Comparison of VEP with contrast sensitivity and other measurements of central visual function

Purpose: In order to evaluate alternative visual acuity testing techniques, especially to discriminate between small changes and for high visual acuity, we conducted a study covering several state‐of‐the‐art techniques. Methods: In this cross‐sectional study, a homogeneous cohort of healthy and young patients (n = 33; 66 eyes) underwent ETDRS vision acuity (VA) testing, testing for contrast sensitivity (CS), VA determination with spatial frequency sweep visual evoked potentials (VEP) and a series of examinations of perifoveal retinal nerve fibre layer thickness (RNFLT) using Spectralis SD‐OCT. To simulate the effect of artificial media opacity, CS, and VEP were repeated with Bangerter foils. Results: We found that Bangerter foils can be used to reduce VA effectively measured by VA testing and VEP VA. CS correlated significantly with VA (correlation coefficients ranging from 0.54 to 0.77). VEP may be used to estimate VA; nevertheless, we found no significant correlation. RNFLT did not correlate significantly with VA. Conclusion: CS seems to correlate well with VA when used for high VA. All other used examinations seem to have difficulties distinguishing between small differences in VA or when the VA is high.     

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     

Measurement of contrast sensitivity function using pattern-reversal visual evoked responses

In order to determine whether pattern-reversal visual evoked response (VER) can be used to measure contrast sensitivity function (CSF), we investigated the effect of change of contrast upon pattern-reversal VER. Contrast thresholds for VER were extrapolated in five spatial frequencies. The CSF curve obtained from the VER showed the inverted U-shape as the psychophysical CSF curve. However, the low frequency fall-off that is usually seen in psychophysical CSF was less evident in the electrophysiological CSF. The difference between the electrophysiological CSF and the psychophysical CSF increased along with the spatial frequency. Proper stimulus conditions would make this method of evaluation of CSF by pattern reversal VER useful in relatively young children in whom psychophysical tests cannot be performed.This study was supported in part by the Massachusetts Lions Eye Research Fund     

Assessment of contrast sensitivity of patients with macular disease using reduced contrast near visual acuity charts

A set of near Bailey-Lovie logMAR letter charts that varied in contrast from 0.40 dB (C = 0.95) to 58.0 dB (C = 0.001) were used to measure the middle and high spatial frequency range of the contrast sensitivity function (CSF) of 15 subjects with age-related maculopathy (ARM) and 15 age-matched normal subjects. The letter charts were shown to have good test-retest reliability. Compared with measures made using CRT generated square wave gratings of variable contrast and spatial frequency they were also shown to provide a valid measure of CSF in the mid to high spatial frequency range. The 20.0 dB letter chart alone was shown to be a good screening device for macular disease. The letter charts do not provide a measure of the peak of the CSF and a supplementary test of contrast sensitivity is needed to quantify contrast sensitivity at a low spatial frequency. Three measures are proposed as necessary to document foveal visual capability of patients with macular disease: distance logMAR visual acuity to measure high spatial frequency resolution, visual acuity with letter charts of 20 dB contrast to assess mid spatial frequency resolution; and contrast sensitivity for the detection of an edge to estimate contrast sensitivity for larger objects.     

Effects of sweep VEP parameters on visual acuity and contrast thresholds in children and adults

Background  

There are many parameters that may impact the thresholds obtained with sweep visually evoked potentials (sVEP), yet a number of these parameters have not been systematically studied, and there is no recognised standard for sVEP recording. In this study, the effects of electrode placement, temporal frequency, sweep direction, presence of a fixation target, stimulus area, and sweep duration on visual acuity (VA) and contrast thresholds of the sVEP were investigated. Additionally, the effect of these parameters on the number of viable threshold readings obtained from five active electrodes was investigated.     

Effects of yellow filters on visual acuity,contrast sensitivity and reading under conditions of forward light scatter

Background  

Yellow filters are sometimes recommended to people with low vision. Our aim was investigate the effects of three commercial yellow filters on visual acuity and contrast sensitivity (with and without glare) and reading (without glare) under conditions of forward light scatter (FLS).     

Sensitivity and specificity of the step VEP in suspected functional visual acuity loss

Background/aim

Early and accurate diagnosis of functional visual loss (FVL) allows optimum management. Visual evoked potentials (VEPs) offer a means of objectively estimating acuity and therefore could assist with early and accurate diagnosis. The aim of this study was to assess the sensitivity and specificity of the step VEP in diagnosing FVL.

