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     

Invariance of the perceived spatial frequency shift of peripherally viewed gratings with manipulations of contrast, duration, and luminance

Gratings appear of higher spatial frequency when they are viewed peripherally rather than foveally. To test the hypothesis that this effect is an artefact of particular laboratory conditions, we manipulated the contrast, luminance and presentation duration, manipulations which have also been shown to increase the apparent spatial frequency of foveally presented gratings. It has been argued that such shifts reflect an attempt to increase sensitivity by changing the receptive field properties of spatially tuned visual channels, while keeping their size labels constant. If so, and peripheral channels are not otherwise mislabelled, it should be possible to find conditions under which the apparent spatial frequency of peripherally viewed gratings matches that of foveal gratings of the same spatial frequency. In this study, manipulations of contrast, luminance, and duration had no effect on the size of the perceived spatial frequency shift in peripheral vision. Thus the putative inappropriate size labelling of peripheral visual channels is constant over a wide range of stimulus values. We speculate that this apparent constant error may result from a mechanism which normally compensates for another factor such as blur, which may otherwise lead to an overestimation of size.     

Contrast sensitivity of the human pattern electroretinogram

Contrast thresholds for the pattern electroretinogram (PERG) were measured using lock-in amplifier retrieval of the retinal signal and a swept contrast display. Contrast sensitivity functions (CSF) developed from these PERG contrast thresholds were compared with those established psychophysically under identical stimulus conditions. Whereas the PERG CSF showed a band-pass characteristic across temporal frequency, the psychophysical CSF (and a temporal CSF developed from visual evoked potential contrast thresholds) had a low-pass pattern. Across spatial frequency, the PERG and psychophysical CSFs had similar shapes, although the PERG CSF peaked at a lower spatial frequency than the psychophysical CSF.     

Reliability and validity of simple photographic plate tests of contrast sensitivity

Sets of edge and square wave grating photographic plates of varying contrasts were used to measure the mid to low spatial frequency range of the contrast sensitivity function (CSF) of 20 subjects with evident ocular disease and 20 age-matched normal subjects. Both plate tests were shown to have good test-retest reliability and to correlate well with electronic cathode ray tube (CRT) measures of CSF. The edge test, when administered in 2-dB steps of ascending contrast, has optimum sensitivity of 0.70 and specificity of 0.84 for detecting the patient with ocular disease with a fail criterion of less than 38 dB. A measure of edge contrast sensitivity was also shown to be a good predictor of the peak of the CSF, which is shown to be largely independent of the visual acuity of the subject. The 2 c/deg and 4 c/deg plates did not provide more information about the visual difficulties of subjects than the edge test. We advocate the use of an edge test as a simple clinical measure of low spatial frequency contrast sensitivity that provides information about visual dysfunction not provided by a measure of visual acuity.     

How does the short-wavelength-sensitive contrast sensitivity function for detection and resolution change with age in the periphery?

To determine the age-related change in the peripheral short-wavelength-sensitive (SWS) grating contrast sensitivity function (CSF), cut-off spatial frequency (acuity) and contrast sensitivity for both a detection and resolution task were measured at 8 degrees eccentricity under conditions of SWS-cone isolation for 51 subjects (19-72 years). The acuity for both the detection and resolution task declined with age, the detection acuity being significantly higher than the resolution acuity at all ages (p<.01). The CSF for both detection and resolution shifted towards lower spatial frequencies with increasing age. The contrast sensitivity for detection remained higher than that for resolution for all ages at the highest spatial frequencies. The age-related loss in the SWS resolution CSF at high spatial frequency probably reflects a loss occurring at the ganglion cells level.     

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.     

Orientation anisotropy: some caveats in interpreting group differences and developmental changes

We tested 220 children aged 7-12 yr and 20 university students for visual contrast sensitivity at spatial frequencies of 1.2-21.9 c/deg for vertical, horizontal and oblique sinusoidal gratings. Males had higher mean sensitivity then females and adults had higher mean sensitivity than children. Sensitivity was greater for cardinal than oblique orientations, and this oblique effect increased with spatial frequency and mean sensitivity, and with age in children. Developmental growth of oblique anisotropy can be explained by growth in contrast sensitivity. Different sizes of oblique effect were found between groups living in different locations, but variations were mainly linked to differences in age and contrast sensitivity. Genetic or environmental interpretations of group differences should be treated with caution. Other anisotropies occurred (horizontal was better than vertical and right than left obliques), but these differences were small and irregular and may represent response biases rather than neurological effects.     

