fMRI responses in medial frontal cortex that depend on the temporal frequency of visual input

Functional networks in the human brain have been investigated using electrophysiological methods (EEG/MEG, LFP, and MUA) and steady-state paradigms that apply periodic luminance or contrast modulation to drive cortical networks. We have used this approach with fMRI to characterize a cortical network driven by a checkerboard reversing at a fixed frequency. We found that the fMRI signals in voxels located in occipital cortex were increased by checkerboard reversal at frequencies ranging from 3 to 14 Hz. In contrast, the response of a cluster of voxels centered on basal medial frontal cortex depended strongly on the reversal frequency, consistently exhibiting a peak in the response for specific reversal frequencies between 3 and 5 Hz in each subject. The fMRI signals at the frontal voxels were positively correlated indicating a homogeneous cluster. Some of the occipital voxels were positively correlated to the frontal voxels apparently forming a large-scale functional network. Other occipital voxels were negatively correlated to the frontal voxels, suggesting a functionally distinct network. The results provide preliminary fMRI evidence that during visual stimulation, input frequency can be varied to engage different functional networks.     

The elicitation of steady‐state visual evoked potentials during sleep

This study confirmed the hypothesis that it is possible to elicit SSVEPs through closed eyelids during NREM sleep. To test this hypothesis, SSVEP amplitudes were measured in eight subjects across two conditions of stimulation (stimulation on and stimulation off) and three brain states (waking, light sleep, and deep sleep). Results showed a significant interaction between stimulation and brain state. In particular, EEG activity at the frequency of stimulation was higher during both light sleep and deep sleep in the stimulation on condition than in the stimulation off condition. The fact that it is possible to elicit SSVEPs during sleep may provide a new way to study how SSVEPs are generated in the brain—one that might help resolve open questions such as identifying the SSVEP activation sequence or deciding if SSVEPs derive from evoked or oscillatory neural processes.     

Functional connectivity and infant spatial working memory: a frequency band analysis

The limited research on the functional meaning of infant EEG frequency bands has used measures of EEG power. The purpose of this study was to examine task‐related changes in frontal EEG coherence measures for three infant EEG frequency bands (2–5 Hz, 6–9 Hz, 10–13 Hz) during a spatial working memory task. Eight‐month‐olds exhibited baseline‐to‐task changes in frontal EEG coherence for all infant frequency bands. Both the 2–5 Hz and the 10–13 Hz bands differentiated frontal functional connectivity during the distinct processing stages, but each band provided unique information. The 10–13 Hz band, however, was the only frequency band to distinguish frontal EEG coherence values during correct and incorrect responses. These data reveal valuable information concerning frontal functional connectivity and the functional meaning of three different infant EEG frequency bands during working memory processing.     

Effects of overt and covert attention on the steady-state visual evoked potential

Flickering stimuli evoke an oscillatory brain response with the same frequency as the driving stimulus, the so-called steady-state visual evoked potential (SSVEP). SSVEPs are robust brain signals whose amplitudes are enhanced with attention and thus play a major role in the development and use of non-invasive Brain–Computer Interfaces (BCIs). We compared the modulation of SSVEP amplitudes when subjects directly gazed at a flickering array of static dots (overt attention) to when they covertly shifted attention to the dots keeping their eyes at central fixation. A discrimination task was performed at the attended location to ensure that subjects shifted attention as instructed. Horizontal eye movements (allowed in overt attention but to be avoided in covert attention) were monitored by the horizontal electrooculogram.     

Functional and effective connectivity of anterior insula in anorexia nervosa and bulimia nervosa

The anterior insula has been proposed to play a crucial role in eating disorders. However, it is still poorly understood how the anterior insula is involved in anorexia nervosa (AN) and bulimia nervosa (BN), which are characterized by opposite motivational responses to food. We applied a cue-reactivity paradigm using blood oxygen level-dependent functional magnetic resonance imaging in women with AN (N=18) and BN (N=20) and age-matched healthy controls (N=20). We defined the left anterior insula as a region-of-interest and performed seed-based functional connectivity and effective connectivity MRI analysis. In response to food images compared to non-food images, both the AN group and BN group demonstrated increased activity in the left anterior insula. In the AN group, the left anterior insula demonstrated significant interactions with the right insula and right inferior frontal gyrus. In the BN group, the left anterior insula demonstrated significant interactions with the medial orbitofrontal cortex. The distinct patterns of functional and effective connectivity of the anterior insula may contribute to the different clinical features of AN and BN.     

