Development of sensory gamma oscillations and cross-frequency coupling from childhood to early adulthood

The Detection of Phase Amplitude Coupling during Sensory Processing

There is increasing interest in understanding how the phase and amplitude of distinct neural oscillations might interact to support dynamic communication within the brain. In particular, previous work has demonstrated a coupling between the phase of low frequency oscillations and the amplitude (or power) of high frequency oscillations during certain tasks, termed phase amplitude coupling (PAC). For instance, during visual processing in humans, PAC has been reliably observed between ongoing alpha (8–13 Hz) and gamma-band (>40 Hz) activity. However, the application of PAC metrics to electrophysiological data can be challenging due to numerous methodological issues and lack of coherent approaches within the field. Therefore, in this article we outline the various analysis steps involved in detecting PAC, using an openly available MEG dataset from 16 participants performing an interactive visual task. Firstly, we localized gamma and alpha-band power using the Fieldtrip toolbox, and extracted time courses from area V1, defined using a multimodal parcelation scheme. These V1 responses were analyzed for changes in alpha-gamma PAC, using four common algorithms. Results showed an increase in alpha (7–13 Hz)–gamma (40–100 Hz) PAC in response to the visual grating stimulus, though specific patterns of coupling were somewhat dependent upon the algorithm employed. Additionally, post-hoc analyses showed that these results were not driven by the presence of non-sinusoidal oscillations, and that trial length was sufficient to obtain reliable PAC estimates. Finally, throughout the article, methodological issues and practical guidelines for ongoing PAC research will be discussed.

Mapping the Consequences of Impaired Synaptic Plasticity in Schizophrenia through Development: An Integrative Model for Diverse Clinical Features

Schizophrenia is associated with alterations in sensory, motor, and cognitive functions that emerge before psychosis onset; identifying pathogenic processes that can account for this multi-faceted phenotype remains a challenge. Accumulating evidence suggests that synaptic plasticity is impaired in schizophrenia. Given the role of synaptic plasticity in learning, memory, and neural circuit maturation, impaired plasticity may underlie many features of the schizophrenia syndrome. Here, we summarize the neurobiology of synaptic plasticity, review evidence that plasticity is impaired in schizophrenia, and explore a framework in which impaired synaptic plasticity interacts with brain maturation to yield the emergence of sensory, motor, cognitive, and psychotic features at different times during development in schizophrenia. Key gaps in the literature and future directions for testing this framework are discussed.

Layer 3 Excitatory and Inhibitory Circuitry in the Prefrontal Cortex: Developmental Trajectories and Alterations in Schizophrenia

Convergent evidence suggests that schizophrenia is a disorder of neurodevelopment with alterations in both early and late developmental processes hypothesized to contribute to the disease process. Abnormalities in certain clinical features of schizophrenia, such as working memory impairments, depend on distributed neural circuitry including the dorsolateral prefrontal cortex (DLPFC) and appear to arise during the protracted maturation of this circuitry across childhood and adolescence. In particular, the neural circuitry substrate for working memory in primates involves the coordinated activity of excitatory pyramidal neurons and a specific population of inhibitory gamma-aminobutyric acid neurons (i.e., parvalbumin-containing basket cells) in layer 3 of the DLPFC. Understanding the relationships between the normal development of-and the schizophrenia-associated alterations in-the DLPFC circuitry that subserves working memory could provide new insights into the nature of schizophrenia as a neurodevelopmental disorder. Consequently, we review the following in this article: 1) recent findings regarding alterations of DLPFC layer 3 circuitry in schizophrenia, 2) the developmental refinements in this circuitry that occur during the period when the working memory alterations in schizophrenia appear to arise and progress, and 3) how various adverse environmental exposures could contribute to developmental disturbances of this circuitry in individuals with schizophrenia.

