Contrast sensitivity: determining the visual quality and function of cataract, intraocular lenses and refractive surgery

PURPOSE OF REVIEW: This review provides an update of recent advances in understanding the quality and functional significance of contrast sensitivity for the clinician regarding cataract, intraocular lenses and refractive surgery that goes beyond the measurement of visual acuity. RECENT FINDINGS: New American National Standards Institute standards for contrast sensitivity based on linear sine-wave gratings are discussed that promise rapid advances of understanding and quantifying visual quality and function by unifying clinical results reported using contrast sensitivity. Increased sensitivity of linear sine-wave gratings over proposed bull's-eye radial gratings is discussed. Digital-image-processing software uses contrast sensitivity data to process images to help understand the quality of what the patient sees. Contrast sensitivity measurement is compared with wavefront aberrometry. Contrast sensitivity measures the total visual system quality in terms of contrast, whereas wavefront aberrometry measures the optical quality in terms of spatial distortion. Both measurements are needed to more fully understand the quality of vision. SUMMARY: Recent advances provide the clinician with an awareness of why the new contrast-sensitivity standards are based on linear sine-wave gratings and how image-processing software can be used to better understand the quality of functional vision of the patient.     

Apparent contrast as a function of modulation depth and spatial frequency: a comparison between perceptual and electrophysiological measures.

The contrast sensitivity function describing the interrelated contrast and spatial response characteristics of the visual system was determined for sine-wave gratings. Three spatial frequencies were then selected for psychophysical scaling of apparent contrast using an intermodal matching technique. The perceptual contrast curves were to a fair approximation power functions of the physical contrast of the striped target. Power transformations as a function of spatial fequency were observed, i.e. with decreasing sensitivity the exponents of the apparent contrast functions increased. A reanalysis of evoked response data published by Campbell and Maffei confirmed these observations.     

Characterization of mouse cortical spatial vision

Little is known about the spatial vision of mice or of the role the visual cortex plays in mouse visual perception. In order to provide baseline information upon which to evaluate the spatial vision of experimentally and genetically altered mice, we used the visual water task to assess the contrast sensitivity and grating acuity of normal C57BL/6 mice. We then ablated striate cortex (V1) bilaterally and re-measured the same visual functions. Intact mice displayed an inverse "U"-shaped contrast sensitivity curve with a maximum sensitivity near 0.2 cycles/degree (c/d). Grating acuity, measured either by discriminating a sine-wave grating from an equiluminant gray, or vertical from horizontal sine wave gratings, was near 0.55 c/d. Grating acuity and contrast sensitivity were reduced significantly following aspiration of V1. The mouse visual system exhibits fundamental mammalian characteristics, including the feature that striate cortex is involved in processing visual information with the highest sensitivity and spatial frequency.     

Single retinal ganglion cell responses in the dark-reared rat: Grating acuity,contrast sensitivity,and defocusing

The visual acuity, refractive state, and depth of focus of the dark-reared hooded rat's dark-adapted eye were determined by recording the responses of retinal ganglion cell axons in the optic tract. The smallest square-wave grating to which the units responded subtended 0.22 c/deg. The contrast sensitivity function for sine-wave gratings peaked at 0.02–0.07 c/deg. Corrective lenses of ± 14 D reduced the responsiveness of optic tract units to 100% contrast square-wave gratings by only 20%, implying (1) that objects from 7 cm in front of the eye to optical infinity are in equivalent focus, and (2) that the depth of focus of the rat's eye is enormous.     

Evoked potential contrast sensitivity in the parafovea: Spatial organization

Visual evoked potential contrast sensitivity functions (VEP/CSFs) were determined for counterphase flickered sine-wave gratings in circular fields up to 8 degrees in diameter centered on the fovea. VEP sources responding to 16 c/deg gratings appeared to be concentrated in the central 2 degrees of the visual field while sources responding to lower spatial frequencies appeared to be distributed over progressively wider areas of the visual field as spatial frequency decreased. It was also found that independently determined VEP/CSFs for non-overlapping annular regions of the visual field centered on the fovea summed to equal the VEP/CSF obtained when both regions were stimulated simultaneously.     

Contrast sensitivity in patients with posterior chamber intraocular lens implants

Divergent contrast sensitivity findings have been reported in patients with intraocular lens implants. In this study, contrast sensitivity to stationary sine-wave gratings of six spatial frequencies from 0.5 to 22.8 cycles/degree was measured psychophysically in 13 patients with posterior chamber intraocular lens implants and in 10 controls. Corrected visual acuity was 0.7 to 1.0 in the intraocular lens group and 0.9 to 1.0 in the reference group. The age, the pupil diameter and the rate of subtle age-related macular changes were equal in the two groups. No statistically significant difference in mean contrast sensitivity between the patients and the controls was observed at any spatial frequency examined.     