Methods

A retrospective audit was conducted in 36 school-aged children presenting with reduced visual acuity and clinical suspicion of FVL. All had undergone step VEP testing as part of their investigation. Medical notes were reviewed, and where necessary, referring centres, general practitioners or electronic clinical portals were consulted to obtain longer-term outcome data.

Results

Twenty-seven of the 36 patients (75 %) were classified as having had FVL: all had a normal step VEP spatial threshold. Nine patients (25 %) had an organic cause for their acuity loss, of whom seven had abnormal step VEP spatial thresholds: the other two patients had some functional overlay to their organic disease. The step VEP sensitivity was 78 % (95 % confidence interval 40–96 %), and specificity was 100 % (95 % confidence interval 84–100 %).

Conclusion

The high specificity of the step VEP for FVL warrants increased suspicion of an organic cause should the step VEP spatial threshold be abnormal.     

视觉对比敏感度下降而视力正常175例中年患者4年随访观察

目的　通过对视觉对比敏感度(contrast sensitivity,CS)下降而视力正常的中年患者进行连续4 a的随访观察，探讨视功能受损患者多种眼病的发病趋势。方法　采用队列研究设计，纳入2014年9至12月在沧州市中心医院体检的CS下降而视力正常的中年患者175例175眼（40~58岁）作为观察组；纳入同期年龄匹配的无CS下降且视力正常体检者167人167眼（40~59岁）作为对照组。对纳入的受检者每年进行CS、最佳矫正视力、直接检眼镜以及光学相干断层扫描(optical coherence tomography，OCT)检测，小部分患者行荧光素眼底血管造影（fundus fluorescein angiography,FFA）检查，连续观察4 a，共实施5次检查，两组受检者均以较差CS值眼纳入研究，评估患者视网膜厚度的动态变化情况，观察特发性黄斑前膜（idiopathic epiretinal membrane,IERM）、白内障、视网膜微血管病变以及CS、视力下降趋势。结果　在连续4 a的动态监测中，观察组视盘全周、视盘上1/4象限视网膜神经纤维层（retinal nerve fiber layer,RNFL）厚度呈进行性下降趋势（均为P<0.01）；观察组2018年视盘全周RNFL厚度（96.335±9.374）μm、视盘上1/4象限RNFL厚度（120.526±9.211）μm均明显低于2014年（102.444±9.351）μm和（124.124±9.178）μm，差异均有统计学意义（均为P<0.01）；经趋势性检验，观察组IERM、白内障、视网膜微血管病变的患病率呈明显上升趋势（χ²=29.267、6.507、4.884，均为P<0.01）,对照组此趋势变化不明显(χ²=3.435、1.404、3.145，均为P>0.05)；观察组、对照组均表现为明显的CS和视力下降趋势（χ²=89.554、17.181,均为P<0.01；χ²=85.970、8.677，均为P<0.01）。结论　CS下降的中年人群虽大部分视力正常，但在随后数年中呈视神经纤维层进行性变薄和多种眼底疾病发病率上升趋势，该人群是多种眼底疾病的高危人群，CS和OCT检查是适合眼科体检筛诊工作的易行、有效手段。     

Behavior of visual acuity, visual fields and contrast sensitivity in simulated cataract

Visual acuity, contrast sensitivity, and visual fields were examined with and without occluders both in a group of healthy subjects and in a group of patients with field defects (mostly caused by glaucoma). The various results were compared with one another as well as with the results obtained from a group of cataract patients examined before and after surgery. The results confirm clinical experience: visual acuity enables some conclusions to be drawn on the degree and progression of opacity of a lens. The changes in contrast sensitivity and in the visual field, however, represent rather the quality of the visual degradation. Contrast sensitivity and diffuse alterations in the visual field have similar progression patterns. The alterations in contrast sensitivity are more pronounced at higher frequency levels. The contrast sensitivity test can be recommended as a simple and complementary functional test, particularly on cataract patients with severe complaints and relatively good visual acuity. The results of occlusive experiments afford some practical conclusions which enhance knowledge and differentiation of visual field changes induced by cataract and glaucoma. These conclusions can provide a basis for further and more detailed investigations.     

