The Effect of Stimulation Frequency and Retinal Stimulus Location on Visual Evoked Potential Topography

The activity of cortical neurons is influenced by retinal stimulus location and temporal modulation. We investigated how reversal frequency of black-and-white checkerboard patterns presented in different parts of the visual field affects evoked potential topography. Visual evoked potentials were recorded from an array of 16 electrodes over the occipital cortex in 12 healthy adults. A checkerboard reversal stimulus (40′ check size) was presented with frequencies between 1.95 reversals/s and 7.81 reversals/s in the center or in the left or right hemiretina. Evoked potential fields displayed the well-known components of pattern reversal evoked activity. Computation of FFT and wavelets displayed electrical brain responses directly related to stimulation frequency. Further analysis showed that both retinal stimulus location and stimulation frequency affected visual evoked activity. Field strength as well as scalp field topography changed significantly with different reversal frequency. In addition, the pattern of lateralization of components also depended on temporal frequency of stimulation.Electrical brain activity elicited by visual stimuli shows globally similar features which are modulated by stimulus location and frequency. Our results indicate that—at least partly—different neuronal assemblies are activated by stimuli of different temporal characteristics.     

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.     

Effects of Attention on Pattern-Reversal Visual Evoked Potentials: Foveal Field Stimulation Versus Peripheral Field Stimulation

The effects of foveal field attention on pattern reversal visual evoked potential (VEP) were investigated in thirteen normal subjects. Conventional monocular VEP was recorded during the three conditions of control, foveal concentration and reaction time task. Three patterns of checkerboard, which were full field (radius, 0-9 degrees), peripheral field (2.5-9 degrees) and foveal field (0-2.5 degrees), were presented for the stimulation in each condition. The P100 and N145 amplitude of the peripheral field VEP were significantly smaller during the concentration and the reaction time task conditions than that in the control session, while the foveal field VEP amplitudes were enhanced in the concentration and reaction time tasks conditions. The full field VEP amplitudes were not significantly changed by the conditions. We concluded that the act of concentration to the foveal field or the task requiring attention to the foveal field, enhanced the VEP response to foveal field stimulation and suppressed the VEP to the peripheral field. A gating effect in area V1 was speculated, although extra-striate cortex might contribute.     

Frequency analysis of the visual steady-state response measured with the fast optical signal in younger and older adults

Relatively high frequency activity (>4 Hz) carries important information about the state of the brain or its response to high frequency events. The electroencephalogram (EEG) is commonly used to study these changes because it possesses high temporal resolution and a good signal-to-noise ratio. However, it provides limited spatial information. Non-invasive fast optical signals (FOS) have been proposed as a neuroimaging tool combining spatial and temporal resolution. Yet, this technique has not been applied to study high frequency brain oscillations because of its relatively low signal-to-noise ratio. Here we investigate the sensitivity of FOS to relatively high-frequency brain oscillations. We measured the steady-state optical response elicited in medial and lateral occipital cortex by checkerboard reversals occurring at 4, 6, and 8 Hz in younger and older adults. Stimulus-dependent oscillations were observed at the predicted stimulation frequency. In addition, in the younger adults the FOS steady-state response was smaller in lateral than medial areas, whereas in the older adults it was reversed in these two cortical regions. This may reflect diminished top-down inhibitory control in the older adults. The results indicate that FOS can be used to study the modulation of relatively high-frequency brain oscillations in adjacent cortical regions.     

