Steady state responses: electrophysiological assessment of sensory function in schizophrenia

Click train elicited local gamma synchrony: differing performance and pharmacological responsivity of primary auditory and prefrontal cortices

Auditory neural networks in the brain naturally entrain to rhythmic stimuli. Such synchronization is an accessible index of local network performance as captured by EEG. Across species, click trains delivered ∼ 40 Hz show strong entrainment with primary auditory cortex (Actx) being a principal source. Imaging studies have revealed additional cortical sources, but it is unclear if they are functionally distinct. Since auditory processing evolves hierarchically, we hypothesized that local synchrony would differ between between primary and association cortices. In female SD rats (N = 12), we recorded 40 Hz click train-elicited gamma oscillations using epidural electrodes situated at two distinct sites; one above the prefrontal cortex (PFC) and another above the Actx, after dosing with saline (1 ml/kg, sc) or the NMDA antagonist, MK801 (0.025, 0.05 or 0.1 mpk), in a blocked crossover design. Post-saline, both regions showed a strong 40 Hz auditory steady state response (ASSR). The latencies for the N1 response were ∼ 16 ms (Actx) and ∼ 34 ms (PFC). Narrow band (38-42 Hz) gamma oscillations appeared rapidly (<40 ms from stim onset at Actx but in a more delayed fashion (∼200 ms) at PFC. MK801 augmented gamma synchrony at Actx while dose-dependently disrupting at the PFC. Event-related gamma (but not beta) coherence, an index of long-distance connectivity, was disrupted by MK801. In conclusion, local network gamma synchrony in a higher order association cortex performs differently from that of the primary auditory cortex. We discuss these findings in the context of evolving sound processing across the cortical hierarchy.

Phenotypic analysis of multielectrode array EEG biomarkers in developing and adult male Fmr1 KO mice

Fragile X Syndrome (FXS) is a leading known genetic cause of intellectual disability with symptoms that include increased anxiety and social and sensory processing deficits. Recent electroencephalographic (EEG) studies in humans with FXS have identified neural oscillation deficits that include increased resting state gamma power, increased amplitude of auditory evoked potentials, and reduced phase locking of sound-evoked gamma oscillations. Similar EEG phenotypes are present in mouse models of FXS, but very little is known about the development of such abnormal responses. In the current study, we employed a 30-channel mouse multielectrode array (MEA) system to record and analyze resting and stimulus-evoked EEG signals in male P21 and P91 WT and Fmr1 KO mice. This led to several novel findings. First, P91, but not P21, Fmr1 KO mice have significantly increased resting EEG power in the low- and high-gamma frequency bands. Second, both P21 and P91 Fmr1 KO mice have markedly attenuated inter-trial phase coherence (ITPC) to spectrotemporally dynamic auditory stimuli as well as to 40 Hz and 80 Hz auditory steady-state response (ASSR) stimuli. This suggests abnormal temporal processing from early development that may lead to abnormal speech and language function in FXS. Third, we found hemispheric asymmetry of fast temporal processing in the mouse auditory cortex in WT but not Fmr1 KO mice. Together, these findings define a set of EEG phenotypes in young and adult mice that can serve as translational targets for genetic and pharmacological manipulation in phenotypic rescue studies.

Therapeutic potential of gamma entrainment using sensory stimulation for cognitive symptoms associated with schizophrenia

Schizophrenia is a complex neuropsychiatric disorder with significant morbidity. Treatment options that address the spectrum of symptoms are limited, highlighting the need for innovative therapeutic approaches. Gamma Entrainment Using Sensory Stimulation (GENUS) is an emerging treatment for neuropsychiatric disorders that uses sensory stimulation to entrain impaired oscillatory network activity and restore brain function. Aberrant oscillatory activity often underlies the symptoms experienced by patients with schizophrenia. We propose that GENUS has therapeutic potential for schizophrenia. This paper reviews the current status of schizophrenia treatment and explores the use of sensory stimulation as an adjunctive treatment, specifically through gamma entrainment. Impaired gamma frequency entrainment is observed in patients, particularly in response to auditory and visual stimuli. Thus, sensory stimulation, such as music listening, may have therapeutic potential for individuals with schizophrenia. GENUS holds novel therapeutic potential to improve the lives of individuals with schizophrenia, but further research is required to determine the efficacy of GENUS, optimize its delivery and therapeutic window, and develop strategies for its implementation in specific patient populations.

Transcranial Alternating Current Stimulation Alters Auditory Steady-State Oscillatory Rhythms and Their Cross-Frequency Couplings

Auditory cortical plasticity deficits in schizophrenia are evidenced with electroencephalographic (EEG)-derived biomarkers, including the 40-Hz auditory steady-state response (ASSR). Aiming to understand the underlying oscillatory mechanisms contributing to the 40-Hz ASSR, we examined its response to transcranial alternating current stimulation (tACS) applied bilaterally to the temporal lobe of 23 healthy participants. Although not responding to gamma tACS, the 40-Hz ASSR was modulated by theta tACS (vs sham tACS), with reductions in gamma power and phase locking being accompanied by increases in theta-gamma phase-amplitude cross-frequency coupling. Results reveal that oscillatory changes induced by frequency-tuned tACS may be one approach for targeting and modulating auditory plasticity in normal and diseased brains.

Effects of Memantine on the Auditory Steady-State and Harmonic Responses to 40 Hz Stimulation Across Species

BACKGROUND

Click trains elicit an auditory steady-state response (ASSR) at the driving frequency (1F) and its integer multiple frequencies (2F, 3F, etc.) called harmonics; we call this harmonic response the steady-state harmonic response (SSHR). We describe the 40 Hz ASSR (1F) and 80 Hz SSHR (2F) in humans and rats and their sensitivity to the uncompetitive NMDA antagonist memantine.

METHODS

In humans (healthy control participants, n = 25; patients with schizophrenia, n = 28), electroencephalography was recorded after placebo or 20 mg memantine in a within-participant crossover design. ASSR used 1 ms, 85-dB clicks presented in 250 40/s 500-ms trains. In freely moving rats (n = 9), electroencephalography was acquired after memantine (0, 0.3, 1, 3 mg/kg) in a within-participant crossover design; 65-dB click trains used 5-mV monophasic, 1-ms square waves (40/s).

RESULTS

Across species, ASSR at 1F generated greater evoked power (EP) than the 2F SSHR. 1F > 2F intertrial coherence (ITC) was also detected in humans, but the opposite relationship (ITC: 2F > 1F) was seen in rats. EP and ITC at 1F were deficient in patients and were enhanced by memantine across species. EP and ITC at 2F were deficient in patients. Measures at 2F were generally insensitive to memantine across species, although in humans the ITC harmonic ratio (1F:2F) was modestly enhanced by memantine, and in rats, both the EP and ITC harmonic ratios were significantly enhanced by memantine.

CONCLUSIONS

ASSR and SSHR are robust, nonredundant electroencephalography signals that are suitable for cross-species analyses that reveal potentially meaningful differences across species, diagnoses, and drugs.

Rodents' visual gamma as a biomarker of pathological neural conditions

Neural gamma oscillations (indicatively 30-100 Hz) are ubiquitous: they are associated with a broad range of functions in multiple cortical areas and across many animal species. Experimental and computational works established gamma rhythms as a global emergent property of neuronal networks generated by the balanced and coordinated interaction of excitation and inhibition. Coherently, gamma activity is strongly influenced by the alterations of synaptic dynamics which are often associated with pathological neural dysfunctions. We argue therefore that these oscillations are an optimal biomarker for probing the mechanism of cortical dysfunctions. Gamma oscillations are also highly sensitive to external stimuli in sensory cortices, especially the primary visual cortex (V1), where the stimulus dependence of gamma oscillations has been thoroughly investigated. Gamma manipulation by visual stimuli tuning is particularly easy in rodents, which have become a standard animal model for investigating the effects of network alterations on gamma oscillations. Overall, gamma in the rodents' visual cortex offers an accessible probe on dysfunctional information processing in pathological conditions. Beyond vision-related dysfunctions, alterations of gamma oscillations in rodents were indeed also reported in neural deficits such as migraine, epilepsy and neurodegenerative or neuropsychiatric conditions such as Alzheimer's, schizophrenia and autism spectrum disorders. Altogether, the connections between visual cortical gamma activity and physio-pathological conditions in rodent models underscore the potential of gamma oscillations as markers of neuronal (dys)functioning.

