Contrast sensitivity for letter and grating targets under various stimulus conditions

Measurement of visual acuity for letters of different contrasts has been suggested as a clinical way to evaluate contrast sensitivity in patients with vision abnormalities. If variable-contrast letter acuity provides information similar to the contrast sensitivity function (CSF), then comparable effects should be seen in stimulus manipulations which simulate decreased vision. Using both our own and published data, we compared the effects of diffusive blur, dioptric blur, and eccentric viewing on contrast sensitivity for letter and grating targets. A diffuser placed close to the eye reduces contrast sensitivity fairly evenly across all spatial frequencies, with similar results for letters and gratings. However, dioptric blur reduces sensitivity substantially more to letters than to comparably fine gratings. Eccentric viewing also produces a larger sensitivity loss for letters than for gratings. Because some stimulus manipulations produce dissimilar changes in contrast sensitivity for letters and gratings, it is questionable whether the results of one measure can be used to draw inferences about the other. It is proposed that local or relative phase discrimination has an important role in explaining the different responses to letter and grating targets.     

Contrast sensitivity of form and motion discrimination during binocular rivalry

Binocular rivalry, which is induced by presenting the two eyes with incompatible stimuli, results in periods where one eye's stimulus is seen and the other stimulus is suppressed. We measured the depth of suppression in two ways, with very different results. First, two similar forms were briefly presented to one eye: the difference in shapes required to discriminate the forms was substantially greater during suppression than during dominance. Second, the two forms were made sufficiently different in shape to be easily distinguishable at high contrast, and contrast was lowered to find the threshold for discrimination of the forms. Contrast sensitivity did not differ between the suppression and dominance states. These results were replicated with a motion discrimination task: suppression markedly worsened the ability to distinguish increases from decreases in speed but did not elevate the minimum contrast required for the same task. We interpret the results in terms of steep contrast-response functions in visual cortex beyond the primary area.     

Contrast sensitivity function as a screening test: a critique

Currently, there is intense clinical, commercial, and academic interest in the potential value of the contrast sensitivity function (CSF) for detecting eye disease. This paper contains an evaluation of the CSF as a screening test. Questions are raised concerning its scoring, accuracy, reliability, and robustness in screening situations. We conclude that the CSF cannot be of much value in visual screening until these questions are answered.     

Sensitization by annular surrounds: the effect of test stimulus size

Studies of scotopic spatial sensitization have shown that the threshold for a small test probe is influenced by the diameter of the background disk against which it is presented. The present study indicates that the diameter of the test probe is also important. As the diameter of the probe is increased from 4.8′ to 2°, sensitization can still be demonstrated. But with increasing test probe size, the magnitude of sensitization decreases and the peak of the sensitization function shifts to a larger diameter of background disk, such that with probes larger than about 40′, the background producing the highest threshold tends to be the same size as the probe. Several possible explanations for the data are considered, including the hypothesis that edge interactions determine thresholds and the alternative suggestion that thresholds are set by size-tuned channels.     

Contrast, spatial frequency and test duration effects on the tilt aftereffect: implications for underlying mechanisms

The tilt aftereffect (TAE) was measured with a forced-choice technique for gratings of different spatial frequencies, contrasts and adapting and test durations. At short test durations, a 2 c/deg grating gave a larger TAE than a 10 c/deg, while at long test durations the opposite occurred. Low contrast gratings tended to give smaller TAEs at short test durations, and larger TAEs at long durations, than high contrast. A longer adapting duration tended to produce larger TAEs at low contrast for any test duration, but larger TAEs at high contrast only at long test durations. We suggest that the spatial frequency effect reflects differential excitation by the test stimuli of transient and sustained channels, and that the contrast effects reflect both a non-linearity in the relationship between excitation and inhibition, and the adapting effects of the test grating.     