Effects of stimulus orientation on spatial frequency function of the visual evoked potential

Visual performance is better in response to vertical and horizontal stimuli than oblique ones in many visual tasks; this is called the orientation effect. In order to elucidate the electrophysiological basis of this psychophysical effect, we studied the effects of stimulus orientation on the amplitudes and latencies of visual evoked potentials (VEPs) over different spatial frequencies of the visual stimulation. VEPs to sinusoidal gratings at four orientations (vertical, horizontal, and oblique at 45 degrees and 135 degrees) with eight spatial frequencies (0.5-10.7 cycles/deg) at reversal rates of 1 Hz and 4 Hz were recorded in nine subjects. At 1-Hz stimulation, the amplitude and latency of P100 were measured. At 4-Hz stimulation, VEPs were Fourier-analyzed to obtain phase and amplitude of the second harmonic response (2F). At 1-Hz stimulation, P100 latencies were decreased for oblique stimuli compared with those for horizontal and vertical stimuli at lower spatial frequencies. Conversely, those for oblique stimuli were increased compared with those for horizontal and vertical stimuli at higher spatial frequencies. At 4-Hz stimulation, spatial tuning observed in 2F amplitude of the oblique gratings shifted to lower spatial frequencies when compared with those of vertical stimulation. The alteration of the VEP spatial frequency function caused by the oblique stimuli was in good agreement with the orientation effect observed in psychophysical studies. Our study may have a clinical implication in that VEP testing with stimuli in more than one orientation at slow and fast temporal modulations can be useful in evaluating neurological disease affecting the visual system.     

Perception of structured optic flow and random visual motion in infants and adults: a high-density EEG study

Electroencephalogram (EEG) was used in 8-month-old infants and adults to study brain electrical activity as a function of perception of structured optic flow and random visual motion. A combination of visual evoked potential (VEP) analyses and analyses of temporal spectral evolution (TSE, time-dependent spectral power) was carried out. Significant differences were found for the N2 component of VEP for optic flow versus random visual motion within and between groups. Both adults and infants showed shorter latencies for structured optic flow than random visual motion, and infants showed longer latencies, particularly for random visual motion, and larger amplitudes than adults. Both groups also showed significant differences in induced activity when TSE of the two motion stimuli (optic flow and random visual motion) was compared with TSE of a static dot pattern. Infants showed an induced decrease in the amplitudes in theta-band frequency, while adults showed an induced increase in beta-band frequency. Differences in induced activity for the two motion stimuli could, however, not be observed. Brain activity related to motion stimuli is different for infants and adults and the differences are observed both in VEPs and in induced activity of the EEG. To investigate how changes in locomotor development are related to accompanying changes in brain activity associated with visual motion perception, more data of infants with different experiences in self-produced locomotion are required.     

Visual evoked potentials induced by illusory outlines (Kanizsa's square)

This report describes studies of visual evoked potentials (VEP) in ten subjects produced in response to Kanizsa's square and a control stimulus which did not involve a visual illusion but which had a similar spatial organization. The results showed that the amplitude-time characteristics of VEP depended on the illusory outlines. Differences in the parameters of VEP produced using the two stimuli were seen in the occipital, parietal, and temporal areas. VEP amplitude differences between the peaks of the N180 and P230 waves increased and the latent period of the N300 wave decreased on presentation of the illusory outlines as compared with the control stimulus. Interstimulus differences in amplitude were seen in the left and right occipital and left temporal areas, while differences in latency were seen in the left occipital lead. The data supported the suggestion that the visual perception system includes two areas encoding illusory outlines, which are associated with different aspects of visual analysis—encoding of individual signs and their complexes (O1 andO2) and comparing sensory codes with codes stored in memory (T5). Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 48, No. 5, pp. 807–815, September–October, 1998.     

Hemodynamic responses to visual stimulation in occipital and frontal cortex of newborn infants: a near-infrared optical topography study

A near-infrared optical topography (OT) was used to reveal spatio-temporal changes in the cerebral oxygenation of newborn infants in response to brief visual stimulation. Newborn infants were presented 3 s stroboscopic light flashing at 14 Hz during spontaneous sleep. Event-related changes in oxy- and deoxyhemoglobin ([oxy-Hb] and [deoxy-Hb]) were observed over the occipital and frontal cortex. The visual stimulus produced statistically significant increases in oxyhemoglobin not only in the occipital cortex but also in the prefrontal cortex. These results suggest that the cerebrovascular coupling is already functioning in newborn’s brain. The prefrontal activation implies that it may contribute to early processing of sensory signals.     