Gamma-band harmonic responses for beta-band auditory steady-state response are intact in patients with early stage schizophrenia

Gamma oscillations, thought to arise from the activity of ɣ-aminobutyric acid (GABA)ergic interneurons, have potential as a biomarker for schizophrenia. Gamma-band auditory steady-state responses (ASSRs) are notably reduced in both chronic and early-stage schizophrenia patients. Furthermore, alterations in gamma-band ASSRs have been demonstrated in animal models through translational research. However, the 40-Hz harmonic responses of the 20-Hz ASSR are not as well-characterized, despite the possibility that these harmonic oscillatory responses may reflect resonant activity in neural circuits. In this study, we investigated the 40-Hz harmonic response to the 20-Hz ASSR in the early stages of schizophrenia. The study recruited 49 participants, including 15 individuals at ultra-high-risk (UHR) for psychosis, 13 patients with first-episode schizophrenia (FES), and 21 healthy controls (HCs). The 40-Hz harmonic responses of the 20-Hz ASSR were evident in all groups. Interestingly, while previous report observed reduced 40-Hz ASSRs, the 40-Hz harmonic responses of the 20-Hz ASSR were not reduced in the UHR or FES groups. These findings suggest that the gamma-band ASSR and its harmonic responses may represent distinct aspects of pathophysiology in the early stages of schizophrenia.

Dysfunction of specific auditory fibers impacts cortical oscillations, driving an autism phenotype despite near-normal hearing

Autism spectrum disorder is discussed in the context of altered neural oscillations and imbalanced cortical excitation-inhibition of cortical origin. We studied here whether developmental changes in peripheral auditory processing, while preserving basic hearing function, lead to altered cortical oscillations. Local field potentials (LFPs) were recorded from auditory, visual, and prefrontal cortices and the hippocampus of BdnfPax2 KO mice. These mice develop an autism-like behavioral phenotype through deletion of BDNF in Pax2+ interneuron precursors, affecting lower brainstem functions, but not frontal brain regions directly. Evoked LFP responses to behaviorally relevant auditory stimuli were weaker in the auditory cortex of BdnfPax2 KOs, connected to maturation deficits of high-spontaneous rate auditory nerve fibers. This was correlated with enhanced spontaneous and induced LFP power, excitation-inhibition imbalance, and dendritic spine immaturity, mirroring autistic phenotypes. Thus, impairments in peripheral high-spontaneous rate fibers alter spike synchrony and subsequently cortical processing relevant for normal communication and behavior.

40-Hz auditory steady-state response deficits are correlated with the severity of persistent auditory verbal hallucination in patients with schizophrenia

BACKGROUND

Abnormal 40 Hz auditory steady-state response (ASSR) has been observed in some psychiatric disorders. Nevertheless, the role of 40 Hz ASSR in persistent auditory verbal hallucinations (pAVHs) schizophrenia (SCZ) is still unknown. This study aims to investigate whether the 40 Hz ASSR impairment is related to pAVHs and can detect pAVHs severity.

METHODS

We analyzed high-density electroencephalography data that from 43 pAVHs patients (pAVH group), 20 moderate auditory verbal hallucinations patients (mid-AVH group), and 24 without auditory verbal hallucinations patients (non-AVH group). Event-related spectral perturbation and inter-trial phase coherence (ITPC) were calculated to quantify dynamic changes of the 40 Hz ASSR power and ITPC, respectively.

RESULTS

Frontal-central, the 40 Hz ASSR power, and ITPC were significantly lower in the pAVH group than in the non-AVH group; There was no significant difference between the pAVH and mid-AVH group. The 40 Hz ASSR was significantly negatively correlated with the severity of pAVHs. The 40 Hz ASSR power, and ITPC could be used as a combinational marker to detect SCZ patients with and without pAVHs.

CONCLUSION

Our findings have shed light on the pathological mechanism of pAVHs in SCZ patients. These results can provide potential avenues for therapeutic intervention of pAVHs.

Click-train evoked steady state harmonic response as a novel pharmacodynamic biomarker of cortical oscillatory synchrony

Sensory networks naturally entrain to rhythmic stimuli like a click train delivered at a particular frequency. Such synchronization is integral to information processing, can be measured by electroencephalography (EEG) and is an accessible index of neural network function. Click trains evoke neural entrainment not only at the driving frequency (F), referred to as the auditory steady state response (ASSR), but also at its higher multiples called the steady state harmonic response (SSHR). Since harmonics play an important and non-redundant role in acoustic information processing, we hypothesized that SSHR may differ from ASSR in presentation and pharmacological sensitivity. In female SD rats, a 2 s-long train stimulus was used to evoke ASSR at 20 Hz and its SSHR at 40, 60 and 80 Hz, recorded from a prefrontal epidural electrode. Narrow band evoked responses were evident at all frequencies; signal power was strongest at 20 Hz while phase synchrony was strongest at 80 Hz. SSHR at 40 Hz took the longest time (∼180 ms from stimulus onset) to establish synchrony. The NMDA antagonist MK801 (0.025-0.1 mg/kg) did not consistently affect 20 Hz ASSR phase synchrony but robustly and dose-dependently attenuated synchrony of all SSHR. Evoked power was attenuated by MK801 at 20 Hz ASSR and 40 Hz SSHR only. Thus, presentation as well as pharmacological sensitivity distinguished SSHR from ASSR, making them non-redundant markers of cortical network function. SSHR is a novel and promising translational biomarker of cortical oscillatory dynamics that may have important applications in CNS drug development and personalized medicine.

Optimal parameters for rapid (invisible) frequency tagging using MEG

Frequency tagging has been demonstrated to be a useful tool for identifying representational-specific neuronal activity in the auditory and visual domains. However, the slow flicker (<30 Hz) applied in conventional frequency tagging studies is highly visible and might entrain endogenous neuronal oscillations. Hence, stimulation at faster frequencies that is much less visible and does not interfere with endogenous brain oscillatory activity is a promising new tool. In this study, we set out to examine the optimal stimulation parameters of rapid frequency tagging (RFT/RIFT) with magnetoencephalography (MEG) by quantifying the effects of stimulation frequency, size and position of the flickering patch. Rapid frequency tagging using flickers above 50 Hz results in almost invisible stimulation which does not interfere with slower endogenous oscillations; however, the signal is weaker as compared to tagging at slower frequencies so certainty over the optimal parameters of stimulation delivery are crucial. The here presented results examining the frequency range between 60 Hz and 96 Hz suggest that RFT induces brain responses with decreasing strength up to about 84 Hz. In addition, even at the smallest flicker patch (2°) focally presented RFT induces a significant and measurable oscillatory brain signal (steady state visual evoked potential/field, SSVEP/F) at the stimulation frequency (66 Hz); however, the elicited response increases with patch size. While focal RFT presentation elicits the strongest response, off-centre presentations do generally mainly elicit a measureable response if presented below the horizontal midline. Importantly, the results also revealed considerable individual differences in the neuronal responses to RFT stimulation. Finally, we discuss the comparison of oscillatory measures (coherence and power) and sensor types (planar gradiometers and magnetometers) in order to achieve optimal outcomes. Based on our extensive findings we set forward concrete recommendations for using rapid frequency tagging in human cognitive neuroscience investigations.

The future of wearable EEG: a review of ear-EEG technology and its applications

Objective.This review paper provides a comprehensive overview of ear-electroencephalogram (EEG) technology, which involves recording EEG signals from electrodes placed in or around the ear, and its applications in the field of neural engineering.Approach.We conducted a thorough literature search using multiple databases to identify relevant studies related to ear-EEG technology and its various applications. We selected 123 publications and synthesized the information to highlight the main findings and trends in this field.Main results.Our review highlights the potential of ear-EEG technology as the future of wearable EEG technology. We discuss the advantages and limitations of ear-EEG compared to traditional scalp-based EEG and methods to overcome those limitations. Through our review, we found that ear-EEG is a promising method that produces comparable results to conventional scalp-based methods. We review the development of ear-EEG sensing devices, including the design, types of sensors, and materials. We also review the current state of research on ear-EEG in different application areas such as brain-computer interfaces, and clinical monitoring.Significance.This review paper is the first to focus solely on reviewing ear-EEG research articles. As such, it serves as a valuable resource for researchers, clinicians, and engineers working in the field of neural engineering. Our review sheds light on the exciting future prospects of ear-EEG, and its potential to advance neural engineering research and become the future of wearable EEG technology.

Alterations of auditory-evoked gamma oscillations are more pronounced than alterations of spontaneous power of gamma oscillation in early stages of schizophrenia

Several animal models of schizophrenia and patients with chronic schizophrenia have shown increased spontaneous power of gamma oscillations. However, the most robust alterations of gamma oscillations in patients with schizophrenia are reduced auditory-oscillatory responses. We hypothesized that patients with early-stage schizophrenia would have increased spontaneous power of gamma oscillations and reduced auditory-oscillatory responses. This study included 77 participants, including 27 ultra-high-risk (UHR) individuals, 19 patients with recent-onset schizophrenia (ROS), and 31 healthy controls (HCs). The auditory steady-state response (ASSR) and spontaneous power of gamma oscillations measured as induced power during the ASSR period were calculated using electroencephalography during 40-Hz auditory click-trains. The ASSRs were lower in the UHR and ROS groups than in the HC group, whereas the spontaneous power of gamma oscillations in the UHR and ROS groups did not significantly differ from power in the HC group. Both early-latency (0-100 ms) and late-latency (300-400 ms) ASSRs were significantly reduced and negatively correlated with the spontaneous power of gamma oscillations in the ROS group. In contrast, UHR individuals exhibited reduced late-latency ASSR and a correlation between the unchanged early-latency ASSR and the spontaneous power of gamma oscillations. ASSR was positively correlated with the hallucinatory behavior score in the ROS group. Correlation patterns between the ASSR and spontaneous power of gamma oscillations differed between the UHR and ROS groups, suggesting that the neural dynamics involved in non-stimulus-locked/task modulation change with disease progression and may be disrupted after psychosis onset.