Appearance of the frequency doubling stimulus in normal subjects and patients with glaucoma

PURPOSE: To determine whether the spatial structure of the frequency doubling technology (FDT) perimetry stimulus is visible at detection-contrast threshold in normal observers and those with glaucoma and to assess its perceived spatial frequency at threshold and suprathreshold contrast. METHODS: Three subject groups were assessed: 10 young normal observers (aged <40 years), 10 older normal observers (aged >50 years), and 10 subjects with glaucoma. Detection thresholds for centrally and eccentrically presented 10 degrees squares, 0.25-cyc/deg, 25-Hz counterphase flicker sine-wave gratings were obtained by using a yes-no staircase procedure. Eccentric locations were in areas of loss of FDT sensitivity (< or =21 degrees ) in subjects with glaucoma, or at 7 degrees or 21 degrees inferonasally in normal observers. Resolution-contrast thresholds were determined by a two-alternative, forced-choice staircase procedure in which subjects selected the orientation of the grating stimulus tilted at +/-45 degrees. Perceived spatial frequency was determined by having subjects alter the spatial frequency of a temporally interleaved stationary sine-wave grating to match the FDT stimulus. RESULTS: No significant difference was found between detection- and resolution-contrast thresholds, implying that spatial structure was visible at detection threshold. In general, subjects perceived the spatial structure to have a spatial frequency closer to doubled than to veridical, although the young normal subjects reported a lower apparent spatial frequency than older individuals. CONCLUSIONS: When instructed as for clinical testing, subjects respond to the presence of the structure of the grating, and perceive the FDT stimulus to have a spatial frequency greater than its true spatial frequency. These findings were consistent across both normal observers and those with glaucoma, at both central and eccentric test locations.     

Discrimination of spatial phase in central and peripheral vision

Sensitivity to relative phase was measured for central and peripheral vision using stimuli comprising 256 harmonics, smoothly filtered in amplitude. With these stimuli, peripheral phase sensitivity was much higher than that previously reported with two-harmonic stimuli. Sensitivity did not depend on the average phase of the stimuli, nor on their second-order statistics, irrespective of the spatial frequency of the stimulus or the position in the visual field. After scaling for size, peripheral sensitivity was as high as central sensitivity. The scaling factor required to equate phase sensitivity was the same as that required to equate contrast sensitivity and grating acuity. These results suggest that phase sensitivity decreases with eccentricity at a similar rate as contrast sensitivity and grating acuity, much more slowly than the positional acuities. This is consistent with the suggestion that phase discrimination is mediated by discriminating the amplitude of the response of quasi-linear filters, and does not require mechanisms that evaluate position. It is suggested that previous measurements on peripheral phase sensitivity may reflect positional uncertainty in the periphery, rather than a deficit in phase sensitivity per se.     

Spatial contrast sensitivity of goldfish: mean luminance, temporal frequency and a new psychophysical technique.

Behavioral contrast sensitivity in goldfish was examined at various mean luminances and stimulus drift rates. Goldfish were classically conditioned to suppress respiration upon presentation of a drifting sinusoidal grating. Contrast threshold at each spatial frequency was determined by means of a new two-alternative forced-choice procedure in which the observer's decision about the presence of the stimulus was based on the animal's respiration pattern. The results show that: (1) as mean luminance decreases, contrast sensitivity to high spatial frequencies decreases and peak sensitivity shifts to lower spatial frequencies; (2) as drift rate increases, contrast sensitivity to low spatial frequencies increases, but sensitivity to high spatial frequencies is relatively unaffected by stimulus drift rate. Both the mean luminance and temporal frequency of the stimulus clearly influence the behavioral contrast sensitivity of the goldfish in ways that would be predicted from behavioral results from other species. We conclude that the mechanisms that mediate contrast sensitivity in goldfish are similar to those that mediate contrast sensitivity in other vertebrates.     