α频率光刺激脑电信号同步化的初步研究

研究α频率（8-13Hz）闪光刺激是否能引起人脑枕区同频率脑电信号的增加。采集15名正常志愿者在平静、光刺激状态下的脑电信号,利用脑地形图、小波分析和功率谱估计的方法对α频率光刺激前后的脑电信号进行处理,将归一化后的数据分为平静-激活组、男女对照组、两个不同电极位置组进行对比分析,对组间信号的差异进行了讨论,并对受试者精神状态的变化进行初步探讨。结果表明：周期性α频率光刺激能引起大脑枕区同频率脑电的显著增加。通过分析实验数据,初步得到了大脑在外部闪光刺激时脑电的变化规律,本实验结果对研究外部刺激对脑电的影响有一定的参考价值。     

Multiple spatial-frequency tuning of electrical responses from human visual cortex

Summary Human occipital potentials evoked by stimulation with a counterphase flickering grating were recorded by a digital narrowband filter technique. The data showed a surprising degree of narrow tuning to particular spatial frequencies in addition to the expected narrow temporal frequency tuning. At each temporal frequency, there could be two or more peaks of response to different spatial frequencies, each distinct from the whole field flicker response. Variations in this multiple spatial frequency tuning were investigated as a function of luminance, electrode location, and temporal frequency for several observers. The results are interpreted in terms of many stimulus-specific resonant neural circuits within the brain, and suggest that it is possible to make a highly detailed exploration of the responses of neural circuits to visual stimulation.Supported by NIH Grants No. 5 R01 EY01582, No. 1 R01 EY02124, NIH General Research Grant No. 5 S01 RR05566 and The Smith-Kettlewell Eye Research Foundation     

Functional imaging of the human lateral geniculate nucleus and pulvinar

In the human brain, little is known about the functional anatomy and response properties of subcortical nuclei containing visual maps such as the lateral geniculate nucleus (LGN) and the pulvinar. Using functional magnetic resonance imaging (fMRI) at 3 tesla (T), collective responses of neural populations in the LGN were measured as a function of stimulus contrast and flicker reversal rate and compared with those obtained in visual cortex. Flickering checkerboard stimuli presented in alternation to the right and left hemifields reliably activated the LGN. The peak of the LGN activation was found to be on average within +/-2 mm of the anatomical location of the LGN, as identified on high-resolution structural images. In all visual areas except the middle temporal (MT), fMRI responses increased monotonically with stimulus contrast. In the LGN, the dynamic response range of the contrast function was larger and contrast gain was lower than in the cortex. Contrast sensitivity was lowest in the LGN and V1 and increased gradually in extrastriate cortex. In area MT, responses were saturated at 4% contrast. Response modulation by changes in flicker rate was similar in the LGN and V1 and occurred mainly in the frequency range between 0.5 and 7.5 Hz; in contrast, in extrastriate areas V4, V3A, and MT, responses were modulated mainly in the frequency range between 7.5 and 20 Hz. In the human pulvinar, no activations were obtained with the experimental designs used to probe response properties of the LGN. However, regions in the mediodorsal right and left pulvinar were found to be consistently activated by bilaterally presented flickering checkerboard stimuli, when subjects attended to the stimuli. Taken together, our results demonstrate that fMRI at 3 T can be used effectively to study thalamocortical circuits in the human brain.     