Unraveling the pathophysiology of schizophrenia: insights from structural magnetic resonance imaging studies

Background

Schizophrenia affects about 1% of the global population. In addition to the complex etiology, linking this illness to genetic, environmental, and neurobiological factors, the dynamic experiences associated with this disease, such as experiences of delusions, hallucinations, disorganized thinking, and abnormal behaviors, limit neurological consensuses regarding mechanisms underlying this disease.

Methods

In this study, we recruited 72 patients with schizophrenia and 74 healthy individuals matched by age and sex to investigate the structural brain changes that may serve as prognostic biomarkers, indicating evidence of neural dysfunction underlying schizophrenia and subsequent cognitive and behavioral deficits. We used voxel-based morphometry (VBM) to determine these changes in the three tissue structures: the gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF). For both image processing and statistical analysis, we used statistical parametric mapping (SPM).

Results

Our results show that patients with schizophrenia exhibited a significant volume reduction in both GM and WM. In particular, GM volume reductions were more evident in the frontal, temporal, limbic, and parietal lobe, similarly the WM volume reductions were predominantly in the frontal, temporal, and limbic lobe. In addition, patients with schizophrenia demonstrated a significant increase in the CSF volume in the left third and lateral ventricle regions.

Conclusion

This VBM study supports existing research showing that schizophrenia is associated with alterations in brain structure, including gray and white matter, and cerebrospinal fluid volume. These findings provide insights into the neurobiology of schizophrenia and may inform the development of more effective diagnostic and therapeutic approaches.

Effects of the mGlu2/3 receptor agonist LY379268 on two models of disturbed auditory evoked brain oscillations in mice

Cognitive impairment is a core feature of schizophrenia and is poorly addressed by currently available medication. This is partly because the underlying circuits are insufficiently understood, and available animal models for brain dysfunction do not adequately mimic human pathology. To improve the translatability of animal studies and complement behavioral data, EEG measurements are being increasingly used in preclinical research. Brain oscillations are similar across species and can be impaired via several means. In this study, we used two approaches to impair early sensory processing and cortical oscillations in mice: a pharmacological model targeting NMDA receptor function in the whole brain via systemic MK-801 application and an optogenetic model targeting parvalbumin-positive (PV+) interneurons locally in the medial prefrontal cortex (mPFC). We evoked brain activity using auditory stimulation, a tool with high translatability from mouse to human. We then investigated the effect of LY379268, an agonist of mGlu2/3 receptors, a potential therapeutic target for schizophrenia, on single neuron and EEG responses. LY379268 was able to rescue MK-801-induced deficits for a variety of clinically relevant early sensory EEG biomarkers. Single neuron recordings revealed a strong effect of LY379268 on the signal-to-noise ratio during auditory stimulation and optogenetic inhibition of PV+ interneurons. Our results contribute to a better understanding of how group II metabotropic glutamate receptors modulate neuronal population and network activity under sensory stimulation while challenged pharmacologically or optogenetically.

Differential Effects of Clozapine and Haloperidol on the 40 Hz Auditory Steady State Response-mediated Phase Resetting in the Prefrontal Cortex of the Female Sprague Dawley Rat

BACKGROUND

Neural synchrony at gamma frequency (~40 Hz) is important for information processing and is disrupted in schizophrenia. From a drug development perspective, molecules that can improve local gamma synchrony are promising candidates for therapeutic development.

HYPOTHESIS

Given their differentiated clinical profile, clozapine, and haloperidol may have distinct effects on local gamma synchrony engendered by 40 Hz click trains, the so-called auditory steady-state response (ASSR).

STUDY DESIGN

Clozapine and haloperidol at doses known to mimic clinically relevant D2 receptor occupancy were evaluated using the ASSR in separate cohorts of female SD rats.

RESULTS

Clozapine (2.5-10 mg/kg, sc) robustly increased intertrial phase coherence (ITC), across all doses. Evoked response increased but less consistently. Background gamma activity, unrelated to the stimulus, showed a reduction at all doses. Closer scrutiny of the data indicated that clozapine accelerated gamma phase resetting. Thus, clozapine augmented auditory information processing in the gamma frequency range by reducing the background gamma, accelerating the gamma phase resetting and improving phase precision and signal power. Modest improvements in ITC were seen with Haloperidol (0.08 and 0.24 mg/kg, sc) without accelerating phase resetting. Evoked power was unaffected while background gamma was reduced at high doses only, which also caused catalepsy.

CONCLUSIONS

Using click-train evoked gamma synchrony as an index of local neural network function, we provide a plausible neurophysiological basis for the superior and differentiated profile of clozapine. These observations may provide a neurophysiological template for identifying new drug candidates with a therapeutic potential for treatment-resistant schizophrenia.

Steady-State Cortico-Cortical Evoked Potential

PURPOSE

The present study evaluated the utility of the steady-state responses of cortico-cortical evoked potentials (SSCCEPs) and compared them with the responses of conventional CCEPs.

METHODS

Eleven patients with medically intractable focal epilepsy who underwent the implantation of subdural electrodes or stereoelectroencephalography were enrolled. Conventional CCEPs were obtained by averaging responses to alternating 1-Hz electrical stimuli, and 5-Hz stimuli were delivered for recording SSCCEPs. The distribution of SSCCEPs was assessed by a frequency analysis of fast Fourier transform and compared with conventional CCEPs.

RESULTS

Steady-state responses of cortico-cortical evoked potentials were successfully recorded in areas consistent with conventional CCEPs in all patients. However, SSCCEPs were more easily disturbed by the 5-Hz stimulation, and small responses had difficulty generating SSCCEPs.

CONCLUSIONS

Steady-state responses of cortico-cortical evoked potentials may be a useful alternative to conventional CCEPs.

Auditory steady-state responses in the auditory cortex of mice during estrus cycle

Auditory-steady state responses (ASSRs) disclose brain's potency to oscillate and have been suggested to serve as biomarkers in various psychiatric disorders. GABAergic neurotransmission, a critical component for brain oscillations, is heavily influenced by sex hormones. In line, the severity of symptoms in psychiatric disorders is linked to changes in sex hormones during menstrual cycle. However, how these sex hormones affect ASSRs remain largely unknown. This was addressed by performing chronic recordings of ASSRs in mice while monitoring its estrus cycle. Here, the stability of ASSRs during long term recordings were validated and showed good reliability. 40 Hz ASSRs showed changes throughout estrus cycle where it decreased in metestrus phase compared to diestrus phase. In contrast, other frequency ASSRs did not show significant changes throughout estrus cycle. Taken together, our findings illustrate that the estrus cycle can influence the generation of ASSRs and the phase of the estrus cycle should be taken into consideration when ASSRs are recorded in females.

Alpha and gamma EEG coherence during on-task and mind wandering states in schizophrenia

OBJECTIVE

Electroencephalographic (EEG) coherence is one of the most relevant physiological measures used to detect abnormalities in patients with schizophrenia. The present study applies a task-related EEG coherence approach to understand cognitive processing in patients with schizophrenia and healthy controls.

METHODS

EEG coherence for alpha and gamma frequency bands was analyzed in a group of patients with schizophrenia and a group of healthy controls during the performance of an ecological task of sustained attention. We compared EEG coherence when participants presented externally directed cognitive states (On-Task) and when they presented cognitive distraction episodes (Mind-Wandering).

RESULTS

Results reflect cortical differences between groups (higher coherence for schizophrenia in the frontocentral and fronto-temporal regions, and higher coherence for healthy-controls in the postero-central regions), especially in the On-Task condition for the alpha band, compared to Mind-Wandering episodes. Few individual differences in gamma coherence were found.

CONCLUSIONS

The current study provides evidence of neurophysiological differences underlying different cognitive states in schizophrenia and healthy controls.

SIGNIFICANCE

Differences between groups may reflect inhibitory processes necessary for the successful processing of information, especially in the alpha band, given its role in cortical inhibition processes. Patients may activate compensatory inhibitory mechanisms when performing the task, reflected in increased coherence in fronto-temporal regions.

