Flicker masking in spatial vision

The effects of uniform field masking at a 6 Hz temporal rate were studied on visual reaction times, response persistence and contrast sensitivity for sinusoidal gratings. The differences in performance between unmasked and masked conditions allowed an analysis of the temporal response properties of spatial frequency channels in human vision. The results of the experiments showed that (a) the temporal sensitivity of high and low spatial frequency channels differs markedly, so that rapid flicker selectively masks low spatial frequencies with little effect on high spatial frequencies; (b) the shift in the distribution of reaction times from a biphasic to a monotonie distribution during masking may be explained by assuming a dichotomy in the response characteristics of transient and sustained mechanisms; (c) transient channels' reponse persistence is shorter than that of sustained channels, and (d) whereas sustained channels preferably respond to the pattern component of gratings, transient channels preferably respond to the transient (on, off) or flicker component; (e) there appears to be considerable overlap in the spatial frequency response characteristics of sustained and transient channels.     

闪烁光ERG和闪烁光VEP对玻璃体混浊患者的视功能评价

对34例玻璃烽混浊患者术前30Hz闪烁光ERG、10HZ闪烁光VEP和30Hz闪烁光VEP的结果，使用离散博里叶分析法，提取波形特征值振幅和相位，与术后视力进行比较．多元回归分析表明：ERG与VEP的振幅都与术后视力呈正相关，复合相关系数r=0.718(P＝0.0076)；相位则无相关。联合应用ERG和VEP检测时术后视力有较好的预测作用。 （中华眼底病杂志，1995，11：159-161）     

Acrylamide effects on the macaque visual system. I. Psychophysics and electrophysiology

Oral acrylamide produces axonal swelling and later degeneration and gliosis in the distal optic tract, especially within the lateral geniculate nucleus, of macaque monkeys. Measures of visual thresholds and cortical-evoked potentials were used to study the time course of visual changes during exposure to acrylamide in macaque monkeys. Contrast sensitivity, visual acuity, and flicker fusion frequency were reduced during exposure, and only flicker fusion recovered rapidly and completely after exposure. Pattern-reversal-evoked responses exhibited increased latency and reduced amplitude during dosing but substantially recovered after exposure. Visual acuity and contrast sensitivity for high spatial frequencies were decreased throughout the 140 days of testing after dosing. These results suggest an acute general depression of visual capacities as the initial effect of acrylamide exposure, whereas later effects were confined to high spatial frequencies.     

弱视儿童瞬态图形视觉诱发电位视力与国际标准视力的比较

目的研究弱视儿童瞬态图形视觉诱发电位（pattern visual evoked potential．PVEP）视力与国际标准视力表视力之间的相关性．探讨用视觉诱发电位技术检测弱视儿童客观视力的可行性及临床实用性。方法检测32例（64眼）3-11岁的弱视患儿的瞬态图形视觉诱发电位视力（PVEP视力）及国际标准视力表视力并进行相关分析。PVEP检测使用美国LKC公司生产的UTAS—E3000电生理诊断系统。使用5个不同空间频率的水平条栅作为刺激,时间频率为2Hz,将能够观察到典型PVEP波形的最高空间频率作为患儿的PVEP视力。结果瞬态图形视觉诱发电位视力与国际标准视力表视力之间有良好的相关性（r=0．712,P〈0．05）;弱视程度不同．瞬态图形视觉诱发电位视力的分布亦有显著差异：国际标准视力表视力与瞬态图形视觉诱发电位视力存在一定数值对应规律。重度弱视组,PVEP视力100％在0．85cpd（最低空间频率）;中度弱视组,PVEP视力分布相对较分散,集中分布在3．40cpd和6．80cpd,无1眼达13．60cpd;轻度弱视组．PVEP视力集中分布在3．40cpd和6．80cpd两个空间频率．其中达6．80cpd者占71．2％;视力≥0．9组．全部PVEP视力≥6．80cpd。四组PVEP视力呈相对集中分布。随国际标准视力提高．PVEP视力也相应提高。结论在无法使用国际标准视力表检查弱视儿童视力时,可以运用瞬态图形视觉诱发电位技术进行客观视力评估．尤其是低龄弱视儿童。     

Apparent fineness of briefly presented gratings: balance between movement and pattern channels.

