Peak Gamma Frequency Research Articles

Gamma Frequency and the Spatial Tuning of Primary Visual Cortex.

Visual stimulation produces oscillatory gamma responses in human primary visual cortex (V1) that also relate to visual perception. We have shown previously that peak gamma frequency positively correlates with central V1 cortical surface area. We hypothesized that people with larger V1 would have smaller receptive fields and that receptive field size, not V1 area, might explain this relationship. Here we set out to test this hypothesis directly by investigating the relationship between fMRI estimated population receptive field (pRF) size and gamma frequency in V1. We stimulated both the near-center and periphery of the visual field using both large and small stimuli in each location and replicated our previous finding of a positive correlation between V1 surface area and peak gamma frequency. Counter to our expectation, we found that between participants V1 size (and not PRF size) accounted for most of the variability in gamma frequency. Within-participants we found that gamma frequency increased, rather than decreased, with stimulus eccentricity directly contradicting our initial hypothesis.

MB-96IMPAIRED NEURAL FUNCTION DURING VISUAL-MOTOR PERFORMANCE IN CHILDREN TREATED FOR BRAIN TUMOURS

MB-96. IMPAIRED NEURAL FUNCTION DURING VISUAL-MOTOR PERFORMANCE IN CHILDREN TREATED FOR BRAIN TUMOURS Sonya Bells1, Colleen Dockstader1, Suzanne Laughlin3, Eric Bouffet2, Jovanka Skocic1, and Donald Mabbott1; The Hospital for Sick Children, Department of Psychology, Toronto, ON, Canada; The Hospital for Sick Children, Department of Haematology/Oncology, Toronto, ON, Canada; The Hospital for Sick Children, Department of Diagnostic Imaging, Toronto, ON, Canada Children treated with cranial radiation therapy for brain tumours exhibit substantial cognitive deficits that may be mediated by impaired neural functioning. We used magnetoencephalography to investigate whether aberrant neural oscillations are present in pediatric brain tumour survivors compared to healthy children during a visual-motor reaction time task. MEG recording (151 channels, VSM MedTech) was acquired for 9 previously irradiated patients and age-matched controls during the visual-motor reaction time task (100 trials in total). Reaction time (RT) was measured along with localized power changes within the visual and motor cortex using beamformer analysis. Patients showed a significantly delayed RT compared to controls (t(2,9) 1⁄4 2.68, p , 0.05). Beamformer analysis revealed that relative to controls, patients had muted event-related desynchrony and synchrony in the beta (13-29 Hz) and alpha (8-12 Hz) bandwidths in motor and visual cortex and potentiated high gamma activity (50-100 Hz) in the contralateral motor cortex. Furthermore, we found differences in peak gamma frequency of the visually-induced oscillations. Peak frequency was measured in the low gamma range (30-50 Hz) in patients and in the high gamma range (50-100 Hz) in controls. Our findings indicate that aberrant neural responses, characterized by reduced alpha and beta power and increased gamma power within the contralateral motor area, and reduced peak gamma in visual areas, underlie impaired visual-motor reaction time in patients. Gamma-band oscillations play an important role in visual processing and are associated with measures of cortical thickness. These oscillatory results may reflect structural differences between these two populations. Neuro-Oncology 18:iii97–iii122, 2016. doi:10.1093/neuonc/now076.92 #The Author(s) 2016. Published by Oxford University Press on behalf of the Society for Neuro-Oncology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.

Superior orientation discrimination and increased peak gamma frequency in autism spectrum conditions.

While perception is recognized as being atypical in individuals with autism spectrum conditions (ASC), the underlying mechanisms for such atypicality are unclear. Here we test the hypothesis that individuals with ASC will show enhanced orientation discrimination compared with neurotypical observers. This prediction is based both on anecdotal report of superior discriminatory skills in ASC and also on evidence in the auditory domain that some individuals with ASC have superior pitch discrimination. In order to establish whether atypical perception might be mediated by an imbalance in the ratio of neural excitation and inhibition (E:I ratio), we also measured peak gamma frequency, which provides an indication of neural inhibition levels. Using a rigorous thresholding method, we found that orientation discrimination thresholds for obliquely oriented stimuli were significantly lower in participants with ASC. Using EEG to measure the visually induced gamma band response, we also found that peak gamma frequency was higher in participants with ASC, relative to a well-matched control group. These novel results suggest that neural inhibition may be increased in the occipital cortex of individuals with ASC. Implications for existing theories of an imbalance in the E:I ratio of ASC are discussed.