基于Granger因果分析的癫痫发作间歇期EEG在δ频段的功能连接模式

目的 应用频域Granger因果分析方法,研究颞叶癫痫发作间歇期16导脑电图(EEG)在与癫痫发作相关的δ频段的过度放电功能连接特性.方法 实验数据来自颞叶癫痫9例患者(6例左颞叶癫痫,3例右颞叶癫痫),9例正常对照受试者.记录每例颞叶癫痫受试者在发作间歇期的痫样放电、非痫样放电以及正常对照组的共3个状态的16导EEG;每个状态下各记录10个EEG数据段,每个数据段长度为20s,采样频率为200 Hz;应用带通滤波提取EEG的δ分量(1～4 Hz).应用频域Granger因果分析方法,分别计算痫样放电组、非痫样放电组和正常组10次记录的16通道EEGδ频段分量之间的频域因果度量平均值Iδ;分析以上3个组颞叶区(左颞叶癫痫:T3、T5,右颞叶癫痫:T4、T6)与额区(Fp1、Fp2、F3、F4)和顶区(C3、C4)之间EEG在δ频段的功能连接模式.结果 痫样放电组:下颞叶区(左下颞叶区T5,右下颞叶区T6)与额区、顶区之间Iδ在0.1323±0.0329～0.1670±0.0289;非痫样放电组:下颞叶区与额区、顶区之间的Iδ在0.0300±0.0130～0.0420±0.0072;正常对照组:下颞叶区与额区、顶区之间的Iδ在0.0153±0.0028～0.0193±0.0057.统计结果表明:痫样放电组下颞叶区与额区、顶区之间的Iδ值与非痫样放电组相比差异有统计学意义(P＜0.05),与正常对照组相比差异有统计学意义(P＜0.01);非痫样放电组下颞叶区与额叶、顶叶之间的Iδ值和正常对照组相比差异无统计学意义(P＞0.05).结论 颞叶癫痫发作间歇期在痫样放电状态下,EEGδ频段在下颞叶区与额区、顶区之间存在较强连接,过度放电从下颞叶区传递到额区和顶区.非痫样放电组和正常组的EEGδ频段,下颞叶区与额叶、顶叶之间连接弱,下颞叶区不是EEG信号传导的起始区.     

Functional connectivity and graph theory in preclinical Alzheimer's disease

Alzheimer's disease (AD) has a long preclinical phase in which amyloid and tau cerebral pathology accumulate without producing cognitive symptoms. Resting state functional connectivity magnetic resonance imaging has demonstrated that brain networks degrade during symptomatic AD. It is unclear to what extent these degradations exist before symptomatic onset. In this study, we investigated graph theory metrics of functional integration (path length), functional segregation (clustering coefficient), and functional distinctness (modularity) as a function of disease severity. Further, we assessed whether these graph metrics were affected in cognitively normal participants with cerebrospinal fluid evidence of preclinical AD. Clustering coefficient and modularity, but not path length, were reduced in AD. Cognitively normal participants who harbored AD biomarker pathology also showed reduced values in these graph measures, demonstrating brain changes similar to, but smaller than, symptomatic AD. Only modularity was significantly affected by age. We also demonstrate that AD has a particular effect on hub-like regions in the brain. We conclude that AD causes large-scale disconnection that is present before onset of symptoms.     

稳态视觉刺激频率与诱发响应相关性的fMRI研究

稳态视觉诱发电位(SSVEP)及脑电逆问题(inverse EEG)的研究显示了大脑皮层对于不同频率的稳态视觉刺激的响应具有幅度及空间位置上的差异.本文利用功能性磁共振成像(fMRI)研究稳态视觉刺激频率与诱发响应之间的相关性.根据SSVEP的实验结果进行真fMRI实验设计,对5名被试者给予不同频率的稳态视觉刺激,同时进行了脑部的fMRI扫描,采用SPM软件进行数据处理和分析.结果显示,不同频率刺激下的响应区域均集中于初级视皮层(V1),并且均具有单侧优势效应,即右侧半球的响应强于左侧半球.该结果显示了初级视皮层单侧优势对于稳态视觉诱发响应的表现形式.本文讨论了单侧优势与刺激性质之间存在相关性的可能,以及对此做进一步研究和应用的前景.     