Gratings presented for 20–60 msec look finer (up to 1–3 times) than when stationary; moreover, coarse gratings reversed in contrast at a rate above 8 Hz appear to double their spatial frequency. The dependence of these “magnification” phenomena on presentation time, contrast and a range of spatial frequency was examined by matching the spatial frequencies of the intermittently and continuously presented gratings.The detection of movement (or flicker) was found to be a necessary condition for both the phenomena to occur. The increase in apparent spatial frequency may reach a factor of two only when the recognition threshold for a grating of doubled spatial frequency is substantially lower than for the fundamental pattern (this occurred for gratings below 1.5 c/deg at a reversal rate of 9 Hz).These observations are consistent with the Reichardt-Rashbass models of movement and flicker detection which suggested a “multiplication” of the input signals or “squaring”. It is concluded that the resulting appearance of patterns depends on some integration of signals from movement and pattern detectors.     

Balance between pattern and flicker sensitivities in the visual fields of ophthalmological patients.

We measured the balance between visual sensitivities to pattern and to flicker rather than measuring absolute sensitivities to pattern or flicker. The test target was a 2-cycle deg-1 sinewave grating that was counterphase modulated at 8 Hz. Seventeen points in the visual field were tested out to eccentricities of 24 degrees. We examined 10 control subjects, 6 patients with glaucoma 10 with ocular hypertension, and 10 with multiple sclerosis. For controls pattern sensitivity was lower than flicker sensitivity in central vision. The converse held in peripheral vision. The balance between pattern sensitivity and flicker sensitivity was markedly abnormal in part or all of the visual field for many patients. There were examples in all patient groups. In some patients flicker sensitivity was depressed relative to pattern. In others the converse was true. Of 10 patients with ocular hypertension and no perimetric field loss 8 had a significantly abnormal ratio between pattern sensitivity and flicker sensitivity at some point in the visual field. The balance between pattern and flicker sensitivity was more sensitive to visual pathology than absolute sensitivity to either pattern or flicker. We conclude that the relationship between pattern and flicker sensitivity may be more sensitive to visual field damage than is conventional perimetry or visual acuity perimetry.     

The pattern electroretinogram by skin electrodes: Effect of spatial frequency and age

Steady-state (8 Hz) pattern electroretinograms in response to sinusoidal gratings were recorded from 18 elder subjects (mean age 62.7 years) and 22 young subjects (mean age 27 years) by means of skin electrodes. All subjects were free from ocular pathology and their corrected visual acuity was 1.0 or better. In young subjects the second harmonic amplitude as a function of spatial frequency is spatially tuned, with a maximum at around 1.7 c/deg. The response of elder subjects is significantly (P < 0.05) reduced over the whole range of spatial frequency as compared with that of younger ones. This decrease is more marked (P < 0.01) at intermediate spatial frequencies, resulting in a shallower tuning. The response phase is not significantly different between young and elder subjects.     

Testing for glaucoma with the frequency-doubling illusion in the whole, macular and eccentric visual fields.

PURPOSE: The frequency doubling (FD) illusion is the basis for new diagnostic methods for glaucoma. The FD illusion is seen when low spatial frequency grating patterns are contrast modulated at high rates. The present experiments examined which spatial frequencies might be optimal and whether high flicker rates are required. METHODS: We determined contrast thresholds for the following: W1, a wide-field 0.25 c/deg grating at 27 Hz contrast reversal; W2, as W1 but no flicker; MAC, 27 Hz, 4 c/deg grating presented to the central 4 degrees; and E1 to E7, seven spatial frequencies in the range 0.063-0.813 c/deg, 27 Hz, presented in a 5 degrees aperture at 15 degrees (nasal) eccentricity. RESULTS: W1 was the best predictor of glaucoma. Of the eccentrically presented stimuli, E6 (0.688 c/deg) was the best predictor of glaucoma while the lower spatial frequencies performed less well. Only MAC was significantly age-dependent.     