Ketamine amplifies induced gamma frequency oscillations in the human cerebral cortex

At subanaesthetic doses, ketamine, an N-methyl-d-aspartate (NMDA) receptor antagonist, has demonstrated remarkable and rapid antidepressant efficacy in patients with treatment-resistant depression. The mechanism of action of ketamine is complex and not fully understood, with altered glutamatergic function and alterations of high-frequency oscillatory power (Wood et al., 2012) noted in animal studies. Here we used magnetoencephalography (MEG) in a single blind, crossover study to assess the neuronal effects of 0.5mg/kg intravenous ketamine on task-related high-frequency oscillatory activity in visual and motor cortices. Consistent with animal findings, ketamine increased beta amplitudes, decreased peak gamma frequency in visual cortex and significantly amplified gamma-band amplitudes in motor and visual cortices. The amplification of gamma-band activity has previously been linked in animal studies to cortical pyramidal cell disinhibition. This study provides direct translatable evidence of this hypothesis in humans, which may underlie the anti-depressant actions of ketamine.

Increased peak gamma frequency in individuals with higher levels of autistic traits.

Individual differences in orientation discrimination threshold are related to both visually-induced peak gamma frequency and the presence of autistic traits. The relationship between peak gamma frequency and orientation discrimination thresholds may be due to both of these factors being mediated by levels of neural inhibition. No study has previously measured the relationship between peak gamma frequency and levels of autistic traits. Thus, this was the aim of the present study. We measured orientation discrimination thresholds and autistic traits in a neurotypical human sample (N=33), and separately recorded electroencephalography to measure visually induced gamma activity. In line with our prediction, we found a significant relationship between peak gamma frequency and level of autistic traits. Consistent with previous work we also found significant relationships between orientation discrimination thresholds and level of autistic traits and between orientation discrimination thresholds and peak gamma frequency. Our results demonstrate that individuals with individuals with higher levels of autistic personality traits have a higher peak-gamma frequency and are better at discriminating between visual stimuli based on orientation. As both higher peak gamma frequency and lower orientation discrimination thresholds have been linked to higher levels of neural inhibition, this suggests that autistic traits co-occur with increased neural inhibition. This discovery is significant as it challenges the currently-held view that autism spectrum conditions are associated with increased neural excitation.

Individual peak gamma frequency predicts switch rate in perceptual rivalry.

Perceptual rivalry-the experience of alternation between two mutually exclusive interpretations of an ambiguous image-provides powerful opportunities to study conscious awareness. It is known that individual subjects experience perceptual alternations for various types of bistable stimuli at distinct rates, and this a stable, heritable trait. Also stable and heritable is the peak frequency of induced gamma-band (30-100 Hz) oscillation of a population-level response in occipital cortex to simple visual patterns, which has been established as a neural correlate of conscious processing. Interestingly, models for rivalry alternation rate and for the frequency of population-level oscillation have both cited inhibitory connections in cortex as crucial determinants of individual differences, and yet the relationship between these two variables has not yet been investigated. Here, we used magnetoencephalography to compare differences in alternation rate for binocular and monocular types of perceptual rivalry to differences in evoked and induced gamma-band frequency of neuromagnetic brain responses to simple nonrivalrous grating stimuli. For both types of bistable images, alternation rate was inversely correlated with the peak frequency of late evoked gamma activity in primary visual cortex (200-400 ms latency). Our results advance models of inhibition that account for subtle variation in normal visual cortex, and shed light on how small differences in anatomy and physiology relate to individual cognition and performance.

Peak frequency of induced gamma-band response to simple stimulus predicts individual switch rate for perceptual rivalry tasks.