Defining the limits of flicker defined form: effect of stimulus size, eccentricity and number of random dots

We investigated the spatio-temporal limits of flicker defined form (FDF) phase contrast thresholds generated from the phantom contour illusion. Random dots (diameter 0.25 degrees, tapered edge) were used throughout the test field. FDF was generated using circular stimuli (temporal frequency 30 Hz, mean background luminance 50 cdm(-2)), the edges being defined by illusory borders generated from the out-of-phase dots within the display. Thresholds improved with increasing stimulus size and number of random dots at all eccentricities. For a constant threshold, fewer random dots were required with increasing eccentricity. Predictive mathematical relationships between contrast threshold, stimulus size and random dot number are discussed.     

Contrast sensitivity,healthy aging and noise

At least three studies have used external noise paradigms to investigate the cause of contrast sensitivity losses due to healthy aging. These studies have used noise that was spatiotemporally localized on the target. Yet, Allard and Cavanagh (2011) have recently shown that the processing strategy can change with localized noise thereby violating the noise-invariant processing assumption and compromising the application of external noise paradigms. The present study reassessed the cause of age-related contrast sensitivity losses using spatiotemporally extended external noise (i.e., full-screen, continuously displayed dynamic noise). Contrast thresholds were measured for young (mean = 24 years) and older adults (mean = 69 years) at 3 spatial frequencies (1, 3 and 9 cpd) and 3 noise conditions (noise-free, local noise and extended noise). At the two highest spatial frequencies, the results were similar with local and extended noise: the sensitivity loss was mainly due to lower calculation efficiency. At the lowest spatial frequency, age-related contrast sensitivity losses were attributed to the internal equivalent noise when using extended noise and, like in previous studies, due to calculation efficiency with local noise. These results show that the interpretation of external noise paradigms can drastically differ depending on the noise type suggesting that external nose paradigms should use external noise that is spatiotemporally extended like internal noise to avoid triggering a processing strategy change. Contrary to previous studies, we conclude that healthy aging does not affect the calculation efficiency of the detection process at low spatial frequencies.     

Contrast sensation: a linear function of stimulus contrast

The method of magnitude estimation was used to determine the relationship between stimulus contrast and contrast sensation for stimulus contrasts from 0.63% to 60% at spatial frequencies of 1, 2, 6.5, and 12 cycles per degree. The relationship between contrast sensation and stimulus contrast was found to be linear over the entire range of stimulus contrasts and spatial frequencies studied. A theoretical function for contrast sensation is proposed and the concept of a contrast sensation surface is introduced. Constant contrast sensation contours derived from this surface are shown to fit data from contrast matching experiments performed in other laboratories.     

The effectiveness of glaucomatous visual field test using a size I stimulus

PURPOSE: Perimetry using size I and III targets was conducted on glaucoma patients. The detectability of the visual field defects using a size I target and abnormal spatial summation was investigated. SUBJECTS & METHODS: Subjects were 14 eyes of 11 glaucoma patients with mild to moderate visual field defects and 11 eyes of 11 normal persons. Program 24-2 of the Humphrey Field Analyzer was applied to each subject. Based on the mean sensitivity and standard deviation at each coordinate in normal persons, the Z-score was calculated in the visual fields of glaucoma patients (absolute scotomas were excluded). RESULTS: The mean Z-score of retinal sensitivity, was less for size III than for size I in abnormal lesions, and less for size I than for size III in normal lesions (paired t-test, p < 0.01). Pathological spatial summation was found more frequently in normal lesions than in abnormal lesions. CONCLUSIONS: In mild visual field defects, size I detects abnormalities with greater sensitivity than size III. Perimetry using the size I target can be effective as a supplementary diagnostic method to detect and follow the early glaucomatous visual field.     