Two distinct processes are evident in rat cone flicker ERG responses at low and high temporal frequencies

The electroretinogram (ERG) provides a noninvasive, objective measure of retinal function, and is one of the most widely used diagnostic tools in the study of visual disorders. Although rodents are often used in the study of retinal disease, the properties of the flicker ERG of the rodent retina have not been fully characterized. Here, we show that the fundamental response of the rat ERG to sine-wave flicker exhibited a low-pass pattern in the frequency range from 2 to 30 Hz, whereas the second harmonic (F2) showed a more complex frequency-response relation. The F2 component represented only a small fraction of the ERG response at low temporal frequencies (below 12 Hz), but it made a substantial contribution to responses at high frequencies. The contrast-response relation was linear when tested with a low-frequency (6 Hz) stimulus, but saturated in response to a high-frequency (20 Hz) stimulus. After intravitreal injection of L-AP4, a specific blocker of the retinal ON pathway, the flicker responses elicited by either 6- or 20-Hz stimuli were greatly reduced in amplitude, whereas only a very slight enhancement was seen after the application of PDA, a drug that blocks retinal OFF-pathway activity. Based on the observed differences in the degree of nonlinearity, and contrast-response properties of the rat flicker ERG at low and high frequencies, as well as the pharmacological results, we postulate that sustained and transient ON bipolar cells generate the flicker ERG responses elicited at low and high temporal frequencies, respectively.     

扫描视觉诱发电位（SVEP）对成年野生型C57BL/6J小鼠客观视力评估的可靠性

目的　评价扫描视觉诱发电位（sweep visual evoked potential,SVEP）对成年野生型C57BL/6J小鼠客观视力评估的可靠性。方法　选取6只鼠龄32~33 d的正常野生型C57BL/6J小鼠,雌雄不限,麻醉小鼠将记录电极 （2 mm直径的钨电极） 放置在λ缝旁3 mm的左皮质（受刺激右眼的对侧）,深度为皮层下400 μm,额部的针状电极作为参考电极,尾部的针状电极作为接地电极,记录过程中遮盖左眼（未受刺激眼）,屏幕放置在小鼠眼前20 cm处,视野范围为100°（水平）×82°（垂直）,平均亮度为25 cd·m^-2;采用垂直方向的正弦波光栅刺激,刺激对比度为100%,包括11个空间频率,时间频率为1 Hz;整个扫描过程需要12 s（11 s空间频率+ 1 s预适应）,数据的误差值稳定后取其平均值。分别记录左右眼对应的视皮层VEP波形。不同刺激频率给予20次刺激,每眼重复测量3~4次,取其平均值,左右眼交替试验。以空间频率对数作为x轴,经离散傅里叶分析所得反应振幅为y轴建立坐标系,选取临近噪点的4个有效测定点行线性回归,回归曲线外推法至振幅为0处的空间频率即为推断的客观视力。结果　空间频率为0.05 c·deg^-1时,SVEP提示N100波振幅达到峰值。随着逐步增加刺激空间频率,SVEP振幅降低﹑潜伏期延迟,空间频率为0.65 c·deg^-1时视皮层电反应几乎无法与噪点区分;6只小鼠右眼空间频率视力为（0.56±0.04）c·deg^-1,左眼为（0.50±0.04）c·deg^-1,两眼空间频率视力比较差异无统计学意义（P>0.05）;12眼空间频率视力为（0.53±0.03）c·deg^-1。结论　可以使用SVEP评估小鼠客观视力,此方法与行为方法测得的视力相近,但SVEP评估小鼠视力结果更为客观精确,所需时间更短。     

Fourier-analysed Steady-state VEPs in Pre-school Children with and without Normal Binocularity

Pre-school children aged 4–5 were examined with steady-state VEP in response to a sinusoidal grating pattern with a spatial frequency of 4 c/deg, reversing at rates 5, 10, and 15 Hz. Normal children (n = 10) were compared with subjects lacking stereo perception (n = 6) and with subjects showing significant unilateral amblyopia with visual acuity in the worse eye <0.5 (n = 7). Fast Fourier Transform was used for analysis of the crude steady-state VEP responses. Compared to normals, the subjects lacking stereo perception showed a significantly lower power of the second harmonic in the response evoked by binocular stimulation with gratings reversed at 15 Hz. The amblyopic group showed a significant difference between the dominant and the non-dominant eye regarding the first harmonic power in the responses evoked by gratings reversed at 5 and 10 Hz. These findings are discussed in relation to the magnocellular and parvocellular visual pathways and suggested models for linear and non-linear processing of visual signals.     