Optimization of SSVEP brain responses with application to eight-command Brain–Computer Interface

This study pursues the optimization of the brain responses to small reversing patterns in a Steady-State Visual Evoked Potentials (SSVEP) paradigm, which could be used to maximize the efficiency of applications such as Brain–Computer Interfaces (BCI). We investigated the SSVEP frequency response for 32 frequencies (5–84 Hz), and the time dynamics of the brain response at 8, 14 and 28 Hz, to aid the definition of the optimal neurophysiological parameters and to outline the onset-delay and other limitations of SSVEP stimuli in applications such as our previously described four-command BCI system. Our results showed that the 5.6–15.3 Hz pattern reversal stimulation evoked the strongest responses, peaking at 12 Hz, and exhibiting weaker local maxima at 28 and 42 Hz. After stimulation onset, the long-term SSVEP response was highly non-stationary and the dynamics, including the first peak, was frequency-dependent. The evaluation of the performance of a frequency-optimized eight-command BCI system with dynamic neurofeedback showed a mean success rate of 98%, and a time delay of 3.4 s. Robust BCI performance was achieved by all subjects even when using numerous small patterns clustered very close to each other and moving rapidly in 2D space. These results emphasize the need for SSVEP applications to optimize not only the analysis algorithms but also the stimuli in order to maximize the brain responses they rely on.     

Activation and deactivation in response to visual stimulation in the occipital cortex of 6-month-old human infants

In an infant's developing cortex, the explanation for the mechanisms underlying the activations and deactivations in response to visual stimuli remains controversial. While previous near-infrared spectroscopy (NIRS) studies in awake infants have demonstrated cortical activations in response to meaningful/attractive visual stimuli, functional magnetic resonance imaging (fMRI) studies performed on sleeping infants showed negative blood oxygenation level-dependent (BOLD) responses to high-luminance unpatterned stimulations, such as a photic stimulation. To examine the effect of the characteristics of visual stimuli on cortical processing in awake infants, we measured cortical hemodynamic responses in 6-month-old infants during the presentation of a high-luminance unpatterned stimulus by using a NIRS system with 94 measurement channels. Results from 35 infants showed dissociated cortical responses between the occipital region and the other parts of the cortex, including the temporal and prefrontal regions. Although the visual stimulus produced sustained increases in oxygenated hemoglobin (oxy-Hb) signals in the temporal and prefrontal regions, it produced a transient increase in oxy-Hb signals followed by a salient decrease in oxy-Hb signals during a trial in a focal region of the occipital visual region. This suggests that the deactivation of the occipital visual region in response to visual stimulation is not a phenomenon that occurs only in the sleeping state, but that a high-luminance unpatterned stimulus can induce deactivation even in the awake infants.     

Spectral analysis of event-related EEG responses during short-term memory performance

Summary Averaged event-related EEG frequency response profiles were generated from normalized spectral analysis data obtained in 2 studies of short-term memory. A continuous performance task was used requiring a keyboard response to targeted letter sequences presented on a video screen at 2 sec. intervals. Seven 2 Hz frequency bands between 5–15 Hz were evaluated. In the first study (n=14) response profiles from control and target stimuli were compared. This analysis disclosed 1) a short latency increase at 5–7 and 7–9 Hz in posterior cortex that was identical in both conditions and was attributed to frequency manifestations of the extrinsic visual evoked response (VEP), 2) a separate long-latency increase of uncertain origin at 5–7 Hz in anterior cortex only in the task condition, and 3) a generalized mid-latency alpha frequency suppression and recovery pattern (event-related desynchronization, or ERD) in all bands during both conditions. The ERD in bands between 9–13 Hz was significantly increased at left central and bilateral parietal cortex during target recall. The second study (n=26) compared good vs. poor performance in the target recall task. Findings disclosed significant differences at 7–9 Hz localized to posterior temporal cortex bilaterally. These differences included reduced magnitude in the VEP component and increased suppression in the ERD component among good performers.This work was supported by the Veterans Administration, the Northrop-Grumman Corporation, and the United States Air Force.     