Functional connectivity in fronto-subcortical circuitry during the resting state in obsessive-compulsive disorder

Obsessions and compulsions mediated by cognitive inflexibility might be associated with abnormal resting state functional connectivity in the default mode network (DMN) that represents intrinsically generated neuronal activity. It was hypothesized that decreased functional connectivity in the DMN would occur in components of fronto-subcortical circuits in patients with obsessive-compulsive disorder (OCD). Twenty-two unmedicated OCD patients and 22 age- and sex-matched healthy controls received resting state functional scanning runs. The posterior cingulate cortex (PCC) region was chosen as the seed region for the connectivity analysis. Correlations between temporal connectivity with the seed region and scores on clinical measures and obsessive-compulsive symptom dimensions were also assessed. OCD patients demonstrated less functional connectivity within the DMN in the anterior cingulate cortex, middle frontal gyrus, and putamen compared to controls. The functional connectivity to the PCC seed region in OCD patients was in the direction opposite to that in the prefrontal areas with regard to scores on cleaning and obsessions/checking dimensions of OCD. These data provide evidence for fronto-subcortical dysfunction in OCD. Results from this study also support the notion that OCD is a heterogeneous disorder mediated by distinct circuits.     

Wavelet transform analysis of visual evoked potentials: some preliminary results

Visual Evoked Potentials (VEP) are analyzed with the continuous wavelet transform (CWT). The modulus of the wavelet transform of the signal is calculated and represented in the time-scale domain. Based on the first-order derivative of the Gaussian function chosen as the wavelet, the modulus representation allows a clear-cut discrimination between normal and pathological cases. By processing a set of data composed of VEPs collected from 30 normal subjects and 20 patients, the method proved to be more accurate than the latency method used in hospitals. In addition, the CWT method is found to yield a fine spectral analysis of VEP data even in the presence of the background EEG signal. As a result, preliminary averaging performed on the data will use a much lower number of responses than those required by the latency method. This allows for a shorter recording session which seems to be very suitable for testing elderly people or children and even restless patients with short attention span.     

Functional connectivity tracks clinical deterioration in Alzheimer's disease

While resting state functional connectivity has been shown to decrease in patients with mild and/or moderate Alzheimer's disease, it is not yet known how functional connectivity changes in patients as the disease progresses. Furthermore, it has been noted that the default mode network is not as homogenous as previously assumed and several fractionations of the network have been proposed. Here, we separately investigated the modulation of 3 default mode subnetworks, as identified with group independent component analysis, by comparing Alzheimer's disease patients to healthy controls and by assessing connectivity changes over time. Our results showed decreased connectivity at baseline in patients versus controls in the posterior default mode network, and increased connectivity in the anterior and ventral default mode networks. At follow-up, functional connectivity decreased across all default mode systems in patients. Our results suggest that earlier in the disease, regions of the posterior default mode network start to disengage whereas regions within the anterior and ventral networks enhance their connectivity. However, as the disease progresses, connectivity within all systems eventually deteriorates.     

Perception of illusory contours forms intermodulation responses of steady state visual evoked potentials as a neural signature of spatial integration

Perception of illusory contours was shown to be a consequence of neural activity related to spatial integration in early visual areas. Candidates for such filling-in phenomena are long-range horizontal connections of neurons in V1/V2, and feedback from higher order visual areas. To get a direct measure of spatial integration in early visual cortex, we presented two differently flickering inducers, which evoked steady-state visual evoked potentials (SSVEPs) while manipulating the formation of an illusory rectangle. As a neural marker of integration we tested differences in amplitudes of intermodulation frequencies i.e. linear combinations of the driving frequencies. These were significantly increased when an illusory rectangle was perceived. Increases were neither due to changes of any of the two driving frequencies nor in the frequency that tagged the processing of the compound object, indicating that results are not a consequence of paying more attention to inducers when the illusory rectangle was visible.     