Accommodation and flicker: evidence of a role for temporal cues in accommodation control?

Static accommodation responses to sinusoidal grating stimuli that displayed temporal modulations in luminance contrast (i.e. contrast flicker) were measured with a laser speckle optometer. The effects of a variety of temporal waveforms were investigated including square-wave modulations, sinusoidal modulations, and band-pass filtered noise. The effects of altering both the amplitude and the temporal frequency (0.4-30 Hz) of the contrast flicker and the spatial frequency of the stimulus (0.77-9.2 c/deg) were also examined. All the flicker waveforms investigated (square wave, sinusoidal and band-pass noise) reduced accommodative accuracy, the effect being most apparent at lower spatial frequencies (0.77-1.15 c/deg). With band-pass filtered noise the effects of flicker were most apparent with frequencies in the range 1-4 Hz, at both lower (0.4 Hz) and higher flicker frequencies (8-32 Hz) accommodations was less affected. It was found that flicker impaired accommodation under conditions where the contrast was at all times suprathreshold. This is incompatible with the proposal that flicker reduces accommodation responses because for part of each flicker cycle the stimulus was below threshold. However, these results are compatible with the alternative hypothesis that flicker impairs the ability of the accommodation system to utilize temporal cues such as those derived from the higher frequency component (1-2 Hz) of accommodative oscillations.     

Ocular refraction and visual contrast sensivity of the rabbit,determined by the VECP

The ocular refraction of the rabbit was determined by inserting spherical lenses of between ?3 and ?4D and measuring the relative VECP amplitude in response to checkerboard patterns of 21 min arc. The rabbit's eye was found to be almost emmetropic (+0.5 D). The contrast sensitivity function of the rabbit's visual system was determined by recording the VECP in response to reversed gratings of different spatial frequencies. The contrast sensitivity was found to be highest at 0.35c/deg with an upper cut-off frequency at 3c/deg. corresponding to a grating acuity of 10 min arc.     

Non-linearities in the focal ERG evoked by pattern and uniform-field stimulation. Their variation in retinal and optic nerve dysfunction

The amplitude of the second harmonic of the focal electroretinogram (ERG) in response to either modulation of the luminance of the uniform-field or the spatial contrast of a patterned field (pattern-reversal ERG) was measured in a group of normal subjects as well as in patients with two different types of unilateral dysfunctions, namely optic atrophy or temporary retinal ischemia. Such patients had a reduced visual acuity in their affected eyes but normal full-field flicker (20 Hz) ERG. In normal eyes, for the same stimulation area and modulation depth, the second harmonic of the uniform-field response is smaller (mean value 62%) than that of the optimal pattern (around 1.5 cycles/degree). The results on patients show that the second harmonic of the pattern response, but not that of the uniform-field response, is reduced in cases of optic atrophy secondary to trauma or optic neuritis. This result suggests generators different, at least in part, for the second harmonic evoked by modulation of either luminance or spatial contrast. By contrast, both responses are reduced in cases of temporary retinal ischemia. These findings are discussed in light of the recent literature on the origin of the pattern ERG. The possible clinical applications of the technique are outlined.     

Inner-retinal contributions to the photopic sinusoidal flicker electroretinogram of macaques. Macaque photopic sinusoidal flicker ERG