Spatial neural modulation transfer function of human foveal visual system for equiluminous chromatic gratings

To determine the spatial modulation transfer function (MTF) of the human foveal visual system for equiluminous chromatic gratings we measured contrast sensitivity as a function of retinal illuminance for spatial frequencies of 0.125-4 c/deg with equiluminous red-green and blue-yellow gratings. Contrast sensitivity for chromatic gratings first increased with luminance, obeying the Rose-DeVries law, but then the increase saturated and contrast sensitivity became independent of light level, obeying Weber's law. Critical retinal illuminance (I(c)) marking the transition point between the laws was found to be independent of spatial frequency at 165 phot. td. According to our detection model of human spatial vision the MTF of the retina and subsequent neural visual pathways (P(c)) is directly proportional to radicalI(c). Hence, P(c) is independent of spatial frequency, reflecting the lack of precortical lateral inhibition for equiluminous chromatic stimuli in spatiochromatically opponent retinal ganglion cells and dLGN neurons.     

Contrast sensitivity in amblyopia. IV. Assessment of vision using vertical and horizontal gratings and optotypes at different contrast levels

The contrast sensitivity of 49 amblyopic children (mean age 9 years) was measured in the beginning and at the end of a treatment period using both vertical gratings and optotypes (LH-4 contrast test) at 2, 3, 5 and 75% contrast. The contrast sensitivity of 37 patients was also measure using horizontal gratings. The contrast sensitivity of the dominant eyes measured with vertical gratings was generally higher than when measured with horizontal gratings. The difference was statistically significant at spatial frequencies 1 and 6 c/deg. A similar difference was present in the amblyopic eyes only at the spatial frequency 1 c/deg both before and after the treatment. Individual variation was great; in a given patient the relationship between the two measurements varied from one spatial frequency to the next. It has been shown in earlier investigations that contrast sensitivity measurements using gratings reveal important new information in the study of amblyopia. Both tests, used in this investigation, measure visual function in the low contrast domain and improve follow-up of the changes in vision during treatment of amblyopia when used in addition to visual acuity measurements. Low contrast optotype test seems to measure contrast sensitivity in amblyopia closely similarly to the grating test. The slight difference in detecting a 'hidden occlusion amblyopia' at the intermediate spatial frequencies needs further study.     

Spatial frequency phase effects in human vision

Color aftereffects (McCollough effects) were generated specific to each member of a pair of vertical gratings which had identical frequency spectra but which differed in the phase angles between their frequency components. The pairs of gratings were either left- and rightfacing sawooth gratings or gratings comprised of the sum of two harmonics—first and second, first and third, or first and fourth. Color aftereffects were readily obtained with sawtooth gratings (which had sharp edges) and with patterns comprised of first and second harmonics; the effects were very weak with the first and third harmonic patterns and almost absent with the first and fourth harmonic patterns. The results suggest that there are phase-sensitive broadband mechanisms within the visual system and that each “spatial frequency channel” cannot be simply represented by a single, symmetric line spread function.     

Reaction times to different spatial frequencies as a function of detectability

Simple reaction time to sine-wave gratings of 1, 4 and 10 c/deg was measured as a function of the detectability of the gratings. Reaction time increased with spatial frequency over a range of detectabilities. Assuming that equally detectable gratings produce equivalent levels of response in the visual system, this increase in reaction time reflects an increase in perceptual latency as spatial frequency is increased.     

Contrast sensitivity function after cataract extraction and intraocular lens implantation

Divergent contrast sensitivity findings have been reported in patients with intraocular lens implants. The purpose of this study was to determine contrast thresholds of patients with good visual acuity after uncomplicated cataract extraction and posterior chamber conventional IOL implantation. Fifty-two eyes of fifty two patients, who had undergone uncomplicated extracapsular cataract extraction and posterior chamber intraocular lens implantation together with 48 eyes of 48 control subjects were tested for contrast sensitivity function. All of the patients had best corrected visual acuity 0.8 (20/25) or better, on the Snellen scale. Patients with concomitant eye disease were excluded. Contrast sensitivity was measured using stationary sine-wave gratings of four spatial frequencies (3.0 to 18.0 cycles/degree), at the testing distance of 8 feet. A loss of contrast sensitivity was found in patients with intraocular lens implants, compared with control subjects of similar age, sex and visual acuity. The loss was statistically significant at intermediate (6 cyc/deg) and high spatial frequencies (12.0 and 18.0 cycles/degree), while it was not statistically significant at low spatial frequencies (3 cyc/deg). This may be the reason of nonspecific visual complaints (‘washed-out images’), despite normal Snellen acuity, after cataract surgery and monofocal IOL implantation.     

