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PubMed2004

Laser in situ keratomileusis for myopia and the contrast sensitivity function.

PURPOSE: To characterize the clinical effects of laser in situ keratomileusis (LASIK) on the cornea and visual performance by the contrast sensitivity function (CSF). SETTING: Clinique d'Ophtalmologie IRIS, Laval, Quebec, Canada. METHODS: Thirty-four patients aged 18 to 50 years volunteered for this prospective study. All patients had bilateral LASIK to correct myopia between -1.00 and -6.75 diopters (D) and refractive astigmatism less than 2.50 D. The mean spherical equivalent (SE) refractive error in the 68 eyes was -3.93 D +/- 1.67 (SD). A Hansatome microkeratome (Bausch & Lomb) and a Technolas 217C excimer laser (Bausch & Lomb) driven by the PlanoScan program were used. The monocular CSF for spatial frequencies of 3, 6, 12, and 18 cycles per degree (cpd) for both day and night vision simulations were made with a CSV-1000E system (Vector Vision) before and 1 month and up to 9 months after LASIK. All patients wore their best spectacle correction for the baseline CSF. RESULTS: The group (n = 68) averaged preoperative and postoperative CSFs did not differ at 1 month (analysis of variance [ANOVA], P>.05). In a subgroup of 11 eyes that had corneal microstriae, however, there was a significant reduction in the photopic and mesopic CSF at 6, 12, and 18 cpd (ANOVA, P<.05) despite normal Snellen visual acuities. The CSF normalized in 6 to 9 months as the microstriae became less visible. CONCLUSIONS: Subtle central corneal microstriae after LASIK can reduce the baseline CSF at medium to high spatial frequencies even with 20/20 visual acuity. The CSF normalizes as the microstriae fade over time.

Adolescent↗

PubMed1991

Comparison of two methods of contrast sensitivity assessment with young and elderly adults.

The sensitivity of the neural channels which process spatial frequency information may be assessed by determining the threshold contrast for a range of spatial frequencies. The stability and reliability of two methods of assessment, von Bekesy tracking and increasing contrast, were examined over 4 sessions with a sample of 12 young and 12 elderly adults. The stimuli were presented as either a static display or in counterphase at a rate of 7.5 Hz. The method of increasing contrast proved to be the more stable and reliable of the two methods. Given the superior psychometric properties of this method and its ease of administration, the method of increasing contrast is recommended as a rapid and reliable screening tool. It should be of particular use for spatial contrast sensitivity assessment of patients who have a limited attention span.

Adolescent↗

PubMed1999

Contrast sensitivity and appearance in briefly presented illusory figures.

We examined the contributions of brightness enhancement, illusory figure formation and figural completion to changes in contrast sensitivity in contour gaps. The brightness on the border of a Kanizsa-square and an outline square was measured as the point of subjective equality with the background (PSE) for small line targets. Increment and decrement thresholds were measured at the same location. We found that contrast thresholds were lower than in a control condition without inducers, and that the threshold reduction was independent of the contrast polarity of the inducers. This reduction cannot be explained by a simple summation of stimulus contrast and induced brightness. In a second experiment the inducers that define the contour of the Kanizsa and the outline square were changed so that the figure was no longer closed, keeping the local stimulus surround constant. Thresholds were equally reduced for all conditions, independently of whether the figure was completed or not, or whether an illusory contour was perceived or not. The results suggest that the reduction of contrast threshold in contour gaps is independent of the brightness perceived in these gaps and of the formation of an illusory figure. Processes that cause contrast threshold reduction in contour gaps also seem to operate independently of figural completion.

Adaptation, Ocular↗

PubMed1997

Human contrast sensitivity in coherent Maxwellian view: effect of coherent noise and comparison with speckle.

Lasers have been used in vision for measuring the neural contrast sensitivity function (CSF) by forming interference fringes on the retina. We distinguish among three kinds of illumination with lasers: incoherent (without noise), Maxwellian or coherent (with coherent noise), and diffuse coherent (with speckle). The three have different characteristics and different CSF's. A coherent imaging system is designed to measure the CSF with fully coherent illumination. This is the CSF of the whole visual system, although it is measured with gratings imaged on the retina. It therefore differs from the neural CSF's measured by other authors with partially coherent illumination. However, the neural CSF's are also obtained in this study with and without noise. The effects of coherent noise and speckle on both the visual system and neural sensitivities are studied and compared. Coherent noise differs from speckle in the following ways: (1) It behaves as a high-pass filter, reducing sensitivity in the low-spatial-frequency range, whereas speckle is a low-pass filter; (2) quantitatively, coherent noise reduces neural sensitivity by a factor k(m) with a maximum value between 4 and 6, whereas speckle reduces neural sensitivity by a factor ks with a maximum value of approximately 25 (1.4 log units) for a 3-mm pupil and up to 35 (1.55 log units) for a 1-mm pupil; (3) the masking effect of the coherent noise is affected by changes in luminance but not by changes in pupil diameter; however, the pupil size is the main parameter affecting the masking effect of the speckle.