Visual and auditory evoked potentials in migraine

We recorded visual (VEP) and brainstem auditory (BAEP) evoked potentials in 50 patients with clinically diagnosed common migraine attended by visual obscuration or sensory symptoms but no neurologic deficit. VEPs were recorded from Oz, 01, and 02 referenced to Fz, with replication of 200 repetitions of 1.88 per second checkerboard stimuli subtending a 56 minute retinal arc. Analysis time was 250 ms., and filter band pass was 1-250 Hz. BAEPs utilized rarefaction stimulation at 70 dB SL, with 150-3,000 Hz filter band pass and 10 ms. analysis time. Two thousand averages were recorded and replicated from Cz-A1 and Cz-A2. VEP N1, P1 and N2 latencies were longer in migraine patients than in controls, and VEP amplitudes were minimally greater. No significant differences were found between patients and controls, however. BAEP I-V and III-V interpeak latencies were significantly prolonged in migraine patients, and the degree of prolongation was greater on the left. Neither VEPs nor BAEPs exceeded clinical norms in migraine patients. VEPs and BAEPs are likely to add little to the clinical assessment of headache patients. BAEP differences may indicate dysfunction of brainstem centers, possibly related to endorphin or serotonin neurotransmission, and possibly related to the pathogenesis of migraine. The left sided asymmetry has been described previously and is of uncertain significance, but may also support a central mechanism for migraine.     

Correlation of visual-evoked hemodynamic responses and potentials in human brain

The interaction of brain hemodynamics and neuronal activity has been intensively studied in recent years to yield better understanding of brain function. We investigated the relationship between visual-evoked hemodynamic responses (HDRs), measured with near-infrared spectroscopy (NIRS), and neuronal activity in humans, approximated with the stimulus train duration or with visual-evoked potentials (VEPs). Concentration changes of oxyhemoglobin (HbO₂) and deoxyhemoglobin (HbR) in tissue and VEPs were recorded simultaneously over the occipital lobe of ten healthy subjects to 3, 6, and 12 s pattern-reversing checkerboard stimulus trains having a reversal frequency of 2 Hz. We found that the area-under-the-curves (\Upsigma)(\Upsigma) of HbO₂ and HbR were linearly correlated with the stimulus train duration and with the \UpsigmaVEP\Upsigma\hbox{VEP} summed over the 3, 6, and 12 s stimulus train durations. The correlation was stronger between the \UpsigmaHbO₂\Upsigma\hbox{HbO}_2 or the \UpsigmaHbR\Upsigma\hbox{HbR} and the \UpsigmaVEP\Upsigma\hbox{VEP} than between the \UpsigmaHbO₂\Upsigma\hbox{HbO}_2 or the \UpsigmaHbR\Upsigma\hbox{HbR} and the stimulus train duration. The \UpsigmaVEPs\Upsigma\hbox{VEP}\hbox{s} explained 55% of the \UpsigmaHbO₂\Upsigma\hbox{HbO}_2 and 74% of the \UpsigmaHbR\Upsigma\hbox{HbR} variance, whereas the stimulus train duration explained only 45% of the \UpsigmaHbO₂\Upsigma\hbox{HbO}_2 and 51% of the \UpsigmaHbR\Upsigma\hbox{HbR} variance. We used \Upsigma\Upsigma of the NIRS responses and VEPs because we wanted to incorporate all possible processes (e.g., attention, habituation, etc.) affecting the responses. The results indicate that the relationship between brain HDRs and VEPs is approximately linear for 3–12 s long stimulus trains consisting of checkerboard patterns reversing at 2 Hz. To interpret hemodynamic responses, the measurement of evoked potentials is beneficial compared to the use of indirect parameters such as the stimulus duration. In addition, interindividual differences in the HbO₂ and HbR responses may be partly explained with differences in the VEPs.     