Modulation of steady-state auditory evoked potentials by cerebellar rTMS

Steady-state auditory evoked responses (SSAER) obtained via electroencephalography (EEG) co-vary in amplitude with blood flow changes in the auditory area of the cerebellum. The aim of the present EEG study was to probe the cerebellar role in the control of such SSAER. For this purpose, we investigated changes in SSAERs due to transient disruption of the cerebellar hemisphere by repetitive transcranial magnetic stimulation (rTMS). SSAERs to click-trains of three different frequencies in the gamma-band (32, 40 and 47 Hz) were recorded from 45 scalp electrodes in six healthy volunteers immediately after 1-Hz rTMS and compared to baseline SSAERs assessed prior to magnetic stimulation. Cerebellar rTMS contralateral to the stimulated ear significantly reduced the amplitude of steady-state responses to 40-Hz click-trains and showed a tendency to reduce the amplitude to 32-Hz click-trains. No effects were observed for 47-Hz click-trains, nor for magnetic stimulation of the cerebellum ipsilateral to auditory stimulation or after sham stimulation. Our results suggest that interference with cerebellar output by rTMS modifies functional activity associated with cortical auditory processing. The finding of maximum effects on 40-Hz SSAERs provides support to the notion that the cerebellum is part of a distributed network involved in the regulation of cortical oscillatory activity and points at some frequency-specificity for the control of auditory-driven neuronal oscillations.     

Evaluating the feasibility of the steady‐state visual evoked potential (SSVEP) to study temporal attention

Improvements in perceptual performance can be obtained when events in the environment are temporally predictable—and temporal predictability improves attention and sensory processing. The amplitude of the steady‐state visual evoked potential (SSVEP) has been shown to correlate with attention paid to a flickering stimulus even if the flickering stimulus is irrelevant for the task. However, to our knowledge, the validity of the SSVEP to study temporal attention has not been established. Therefore, we designed an SSVEP temporal attention task to evaluate whether the SSVEP and its temporal dynamics can be used to study temporal attention. We used a forced‐choice perceptual detection task while presenting task‐irrelevant visual flicker at alpha (10 Hz) and two surrounding frequencies (6 or 15 Hz). Temporal predictability was manipulated by having the interstimulus intervals (ISI) be constant or variable. Behavioral results replicated previous studies confirming the benefits of temporal expectations on performance for trials with constant ISI. EEG analyses revealed robust SSVEP amplitudes for all flicker frequencies, although a main effect of temporal expectations on SSVEP amplitude was not significant. Additional analyses revealed temporal predictability‐related modulations of SSVEP amplitude at 10 Hz and its second harmonic (20 Hz). The effect of temporal predictability was also observed for the 6 Hz flicker, but not for 15 Hz for any ISI condition. These results provide some evidence for the feasibility of the SSVEP technique to study temporal attention for stimuli with flicker frequencies around the alpha band.     

下肢临近关节稳态体感诱发电位的提取和分析

稳态体感诱发电位（SSSEP）在提升运动想象脑机接口分类性能方面有着显著作用,因此如何有效提取SSSEP是研究的重点。为有效提取下肢膝-踝临近关节处的SSSEP,设计体感刺激参数筛选试验,分别采集10名受试者在右腿腘窝和右脚踝内侧处10种不同电刺激频率下的脑电信号,使用快速傅里叶变换、时频图谱和脑地形图进行综合分析。结果显示,受试者在右脚踝内侧的频谱特征优于右腿腘窝,33 Hz为最优刺激频率,能用于下肢的电刺激强度为10~20 mA。     

The dynamic allocation of attention to emotion: Simultaneous and independent evidence from the late positive potential and steady state visual evoked potentials