A phase-specific adaptation effect of the square-wave grating

The existence of wide-band, phase-specific channels in the human visual system has been suggested in recent investigations. We used a number of adapting gratings to test for phase-specific adaptation effects. Observers were asked to discriminate between a simple 3 cpd sine-wave grating, (3), and a complex grating composed of this (3) plus a 9 cpd grating combined in one of two phases: peaks-subtract, (3,9:0), or peaks-add, (3,9:pi). The results show a significant phase-specific adaptation effect. That is, following adaptation to a square-wave grating, discrimination performance for (3,9:0) vs. (3) deteriorated significantly more than for (3,9:pi) vs. (3). Adaptation to the first two harmonics of the square-wave, (3,9:0), or a 3 cpd triangle-wave grating failed to produce phase-specific adaptation effects that reached significance.     

Aging,perceptual learning,and changes in efficiency of motion processing

In the present study we examined the use of perceptual learning to improve motion processing in older and younger individuals. Using the Perceptual Template Model (Lu and Dosher, 1998, Lu and Dosher, 1999), age-related differences in baseline perceptual inefficiencies and changes due to training were assessed for additive internal noise, tolerance to external noise, and internal multiplicative noise. In Experiments 1 and 2 we trained participants by manipulating contrast in noise embedded sine-wave gratings and Random Dot Cinematograms (RDCs). The results indicate that older observers have higher additive internal noise and lower tolerance to external noise compared to younger observers. The rate of perceptual learning in older observers was found to be similar to that of younger observers suggesting that plasticity of motion processing mechanisms is well preserved in advancing age. Transfer of learning between sine-wave gratings and RDCs for both older and younger observers was examined in an analysis of pre/post-test measurements. The results indicate that transfer of learning occurred for both age groups. This suggests that older individuals maintain a sufficient degree of plasticity to allow generalization between sine-wave gratings and RDCs. In addition, training with RDCs was found to produce greater perceptual learning than training with sine-wave gratings. These experiments provide important findings regarding changes in perceptual efficiency for motion perception in older adults and suggest that perceptual learning is an effective approach for recovering from age-related declines in visual processing.     

VEP contrast sensitivity responses reveal reduced functional segregation of mid and high filters of visual channels in autism

Despite the vast amount of behavioral data showing a pronounced tendency in individuals with autism spectrum disorder (ASD) to process fine visual details, much less is known about the neurophysiological characteristics of spatial vision in ASD. Here, we address this issue by assessing the contrast sensitivity response properties of the early visual-evoked potentials (VEPs) to sine-wave gratings of low, medium and high spatial frequencies in adults with ASD and in an age- and IQ-matched control group. Our results show that while VEP contrast responses to low and high spatial frequency gratings did not differ between ASD and controls, early VEPs to mid spatial frequency gratings exhibited similar response characteristics as those to high spatial frequency gratings in ASD. Our findings show evidence for an altered functional segregation of early visual channels, especially those responsible for processing mid- and high-frequency spatial scales.     

Greater losses in sensitivity to second-order local motion than to first-order local motion after early visual deprivation in humans

We compared sensitivity to first-order versus second-order local motion in patients treated for dense central congenital cataracts in one or both eyes. Amplitude modulation thresholds were measured for discriminating the direction of motion of luminance-modulated (first-order) and contrast modulated (second-order) horizontal sine-wave gratings. Early visual deprivation, whether monocular or binocular, caused losses in sensitivity to both first- and second-order motion, with greater losses for second-order motion than for first-order motion. These findings are consistent with the hypothesis that the two types of motion are processed by different mechanisms and suggest that those mechanisms are differentially sensitive to early visual input.     

Contrast sensitivity at low spatial frequencies in the hooded rat

Behavioural methods were used to measure spatial contrast sensitivity, to vertical sine-wave luminance profile gratings, in hooded rats. Low frequency attenuation of sensitivity was observed at spatial frequencies below 0.lc/deg. This attenuation exceeded that produced by varying the number of periods in the grating, indicating that it was a function of spatial frequency as well as number of periods.     