Synchronized Auditory Gamma Response to Frontal Transcranial Direct Current Stimulation (tDCS) and its Inter-Individual Variation in Healthy Humans

In schizophrenia, a disorder associated with N-methyl-D-aspartate receptor (NMDAR) hypofunction, auditory cortical plasticity deficits have been indexed by the synchronized electroencephalographic (EEG) auditory steady-state gamma-band (40-Hz) response (ASSR) and the early auditory evoked gamma-band response (aeGBR), both considered to be target engagement biomarkers for NMDAR function, and potentially amenable to treatment by NMDAR modulators. As transcranial direct current stimulation (tDCS) is likely dependent on NMDAR neurotransmission, this preliminary study, conducted in 30 healthy volunteers, assessed the off-line effects of prefrontal anodal tDCS and sham (placebo) treatment on 40-Hz ASSR and aeGBR. Anodal tDCS failed to alter aeGBR but increased both 40-Hz ASSR power, as measured by event-related spectral perturbations (ERSP), and phase locking, as measured by inter-trial phase consistency (ITPC). Inter-individual differences in tDCS-induced increases in ERSP were negatively related to baseline ERSPs. These findings provide tentative support for further study of tDCS as a potential NMDAR neuromodulatory intervention for synchronized auditory gamma response deficits.

Mental fatigue decreases complexity: Evidence from multiscale entropy analysis of instantaneous frequency variation in alpha rhythm

Mental fatigue (MF) jeopardizes performance and safety through a variety of cognitive impairments and according to the complexity loss theory, should represent "complexity loss" in electroencephalogram (EEG). However, the studies are few and inconsistent concerning the relationship between MF and loss of complexity, probably because of the susceptibility of brain waves to noise. In this study, MF was induced in thirteen male college students by a simulated flight task. Before and at the end of the task, spontaneous EEG and auditory steady-state response (ASSR) were recorded and instantaneous frequency variation (IFV) in alpha rhythm was extracted and analyzed by multiscale entropy (MSE) analysis. The results show that there were significant differences in IFV in alpha rhythm either from spontaneous EEG or from ASSR for all subjects. Therefore, the proposed method can be effective in revealing the complexity loss caused by MF in spontaneous EEG and ASSR, which may serve as a promising analyzing method to mark mild mental impairments.

Managing electromyogram contamination in scalp recordings: An approach identifying reliable beta and gamma EEG features of psychoses or other disorders

OBJECTIVE

In publications on the electroencephalographic (EEG) features of psychoses and other disorders, various methods are utilized to diminish electromyogram (EMG) contamination. The extent of residual EMG contamination using these methods has not been recognized. Here, we seek to emphasize the extent of residual EMG contamination of EEG.

METHODS

We compared scalp electrical recordings after applying different EMG-pruning methods with recordings of EMG-free data from 6 fully paralyzed healthy subjects. We calculated the ratio of the power of pruned, normal scalp electrical recordings in the six subjects, to the power of unpruned recordings in the same subjects when paralyzed. We produced "contamination graphs" for different pruning methods.

RESULTS

EMG contamination exceeds EEG signals progressively more as frequencies exceed 25 Hz and with distance from the vertex. In contrast, Laplacian signals are spared in central scalp areas, even to 100 Hz.

CONCLUSION

Given probable EMG contamination of EEG in psychiatric and other studies, few findings on beta- or gamma-frequency power can be relied upon. Based on the effectiveness of current methods of EEG de-contamination, investigators should be able to reanalyze recorded data, reevaluate conclusions from high-frequency EEG data, and be aware of limitations of the methods.

Auditory driven gamma synchrony is associated with cortical thickness in widespread cortical areas

OBJECTIVE

Gamma synchrony is a fundamental functional property of the cerebral cortex, impaired in multiple neuropsychiatric conditions (i.e. schizophrenia, Alzheimer's disease, stroke etc.). Auditory stimulation in the gamma range allows to drive gamma synchrony of the entire cortical mantle and to estimate the efficiency of the mechanisms sustaining it. As gamma synchrony depends strongly on the interplay between parvalbumin-positive interneurons and pyramidal neurons, we hypothesize an association between cortical thickness and gamma synchrony. To test this hypothesis, we employed a combined magnetoencephalography (MEG) - Magnetic Resonance Imaging (MRI) study.

METHODS

Cortical thickness was estimated from anatomical MRI scans. MEG measurements related to exposure of 40 Hz amplitude modulated tones were projected onto the cortical surface. Two measures of cortical synchrony were considered: (a) inter-trial phase consistency at 40 Hz, providing a vertex-wise estimation of gamma synchronization, and (b) phase-locking values between primary auditory cortices and whole cortical mantle, providing a measure of long-range cortical synchrony. A correlation between cortical thickness and synchronization measures was then calculated for 72 MRI-MEG scans.

RESULTS

Both inter-trial phase consistency and phase locking values showed a significant positive correlation with cortical thickness. For inter-trial phase consistency, clusters of strong associations were found in the temporal and frontal lobes, especially in the bilateral auditory and pre-motor cortices. Higher phase-locking values corresponded to higher cortical thickness in the frontal, temporal, occipital and parietal lobes.

DISCUSSION AND CONCLUSIONS

In healthy subjects, a thicker cortex corresponds to higher gamma synchrony and connectivity in the primary auditory cortex and beyond, likely reflecting underlying cell density involved in gamma circuitries. This result hints towards an involvement of gamma synchrony together with underlying brain structure in brain areas for higher order cognitive functions. This study contributes to the understanding of inherent cortical functional and structural brain properties, which might in turn constitute the basis for the definition of useful biomarkers in patients showing aberrant gamma synchronization.

Evaluating delay of gamma oscillations in patients with schizophrenia using evoked response audiometry system

Impaired gamma oscillations found in a 40-Hz auditory steady-state response (ASSR) in patients with schizophrenia are the robust findings that can be used for future biomarker-based therapeutics. To apply these significant observations into the clinical practice, a clinical system for evoked response audiometry (ERA) may be available. In this study, the delayed 40-Hz ASSR, which was reported as a potent biomarker for schizophrenia, was examined using the ERA system in patients with schizophrenia and its clinical relevance was investigated. The phase of ASSR was significantly delayed in patients with schizophrenia compared with the healthy subjects. The delayed phase was associated with severity of the disease symptoms in the patients. A phase delay with aging was found in healthy subjects, but not in patients with schizophrenia. These findings show availability of the ERA system to identify the delayed 40-Hz ASSR and its clinical implication in patients with schizophrenia. Further applications of the ERA system in clinical psychiatry are warranted in developing biological assessments of schizophrenia with 40-Hz ASSR.

The Full Informational Spectral Analysis for Auditory Steady-State Responses in Human Brain Using the Combination of Canonical Correlation Analysis and Holo-Hilbert Spectral Analysis

Auditory steady-state response (ASSR) is a translational biomarker for several neurological and psychiatric disorders, such as hearing loss, schizophrenia, bipolar disorder, autism, etc. The ASSR is sinusoidal electroencephalography (EEG)/magnetoencephalography (MEG) responses induced by periodically presented auditory stimuli. Traditional frequency analysis assumes ASSR is a stationary response, which can be analyzed using linear analysis approaches, such as Fourier analysis or Wavelet. However, recent studies have reported that the human steady-state responses are dynamic and can be modulated by the subject’s attention, wakefulness state, mental load, and mental fatigue. The amplitude modulations on the measured oscillatory responses can result in the spectral broadening or frequency splitting on the Fourier spectrum, owing to the trigonometric product-to-sum formula. Accordingly, in this study, we analyzed the human ASSR by the combination of canonical correlation analysis (CCA) and Holo-Hilbert spectral analysis (HHSA). The CCA was used to extract ASSR-related signal features, and the HHSA was used to decompose the extracted ASSR responses into amplitude modulation (AM) components and frequency modulation (FM) components, in which the FM frequency represents the fast-changing intra-mode frequency and the AM frequency represents the slow-changing inter-mode frequency. In this paper, we aimed to study the AM and FM spectra of ASSR responses in a 37 Hz steady-state auditory stimulation. Twenty-five healthy subjects were recruited for this study, and each subject was requested to participate in two auditory stimulation sessions, including one right-ear and one left-ear monaural steady-state auditory stimulation. With the HHSA, both the 37 Hz (fundamental frequency) and the 74 Hz (first harmonic frequency) auditory responses were successfully extracted. Examining the AM spectra, the 37 Hz and the 74 Hz auditory responses were modulated by distinct AM spectra, each with at least three composite frequencies. In contrast to the results of traditional Fourier spectra, frequency splitting was seen at 37 Hz, and a spectral peak was obscured at 74 Hz in Fourier spectra. The proposed method effectively corrects the frequency splitting problem resulting from time-varying amplitude changes. Our results have validated the HHSA as a useful tool for steady-state response (SSR) studies so that the misleading or wrong interpretation caused by amplitude modulation in the traditional Fourier spectrum can be avoided.