Contrast Sensitivity↗

PubMed1987

Preliminary findings suggesting cyclic changes in visual contrast sensitivity during the menstrual cycle.

Visual contrast thresholds to both stationary and moving gratings of three spatial frequencies (2, 4, and 16 cyc/deg) were measured over a 32-day period in two women displaying normal menstrual cycles and in two noncycling control subjects. The time-series data of each subject in each condition were Fourier analyzed and the resulting amplitude spectra showed differences between the two sets of subjects. The spectra of the control subjects were relatively flat, whereas those of the experimental subjects showed a number of peaks at several harmonics (periods). Conservative significance tests suggested that the peaks in the spectra of the cycling women were larger than might be expected by chance. The data also suggested that changes in sensitivity were greatest for 4-cyc/deg gratings, those nearest the peak of the normal contrast sensitivity function.

Adult↗

PubMed1983

The effect of temporal frequency variation on threshold contrast sensitivity deficits in optic neuritis.

The effect of increasing temporal frequency on contrast sensitivity anomalies in unilateral optic neuritis has been investigated. For 4 c/deg gratings no change in the deficit was observed at any temporal frequency whereas there was a tendency for the deficit to decrease with increase in temporal frequency for 0-5 c/deg gratings. The latter effect was not observed in a case of optic neuritis with severe deficit and not in two cases of other optic nerve pathology. An explanation based on the assumption that "threshold scotomata" might be present in cases of demyelinating optic nerve disease is proposed.

Adult↗

PubMed1995

Contrast sensitivity evaluation in eyes predisposed to age-related macular degeneration and presenting normal visual acuity.

In order to test the validity of contrast sensitivity (CS) measurements in the early detection of visual impairment in age-related macular degeneration (AMD), we have evaluated the findings of CS in patients with drusen and normal visual acuity (17 eyes), as well as in the contralateral 'healthy' eye of patients with AMD (14 eyes). We also tried to estimate the validity of CS measurements in the prognosis of neovascular macular degeneration. The CS findings were evaluated in comparison to the findings of CS measurements in age-matched controls (32 eyes). CS loss is a constant finding in eyes with drusen and normal visual acuity. CS loss is more important at the middle range and high spatial frequencies. CS loss and degree of CS loss are not prognostic indicators of neovascular macular degeneration.

Adult↗

PubMed2006

Treated amblyopes remain deficient in spatial vision: a contrast sensitivity and external noise study.

To evaluate residual spatial vision deficits in treated amblyopia, we recruited five clinically treated amblyopes (mean age=10.6 years). Contrast sensitivity functions (CSF) in both the previously amblyopic eyes (pAE; visual acuity=0.944+/-0.019 MAR) and fellow eyes (pFE; visual acuity=0.936+/-0.021 MAR) were measured using a standard psychophysical procedure for all the subjects. The results indicated that the treated amblyopes remained deficient in spatial vision, especially at high spatial frequencies, although their Snellen visual acuity had become normal in the pAEs. To identify the mechanisms underlying spatial vision deficits of treated amblyopes, threshold vs external noise contrast (TvC) functions--the signal contrast necessary for the subject to maintain a threshold performance level in varying amounts of external noise ("TV snow")--were measured in both eyes of four of the subjects in a sine-wave grating detection task at several spatial frequencies. Two mechanisms of amblyopia were identified: increased internal noise at low to medium spatial frequencies, and both increased internal noise and increased impact of external noise at high spatial frequencies. We suggest that, in addition to visual acuity, other tests of spatial vision (e.g., CSF, TvC) should be used to assess treatment outcomes of amblyopia therapies. Training in intermediate and high spatial frequencies may be necessary to fully recover spatial vision in amblyopia in addition to the occlusion therapy.

Adolescent↗

PubMed1995

Zero frequency masking and a model of contrast sensitivity.