The peak frequency of an induced gamma-band response to a simple stimulus is known to vary across individuals (Muthukumaraswamy et al., 2010) and is thought to be shaped by differences in the extent of inhibitory connections in visual cortex (Brunel & Wang, 2003). In support of this, peak gamma frequency has been found to predict individual levels of resting state GABA in visual cortex (Edden et al., 2009; Muthukumaraswamy eta l., 2009). Also varying reliably across normal populations is the rate of alternation for perceptual rivalry tasks including binocular rivalry and monocular pattern rivalry (Carter & Pettigrew, 2003; Miller et al., 2010). Models of perceptual rivalry include mutual inhibition as a constraint on switching dynamics (e.g., Wilson 2003), but do the differences in switch rate reflect individual differences in cortical inhibition? If so, individual switch rate should be inversely correlated with peak gamma frequency, yet this prediction has not been tested. We used magnetoencephalography (MEG) to compare the peak gamma-band frequency of neuromagnetic responses of 12 healthy adults (6 female) to the onset of simple grating stimuli with their individual switch rates for binocular and monocular rivalry tasks. We computed Morlet wavelet analyses for left and right V1, V2, and MT+ of individual source data based on minimum norm estimates for each participant. Peak frequency was determined for three latency ranges: evoked (10-150ms), early induced (200-450ms) and late induced (500-800ms). Results show significant inverse correlations between peak frequency of early induced gamma in V1 and switch rate for both rivalry tasks, compatible with models that propose inhibitory connections in visual cortex are crucial for tuning gamma-band frequency as well as perceptual alternation rate. Our study suggests that subtle variations of behavior in a normal population can directly advance our understanding of visual function by encouraging models at increasingly finer scales. Meeting abstract presented at VSS 2014

GABA level, gamma oscillation, and working memory performance in schizophrenia.

A relationship between working memory impairment, disordered neuronal oscillations, and abnormal prefrontal GABA function has been hypothesized in schizophrenia; however, in vivo GABA measurements and gamma band neural synchrony have not yet been compared in schizophrenia. This case–control pilot study (N = 24) compared baseline and working memory task-induced neuronal oscillations acquired with high-density electroencephalograms (EEGs) to GABA levels measured in vivo with magnetic resonance spectroscopy. Working memory performance, baseline GABA level in the left dorsolateral prefrontal cortex (DLPFC), and measures of gamma oscillations from EEGs at baseline and during a working memory task were obtained. A major limitation of this study is a relatively small sample size for several analyses due to the integration of diverse methodologies and participant compliance. Working memory performance was significantly lower for patients than for controls. During the working memory task, patients (n = 7) had significantly lower amplitudes in gamma oscillations than controls (n = 9). However, both at rest and across working memory stages, there were significant correlations between gamma oscillation amplitude and left DLPFC GABA level. Peak gamma frequency during the encoding stage of the working memory task (n = 16) significantly correlated with GABA level and working memory performance. Despite gamma band amplitude deficits in patients across working memory stages, both baseline and working memory-induced gamma oscillations showed strong dependence on baseline GABA levels in patients and controls. These findings suggest a critical role for GABA function in gamma band oscillations, even under conditions of system and cognitive impairments as seen in schizophrenia.

Gamma Oscillations and Neural Field DCMs Can Reveal Cortical Excitability and Microstructure

This paper shows how gamma oscillations can be combined with neural population models and <em>dynamic causal modeling</em> (DCM) to distinguish among alternative hypotheses regarding cortical excitability and microstructure. This approach exploits inter-subject variability and trial-specific effects associated with modulations in the peak frequency of gamma oscillations. Neural field models are used to evaluate model evidence and obtain parameter estimates using invasive and non-invasive gamma recordings. Our overview comprises two parts: in the first part, we use neural fields to <em>simulate</em> neural activity and distinguish the effects of post synaptic filtering on predicted responses in terms of synaptic rate constants that correspond to different timescales and distinct neurotransmitters. We focus on model predictions of conductance and convolution based field models and show that these can yield spectral responses that are sensitive to biophysical properties of local cortical circuits like synaptic kinetics and filtering; we also consider two different mechanisms for this filtering: a nonlinear mechanism involving specific conductances and a linear convolution of afferent firing rates producing post synaptic potentials. In the second part of this paper, we use neural fields <em>quantitatively</em>—to <em>fit</em> empirical data recorded during visual stimulation. We present two studies of spectral responses obtained from the visual cortex during visual perception experiments: in the first study, MEG data were acquired during a task designed to show how activity in the gamma band is related to visual perception, while in the second study, we exploited high density electrocorticographic (ECoG) data to study the effect of varying stimulus contrast on cortical excitability and gamma peak frequency.