By our bootstraps: Comparing methods for measuring auditory 40 Hz steady-state neural activity

Although the 40 Hz auditory steady-state response (ASSR) is of clinical interest, the construct validity of EEG and MEG measures of 40 Hz ASSR cortical microcircuits is unclear. This study evaluated several MEG and EEG metrics by leveraging findings of (a) an association between the 40 Hz ASSR and age in the left but not right hemisphere, and (b) right- > left-hemisphere differences in the strength of the 40 Hz ASSR. The contention is that, if an analysis method does not demonstrate a left 40 Hz ASSR and age relationship or hemisphere differences, then the obtained measures likely have low validity. Fifty-three adults were presented 500 Hz stimuli modulated at 40 Hz while MEG and EEG were collected. ASSR activity was examined as a function of phase similarity (intertrial coherence) and percent change from baseline (total power). A variety of head models (spherical and realistic) and a variety of dipole source modeling strategies (dipole source localization and dipoles fixed to Heschl's gyri) were compared. Several sensor analysis strategies were also tested. EEG sensor measures failed to detect left 40 Hz ASSR and age associations or hemisphere differences. A comparison of MEG and EEG head-source models showed similarity in the 40 Hz ASSR measures and in estimating age and left 40 Hz ASSR associations, indicating good construct validity across models. Given a goal of measuring the 40 Hz ASSR cortical microcircuits, a source-modeling approach was shown to be superior in measuring this construct versus methods that rely on EEG sensor measures.

Neurobiological bases of reading disorder Part I: Etiological investigations

While many studies have focused on identifying the neural and behavioral characteristics of decoding-based reading disorder (RD, aka developmental dyslexia), the etiology of RD remains largely unknown and understudied. Because the brain plays an intermediate role between genetic factors and behavioral outcomes, it is promising to address causality from a neural perspective. In the current, Part I of the two-part review, we discuss neuroimaging approaches to addressing the causality issue and review the results of studies that have employed these approaches. We assume that if a neural signature were associated with RD etiology, it would (a) manifest across comparisons in different languages, (b) be experience independent and appear in comparisons between RD and reading-matched controls, (c) be present both pre- and post-intervention, (d) be found in at-risk, pre-reading children and (e) be associated with genetic risk. We discuss each of these five characteristics in turn and summarize the studies that have examined each of them. The available literature provides evidence that anomalies in left temporo-parietal cortex, and possibly occipito-temporal cortex, may be closely related to the etiology of RD. Improved understanding of the etiology of RD can help improve the accuracy of early detection and enable targeted intervention of cognitive processes that are amenable to change, leading to improved outcomes in at-risk or affected populations.

Development of cortical motor circuits between childhood and adulthood: A navigated TMS-HdEEG study

Motor functions improve during childhood and adolescence, but little is still known about the development of cortical motor circuits during early life. To elucidate the neurophysiological hallmarks of motor cortex development, we investigated the differences in motor cortical excitability and connectivity between healthy children, adolescents, and adults by means of navigated suprathreshold motor cortex transcranial magnetic stimulation (TMS) combined with high-density electroencephalography (EEG). We demonstrated that with development, the excitability of the motor system increases, the TMS-evoked EEG waveform increases in complexity, the magnitude of induced activation decreases, and signal spreading increases. Furthermore, the phase of the oscillatory response to TMS becomes less consistent with age. These changes parallel an improvement in manual dexterity and may reflect developmental changes in functional connectivity. Hum Brain Mapp 38:2599-2615, 2017. © 2017 Wiley Periodicals, Inc.

It's All in the Rhythm: The Role of Cannabinoids in Neural Oscillations and Psychosis

Evidence has accumulated over the past several decades suggesting that both exocannabinoids and endocannabinoids play a role in the pathophysiology of schizophrenia. The current article presents evidence suggesting that one of the mechanisms whereby cannabinoids induce psychosis is through the alteration in synchronized neural oscillations. Neural oscillations, particularly in the gamma (30-80 Hz) and theta (4-7 Hz) ranges, are disrupted in schizophrenia and are involved in various areas of perceptual and cognitive function. Regarding cannabinoids, preclinical evidence from slice and local field potential recordings has shown that central cannabinoid receptor (cannabinoid receptor type 1) agonists decrease the power of neural oscillations, particularly in the gamma and theta bands. Further, the administration of cannabinoids during critical stages of neural development has been shown to disrupt the brain's ability to generate synchronized neural oscillations in adulthood. In humans, studies examining the effects of chronic cannabis use (utilizing electroencephalography) have shown abnormalities in neural oscillations in a pattern similar to those observed in schizophrenia. Finally, recent studies in humans have also shown disruptions in neural oscillations after the acute administration of delta-9-tetrahydrocannabinol, the primary psychoactive constituent in cannabis. Taken together, these data suggest that both acute and chronic cannabinoids can disrupt the ability of the brain to generate synchronized oscillations at functionally relevant frequencies. Hence, this may represent one of the primary mechanisms whereby cannabinoids induce disruptions in attention, working memory, sensory-motor integration, and many other psychosis-related behavioral effects.