The purpose of this study was to examine inner-retinal contributions to the photopic sinusoidal flicker ERG. ERGs were recorded from 5 anesthetized monkeys to sinusoidally modulated (100%, 0.5–120 Hz) red full field flicker at L_mean of 3.2 log phot td on a rod saturating blue background (3.7 log scot td; 3.0 log phot td) before and after intravitreal injections of tetrodotoxin (TTX) to block Na⁺-dependent spikes of retinal ganglion and amacrine cells, followed by N-methyl-D-aspartate (NMDLA) to suppress all activity of these cells. Recordings also were made after blocking bipolar (and horizontal) cell responses with L-2-amino-4-phosphonobutyric acid (APB) and 2-cis-piperidine-2,3-dicarboxylic acid (PDA) or 6-cyano-nitroquinoxaline-2,3-dione (CNQX). Control fundamental (F1) and second harmonic (F2) amplitudes were large and variable at temporal frequencies up to 2 Hz. At higher frequencies, F1 amplitude was minimal with a phase step at a frequency between 13 and 19 Hz and maximal at 27–33 Hz. F2 was minimal at 2–3 Hz and maximal at 6–8 Hz, again with a phase step near the minimum. TTX, or NMDLA, produced small changes in F1 that shifted the amplitude minimum to a lower and the maximum to a higher frequency. In contrast, F2 was more strongly affected; both the amplitude minimum (and phase step) and maximum were greatly attenuated, leaving a moderate response from 0.5 to 8 Hz, which then declined as frequency was increased to 30 HZ. After APB and PDA or CNQX, F1 decreased continuously with increasing frequency and F2 was generally much smaller. The nearly linear F1 phase plot was consistent with the presence of a single mechanism (i.e. photoreceptors). Inner-retinal neurons contribute to the photopic sinusoidal flicker ERG. Whereas for F1, inner-retinal contributions are small relative to those from bipolar cells; for F2, they are equal or greater between 2 and 16 Hz.     

Accommodation responses to flickering stimuli

It has been suggested that accommodation to visual display unit (VDU) symbols or to stimuli lit by fluorescent tubes or similar temporally modulated light sources may be less accurate due to flicker. Furthermore, if the microfluctuations of accommodation play a part in the control of steady-state accommodation, the effects of stimulus flicker might affect the quality of this control. Experiments are described in which monocular accommodation stimulus-response curves for sinusoidal grating stimuli (1.0, 5.3 and 8.9 c/deg) were measured under the following conditions of temporal modulation: (1) square-wave on-off modulation to give spatial modulation varying between 0 and 37%; (2) sinusoidal modulation to give spatial modulation varying between 0 and 37%; (3) sinusoidal modulation to give spatial modulation varying between 16 and 32%; and (4) on-off modulation with varying mark/space ratio. Temporal modulation frequencies were between 0.5 and 7.8 Hz and above the critical fusion frequency (CFF) i.e. 40 Hz. In general, the results suggest that while lower-frequency flicker may adversely affect the accuracy and stability of the accommodation response, the latter are very little affected by flicker at frequencies ≥ 40 Hz. Thus flicker from fluorescent lamps or VDUs is unlikely to cause systematic accommodation difficulties. The possible relevance of the results to theories which ascribe a role for the higher-frequency (> 0.5 Hz) accommodative microfluctuations in the control of the response is considered.     

Objective measurement of contrast sensitivity function using contrast sweep visual evoked responses

AIM/BACKGROUND—The contrast sensitivity function (CSF) measurement provides information that is not accessible by standard visual acuity determinations. The contrast sweep pattern reversal visual evoked responses (CSVER) technique was used to objectively measure the CSF in clinical practice.
METHODS—The contrast thresholds were measured at five spatial frequencies in 10 normal subjects. The CSVER were recorded with sinusoidally modulated vertical gratings at 10 contrast levels (96, 64, 48, 32, 16, 8, 4, 2, 1, and 0.5%) presented in five spatial frequencies (0.5, 1.0, 2.0, 4.0, and 8.0 cycles per degree). Each of 10 contrast levels was displayed for 2 seconds at the desired spatial frequency. The CSVER amplitudes at the second harmonic were calculated by discrete Fourier transform. The results were compared with those obtained using a psychophysical method.
RESULTS—An inverted U-shaped CSF which peaked at 2.0 cycles per degree with a contrast sensitivity of 34.5 (contrast, 2.9%) was observed. The CSF assessed electrophysiologically was 0.62 to 0.79 log units lower than the sensitivity measured using the psychophysical method. However, the overall shapes were highly correlated.
CONCLUSION—One can objectively measure CSF with CSVER and this may be useful in patients in whom the psychophysical method is limited.