Aberrant attentional modulation of the auditory steady state response (ASSR) is related to auditory hallucination severity in the first-episode schizophrenia-spectrum

The 40-Hz auditory steady state response (ASSR) is reduced early in schizophrenia, with differences evident even at the first episode of schizophrenia-spectrum psychosis (FESz). Although robust, there is high variability in effect size across studies, possibly due to differences in experimental control of attention and heterogeneity of symptom profiles across studies, both of which may affect the ASSR. We investigated the relationships among ASSR deficits, attention-mediated sensory gain, and auditory hallucinations in 25 FESz (15 male; 23.3 ± 4.5 years) and 32 matched healthy comparison subjects (HC, 22 male; 24.7 ± 5.8 years). ASSR was measured to 40-Hz click trains at three intensities (75, 80, and 85 dB) while participants attended or ignored stimuli. ASSR evoked power and inter-trial phase coherence (ITPC) were measured using the Morlet wavelet transform. FESz did not show overall ASSR power reduction (p > 0.1), but power was significantly increased with attention in HC (p < 0.01), but not in FESz (p > 0.1). Likewise, FESz did not evince overall ASSR ITPC reduction (p > 0.1), and ITPC was significantly increased with attention in HC (p < 0.01), but not in FESz (p > 0.09). Attention-related change in ASSR correlated with auditory hallucination severity for power (r = -0.49, p < 0.05) and ITPC (r = -0.58, p < 0.01). FESz with auditory hallucinations may have pathologically increased basal excitability of auditory cortex and consequent reduced ability to further increase auditory cortex sensory gain with focused attention. These findings indicate hallucination-related pathophysiology early in schizophrenia and may guide novel intervention strategies aimed to modulate basal activity levels.

Comparison of non-invasive, scalp-recorded auditory steady-state responses in humans, rhesus monkeys, and common marmosets

Auditory steady-state responses (ASSRs) are basic neural responses used to probe the ability of auditory circuits to produce synchronous activity to repetitive external stimulation. Reduced ASSR has been observed in patients with schizophrenia, especially at 40 Hz. Although ASSR is a translatable biomarker with a potential both in animal models and patients with schizophrenia, little is known about the features of ASSR in monkeys. Herein, we recorded the ASSR from humans, rhesus monkeys, and marmosets using the same method to directly compare the characteristics of ASSRs among the species. We used auditory trains on a wide range of frequencies to investigate the suitable frequency for ASSRs induction, because monkeys usually use stimulus frequency ranges different from humans for vocalization. We found that monkeys and marmosets also show auditory event-related potentials and phase-locking activity in gamma-frequency trains, although the optimal frequency with the best synchronization differed among these species. These results suggest that the ASSR could be a useful translational, cross-species biomarker to examine the generation of gamma-band synchronization in nonhuman primate models of schizophrenia.

Effect of dopamine receptor-related compounds on naive common marmosets for auditory steady state response

Abnormalities of auditory steady state responses (ASSR) and the effects of antipsychotic drugs on ASSR have been investigated in patients with schizophrenia. It is presumed that effects of drugs do not directly reflect on ASSR, because of ASSR abnormalities associated with schizophrenia. Therefore, to investigate the direct effect of drugs on ASSR, we established an ASSR evaluation system for common marmosets in a naïve state. Dopamine D1 receptor stimulation (SKF-81297, 2 mg/kg, intraperitoneal) significantly increased evoked power (EP) at 40 Hz. The phase locking factor (PLF) was increased significantly at 20, 30, 40, and 80 Hz. However, the administration of a dopamine D1 receptor antagonist (SCH-39166, 0.3 mg/kg intraperitoneal) resulted in a significant decrease in EP and PLF at 30 Hz. Dopamine D2 receptor stimulation (quinpirole, 1 mg/kg, intramuscular) tended to increase EP and induced power (IP) at all frequencies, and a significant difference was observed at 30 Hz IP. There was no change in PLF at all frequencies. In addition, dopamine D2 receptor blockade (raclopride, 3 mg/kg, intraperitoneal) reduced EP and PLF at 30 Hz. Subcutaneous administration of the serotonin dopamine antagonist, risperidone (0.3 mg/kg), tended to increase IP and decrease PLF, but not significantly. Taken together, it is possible to compare the differences in the mode of action of drugs on ASSR using naïve non-human primates.

Subcortical control of the default mode network: Role of the basal forebrain and implications for neuropsychiatric disorders

The precise interplay between large-scale functional neural systems throughout the brain is essential for performance of cognitive processes. In this review we focus on the default mode network (DMN), one such functional network that is active during periods of quiet wakefulness and believed to be involved in introspection and planning. Abnormalities in DMN functional connectivity and activation appear across many neuropsychiatric disorders, including schizophrenia. Recent evidence suggests subcortical regions including the basal forebrain are functionally and structurally important for regulation of DMN activity. Within the basal forebrain, subregions like the ventral pallidum may influence DMN activity and the nucleus basalis of Meynert can inhibit switching between brain networks. Interactions between DMN and other functional networks including the medial frontoparietal network (default), lateral frontoparietal network (control), midcingulo-insular network (salience), and dorsal frontoparietal network (attention) are also discussed in the context of neuropsychiatric disorders. Several subtypes of basal forebrain neurons have been identified including basal forebrain parvalbumin-containing or somatostatin-containing neurons which can regulate cortical gamma band oscillations and DMN-like behaviors, and basal forebrain cholinergic neurons which might gate access to sensory information during reinforcement learning. In this review, we explore this evidence, discuss the clinical implications on neuropsychiatric disorders, and compare neuroanatomy in the human vs rodent DMN. Finally, we address technological advancements which could help provide a more complete understanding of modulation of DMN function and describe newly identified BF therapeutic targets that could potentially help restore DMN-associated functional deficits in patients with a variety of neuropsychiatric disorders.

Loss of central mineralocorticoid or glucocorticoid receptors impacts auditory nerve processing in the cochlea

The key auditory signature that may associate peripheral hearing with central auditory cognitive defects remains elusive. Suggesting the involvement of stress receptors, we here deleted the mineralocorticoid and glucocorticoid receptors (MR and GR) using a CaMKIIα-based tamoxifen-inducible Cre^ERT2/loxP approach to generate mice with single or double deletion of central but not cochlear MR and GR. Hearing thresholds of MRGR^{CaMKIIαCreERT2} conditional knockouts (cKO) were unchanged, whereas auditory nerve fiber (ANF) responses were larger and faster and auditory steady state responses were improved. Subsequent analysis of single MR or GR cKO revealed discrete roles for both, central MR and GR on cochlear functions. Limbic MR deletion reduced inner hair cell (IHC) ribbon numbers and ANF responses. In contrast, GR deletion shortened the latency and improved the synchronization to amplitude-modulated tones without affecting IHC ribbon numbers. These findings imply that stress hormone-dependent functions of central MR/GR contribute to “precognitive” sound processing in the cochlea.

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Top-down MR/GR signaling differentially contributes to cochlear sound processing

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Limbic MR stimulates auditory nerve fiber discharge rates

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Central GR deteriorates auditory nerve fiber synchrony

Associations between polygenic risk scores and amplitude of low-frequency fluctuation of inferior frontal gyrus in schizophrenia

Schizophrenia (SCZ) is a serious and complex mental disorder with high heritability. Polygenic risk score (PRS) is a useful tool calculating the accumulating effects of multiple common genetic variants of schizophrenia. The amplitude of low-frequency fluctuation (ALFF) is an efficient index to reflect spontaneous, intrinsic neuronal activity. Aberrant ALFF of brain regions were reported in schizophrenia frequently, but the relationship between PRS and ALFF has not been studied. In the present study, we compared PRS and ALFF in 101 schizophrenia patients and 106 age-matched healthy controls to test their associations with schizophrenia. Then, the correlation of PRS with ALFF was measured to reveal the effect of polygenic risk on brain activity in schizophrenia. We found that schizophrenia patients showed significant differences in PRS and ALFF compared with controls. Twenty-six brain regions showed significant difference of ALFF between schizophrenia cases and controls, of which left inferior frontal gyrus, triangular part (IFGtriang.L) showed increased activity in schizophrenia. PRS-SCZ was positively correlated with ALFF in IFGtriang.L in 57 non-chronic patients. Genes involved in synaptic organization and transmission, especially in glutamatergic synapse, were highly enriched in PRS-SCZ genes, suggesting the dysfunction of synapses in schizophrenia. These results help to understand the molecular mechanism underlying schizophrenia and related brain dysfunction.