A static perimetric technique believed to test receptive field properties: Responses near visual field lesions with sharp borders

The organization of the ‘Westheimer function’ in the retina has been further analysed. Patients with sharp visual field cuts caused by chiasmal and cortical lesions have been studied. Data were generally obtained from points approximately 10°–15° from the fovea (where areas of spatial interaction and the magnitude of the inhibitory component were large). The center of the three field display was placed just within the boundary of the sighted area of the visual field. Hence, a substantial part of the background field fell on nonsighted areas, which presumably retained retinal function. If the inhibitory part of the function originates proximal to the retinal ganglion cell layer (or the underlying retina was nonfunctional), then the basic ‘Westheimer function’ probably would be altered when these data are compared with findings obtained at a normal point in the visual field of the affected eye. All cases measured to date exhibited essentially unaltered functions. This finding, plus previous studies of patients with inner retinal diseases, places the locus of the measured inhibitory component of the Westheimer function most probably in the inner retina. This research has been supported in part by Research Grants No. EY 00204 and EY 00233 and Career Development Award No. K3 EY 15138 (to J.E.) of the National Eye Institute, National Institutes of Health, Bethesda, Maryland.     

Losses of flicker sensitivity during dark adaptation: effects of test size and wavelength.

The losses of flicker sensitivity that occur during dark adaptation following extinction of a bleach depend greatly on the test stimuli that are used to measure those losses. Generally, the larger the test size is for a given test wavelength, the later is the loss of flicker sensitivity; for sufficiently large tests, losses become progressively more gradual. Similarly, the shorter the test wavelength is (between about 660 and 580 nm) for a given test size, the later is the loss of flicker sensitivity; for sufficiently short test wavelengths, losses become progressively more gradual. Although the pattern of flicker sensitivity loss as a function of test size is the same both foveally and parafoveally, foveal losses occur later and/or more gradually for a given test stimulus. The results suggest that losses of flicker sensitivity occur when spectrally antagonistic response(s) to the test are no longer adequate to maintain the flicker-response to that test. The results suggest also that the adequacy of such spectral antagonism depends on the local adaptation state of the retina, which becomes increasingly more light-adapted with increasingly large test size.     

Perceived contrast and stimulus size: experiment and simulation

Perceived contrast functions were determined for three different Gabor patch sizes using magnitude estimation and verified by contrast matching. While thresholds show a significant decrease with decreasing patch size, perceived contrasts are equal and independent of patch size for contrasts above 0.06. Contrast matching was also used to study the apparent contrast of two other spatially limited stimuli; the sum of two orthogonal 4 c/deg sine waves multiplied by a gaussian envelope and the sum of spatially adjacent positive and negative gaussians. Models of contrast perception, based on tuned Gabor spatial filters, were formulated and tested for agreement with our experimental data. A model that pools filter responses across spatial frequencies and orientations was found to be more in agreement with our data than a model that simply uses the response of a single, maximally excited, mechanism to mediate contrast perception. Optimum filter bandwidth was found to be about 1.1 octaves.     

Contrast sensitivity: Psychophysical and evoked potential methods compared

Contrast sensitivity functions derived from steady state evoked potential thresholds show high correlation with psychophysically derived threshold contrast sensitivity functions. This correlation holds as flicker rate, type of flicker, optical correction and visibility conditions are changed over a substantial range, provided that VEP and psychophysical data are recorded under the same experimental conditions.     

Contrast sensitivity, acuity, and the perception of ''real-world'' targets.

A major assumption underlying the use of contrast sensitivity testing is that it predicts whether a patient has difficulty seeing objects encountered in everyday life. However, there has been no large-scale attempt to examine whether this putative relationship actually exists. We have examined this assumption using a clinic based sample of adults aged 20-77 years. Contrast thresholds were measured for both: (1) gratings of 0.5-22.8 cycles/degree; and (2) real-world targets (faces, road signs, objects). Multiple regression techniques indicated that the best predictors of thresholds for real-world targets were age and middle to low spatial frequencies. Models incorporating these variables accounted for 25-40% of the variance. Although acuity significantly correlated with thresholds for real-world targets, the inclusion of acuity as a predictor variable did not improve the model. These data provide direct evidence that spatial contrast sensitivity can effectively predict how well patients see targets typical of everyday life.