Keywords: contrast sensitivity function; electrophysiology; psychophysics; visual evoked potential     

Does visual flicker phase at gamma frequency modulate neural signal propagation and stimulus selection?

Oscillatory synchronization of neuronal populations has been proposed to play a role in perceptual integration and attentional processing. However, some conflicting evidence has been found with respect to its causal relevance for sensory processing, particularly when using flickering visual stimuli with the aim of driving oscillations. We tested psychophysically whether the relative phase of gamma frequency flicker (60 Hz) between stimuli modulates well-known facilitatory lateral interactions between collinear Gabor patches (Experiment 1) or crowding of a peripheral target by irrelevant distractors (Experiment 2). Experiment 1 assessed the impact of suprathreshold Gabor flankers on detection of a near-threshold central Gabor target (“Lateral interactions paradigm”). The flanking stimuli could flicker either in phase or in anti-phase with each other. The typical facilitation of target detection was found with collinear flankers, but this was unaffected by flicker phase. Experiment 2 employed a “crowding” paradigm, where orientation discrimination of a peripheral target Gabor patch is disrupted when surrounded by irrelevant distractors. We found the usual crowding effect, which declined with spatial separation, but this was unaffected by relative flicker phase between target and distractors at all separations. These results imply that externally driven manipulations of gamma frequency phase cannot modulate perceptual integration in vision.     

Nonlinearities in binocular visual evoked potentials in children

Evidence is controversial in respect to the optimal conditions in which visual evoked potentials provide an objective measure of binocular visual function, related and unrelated to stereopsis, and there is little emphasis on the type of stimulus that produces facilitation in binocular recording. We investigated the effects of stimulus type (flicker or pattern), contrast, and temporal modulation on facilitation, which was defined as a binocular response greater than sum of monocular responses. Monocular and binocular responses to sinusoidally modulated flicker and grating patterns were recorded in children and Fourier analyzed. The relationship of the fundamental Fourier component for flicker and the second harmonic component for pattern were each examined as function of temporal modulation at two levels of contrast for monocular and binocular visual evoked potentials. Binocular facilitation was found across all conditions for flicker. Data suggest that processing of pattern and flicker has different sites of origin within the visual system. Facilitation in binocular visual evoked potentials also indicates that they are not a result of simple summation of monocular responses, since there appears a nonlinear component to such interaction.     

ON-pathway dysfunction and timing properties of the flicker ERG in carriers of X-linked retinitis pigmentosa

PURPOSE: Carriers of X-linked retinitis pigmentosa (XLRP) frequently show prolonged implicit times of the flicker electroretinogram (ERG). This study tested the hypothesis that a preferential response attenuation within the cone depolarizing (ON) bipolar cell (DBC) pathway is a major contributing factor. METHODS: Light-adapted, full-field ERGs were recorded from 10 XLRP carriers and 12 visually normal control subjects. Fundamental amplitudes and phases of ERG responses to sinusoidally flickering stimuli at temporal frequencies ranging from 8 to 96 Hz were analyzed within the framework of a recent vector summation model of the cone system ERG to test for evidence of a response attenuation within the DBC pathway. In addition, ERG responses to sawtooth flicker were examined for a reduced b- to d-wave amplitude ratio, indicative of ON pathway dysfunction. RESULTS: The carriers' fundamental response phases at 32 Hz correlated significantly with their log ratios of response amplitudes at 32 versus 12 Hz (r = 0.89, P < 0.001) and with their log b- to d-wave amplitude ratios (r = 0.71, P < 0.05), both of which were used as indices of response attenuation within the DBC pathway. A control experiment demonstrated that a reduced sensitivity of cone phototransduction made at most only a minimal contribution to the timing changes in the carriers' flicker ERG responses. CONCLUSIONS: The overall pattern of results indicates that a preferential response attenuation within the DBC pathway is the primary source of timing changes in the flicker ERGs of these carriers of XLRP. These findings illustrate the value of analyzing ERG responses to flickering stimuli at multiple temporal frequencies to evaluate mechanisms of disease action in photoreceptor degenerations.