Visual resolution and sensitivity in a nocturnal primate (galago) measured with visual evoked potentials

Visual resolution and contrast sensitivity were examined in anesthetized, paralyzed galagos using visual evoked potentials (VEPs) resulting from stimulation with phase-reversed sinewave gratings. Spatial frequency vs contrast response functions were band-pass with peak sensitivity at 0.2-0.4 c/deg and a high frequency cut-off between 1.6 and 3 c/deg. Peak contrast sensitivities (estimated from extrapolation of contrast response functions) varied across animals from 10 to 170. Variation of the stimulus modulation rate showed that best responses occurred at 1 Hz with an upper limit of 6-16 Hz. As in other primates, an oblique effect was seen in 6 of 8 animals. The contrast sensitivity function (CSF) determined from cortical VEPs agrees well with the CSFs of cells in the lateral geniculate nucleus, but peak sensitivity and spatial frequency are slightly lower than found for the behavioral CSF. Overall visual performance resembled closely that of another nocturnal species, the cat.     

Contrast sensitivity function of preschool children.

A procedure specifically adapted for children of preschool age has been used to measure contrast sensitivity in emmetropic children aged 3 to 5 years. Mothers of the children acted as adult observers using the same procedure. The results show that the contrast sensitivity function of adults and children is very similar, sensitivity for the children being slightly lower than that for adults at all spatial frequencies. The sensory and cognitive factors involved in these differences are discussed.     

The contrast sensitivity function for detection and resolution of blue-on-yellow gratings in foveal and peripheral vision

Previous studies using polychromatic gratings have shown that the peripheral grating contrast sensitivity function is significantly different when the task is resolution rather than detection. Specifically, in the middle frequency range, while resolution acuity drops suddenly to zero, detection performance continues up to much higher frequencies, accompanied by observations of aliasing. We wanted to determine if the same holds true for blue-cone isolating gratings in either foveal or peripheral vision. Contrast sensitivity function (CSFs) were measured at the fovea and 20 degrees eccentricity in the temporal retina under conditions of short-wavelength-sensitive (SWS)-cone pathway isolation using a two-alternative forced choice paradigm. The detection and resolution CSF were identical at the low frequency end but at higher frequencies resolution sensitivity falls abruptly while contrast detection remained possible till higher frequencies [cut-off frequencies: fovea detection 6.0 cycles (degree)(-1), resolution 4.6 cycles (degree)(-1); periphery detection 1.6 cycles (degree)(-1), resolution 1.05 cycles (degree)(-1)]. Aliasing was observable when spatial frequency exceeded the resolution limit. Medium/high contrast blue-cone-mediated resolution acuity is sampling limited in both the fovea and periphery. Previous studies of blue-cone contrast sensitivity which employed a detection task do not reflect the true resolution limit.     

弱视治疗前后对比敏感度的分析

目的：分析弱视患者治疗前后的对比敏感度的变化。方法随机选取40例（80眼）健康体检儿童（视力≥1.0）为正常对照组,47（79眼）例弱视儿童为病人组。利用美国 STEREO 公司生产的 OPTEC 6500视功能测试仪分别测量正常对照组和病人组治疗前、综合治疗5个月时5种空间频率[1.5、3、6、12和18周/度（cycles/degree, c/d）]的对比敏感度（contrast sensitivity , CS）。所有检查对象在正常光线下,采用自然瞳孔测试,病人组先检查患者视力较好的眼,后检查视力较差的眼,将测试结果存储,通过相应软件处理,自动生成资料,即时呈现出对比敏感度函数（contrast sensitivity function, CSF）曲线图。用 SPSS17.0统计软件对比正常组、弱视组治疗前后不同空间频率对比敏感度数值有无差异。结果（1）正常儿童的 CSF 曲线其峰值位于空间频率6 c/d 附近,在较低、较高空间频率,CSF均降低,曲线呈一倒“ U“字形。曲线向高空间频率端外推,得截止率为25c/d。（2）轻度弱视组：治疗前 CS 在高空间频率时,振幅比正常对照组降低明显,,两组差异有统计学意义（P ＜0.05）。治疗后 CS 较治疗前升高,治疗前后差异有统计学意义（P＜0.05）,治疗后 CS 与正常对照组差异无统计学意义。（3）中度弱视组：治疗前 CS 在中、高空间频率比正常对照组降低,和正常对照组之间差异有统计学意义（P ＜0.05）。治疗后 CS 较治疗前升高,差异有统计学意义（P＜0.05）,治疗后 CS 与正常对照组差异无统计学意义。（4）重度弱视组：治疗前 CS 在低、中、高空间频率均比正常对照组降低,在中、高空间频率和正常对照组之间差异有统计学意义,但在低空间频率差异无统计学意义。治疗后CS 较治疗前升高,差异无统计学意义,和正常对照组差异亦无统计学意义。（5）弱视程度与疗效：弱视越重,疗效     