Correlation of spatial contrast sensitivity and visual fields in glaucoma

Perimetry is the standard examination in glaucoma, whereas contrast sensitivity is often proposed as a complementary test. This study investigates the extent to which the outcomes of the two tests correlate. One eye each of 44 glaucoma patients and 16 glaucoma suspects was tested with the Haag-Streit Visometer and the Octopus 201 perimeter (program G1). Contrast sensitivities correlated highest with the center of the visual field (Spearman's rho = 0.76;P < 0.001) and slightly less with the mean sensitivity of the entire visual field (rho = 0.63;P < 0.001). Mean contrast sensitivity correlated weakly with the square root of corrected loss variance (rho= –0.45;P < 0.001) and short-term fluctuation (rho = –0.34;P < 0.007). The two tests differed considerably only in very few patients. We recommend careful evaluation of the central area of glaucomatous visual fields whenever contrast sensitivity testing is not available.Mario Zulauf is a visiting research scientist at the Yale Eye Center, supported by the Florian Verry Foundation, Lausanne, Switzerland, and the Swiss National Fund.     

VEPs in humans reveal high and low spatial contrast mechanisms

The effect of contrast on visual evoked potential (VEP) amplitude was examined in nine observers. A 6.0 cycles/deg (cpd) grating was modulated in an "on-off" mode at 7.5 Hz. The VEP response contains significant first and second harmonic components: their growth with contrast is parallel, each function consisting of two limbs. The data are consistent with the hypothesis that the pattern VEP obtained with "on-off" presentation may reflect the contributions of "low" and "high" contrast neuronal populations demonstrated in physiological studies of the primate.     

Multifocal and monofocal implant. Comparative functional results (contrast sensitivity and dynamic visual acuity)]

The functional results (contrast sensitivity and dynamic visual acuity) of 19 multifocal (3 M design) and 14 monofocal intraocular (IOL) were compared. Best corrected visual acuity was > or = 8/10 P 2. Major differences of functional performance in favour of monofocal IOL's were found under standard conditions of vision (low contrast and illumination levels). Dynamic visual acuity allowed accurate evaluation of the difference of performance between these two models of implant. The indications for multifocal IOLs were reconsidered in the light of these results.     

Transfer of contrast sensitivity in linear visual networks.

Contrast sensitivity is a useful measure of the ability of an observer to distinguish contrast signals from noise. Although usually applied to human observers, contrast sensitivity can also be defined operationally for individual visual neurons. In a model linear neuron consisting of a filter and noise source, this operational measure is a function of filter gain, noise power spectrum, signal duration, and a performance criterion. This definition allows one to relate the sensitivities of linear neurons at different levels in the visual pathway. Mathematical formulae describing these relationships are derived, and the general model is applied to the specific problem of relating the sensitivities of parvocellular LGN neurons and cortical simple cells in the primate.     

Achromatic and chromatic perceived contrast are reduced in the visual periphery

The loss of contrast sensitivity with eccentricity is well documented, and is steeper for higher spatial frequencies, and for L/M cone-opponent stimuli compared to achromatic or S-cone-opponent. Here, we ask how perceived contrast depends on eccentricity when stimuli are presented at suprathreshold contrasts, and test two opposing predictions. Contrast constancy predicts no loss in perceived contrast across the visual field regardless of changes in detection threshold – appearance depends only on physical contrast. Conversely, perceived contrast may be scaled in the same way as detection threshold, reflecting the proportional increase in stimulus contrast above threshold. We measured perceived contrast for L/M cone-opponent, S-cone opponent, and Ach stimuli up to 18 degrees of eccentricity using a 2AFC contrast matching method between fovea and periphery. We tested a range of reference contrasts from low (close to detection threshold) to high suprathreshold contrasts and we relate suprathreshold perceived contrast to measured detection thresholds. We find evidence for a hybrid model in which apparent contrast is reduced with eccentricity for stimuli in the low and mid contrast range, with contrast constancy only attained at the highest contrasts. When equated for similar sensitivity losses, we find no difference between chromatic and Ach contrast responses.     

Texture segregation is processed by primary visual cortex in man and monkey. Evidence from VEP experiments.

We investigated whether the process of texture segregation can be allocated to a specific visual cortical area. We designed a stimulus to reveal the presence of a mechanism, which is specifically sensitive to a checkerboard, that is solely defined by textures segregating due to orientation differences of the constituting line segments. We recorded evoked potentials to this stimulus in man and awake monkey. A difference component, signalling texture segregation sensitivity, could be recorded from both types of subjects. Its presence depended on the spatial extent of the textures, in a manner correlating with the perceptibility of the checkerboard. This difference response could be localized in primary visual cortex by means of equivalent dipole estimations.