Visual Cortical Gamma-Band Activity During Free Viewing of Natural Images

Gamma-band activity in visual cortex has been implicated in several cognitive operations, like perceptual grouping and attentional selection. So far, it has been studied primarily under well-controlled visual fixation conditions and using well-controlled stimuli, like isolated bars or patches of grating. If gamma-band activity is to subserve its purported functions outside of the laboratory, it should be present during natural viewing conditions. We recorded neuronal activity with a 252-channel electrocorticographic (ECoG) grid covering large parts of the left hemisphere of 2 macaque monkeys, while they freely viewed natural images. We found that natural viewing led to pronounced gamma-band activity in the visual cortex. In area V1, gamma-band activity during natural viewing showed a clear spectral peak indicative of oscillatory activity between 50 and 80 Hz and was highly significant for each of 65 natural images. Across the ECoG grid, gamma-band activity during natural viewing was present over most of the recorded visual cortex and absent over most remaining cortex. After saccades, the gamma peak frequency slid down to 30–40 Hz at around 80 ms postsaccade, after which the sustained 50- to 80-Hz gamma-band activity resumed. We propose that gamma-band activity plays an important role during natural viewing.

Visual stimulus eccentricity affects human gamma peak frequency

The peak frequency of neuronal gamma-band synchronization has received much attention in recent years. Gamma peak frequency shifts to higher frequency values for higher contrast, faster moving, and attended stimuli. In monkey V1, gamma peak frequency for a drifting grating is higher for a parafoveal as compared to an eccentric stimulus (Lima et al., 2010). This effect might be due to the cortical magnification factor: the higher cortical magnification for parafoveal stimuli increases the velocity with which the cortical representations of the moving grating stripes move across the cortical surface. Since faster moving stimuli lead to higher gamma frequency, a faster moving cortical representation might do the same. This explanation predicts that the eccentricity effect on gamma peak frequency is absent for stationary stimuli. To test this, we investigated the effect of eccentricity on gamma peak frequency by recording magnetoencephalography in human subjects while they viewed moving or stationary gratings. We found that both the moving and the stationary stimuli induced lower peak frequencies for larger eccentricities, arguing against an explanation based on the cortical magnification factor. We further investigated whether this eccentricity effect was explained by differences in the size or the spatial frequency of the expected cortical activation. Neither of those explained the eccentricity effect. We propose that the different stimulus and top-down factors leading to higher gamma peak frequency all result in higher stimulus salience, that salience is translated into gamma peak frequency, and that gamma peak frequency might subserve the preferential processing of neuronal activity induced by salient stimuli.

No Consistent Relationship between Gamma Power and Peak Frequency in Macaque Primary Visual Cortex

Neural activity in the gamma frequency range ("gamma") is elevated during active cognitive states. Gamma has been proposed to play an important role in cortical function, although this is debated. Understanding what function gamma might fulfill requires a better understanding of its properties and the mechanisms that generate it. Gamma is characterized by its spectral power and peak frequency, and variations in both parameters have been associated with changes in behavioral performance. Modeling studies suggest these properties are co-modulated, but this has not been established. To test the relationship between these properties, we measured local field potentials (LFPs) and neuronal spiking responses in primary visual cortex of anesthetized monkeys, for drifting sinusoidal gratings of different sizes, contrasts, orientations and masked with different levels of noise. We find that there is no fixed relationship between LFP gamma power and peak frequency, and neither is related to the strength of spiking activity. We propose a simple model that can account for the complex stimulus dependence we observe, and suggest that separate mechanisms determine gamma power and peak frequency.

Dynamic causal modelling of lateral interactions in the visual cortex

This paper presents a dynamic causal model based upon neural field models of the Amari type. We consider the application of these models to non-invasive data, with a special focus on the mapping from source activity on the cortical surface to a single channel. We introduce a neural field model based upon the canonical microcircuit (CMC), in which neuronal populations are assigned to different cortical layers. We show that DCM can disambiguate between alternative (neural mass and field) models of cortical activity. However, unlike neural mass models, DCM with neural fields can address questions about neuronal microcircuitry and lateral interactions. This is because they are equipped with interlaminar connections and horizontal intra-laminar connections that are patchy in nature. These horizontal or lateral connections can be regarded as connecting macrocolumns with similar feature selectivity. Crucially, the spatial parameters governing horizontal connectivity determine the separation (width) of cortical macrocolumns. Thus we can estimate the width of macro columns, using non-invasive electromagnetic signals. We illustrate this estimation using dynamic causal models of steady-state or ongoing spectral activity measured using magnetoencephalography (MEG) in human visual cortex. Specifically, we revisit the hypothesis that the size of a macrocolumn is a key determinant of neuronal dynamics, particularly the peak gamma frequency. We are able to show a correlation, over subjects, between columnar size and peak gamma frequency — that fits comfortably with established correlations between peak gamma frequency and the size of visual cortex defined retinotopically. We also considered cortical excitability and assessed its relative influence on observed gamma activity. This example highlights the potential utility of dynamic causal modelling and neural fields in providing quantitative characterisations of spatially extended dynamics on the cortical surface — that are parameterised in terms of horizontal connections, implicit in the cortical micro-architecture and its synaptic parameters.