The effect of contrast on sustained detection

During a l min observation, the percentage of time a high spatial frequency grating can be detected is influenced by the contrast and spatial frequency of a second, superimposed and orthogonally-oriented sine wave grating. Increasing contrast of the second pattern aids detection of the first by providing a more effective accommodative stimulus. The function relating spatial frequency and the minimum contrast needed to activate accommodation is similar in shape to the classical contrast sensitivity function. However, an order of magnitude more contrast is required to stabilize accommodation than is required to simply detect a pattern. These results suggest that performance on visual tasks requiring sustained rather than brief detection may be markedly impaired under low contrast conditions.     

Spatial-frequency cutoff requirements for pattern recognition in central and peripheral vision

It is well known that object recognition requires spatial frequencies exceeding some critical cutoff value. People with central scotomas who rely on peripheral vision have substantial difficulty with reading and face recognition. Deficiencies of pattern recognition in peripheral vision, might result in higher cutoff requirements, and may contribute to the functional problems of people with central-field loss. Here we asked about differences in spatial-cutoff requirements in central and peripheral vision for letter and face recognition.The stimuli were the 26 letters of the English alphabet and 26 celebrity faces. Each image was blurred using a low-pass filter in the spatial frequency domain. Critical cutoffs (defined as the minimum low-pass filter cutoff yielding 80% accuracy) were obtained by measuring recognition accuracy as a function of cutoff frequency (in cycles per object).Our data showed that critical cutoffs increased from central to peripheral vision by 20% for letter recognition and by 50% for face recognition. We asked whether these differences could be accounted for by central/peripheral differences in the contrast sensitivity function (CSF). We addressed this question by implementing an ideal-observer model which incorporates empirical CSF measurements and tested the model on letter and face recognition. The success of the model indicates that central/peripheral differences in the cutoff requirements for letter and face recognition can be accounted for by the information content of the stimulus limited by the shape of the human CSF, combined with a source of internal noise and followed by an optimal decision rule.     

Spatial-frequency characteristics of letter identification in central and peripheral vision

Spatial-frequency characteristics of letter identification are much better understood in the fovea than in the periphery. The purpose of this study was to compare the spatial-frequency characteristics of letter identification in central and peripheral vision. We measured contrast thresholds for identifying single, Times-Roman lower-case letters that were spatially band-pass filtered. Each of the 26 letters was digitally filtered with a set of nine cosine log filters, with peak object spatial frequencies ranging from 0.63 to 10 c/letter, in half-octave steps. Bandwidth of the filters was 1 octave. Three observers with normal vision were each tested monocularly at the fovea, and at 5 degrees and 10 degrees in the inferior visual field. Letter sizes were 0.2, 0.4 and 0.6 log units larger than high contrast, unfiltered acuity letters. Plots of contrast sensitivity for letter identification vs. frequency of the band-pass filters exhibit spatial tuning. In general, the spatial-frequency characteristics of letter identification are fundamentally identical between central and peripheral vision. These characteristics include the scaling of the peak frequency of the spatial-tuning functions with letter size and the bandwidth of the tuning functions. The only difference between the fovea and the periphery is that for the same physical letter size, peak sensitivity of the spatial-tuning functions occurs at a higher retinal frequency at the fovea than in the periphery. To test whether or not the contrast sensitivity function (CSF) can account for the differences in the spatial-frequency characteristics of letter identification between central and peripheral vision, we incorporated a human CSF into an ideal-observer model, and tested the performance of this ideal-observer on the same letter identification task used with the human observers. Data from this CSF-ideal-observer resemble closely those of human observers, suggesting that the spatial-frequency characteristics of human letter identification can be accounted for by the CSF and the letter-identity information, without invoking selection among narrow-band spatial-frequency channels.     