Neocortical gamma oscillations in idiopathic generalized epilepsy

OBJECTIVE

Absence seizures in patients with idiopathic generalized epilepsy (IGE) may in part be explained by a decrease in phasic GABAA (type-A γ-aminobutyric acid) receptor function, but the mechanisms are only partly understood. Here we studied the relation between ictal and interictal spike-wave discharges (SWDs) and electroencephalography (EEG) gamma oscillatory activity (30-60 Hz) in patients with IGE.

METHODS

EEG recordings were obtained of 14 children with IGE (mean age, 8.5 ± 5 years) and 14 age- and sex-matched controls. Time-frequency analysis of each seizure and seizure-free control epochs was performed and cross-coherences of neocortical gamma oscillations were calculated to describe interictal and ictal characteristics of generalized seizures.

RESULTS

SWDs were characterized with an abrupt increase of oscillatory activity of 3-4 and 13-60 Hz, peaking at 3-4 and 30-60 Hz, and with a simultaneous decrease in the 8-12 Hz frequency band. The rise in EEG gamma oscillations was short-lasting and decreased before activity declined at lower frequency ranges. Compared to control patients, patients with epilepsy also showed higher interictal values of mean coherence of gamma activity, but this interictal increase was not significant after post hoc analysis.

SIGNIFICANCE

Our data support the hypothesis that gamma oscillatory activity increase concomitantly with rises in activity of lower EEG frequencies during absence seizures and that the activity starts to cease earlier than lower EEG frequencies. The data did not support a change in gamma activity preceding the 3-4 Hz SWDs. SWDs are hypothetically generated by the synchronous interaction between the thalamus and the cortex, whereas the production of gamma activity is the result of activity in local inhibitory networks. Thus, the modification of SWD by gamma activity may be understood in terms of the cellular and synaptic mechanisms involved.

Pathophysiological implication of CaV3.1 T-type Ca2+ channels in trigeminal neuropathic pain

A crucial pathophysiological issue concerning central neuropathic pain is the modification of sensory processing by abnormally increased low-frequency brain rhythms. Here we explore the molecular mechanisms responsible for such abnormal rhythmicity and its relation to neuropathic pain syndrome. Toward this aim, we investigated the behavioral and electrophysiological consequences of trigeminal neuropathic pain following infraorbital nerve ligations in CaV3.1 T-type Ca(2+) channel knockout and wild-type mice. CaV3.1 knockout mice had decreased mechanical hypersensitivity and reduced low-frequency rhythms in the primary somatosensory cortex and related thalamic nuclei than wild-type mice. Lateral inhibition of gamma rhythm in primary somatosensory cortex layer 4, reflecting intact sensory contrast, was present in knockout mice but severely impaired in wild-type mice. Moreover, cross-frequency coupling between low-frequency and gamma rhythms, which may serve in sensory processing, was pronounced in wild-type mice but not in CaV3.1 knockout mice. Our results suggest that the presence of CaV3.1 channels is a key element in the pathophysiology of trigeminal neuropathic pain.

Translating Adult Electrophysiology Findings to Younger Patient Populations: Difficulty Measuring 40-Hz Auditory Steady-State Responses in Typically Developing Children and Children with Autism Spectrum Disorder

BACKGROUND

x03B3; (∼30-80 Hz) brain rhythms are thought to be abnormal in neurodevelopmental disorders such as schizophrenia and autism spectrum disorder (ASD). In adult populations, auditory 40-Hz click trains or 40-Hz amplitude-modulated tones are used to assess the integrity of superior temporal gyrus (STG) 40-Hz x03B3;-band circuits. As STG 40-Hz auditory steady-state responses (ASSRs) are not fully developed in children, tasks using these stimuli may not be optimal in younger patient populations. The present study examined this issue in typically developing (TD) children as well as in children with ASD, using source localization to directly assess activity in the principal generators of the 40-Hz ASSR in the left and right primary/secondary auditory cortices.