Tensor Decomposition of Human Narrowband Oscillatory Brain Activity in Frequency, Space and Time

Many brain processes in health and disease are associated with modulation of narrowband brain oscillations (NBOs) in the scalp-recorded EEG, which exhibit specific frequency spectra and scalp topography. Isolating and tracking NBOs over time using algorithms is useful in domains such as brain-computer interfaces or when measuring the EEG effects of experimental manipulations. Previously, we successfully applied modified tensor methods for identifying and tracking NBO activity over time or conditions. We introduced frequency and spatial constraints that greatly improved their physiological plausibility. In this paper we rigorously demonstrate the power and precision of tensor methods to separate, isolate and track NBOs using sources simulated with forward models. This allows us to control the attributes of NBOs and validate tensor solutions. We find that tensor methods can accurately identify, separate and track NBOs over time, using realistic sources either alone or in combination, and compare favorably to well-known spatio-spectral decomposition method for NBO estimation.

Test-retest reliability of mismatch negativity and gamma-band auditory steady-state response in patients with schizophrenia

Patients with schizophrenia show widespread impairments in clinical, cognitive and psychosocial functioning. Mismatch negativity (MMN) and gamma-band auditory steady-state response (ASSR) are two neurophysiological biomarkers widely used to inform diagnosis, guide treatments and track response to interventions in schizophrenia. However, evidence for the test-retest reliability of these indices across multiple sessions in schizophrenia patients remains scarce. In the present study, we included 34 schizophrenia patients (17 females) and obtained duration MMN (dMMN), frequency MMN (fMMN) and 40-Hz ASSR data across three sessions with intervals of 2 days. Event-related spectrum perturbation (ERSP) and inter-trial coherence (ITC) were calculated following Morlet wavelet time-frequency decomposition of ASSR data. The intra-class correlation coefficient (ICC) was used to quantify the reliability of MMN and ASSR measures among the three sessions. We found fair to good reliability for dMMN amplitudes but poor reliability for fMMN amplitudes. For the ASSR measures, ERSP showed good to excellent test-retest reliability while ITC had poor to fair test-retest reliability. In addition, the average of dMMN amplitudes was significantly correlated with that of ERSP across the three sessions. In summary, we established for the first time the short-term test-retest reliability of MMN and ASSR measures in schizophrenia patients. These findings demonstrate that dMMN amplitudes and ERSP of ASSR are reliable indices which may be used in longitudinal observational studies.

Decreased BOLD signals elicited by 40-Hz auditory stimulation of the right primary auditory cortex in bipolar disorder: An fMRI study

BACKGROUND

A number studies have been conducted on abnormalities in the cortical circuitry of gamma oscillations, including deficit in auditory steady-state response (ASSR) to gamma-frequency (≧ 30-Hz) stimulation, in patients with bipolar disorder (BD). In the current study, we investigated neural responses during click stimulation by blood oxygen level-dependent (BOLD) signals. We focused on Broadman 41 and 42, the main sources of ASSR.

MATERIALS AND METHODS

We acquired BOLD responses elicited by click trains of 80-, 40-, 30- and 20-Hz frequencies from 25 patients with BD to 27 healthy controls (HC) with normal hearing between 22 and 59 years of age assessed via a standard general linear-model-based analysis. We extracted contrast values by identifying the primary auditory cortex and Brodmann areas 41 and 42 as regions of interest (ROI)s.

RESULTS

BD group showed significantly decreased ASSR-BOLD signals in response to 40-Hz stimuli compared to the HC group in the right Brodmann areas 41 and 42. We found significant negative correlations between the BOLD change in the right Brodmann areas 41 and 42 and Structured Interview Guide for the Hamilton Depression Rating Scale (SIGH-D) scores, also the BOLD change in the right Brodmann areas 41 and 42 and the Positive and Negative Syndrome Scale (PANSS)-Negative scores.

CONCLUSION

The observed decrease in BOLD signal patterns in the right primary auditory cortex during 40-Hz ASSR may be a potential biomarker option for bipolar disorder.

The beta component of gamma-band auditory steady-state responses in patients with schizophrenia

The mechanisms underlying circuit dysfunctions in schizophrenia (SCZ) remain poorly understood. Auditory steady-state responses (ASSRs), especially in the gamma and beta band, have been suggested as a potential biomarker for SCZ. While the reduction of 40 Hz power for 40 Hz drive has been well established and replicated in SCZ patients, studies are inconclusive when it comes to an increase in 20 Hz power during 40 Hz drive. There might be several factors explaining the inconsistencies, including differences in the sensitivity of the recording modality (EEG vs MEG), differences in stimuli (click-trains vs amplitude-modulated tones) and large differences in the amplitude of the stimuli. Here, we used a computational model of ASSR deficits in SCZ and explored the effect of three SCZ-associated microcircuit alterations: reduced GABA activity, increased GABA decay times and NMDA receptor hypofunction. We investigated the effect of input strength on gamma (40 Hz) and beta (20 Hz) band power during gamma ASSR stimulation and saw that the pronounced increase in beta power during gamma stimulation seen experimentally could only be reproduced in the model when GABA decay times were increased and only for a specific range of input strengths. More specifically, when the input was in this specific range, the rhythmic drive at 40 Hz produced a strong 40 Hz rhythm in the control network; however, in the 'SCZ-like' network, the prolonged inhibition led to a so-called 'beat-skipping', where the network would only strongly respond to every other input. This mechanism was responsible for the emergence of the pronounced 20 Hz beta peak in the power spectrum. The other two microcircuit alterations were not able to produce a substantial 20 Hz component but they further narrowed the input strength range for which the network produced a beta component when combined with increased GABAergic decay times. Our finding that the beta component only existed for a specific range of input strengths might explain the seemingly inconsistent reporting in experimental studies and suggests that future ASSR studies should systematically explore different amplitudes of their stimuli. Furthermore, we provide a mechanistic link between a microcircuit alteration and an electrophysiological marker in schizophrenia and argue that more complex ASSR stimuli are needed to disentangle the nonlinear interactions of microcircuit alterations. The computational modelling approach put forward here is ideally suited to facilitate the development of such stimuli in a theory-based fashion.

Disorganization of Oscillatory Activity in Animal Models of Schizophrenia

Schizophrenia is a chronic, debilitating disorder with diverse symptomatology, including disorganized cognition and behavior. Despite considerable research effort, we have only a limited understanding of the underlying brain dysfunction. In this article, we review the potential role of oscillatory circuits in the disorder with a particular focus on the hippocampus, a region that encodes sequential information across time and space, as well as the frontal cortex. Several mechanistic explanations of schizophrenia propose that a loss of oscillatory synchrony between and within these brain regions may underlie some of the symptoms of the disorder. We describe how these oscillations are affected in several animal models of schizophrenia, including models of genetic risk, maternal immune activation (MIA) models, and models of NMDA receptor hypofunction. We then critically discuss the evidence for disorganized oscillatory activity in these models, with a focus on gamma, sharp wave ripple, and theta activity, including the role of cross-frequency coupling as a synchronizing mechanism. Finally, we focus on phase precession, which is an oscillatory phenomenon whereby individual hippocampal place cells systematically advance their firing phase against the background theta oscillation. Phase precession is important because it allows sequential experience to be compressed into a single 120 ms theta cycle (known as a 'theta sequence'). This time window is appropriate for the induction of synaptic plasticity. We describe how disruption of phase precession could disorganize sequential processing, and thereby disrupt the ordered storage of information. A similar dysfunction in schizophrenia may contribute to cognitive symptoms, including deficits in episodic memory, working memory, and future planning.

Global and Parallel Cortical Processing Based on Auditory Gamma Oscillatory Responses in Humans

Gamma oscillations are physiological phenomena that reflect perception and cognition, and involve parvalbumin-positive γ-aminobutyric acid-ergic interneuron function. The auditory steady-state response (ASSR) is the most robust index for gamma oscillations, and it is impaired in patients with neuropsychiatric disorders such as schizophrenia and autism. Although ASSR reduction is known to vary in terms of frequency and time, the neural mechanisms are poorly understood. We obtained high-density electrocorticography recordings from a wide area of the cortex in 8 patients with refractory epilepsy. In an ASSR paradigm, click sounds were presented at frequencies of 20, 30, 40, 60, 80, 120, and 160 Hz. We performed time-frequency analyses and analyzed intertrial coherence, event-related spectral perturbation, and high-gamma oscillations. We demonstrate that the ASSR is globally distributed among the temporal, parietal, and frontal cortices. The ASSR was composed of time-dependent neural subcircuits differing in frequency tuning. Importantly, the frequency tuning characteristics of the late-latency ASSR varied between the temporal/frontal and parietal cortex, suggestive of differentiation along parallel auditory pathways. This large-scale survey of the cortical ASSR could serve as a foundation for future studies of the ASSR in patients with neuropsychiatric disorders.