Attentional Stimulus Selection through Selective Synchronization between Monkey Visual Areas

A central motif in neuronal networks is convergence, linking several input neurons to one target neuron. In visual cortex, convergence renders target neurons responsive to complex stimuli. Yet, convergence typically sends multiple stimuli to a target, and the behaviorally relevant stimulus must be selected. We used two stimuli, activating separate electrocorticographic V1 sites, and both activating an electrocorticographic V4 site equally strongly. When one of those stimuli activated one V1 site, it gamma synchronized (60-80 Hz) to V4. When the two stimuli activated two V1 sites, primarily the relevant one gamma synchronized to V4. Frequency bands of gamma activities showed substantial overlap containing the band of interareal coherence. The relevant V1 site had its gamma peak frequency 2-3 Hz higher than the irrelevant V1 site and 4-6 Hz higher than V4. Gamma-mediated interareal influences were predominantly directed from V1 to V4. We propose that selective synchronization renders relevant input effective, thereby modulating effective connectivity.

Individual Differences in Subconscious Motor Control Predicted by GABA Concentration in SMA

Subliminal visual stimuli affect motor planning, but the size of such effects differs greatly between individuals. Here, we investigated whether such variation may be related to neurochemical differences between people. Cortical responsiveness is expected to be lower under the influence of more of the main inhibitory neurotransmitter, GABA. Thus, we hypothesized that, if an individual has more GABA in the supplementary motor area (SMA)--a region previously associated with automatic motor control--this would result in smaller subliminal effects. We measured the reversed masked prime--or negative compatibility--effect, and found that it correlated strongly with GABA concentration, measured with magnetic resonance spectroscopy. This occurred specifically in the SMA region, and not in other regions from which spectroscopy measurements were taken. We replicated these results in an independent cohort: more GABA in the SMA region is reliably associated with smaller effect size. These findings suggest that, across individuals, the responsiveness of subconscious motor mechanisms is related to GABA concentration in the SMA.

The level of GABAergic inhibition predicts peak gamma frequency and fMRI amplitude in humans

The level of GABAergic inhibition predicts peak gamma frequency and fMRI amplitude in humans

Resting GABA concentration predicts peak gamma frequency and fMRI amplitude in response to visual stimulation in humans

Functional imaging of the human brain is an increasingly important technique for clinical and cognitive neuroscience research, with functional MRI (fMRI) of the blood oxygen level-dependent (BOLD) response and electroencephalography or magnetoencephalography (MEG) recordings of neural oscillations being 2 of the most popular approaches. However, the neural and physiological mechanisms that generate these responses are only partially understood and sources of interparticipant variability in these measures are rarely investigated. Here, we test the hypothesis that the properties of these neuroimaging metrics are related to individual levels of cortical inhibition by combining magnetic resonance spectroscopy to quantify resting GABA concentration in the visual cortex, MEG to measure stimulus-induced visual gamma oscillations and fMRI to measure the BOLD response to a simple visual grating stimulus. Our results demonstrate that across individuals gamma oscillation frequency is positively correlated with resting GABA concentration in visual cortex (R = 0.68; P < 0.02), BOLD magnitude is inversely correlated with resting GABA (R = -0.64; P < 0.05) and that gamma oscillation frequency is strongly inversely correlated with the magnitude of the BOLD response (R = -0.88; P < 0.001). Our results are therefore supportive of recent theories suggesting that these functional neuroimaging metrics are dependent on the excitation/inhibition balance in an individual's cortex and have important implications for the interpretation of functional imaging results, particularly when making between-group comparisons in clinical research.