METHODS

40-Hz amplitude-modulated tones of 1 s duration were binaurally presented while magnetoencephalography data were obtained from 48 TD children (45 males; 7-14 years old) and 42 ASD children (38 males; 8-14 years old). T1-weighted structural MRI was obtained. Using single dipoles anatomically constrained to each participant's left and right Heschl's Gyrus, left and right 40-Hz ASSR total power (TP) and intertrial coherence (ITC) measures were obtained. Associations between 40-Hz ASSR TP, ITC and age as well as STG gray matter cortical thickness (CT) were assessed. Group STG function and structure differences were also examined.

RESULTS

TD and ASD did not differ in 40-Hz ASSR TP or ITC. In TD and ASD, age was associated with left and right 40-Hz ASSR ITC (p < 0.01). The interaction term was not significant, indicating in both groups a ∼0.01/year increase in ITC. 40-Hz ASSR TP and ITC were greater in the right than left STG. Groups did not differ in STG CT, and no associations were observed between 40-Hz ASSR activity and STG CT. Finally, right STG transient x03B3; (50-100 ms and 30-50 Hz) was greater in TD versus ASD (significant for TP, trend for ITC).

CONCLUSIONS

The 40-Hz ASSR develops, in part, via an age-related increase in neural synchrony. Greater right than left 40-Hz ASSRs (ITC and TP) suggested earlier maturation of right versus left STG neural network(s). Given a ∼0.01/year increase in ITC, 40-Hz ASSRs were weak or absent in many of the younger participants, suggesting that 40-Hz driving stimuli are not optimal for examining STG 40-Hz auditory neural circuits in younger populations. Given the caveat that 40-Hz auditory steady-state neural networks are poorly assessed in children, the present analyses did not point to atypical development of STG 40-Hz ASSRs in higher-functioning children with ASD. Although groups did not differ in 40-Hz auditory steady-state activity, replicating previous studies, there was evidence for greater right STG transient x03B3; activity in TD versus ASD.

Electrophysiological Endophenotypes for Schizophrenia

Endophenotypes are quantitative, heritable traits that may help to elucidate the pathophysiologic mechanisms underlying complex disease syndromes, such as schizophrenia. They can be assessed at numerous levels of analysis; here, we review electrophysiological endophenotypes that have shown promise in helping us understand schizophrenia from a more mechanistic point of view. For each endophenotype, we describe typical experimental procedures, reliability, heritability, and reported gene and neurobiological associations. We discuss recent findings regarding the genetic architecture of specific electrophysiological endophenotypes, as well as converging evidence from EEG studies implicating disrupted balance of glutamatergic signaling and GABAergic inhibition in the pathophysiology of schizophrenia. We conclude that refining the measurement of electrophysiological endophenotypes, expanding genetic association studies, and integrating data sets are important next steps for understanding the mechanisms that connect identified genetic risk loci for schizophrenia to the disease phenotype.

Frequency of gamma oscillations in humans is modulated by velocity of visual motion

Gamma oscillations are generated in networks of inhibitory fast-spiking (FS) parvalbumin-positive (PV) interneurons and pyramidal cells. In animals, gamma frequency is modulated by the velocity of visual motion; the effect of velocity has not been evaluated in humans. In this work, we have studied velocity-related modulations of gamma frequency in children using MEG/EEG. We also investigated whether such modulations predict the prominence of the "spatial suppression" effect (Tadin D, Lappin JS, Gilroy LA, Blake R. Nature 424: 312-315, 2003) that is thought to depend on cortical center-surround inhibitory mechanisms. MEG/EEG was recorded in 27 normal boys aged 8-15 yr while they watched high-contrast black-and-white annular gratings drifting with velocities of 1.2, 3.6, and 6.0°/s and performed a simple detection task. The spatial suppression effect was assessed in a separate psychophysical experiment. MEG gamma oscillation frequency increased while power decreased with increasing velocity of visual motion. In EEG, the effects were less reliable. The frequencies of the velocity-specific gamma peaks were 64.9, 74.8, and 87.1 Hz for the slow, medium, and fast motions, respectively. The frequency of the gamma response elicited during slow and medium velocity of visual motion decreased with subject age, whereas the range of gamma frequency modulation by velocity increased with age. The frequency modulation range predicted spatial suppression even after controlling for the effect of age. We suggest that the modulation of the MEG gamma frequency by velocity of visual motion reflects excitability of cortical inhibitory circuits and can be used to investigate their normal and pathological development in the human brain.