Stimulating the visual system tends to desensitize it to certain stimulus properties. Such desensitization is usually called adaptation or masking, but the distinction between the two is unclear. Nonspecific desensitization by light is usually regarded as adaptation, whereas pattern-specific desensitization is typically considered masking. Here we unify the treatment of such desensitizing phenomena by handling both in the spatial frequency domain. The amount of adapting light in a stimulus is represented in the spatial frequency domain by the component at zero frequency. To determine whether such adapting light acts like other components in the spatial frequency domain, we compared the effect of masking by the zero frequency component with the effects of masking by components at other frequencies. We show that the zero frequency component acts like other masking components, decreasing sensitivity to nearby test frequencies and thereby producing the insensitivity to low spatial frequencies that gives the contrast sensitivity curve its band-pass shape at high light levels. Treating light adaptation as masking by the zero frequency component leads to a general model that describes visual sensitivity to test gratings of varying spatial frequency at varying mean luminance, in the presence (or absence) of masking gratings of varying spatial frequency. Individual components of the model provide insight into visual processing at the system level.

Adaptation, Ocular↗

PubMed2006

Measuring contrast sensitivity under different lighting conditions: comparison of three tests.

PURPOSE: The purpose of this study was to evaluate three psychophysical tests for the measurement of contrast sensitivity (CS) and disability glare (DG) at different luminance levels. METHODS: In 60 eyes of 60 individuals (group 1: 20 healthy eyes of young individuals; group 2: 20 healthy eyes of elderly subjects; group 3: 20 eyes with nuclear cataract), CS with best correction was measured twice with the Frankfurt-Freiburg Contrast and Acuity Test System (FF-CATS) and the Functional Acuity Contrast Test (FACT, 1.5 cycles per degree [cpd]) at 167 cd/m2 and 0.167 cd/m2, and with the Pelli-Robson Chart (PRC) at 100 cd/m2 with and without glare. Repeatability of test and retest, and discriminative ability between the different subgroups, were assessed for CS values. RESULTS: Maximum CS values varied across tests. In all groups, highest CS values were obtained with the photopic FF-CATS. For FACT scores at 1.5 cpd, there was a ceiling effect for young subjects. CS scores obtained with the PRC were the lowest. The PRC had the best test-retest repeatability of all tests. Under mesopic conditions with glare, reliability was generally lower; the FF-CATS had the highest repeatability of the mesopic tests. The FF-CATS discriminated best between the different groups for all conditions. CONCLUSIONS: There are large discrepancies in the test results between CS testing methods, especially under different lighting conditions. Results from different CS tests are not interchangeable.

Adult↗

PubMed1994

Luminance effects on visual acuity and small letter contrast sensitivity.

The purpose of this study was to evaluate the effects of luminance on visual acuity (VA) and small letter contrast sensitivity (SLCS). Computer-generated letter charts were used to measure VA and SLCS [6/7.5 (20/25) Snellen equivalent] as a function of stimulus luminance. Letter size (VA) and contrast (SLCS) were varied in equal logarithmic steps, making the task and scoring procedure comparable for the two types of measurement. Both VA and SLCS decreased with decreasing luminance, but the effect was far greater in the contrast domain. Reducing luminance from 116 cd/m2 to 0.23 cd/m2 produced a 3 x reduction in VA, but a 17 x reduction in SLCS. The greater sensitivity of SLCS to luminance endured even after correction for greater measurement variability. SLCS is a sensitive approach for detecting resolution loss undisclosed by standard measures of VA. It may be useful for monitoring visual loss from light attenuation in early cataracts, and for detecting subtle resolution loss from neural or pathologic factors in ocular and neuro-ophthalmologic disease.

Adult↗

PubMed2003

The scotopic low-frequency spatial contrast sensitivity develops in children between the ages of 5 and 14 years.

The purpose of this study was to investigate the development of visual contrast sensitivity (CS) in children between 5 and 14 years of age. Six spatial frequencies and static (0 Hz) and dynamic (8 Hz) conditions were used at photopic and scotopic luminance levels. The results revealed significant maturation of CS, which reached the adult-like values by 11-12 years of age. The development was more pronounced at low spatial frequencies (<2 cycles/degree) and in the dynamic condition. The scotopic CS exhibited slower development than the photopic CS. These results suggest the late maturation of the magnocellular visual pathway.

Adolescent↗

PubMed1996

Eye movement patterns in reading as a function of visual field defects and contrast sensitivity loss.

Saccadic eye movements during reading were examined as a function of the side of visual field cut and the impairment of visual contrast sensitivity Five patients with various visual field defects were compared to five age-matched controls. Patients with right visual field defect showed an increase in the number of rightward saccades and a decrease in their amplitude, and patients with left visual field defects showed a pattern more similar to that of the control subjects. Two patients showed a selective deficit in the range of medium-high spatial frequency; they were the only patients to show a lengthening of mean fixation time during reading. These results indicate selective effects on reading depending on primary sensory defects. The clinical relevance of these findings is briefly discussed.