A Role for Somatostatin-Positive Interneurons in Neuro-Oscillatory and Information Processing Deficits in Schizophrenia

Alterations in neocortical GABAergic interneurons (INs) have been affiliated with neuropsychiatric diseases, including schizophrenia (SZ). Significant progress has been made linking the function of a specific subtype of GABAergic cells, parvalbumin (PV) positive INs, to altered gamma-band oscillations, which, in turn, underlie perceptual and feedforward information processing in cortical circuits. Here, we review a smaller but growing volume of literature focusing on a separate subtype of neocortical GABAergic INs, somatostatin (SST) positive INs. Despite sharing similar neurodevelopmental origins, SSTs exhibit distinct morphology and physiology from PVs. Like PVs, SSTs are altered in postmortem brain samples from multiple neocortical regions in SZ, although basic and translational research into consequences of SST dysfunction has been relatively sparse. We highlight a growing body of work in rodents, which now indicates that SSTs may also underlie specific aspects of cortical circuit function, namely low-frequency oscillations, disinhibition, and mediation of cortico-cortical feedback. SSTs may thereby support the coordination of local cortical information processing with more global spatial, temporal, and behavioral context, including predictive coding and working memory. These functions are notably deficient in some cases of SZ, as well as other neuropsychiatric disorders, emphasizing the importance of focusing on SSTs in future translational studies. Finally, we highlight the challenges that remain, including subtypes within the SST class.

Test-retest reliability of tone- and 40 Hz train-evoked gamma oscillations in female rats and their sensitivity to low-dose NMDA channel blockade

RATIONALE

Schizophrenia patients consistently show deficits in sensory-evoked broadband gamma oscillations and click-evoked entrainment at 40 Hz, called the 40-Hz auditory steady-state response (ASSR). Since such evoked oscillations depend on cortical N-methyl D-aspartic acid (NMDA)-mediated network activity, they can serve as pharmacodynamic biomarkers in the preclinical and clinical development of drug candidates engaging these circuits. However, there are few test-retest reliability data in preclinical species, a prerequisite for within-subject testing paradigms.

OBJECTIVE

We investigated the long-term psychometric stability of these measures in a rodent model.

METHODS

Female rats with chronic epidural implants were used to record tone- and 40 Hz click-evoked responses at multiple time points and across six sessions, spread over 3 weeks. We assessed reliability using intraclass correlation coefficients (ICC). Separately, we used mixed-effects ANOVA to examine time and session effects. Individual subject variability was determined using the coefficient of variation (CV). Lastly, to illustrate the importance of long-term measure stability for within-subject testing design, we used low to moderate doses of an NMDA antagonist MK801 (0.025-0.15 mg/kg) to disrupt the evoked response.

RESULTS

We found that 40-Hz ASSR showed good reliability (ICC=0.60-0.75), while the reliability of tone-evoked gamma ranged from poor to good (0.33-0.67). We noted time but no session effects. Subjects showed a lower variance for ASSR over tone-evoked gamma. Both measures were dose-dependently attenuated by NMDA antagonism.

CONCLUSION

Overall, while both evoked gamma measures use NMDA transmission, 40-Hz ASSR showed superior psychometric properties of higher ICC and lower CV, relative to tone-evoked gamma.

Optogenetic manipulation of an ascending arousal system tunes cortical broadband gamma power and reveals functional deficits relevant to schizophrenia

Increases in broadband cortical electroencephalogram (EEG) power in the gamma band (30-80 Hz) range have been observed in schizophrenia patients and in mouse models of schizophrenia. They are also seen in humans and animals treated with the psychotomimetic agent ketamine. However, the mechanisms which can result in increased broadband gamma power and the pathophysiological implications for cognition and behavior are poorly understood. Here we report that tonic optogenetic manipulation of an ascending arousal system bidirectionally tunes cortical broadband gamma power, allowing on-demand tests of the effect on cortical processing and behavior. Constant, low wattage optogenetic stimulation of basal forebrain (BF) neurons containing the calcium-binding protein parvalbumin (PV) increased broadband gamma frequency power, increased locomotor activity, and impaired novel object recognition. Concomitantly, task-associated gamma band oscillations induced by trains of auditory stimuli, or exposure to novel objects, were impaired, reminiscent of findings in schizophrenia patients. Conversely, tonic optogenetic inhibition of BF-PV neurons partially rescued the elevated broadband gamma power elicited by subanesthetic doses of ketamine. These results support the idea that increased cortical broadband gamma activity leads to impairments in cognition and behavior, and identify BF-PV activity as a modulator of this activity. As such, BF-PV neurons may represent a novel target for pharmacotherapy in disorders such as schizophrenia which involve aberrant increases in cortical broadband gamma activity.

Effect of locomotion on the auditory steady state response of head-fixed mice

OBJECTIVES

Electroencephalographic (EEG) examinations of the auditory steady-state response (ASSR) can non-invasively probe cortical function to generate the gamma-band (40 Hz) oscillation, which is increasingly applied to the neurophysiological studies on the rodent models of psychiatric disorders. Though, it has been well established that the brain activities are significantly modulated by the behavioural state (such as locomotion), how the ASSR is affected remains unclear.

METHODS

We investigated the effect of locomotion by recording local field potential (LFP) evoked by 40-Hz click-train from multiple brain areas: auditory cortex (AC), medial geniculate body (MGB), hippocampus (HP) and prefrontal cortex (PFC), in head-fixed mice free to run on a treadmill. Comparisons were conducted on the LFPs during spontaneous movement and stationary conditions.

RESULTS

We found that in both the auditory (AC and MGB) and non-auditory areas (HP and PFC), locomotion reduced the initial negative deflection of LFP (early response during 0-100 ms from stimulus onset), and had no significant effect on the ASSR phase-locking to the late stimulus (100-500 ms).

CONCLUSIONS

Our results suggest that different neural mechanisms contribute to the early response and ASSR, and the ASSR is a more robust biomarker to investigate the pathogenesis of neuropsychiatric disorders.

Altered neural oscillations and behavior in a genetic mouse model of NMDA receptor hypofunction

Abnormalities in electroencephalographic (EEG) biomarkers occur in patients with schizophrenia and those clinically at high risk for transition to psychosis and are associated with cognitive impairment. Converging evidence suggests N-methyl-D-aspartate receptor (NMDAR) hypofunction plays a central role in the pathophysiology of schizophrenia and likely contributes to biomarker impairments. Thus, characterizing these biomarkers is of significant interest for early diagnosis of schizophrenia and development of novel treatments. We utilized in vivo EEG recordings and behavioral analyses to perform a battery of electrophysiological biomarkers in an established model of chronic NMDAR hypofunction, serine racemase knockout (SRKO) mice, and their wild-type littermates. SRKO mice displayed impairments in investigation-elicited gamma power that corresponded with reduced short-term social recognition and enhanced background (pre-investigation) gamma activity. Additionally, SRKO mice exhibited sensory gating impairments in both evoked-gamma power and event-related potential amplitude. However, other biomarkers including the auditory steady-state response, sleep spindles, and state-specific power spectral density were generally neurotypical. In conclusion, SRKO mice demonstrate how chronic NMDAR hypofunction contributes to deficits in certain translationally-relevant EEG biomarkers altered in schizophrenia. Importantly, our gamma band findings suggest an aberrant signal-to-noise ratio impairing cognition that occurs with NMDAR hypofunction, potentially tied to impaired task-dependent alteration in functional connectivity.

Binding Mechanisms in Visual Perception and Their Link With Neural Oscillations: A Review of Evidence From tACS

Neurophysiological studies in humans employing magneto- (MEG) and electro- (EEG) encephalography increasingly suggest that oscillatory rhythmic activity of the brain may be a core mechanism for binding sensory information across space, time, and object features to generate a unified perceptual representation. To distinguish whether oscillatory activity is causally related to binding processes or whether, on the contrary, it is a mere epiphenomenon, one possibility is to employ neuromodulatory techniques such as transcranial alternating current stimulation (tACS). tACS has seen a rising interest due to its ability to modulate brain oscillations in a frequency-dependent manner. In the present review, we critically summarize current tACS evidence for a causal role of oscillatory activity in spatial, temporal, and feature binding in the context of visual perception. For temporal binding, the emerging picture supports a causal link with the power and the frequency of occipital alpha rhythms (8-12 Hz); however, there is no consistent evidence on the causal role of the phase of occipital tACS. For feature binding, the only study available showed a modulation by occipital alpha tACS. The majority of studies that successfully modulated oscillatory activity and behavioral performance in spatial binding targeted parietal areas, with the main rhythms causally linked being the theta (~7 Hz) and beta (~18 Hz) frequency bands. On the other hand, spatio-temporal binding has been directly modulated by parieto-occipital gamma (~40-60 Hz) and alpha (10 Hz) tACS, suggesting a potential role of cross-frequency coupling when binding across space and time. Nonetheless, negative or partial results have also been observed, suggesting methodological limitations that should be addressed in future research. Overall, the emerging picture seems to support a causal role of brain oscillations in binding processes and, consequently, a certain degree of plasticity for shaping binding mechanisms in visual perception, which, if proved to have long lasting effects, can find applications in different clinical populations.