Emotional stimuli capture and hold attention without explicit instruction. The late positive potential (LPP) component of the event related potential can be used to track motivated attention toward emotional stimuli, and is larger for emotional compared to neutral pictures. In the frequency domain, the steady state visual evoked potential (ssVEP) has also been used to track attention to stimuli flickering at a particular frequency. Like the LPP, the ssVEP is also larger for emotional compared to neutral pictures. Prior work suggests that both the LPP and ssVEP are sensitive to “top-down” manipulations of attention, however the LPP and ssVEP have not previously been examined using the same attentional manipulation in the same participants. In the present study, LPP and ssVEP amplitudes were simultaneously elicited by unpleasant and neutral pictures. Partway through picture presentation, participants’ attention was directed toward an arousing or non-arousing region of unpleasant pictures. In line with prior work, the LPP was reduced when attention was directed toward non-arousing compared to arousing regions of unpleasant pictures; similar results were observed for the ssVEP. Thus, both electrocortical measures index affective salience and are sensitive to directed (here: spatial) attention. Variation in the LPP and ssVEP was unrelated, suggesting that these measures are not redundant with each other and may capture different neurophysiological aspects of affective stimulus processing and attention.     

Objective measurement of human visual acuity by visual evoked potentials

Electrophysiological studies were performed to measure the threshold (upper end of range) spatial frequency using visual evoked potentials and comparison with visual acuity neuron 26 healthy subjects. The aim of the present work was to create a method for objective measurement of visual acuity. This was addressed by initial measurements using a universally accepted method of visual stimulation and processing of electroencephalograms, which allows errors due to individual differences in visual system function to be minimized. These experiments yielded a strong correlation between the threshold spatial frequency of the test grid yielding an evoked potential on presentation and visual acuity, in degrees, expressed as the resolving ability of the visual system for this optotype. A logarithmic relationship was found between these values and an equation allowing automated calculation of visual acuity (resolving ability) from electrophysiological data was derived. The results were independent of the subject’s responses and therefore provides a maximally objective assessment of visual acuity. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 91, No. 8, pp. 956–969, August, 2005.     

Perception of visual apparent movement and topographic analysis of related evoked potentials

Summary Most of the studies in long-range apparent movement have been concerned with the psychophysical properties in central vision. However, apparent movement in peripheral vision is not clearly understood. This study focuses on the properties of the perception of apparent movement between central and peripheral vision. The results show that judgments of apparent movement in central vision indicate high performance at less than 4° of spatial separation, and that judgments in peripheral vision indicate high performance at greater than 4°. Evoked potentials associated with long-range apparent movement were also measured, and topographic maps were constructed to evaluate the active areas corresponding to visual apparent movement. The topographic maps show that the central regions are activated above 300 msec after the onset of the second apparent-movement stimulation. The results suggest that visual apparent movement occurs in the visual association area, or at higher levels in the central nervous system.     

Reliability of event‐related EEG functional connectivity during visual entrainment: Magnitude squared coherence and phase synchrony estimates

There is an increasing trend towards using noninvasive electroencephalography (EEG) to quantify functional brain connectivity. However, little is known about the psychometrics of commonly used functional connectivity indices. We examined the internal consistency of two different connectivity metrics: magnitude squared coherence and phase synchrony. EEG was recorded during visual entrainment to elicit a strong oscillatory component of known frequency. We found acceptable to good split‐half reliability for the connectivity metrics when computing all possible pairwise interactions and after selecting an a priori seed reference. We also compared reliability estimates when using average referenced sensor versus reference independent current source density EEG data. Additional considerations were given to determining how reliability was influenced by factors including trial number, signal‐to‐noise ratio, and frequency content.     

Spinal evoked potentials evaluated with two relevant electrode types

The spinal evoked potentials (SEPs) were recorded at the level of the 7th lumbar segment in five cats by stimulating the popliteal nerve separately on both sides. The recordings were performed both with silver-silver chloride cup electrodes and stainless steel needle electrodes. The shapes and latencies of the responses were highly similar, when comparing the responses recorded with the two electrode types. These similarities are explained by similar recording properties of both electrode types as shown by means of transfer function: attenuation and phase curves for both electrode types are highly similar in the bandpass used in the present study. It is concluded that the properties of stainless steel needle electrodes are highly correspondent with the conventional silver-silver chloride electrodes when somatosensory evoked responses are recorded.