非球面人工晶状体植入眼的视觉质量分析

目的 观察非球面人工晶状体(IOL)、球面IOL植入眼与正常中青年人眼的波前像差和对比敏感度差异.方法 46例(49只眼)老年性白内障患者随机分成两组,分别植入非球面IOL Acrysof IQ(SN60WF)和球面IOL Acrysof Nature(SN60AT).随机选择同期正常中青年人(年龄20~40岁)作为对照组.分别对术后3月的患者和对照组进行最佳矫正视力、像差和对比敏感度检查.结果 IQ组的球差显著小于Natural组(P ＜0.05),而和正常中青年组比较无统计学意义(P ＞0.05);两IOL组的高阶像差和慧差均显著大于正常中青年组(P ＜0.05).非眩光状态下,IQ组在低频时(6.3degrees)的对比敏感度显著高于Natural组(P ＜0.05),和正常中青年组比较无统计学意义(P ＞0.05);眩光状态下,IQ组在低频时(6.3和4.0degrees)的对比敏感度显著高于Natural组(P ＜0.05),两IOL组在所有视角下的对比敏感度均显著低于正常中青年组(P ＜0.05).结论 非球面IOL植入能使白内障患者术后眼内球差明显减小,非眩光状态下低频时的对比敏感度明显提高,更接近正常中青年人眼水平,具有比球面IOL更佳的视觉质量.     

矢量法评价Epi-LASIK对散光的矫正效果及其与视觉质量变化的关系

目的:采用标准矢量分析法评价准分子激光角膜上皮瓣下磨镶术(epipolis laser in situ keratomileusis,Epi-LASIK)对散光的矫正效果,并探讨其与明视无眩光下对比敏感度函数(contrast sensitivity function,CSF)变化的关系。方法:选取行Epi-LASIK术矫正近视散光的68眼,近视屈光度为-5.00～-9.00D,散光屈光度为-1.00～-3.00D,术后随访6mo。运用美国国家标准学会推荐的标准矢量分析法评估Epi-LASIK术后1,6mo散光的矫正效果。采用CSV-1000E对比敏感度仪检测手术前后最佳矫正视力下明视无眩光时的CSF。结果:术后1mo各空间频率(单位为cycles per degree,c/d)的CSF低于术前(P<0.01),术后6mo时12c/d和18c/d的CSF分别低于术前(P<0.05),术后1mo时的3c/d,6c/d和12c/d CSF分别低于术后6mo(P<0.05);术后1,6mo时的|SIRC|与18c/d的CSF下降呈正相关(r=0.537,0.674,P<0.01);术后1mo的EM为-0.21±0.29,6mo的EM为-0.12±0.31。结论:Epi-LASIK术后6mo内,中高度近视合并中低度散光患者可出现明视无眩光下高频段CSF的下降,其下降原因可能与手术引起的散光矫正量有关。     

Characterization of spatial aliasing and contrast sensitivity in peripheral vision

Psychometric performance was measured for contrast detection and spatial resolution tasks in foveal and peripheral vision. Objective evidence was obtained for a quantitative difference between resolution acuity and detection acuity in the peripheral field. These two types of spatial acuity differed by up to an order of magnitude (3 vs 30 c/deg at 30 deg eccentricity) and they varied with stimulus contrast in distinctly different ways. Contrast sensitivity at the resolution limit was an order of magnitude above the absolute threshold of unity and the shape of the contrast sensitivity function was significantly different from that measured for foveal vision. The results suggest that current models of eccentricity scaling of contrast sensitivity be re-evaluated to take account of the extensive aliasing zone of spatial frequencies which becomes functional in peripheral vision when the retinal image is well focused.