Modulation of visual cortical excitability in migraine with aura: effects of 1 Hz repetitive transcranial magnetic stimulation

Recent studies showed hyperexcitability of the occipital cortex in subjects affected by migraine with aura. It has been shown that 1 Hz repetitive transcranial magnetic stimulation (rTMS) reduces excitability of visual cortex in normal subjects. The aim of the study was to investigate the effects of low frequency (1 Hz) rTMS on visual cortical excitability by measuring changes in phosphene threshold (PT) in subjects with migraine with aura. Thirteen patients with migraine with aura and 15 healthy controls were examined. Using a standardized transcranial magnetic stimulation protocol of the occipital cortex, we assessed the PT (the lowest magnetic stimulation intensity at which subjects just perceived phosphenes) before and after a 1-Hz rTMS train delivered at PT intensity for 15 min. The difference in the proportion of subjects reporting phosphenes in migrainer and control groups was significant (migrainers: 100% vs controls 47%; P<0.05), and 1 Hz rTMS over the occipital cortex led to a significantly increased visual cortex excitability expressed as a decrease in PT in subjects affected by migraine with aura. Conversely, after a 1-Hz TMS train normal subjects showed increased PT values, which suggests a decreased visual cortex excitability. Our findings confirm that the visual cortex is hyperexcitable in migrainers and suggest a failure of inhibitory circuits, which are unable to be upregulated by low frequency rTMS.     

稳态视觉诱发电位频率响应特性研究

目的稳态视觉诱发电位(steady-state visual evoked potential,SSVEP)是大脑对周期性视觉刺激产生的响应,已广泛应用于基于脑电(electroencephalogram,EEG)的脑-机接口(brain-computer interface,BCI)。SSVEP频率响应曲线通常是以发光二极管(light emitting diode,LED)作为视觉刺激器的方式获得的。近年来,计算机显示器广泛用于产生闪烁刺激,然而基于计算机显示器的SSVEP频率响应曲线少有研究。为此,本文研究了基于计算机显示器的SSVEP频率响应特性。方法利用采样正弦编码方法在普通LCD显示器上产生了42个刺激频率(频率范围4~45 Hz),并收集了10位健康受试者的脑电数据,以研究SSVEP幅值/信噪比(signal-to-noise ratio,SNR)与刺激频率的关系。结果较强SSVEP响应出现在大脑枕区。SSVEP基频幅值的峰值出现在10 Hz处,且第二峰值出现在20 Hz处。SSVEP二次谐波幅值的峰值出现在6 Hz且在高刺激频率处幅值较小。低、中频段的SSVEP基频信噪比处于相当的水平。结论本文的实验结果可以为基于计算机显示器的SSVEP-BCIs的频率选择提供依据。     

视听觉刺激下大脑头皮电位空间变化特性

目的:脑机接口（BCI）技术可以为肢体残障人士提供一种新的交流方式,在医疗康复领域具有很好的应用前景。目前单一视觉刺激的BCI系统难以适用于实际场合中多感觉信息输入的情况,需要进一步了解听觉刺激对视觉诱发电位的影响,为视听混合刺激下的BCI技术研究提供依据。 方法:在闪光刺激为12和42 Hz条件下分别加入12和42 Hz的听觉刺激,研究听觉刺激的加入对视觉刺激下大脑头表额、枕、中央、顶、颞5个空间点脑电功率的影响。 结果:视听脉冲同时刺激条件下,枕区脑电功率最大,其余空间点的功率随测试点到枕区距离的增加而减少;与单一视觉刺激下空间某点的脑电功率相比,听觉刺激的加入对该点脑电功率起增强还是抑制作用,主要取决于该点的空间位置。 结论:研究结果为听觉刺激和视觉刺激在BCI中的整合及多模态脑机接口的研究提供有意义的实验依据。     

Pattern reversal visual evoked potentials in fencers.

Visual evoked potentials were recorded from occipital and temporal leads in the two cerebral hemispheres of eight fencers and eight control subjects. The stimulus was a checkerboard subtending a small (1 degree) or large (30 degrees) visual field. Significant differences in P60, N75 and P100 latency and amplitude were found between the two subject groups, especially during the processing of the large visual field. In fencers and left-handers shorter latencies were found for the large visual field condition, whereas right-handers showed an opposite trend. The results give further evidence of special patterns of visual processing in athletes, like fencers, in agreement with the literature.