Adult↗

PubMed1990

Binocular contrast sensitivity with monocular glare disability.

Glare disability plays an important part in the investigation and assessment of cataract. We investigated the effect of monocular glare disability on binocular contrast sensitivity. The magnitude of binocular summation was measured at different degrees of monocular glare disability. In the absence of glare, maximum binocular summation was shown. With increasing glare disability, the degree of binocular summation decreased steadily until binocular inhibition was produced. Binocular inhibition is defined as a reduction in binocular sensitivity to reach a level below the monocular. The clinical implications of binocular inhibition with monocular glare disability are discussed.

Adult↗

PubMed1999

Factors limiting contrast sensitivity in experimentally amblyopic macaque monkeys.

Contrast detection is impaired in amblyopes. To understand the contrast processing deficit in amblyopia, we studied the effects of masking noise on contrast threshold in amblyopic macaque monkeys. Amblyopia developed as a result of either experimentally induced strabismus or anisometropia. We used random spatiotemporal broadband noise of varying contrast power to mask the detection of sinusoidal grating patches. We compared masking in the amblyopic and non-amblyopic eyes. From the masking functions, we calculated equivalent noise contrast (the noise power at which detection threshold was elevated by square root of 2) and signal-to-noise ratio (the ratio of threshold contrast to noise contrast at high noise power). The relation between contrast threshold and masking noise level was similar for amblyopic and non-amblyopic eyes. Although in most cases there was some elevation in equivalent noise for amblyopic compared to fellow eyes, signal-to-noise ratio showed greater variation with the extent of amblyopia. These results support the idea that the contrast detection deficit in amblyopia is a cortical deficit.

Amblyopia↗

PubMed1993

Contrast sensitivity to patch stimuli: effects of spatial bandwidth and temporal presentation.

Models of the spatial response of human vision are important for applied work, but the available contrast sensitivity function (CSF) data vary widely due to the diverse spatiotemporal stimuli used over the years. To assist selection, this paper: (1) reports measurements of the effects on the CSF of varying the spatial and temporal windows of grating patches; (2) demonstrates that the widely discrepant CSFs from previous studies can be accounted for by using these results; and (3) discusses simple criteria for choosing CSFs for practical applications. CSFs were measured for several combinations of spatial and temporal waveforms, using the same subjects under otherwise identical conditions. The CSF was measured over the range of 0.5-10 c/deg using Gabor-type patches of 1.0-, 0.5-, 0.25-, and 0.125-octave spatial bandwidths using both abrupt and gradual temporal presentations. The results were compared with the CSF obtained with a fixed aperture (4 deg x 4 deg) grating pattern. Increasing the number of cycles resulted in increased sensitivity at intermediate frequencies, changing the CSF to a narrower bandpass shape. For each patch bandwidth, the gradual presentation CSF had a narrower spatial pass band than with the abrupt presentation. The relevance of the large differences in the CSFs obtained with different stimuli to our understanding of visual performance is discussed.

Contrast Sensitivity↗

PubMed1983

Contrast sensitivity for vertically and obliquely oriented gratings as a function of grating area.

Contrast sensitivity for both vertically and obliquely oriented gratings increased with increases in stimulus area to an asymptotic value. Sensitivity grew more slowly for oblique gratings and reached an asymptotic value at a larger area than for vertical gratings. For equal areas, oblique gratings always yielded poorer sensitivity. The results suggest a larger spatial summation area for obliquely oriented gratings.

Adult↗

PubMed1991

Macular colour contrast sensitivity in ocular hypertension and glaucoma: evidence for two types of defect.

Colour contrast sensitivity (CCS) of a large cohort of glaucomatous patients, ocular hypertensive patients (OH), and normal persons was measured at six-month intervals during a two-year period. The OHs were graded into high, medium, and low risk groups. 69% of glaucomatous patients and 32% of all OHs had CCS thresholds greater than the mean plus 2 SDs of the controls. Satisfactory specificity and sensitivity could not be obtained by adjusting the criterion of threshold. In abnormal eyes, progressive small increases of threshold occurred during the study, but glaucomatous eyes with normal thresholds on the first visit retained normal thresholds in the subsequent visits. Although our system is very sensitive and precise, the proportion of abnormalities detected is no greater than with other techniques. In some glaucomatous patients there is a true preservation of colour vision which does not merely reflect the limitations of the test employed.

Color Perception↗