Laminar Profile of Auditory Steady-State Response in the Auditory Cortex of Awake Mice

Objective

Auditory steady-state response (ASSR) is a gamma oscillation evoked by periodic auditory stimuli, which is commonly used in clinical electroencephalographic examination to evaluate the neurological functions. Though it has been suggested that auditory cortex is the origin of ASSR, how the laminar architecture of the neocortex contributes to the ASSR recorded from the brain surface remains unclear.

Methods

We used a 16-channel silicon probe to record the local field potential and the single-unit spike activity in the different layers of the auditory cortex of unanesthetized mice. Click-trains with a repetition rate at 40-Hz were present as sound stimuli to evoke ASSR.

Results

We found that the LFPs of all cortical layers showed a stable ASSR synchronizing to the 40-Hz click stimuli, while the ASSR was strongest in the granular (thalamorecipient) layer. Furthermore, time-frequency analyses also revealed the strongest coherence between the signals recorded from the granular layer and pial surface.

Conclusion

Our results reveal that the 40-Hz ASSR primarily shows the evoked gamma oscillation of thalamorecipient layers in the neocortex, and that the ASSR may be a biomarker to detect the cognitive deficits associated with impaired thalamo-cortical connection.

Extracting human cortical responses to sound onsets and acoustic feature changes in real music, and their relation to event rate

Evoked cortical responses (ERs) have mainly been studied in controlled experiments using simplified stimuli. Though, an outstanding question is how the human cortex responds to the complex stimuli encountered in realistic situations. Few electroencephalography (EEG) studies have used Music Information Retrieval (MIR) tools to extract cortical P1/N1/P2 to acoustical changes in real music. However, less than ten events per music piece could be detected leading to ERs due to limitations in automatic detection of sound onsets. Also, the factors influencing a successful extraction of the ERs have not been identified. Finally, previous studies did not localize the sources of the cortical generators. This study is based on an EEG/MEG dataset from 48 healthy normal hearing participants listening to three real music pieces. Acoustic features were computed from the audio signal of the music with the MIR Toolbox. To overcome limits in automatic methods, sound onsets were also manually detected. The chance of obtaining detectable ERs based on ten randomly picked onset points was less than 1:10,000. For the first time, we show that naturalistic P1/N1/P2 ERs can be reliably measured across 100 manually identified sound onsets, substantially improving the signal-to-noise level compared to <10 trials. More ERs were measurable in musical sections with slow event rates (0.2 Hz-2.5 Hz) than with fast event rates (>2.5 Hz). Furthermore, during monophonic sections of the music only P1/P2 were measurable, and during polyphonic sections only N1. Finally, MEG source analysis revealed that naturalistic P2 is located in core areas of the auditory cortex.

Envelope Following Response to 440 Hz Carrier Chirp-Modulated Tones Show Clinically Relevant Changes in Schizophrenia

The 40 Hz auditory steady-state response (ASSR) impairment is suggested as an electrophysiological biomarker of schizophrenia; however, existing data also points to the deficiency of low and high frequency ASSR responses. In order to obtain the full picture of potential impairment in schizophrenia, it is important to test responses at different frequencies. The current study aims to evaluate a wide frequency range (1-120 Hz) in response to brief low-frequency carrier chirp-modulated tones in a group of patients with schizophrenia. The EEG-derived envelope following responses (EFRs) were obtained in a group of male patients with schizophrenia (N = 18) and matched controls (N = 18). While subjects were watching silent movies, 440 Hz carrier chirp-modulated at 1-120 Hz tones were presented. Phase-locking index and evoked amplitude in response to stimulation were assessed and compared on point-to-point basis. The peak frequency of the low gamma response was estimated. Measures were correlated with psychopathology-positive, negative, total scores of the Positive and Negative Syndrome Scale (PANSS), and hallucination subscale scores. In comparison to controls, patients showed (1) reduced power of theta-beta (4-18 Hz) responses, (2) intact but slower low gamma (30-60 Hz), and (3) reduced high gamma (95-120 Hz) responses. No correlation survived the Bonferroni correction, but a sign of positive association between low gamma phase-locking and the prevalence of hallucinations, and a sign of negative association between high gamma phase-locking and the total PANSS scores were observed. Brain networks showed impaired capabilities to generate EFRs at different frequencies in schizophrenia; moreover, even when responses of patients did not significantly differ from controls on the group level, they still showed potentially clinically relevant variability.

Differential Plasticity in Auditory and Prefrontal Cortices, and Cognitive-Behavioral Deficits Following Noise-Induced Hearing Loss

Excessive exposure to loud noise causes hearing loss and neural plasticity throughout the auditory pathway. Recent studies have identified that non-auditory regions, such as the hippocampus, are also susceptible to noise exposure; however, the electrophysiological and behavioral consequences of noise-induced hearing loss on the prefrontal cortex (PFC) are unclear. Using chronically-implanted electrodes in awake rats, we investigated neural plasticity in the auditory and prefrontal cortices in the days following noise exposure via metrics associated with spontaneous neural oscillations and the 40-Hz auditory steady-state response (ASSR). Noise exposure did not alter the profile of spontaneous oscillations in either of the cortices, yet it caused a differential plasticity in the sound-evoked activity, which was characterized by enhanced event-related potentials (ERPs) in the auditory cortex (i.e., central gain), and decreased inter-trial coherence (ITC) of the 40-Hz ASSR within the PFC. Moreover, phase synchrony between auditory and prefrontal cortices was decreased post-exposure, suggesting a reduction in functional connectivity. Cognitive-behavioral testing using the Morris water maze and a series of lever-pressing tasks revealed that noise exposure impaired spatial learning and reference memory, as well as stimulus-response habit learning, whereas cognitive flexibility tasks requiring set-shifting and reversal learning appeared unaffected. Collectively, our findings identify the complex and region-specific cortical plasticity associated with noise-induced hearing loss, and highlight the varying degrees of susceptibility of non-auditory, cognitive tasks of learning, memory and executive function to noise exposure.

Gamma power abnormalities in a Fmr1-targeted transgenic rat model of fragile X syndrome

Fragile X syndrome (FXS) is characteristically displayed intellectual disability, hyperactivity, anxiety, and abnormal sensory processing. Electroencephalography (EEG) abnormalities are also observed in subjects with FXS, with many researchers paying attention to these as biomarkers. Despite intensive preclinical research using Fmr1 knock out (KO) mice, an effective treatment for FXS has yet to be developed. Here, we examined Fmr1-targeted transgenic rats (Fmr1-KO rats) as an alternative preclinical model of FXS. We characterized the EEG phenotypes of Fmr1-KO rats by measuring basal EEG power and auditory steady state response (ASSR) to click trains of stimuli at a frequency of 10–80 Hz. Fmr1-KO rats exhibited reduced basal alpha power and enhanced gamma power, and these rats showed enhanced locomotor activity in novel environment. While ASSR clearly peaked at around 40 Hz, both inter-trial coherence (ITC) and event-related spectral perturbation (ERSP) were significantly reduced at the gamma frequency band in Fmr1-KO rats. Fmr1-KO rats showed gamma power abnormalities and behavioral hyperactivity that were consistent with observations reported in mouse models and subjects with FXS. These results suggest that gamma power abnormalities are a translatable biomarker among species and demonstrate the utility of Fmr1-KO rats for investigating drugs for the treatment of FXS.

Abnormal visual representations associated with confusion of perceived facial expression in schizophrenia with social anxiety disorder

Deficits in social functioning are especially severe amongst schizophrenia individuals with the prevalent comorbidity of social anxiety disorder (SZ&SAD). Yet, the mechanisms underlying the recognition of facial expression of emotions-a hallmark of social cognition-are practically unexplored in SZ&SAD. Here, we aim to reveal the visual representations SZ&SAD (n = 16) and controls (n = 14) rely on for facial expression recognition. We ran a total of 30,000 trials of a facial expression categorization task with Bubbles, a data-driven technique. Results showed that SZ&SAD's ability to categorize facial expression was impared compared to controls. More severe negative symptoms (flat affect, apathy, reduced social drive) was associated with more impaired emotion recognition ability, and with more biases in attributing neutral affect to faces. Higher social anxiety symptoms, on the other hand, was found to enhance the reaction speed to neutral and angry faces. Most importantly, Bubbles showed that these abnormalities could be explained by inefficient visual representations of emotions: compared to controls, SZ&SAD subjects relied less on fine facial cues (high spatial frequencies) and more on coarse facial cues (low spatial frequencies). SZ&SAD participants also never relied on the eye regions (only on the mouth) to categorize facial expressions. We discuss how possible interactions between early (low sensitivity to coarse information) and late stages of the visual system (overreliance on these coarse features) might disrupt SZ&SAD's recognition of facial expressions. Our findings offer perceptual mechanisms through which comorbid SZ&SAD impairs crucial aspects of social cognition, as well as functional psychopathology.