–63 Hz) and significantly decreased sustained alpha-band (8–12 Hz) activity during stimulus presentation, compared to baseline activity (False-Discovery-Rate corrected for multiple comparisons). Subsequently tested group effects (UHRs vs. CONs) across these 14 visual regions and across a time interval between 200 and 1200 ms postgrating onset revealed significantly decreased gamma-band responses for UHR, but no group differences in alpha-band responses. In addition, UHR participants’ performance was characterized by elevated error rates, compared to CON participants. Conclusion: Gamma-band visual-cortex responses during perception are already impaired in individuals at UHR for psychosis development and therefore represent a potential candidate biomarker for psychosis prediction.

Speech-specific categorical perception deficit in autism: An Event-Related Potential study of lexical tone processing in Mandarin-speaking children

Recent studies reveal that tonal language speakers with autism have enhanced neural sensitivity to pitch changes in nonspeech stimuli but not to lexical tone contrasts in their native language. The present ERP study investigated whether the distinct pitch processing pattern for speech and nonspeech stimuli in autism was due to a speech-specific deficit in categorical perception of lexical tones. A passive oddball paradigm was adopted to examine two groups (16 in the autism group and 15 in the control group) of Chinese children’s Mismatch Responses (MMRs) to equivalent pitch deviations representing within-category and between-category differences in speech and nonspeech contexts. To further examine group-level differences in the MMRs to categorical perception of speech/nonspeech stimuli or lack thereof, neural oscillatory activities at the single trial level were further calculated with the inter-trial phase coherence (ITPC) measure for the theta and beta frequency bands. The MMR and ITPC data from the children with autism showed evidence for lack of categorical perception in the lexical tone condition. In view of the important role of lexical tones in acquiring a tonal language, the results point to the necessity of early intervention for the individuals with autism who show such a speech-specific categorical perception deficit.

Amplified induced neural oscillatory activity predicts musicians’ benefits in categorical speech perception

Event-related brain potentials (ERPs) reveal musical experience refines neural encoding and confers stronger categorical perception (CP) and neural organization for speech sounds. In addition to evoked brain activity, the human EEG can be decomposed into induced (non-phase-locked) responses whose various frequency bands reflect different mechanisms of perceptual-cognitive processing. Here, we aimed to clarify which spectral properties of these neural oscillations are most prone to music-related neuroplasticity and which are linked to behavioral benefits in the categorization of speech. We recorded electrical brain activity while musicians and nonmusicians rapidly identified speech tokens from a sound continuum. Time-frequency analysis parsed evoked and induced EEG into alpha- (∼10Hz), beta- (∼20Hz), and gamma- (>30Hz) frequency bands. We found that musicians’ enhanced behavioral CP was accompanied by improved evoked speech responses across the frequency spectrum, complementing previously observed enhancements in evoked potential studies (i.e., ERPs). Brain-behavior correlations implied differences in the underlying neural mechanisms supporting speech CP in each group: modulations in induced gamma power predicted the slope of musicians’ speech identification functions whereas early evoked alpha activity predicted behavior in nonmusicians. Collectively, findings indicate that musical training tunes speech processing via two complementary mechanisms: (i) strengthening the formation of auditory object representations for speech signals (gamma-band) and (ii) improving network control and/or the matching of sounds to internalized memory templates (alpha/beta-band). Both neurobiological enhancements may be deployed behaviorally and account for musicians’ benefits in the perceptual categorization of speech.

Oscillatory Neural Activity during Performance of a Cognitive Task in the Presence of Fluctuating Ambient Noise

Oscillatory Neural Activity during Performance of a Cognitive Task in the Presence of Fluctuating Ambient Noise

Beta-band activity represents the recent past during episodic encoding

While much research has focused on understanding how individual stimuli are encoded in episodic memory, less is known about how a series of events is bound into a coherent episode. Cognitive models of episodic memory propose that information about presented stimuli is integrated into a composite representation reflecting one's past experience, allowing events separated in time to become associated. Recent evidence suggests that neural oscillatory activity may be critically involved in this process. To examine how oscillatory activity contributes to binding of information across events, we measured scalp EEG as participants studied categorized lists of people, places, and objects. We assessed their memory for the lists using free recall, allowing us to characterize the temporal and semantic organization of the studied items in memory. Using pattern classification, we identified EEG activity during encoding at a range of frequencies and scalp locations that was sensitive to the category of presented stimuli. In the beta band (16–25Hz) at right posterior electrodes, we observed activity that was also sensitive to the category of recently presented stimuli. This neural activity showed two characteristics consistent with a representation of the recent past: It became stronger when multiple items from the same category were presented in succession, and it contained a fading trace of the previous category after a category shift. When items were separated by an inter-item distraction task, this integrative beta-band activity was disrupted. Distraction also led to decreased semantic organization of the studied materials without affecting their temporal organization; this suggests that distraction disrupts the integration of semantic information over time, preventing encoding of items in terms of the semantic context of earlier items. Our results provide evidence that beta-band activity is involved in maintaining information about recent events, allowing construction of a coherent representation of a temporally extended episode in memory.