Differential Alterations of Auditory Gamma Oscillatory Responses Between Pre-Onset High-Risk Individuals and First-Episode Schizophrenia

Alterations in gamma-band auditory steady-state response (ASSR) are the most robust finding of abnormal neural oscillations in patients with first-episode (FES) and chronic schizophrenia. Gamma-band ASSRs may indicate GABAergic interneuron dysfunction. Nevertheless, it is unknown whether abnormal gamma-band ASSRs are present before the onset of psychosis. Subjects were 15 ultra-high-risk (UHR) individuals, 13 FES patients, and 21 healthy control (HC) subjects. We performed electroencephalogram recordings and measured ASSRs in each group as they were presented with click trains at 20, 30, and 40 Hz. We then conducted time-frequency analyses and calculated intertrial phase coherence and event-related spectral perturbation. The time course of gamma-band ASSRs showed significantly different features among groups. Compared with the HC group, the UHR group was characterized by intact early-latency (0-100 ms) and reduced late-latency (300-500 ms) ASSRs. In contrast, both early- and late-latency ASSRs were significantly reduced in the FES group. Gamma-band ASSRs were correlated with clinical symptoms and attentional functioning in FES (|rs| > 0.70). These results suggest differential alterations of gamma-band ASSRs between UHR and FES groups. The late-latency ASSR alteration may represent a biomarker for early detection of psychosis, while the early-latency ASSR abnormality may develop through the onset of psychosis.

γ-band abnormalities as markers of autism spectrum disorders

Autism is a behaviorally diagnosed neurodevelopmental disorder with no current biomarkers with high specificity and sensitivity. γ-band abnormalities have been reported in many studies of autism spectrum disorders. γ-band activity is associated with perceptual and cognitive functions that are compromised in autism. Some γ-band deficits have also been seen in unaffected first-degree relatives, suggesting heritability of these findings. This review covers the published literature on γ abnormalities in autism, the proposed mechanisms underlying the deficits and the potential for translation into new treatments. Although the utility of γ-band metrics as diagnostic biomarkers is currently limited, such changes in autism are also useful as endophenotypes, for evaluating potential neural mechanisms, and for use as surrogate markers of treatment response to interventions.

Functional Maturation of GABA Synapses During Postnatal Development of the Monkey Dorsolateral Prefrontal Cortex

Development of inhibition onto pyramidal cells may be crucial for the emergence of cortical network activity, including gamma oscillations. In primate dorsolateral prefrontal cortex (DLPFC), inhibitory synaptogenesis starts in utero and inhibitory synapse density reaches adult levels before birth. However, in DLPFC, the expression levels of γ-aminobutyric acid (GABA) synapse-related gene products changes markedly during development until young adult age, suggesting a highly protracted maturation of GABA synapse function. Therefore, we examined the development of GABA synapses by recording GABAAR-mediated inhibitory postsynaptic currents (GABAAR-IPSCs) from pyramidal cells in the DLPFC of neonatal, prepubertal, peripubertal, and adult macaque monkeys. We found that the decay of GABAAR-IPSCs, possibly including those from parvalbumin-positive GABA neurons, shortened by prepubertal age, while their amplitude increased until the peripubertal period. Interestingly, both GABAAR-mediated quantal response size, estimated by miniature GABAAR-IPSCs, and the density of GABAAR synaptic appositions, measured with immunofluorescence microscopy, were stable with age. Simulations in a computational model network with constant GABA synapse density showed that the developmental changes in GABAAR-IPSC properties had a significant impact on oscillatory activity and predicted that, whereas DLPFC circuits can generate gamma frequency oscillations by prepubertal age, mature levels of gamma band power are attained at late stages of development.