Space domain properties of a spatial frequency channel in human vision

Many properties of contrast detection in human vision may be described with reference to a set of tuned spatial frequency channels. The spatial sensitivity of the channel with optimal sensitivity at 3.0 c/deg was studied by measuring threshold as a function of the width of truncated 3.0 c/deg sine-wave gratings that ranged from 2.3′ to 4.6°. Three strategies were used to isolate the threshold response of the channel: (1) The channel at 3.0 c/deg was chosen because of its position at the peak of the contrast sensitivity function. (2) A discrimination paradigm was used in which test stimuli were superimposed on a low contrast grating which was shown to selectively facilitate their detection. (3) The detecting channel was more sensitive to the sine-wave configuration of the test stimuli than to more conventional spatial summation stimuli, such as rectangular bars. Results of the main experiment showed that threshold contrasts for the truncated sine-wave stimuli declined in two stages. From 2.3′ to 40′, the threshold decline was steep, with a plateau at 10′. From 40′ to 4.6°, threshold declined as a power function of stimulus width with an exponent of ?0.35. The data of the main experiment were used to derive the spatial receptive field sensitivity for the channel at 3.0 c/deg. The data were accounted for by spatial summation within a receptive field, and probability summation in space across receptive fields.     

The direction-selective contrast response of area 18 neurons is different for first- and second-order motion

Cortical neurons selective for the direction of motion often exhibit some limited response to motion in their nonpreferred directions. Here we examine the dependence of neuronal direction selectivity on stimulus contrast, both for first-order (luminance-modulated, sine-wave grating) and second-order (contrast-modulated envelope) stimuli. We measured responses from single neurons in area 18 of cat visual cortex to both kinds of moving stimuli over a wide range of contrasts (1.25-80%). Direction-selective contrast response functions (CRFs) were calculated as the preferred-minus-null difference in average firing frequency as a function of contrast. We also applied receiver operating characteristic analysis to our CRF data to obtain neurometric functions characterizing the potential ability of each neuron to discriminate motion direction at each contrast level tested. CRFs for sine-wave gratings were usually monotonic; however, a substantial minority of neurons (35%) exhibited nonmonotonic CRFs (such that the degree of direction selectivity decreased with increasing contrast). The underlying preferred and nonpreferred direction CRFs were diverse, often having different shapes in a given neuron. Neurometric functions for direction discrimination showed a similar degree of heterogeneity, including instances of nonmonotonicity. For contrast-modulated stimuli, however, CRFs for either carrier or envelope contrast were always monotonic. In a given neuron, neurometric thresholds were typically much higher for second- than for first-order stimuli. These results demonstrate that the degree of a cell's direction selectivity depends on the contrast at which it is measured, and therefore is not a characteristic parameter of a neuron. In general, contrast response functions for first-order stimuli were very heterogeneous in shape and sensitivity, while those for second-order stimuli showed less sensitivity and were quite stereotyped in shape.     

Contrast sensitivity

Spatial contrast is a physical dimension referring to the light-dark transition of a border or an edge in an image that delineates the existence of a pattern or an object. Contrast sensitivity refers to a measure of how much contrast a person requires to see a target. Contrast-sensitivity measurements differ from acuity measurements; acuity is a measure of the spatial-resolving ability of the visual system under conditions of very high contrast, whereas contrast sensitivity is a measure of the threshold contrast fur seeing a target. Today the most common methods for measuring contrast sensitivity are chart-based systems that can be mounted on the wall. These charts use test targets that are either sine-wave gratings or letters. Which specific chart a clinician selects should be guided by his or her purpose in using contrast sensitivity for patient management. In the research setting, chart selection should rest on ensuring that the scientific aims of the study are met. Contrast-sensitivity tests can provide useful information by revealing in some conditions visual loss not identifiable through visual acuity tests, by providing another method of monitoring treatments, and by providing a better understanding of visual performance problems faced by persons with vision impairment.     

Flicker sensitivity as a function of target area with and without temporal noise

Flicker sensitivities (1–30 Hz) in foveal, photopic vision were measured as functions of stimulus area with and without strong external white temporal noise. Stimuli were circular, sinusoidally flickering sharp-edged spots of variable diameters (0.25–4°) but constant duration (2 s), surrounded by a uniform equiluminant field. The data was described with a model comprising (i) low-pass filtering in the retina (R), with a modulation transfer function (MTF) of a form derived from responses of cones; (ii) normalisation of the temporal luminance distribution by the average luminance; (iii) high-pass filtering by postreceptoral neural pathways (P), with an MTF proportional to temporal frequency; (iv) addition of internal white neural noise (N_i); (v) integration over a spatial window; and (vi) detection by a suboptimal temporal matched filter of efficiency η. In strong external noise, flicker sensitivity was independent of spot area. Without external noise, sensitivity increased with the square root of stimulus area (Piper's law) up to a critical area (A_c), where it reaches a maximum level (S_max). Both A_c and η were monotonic functions of temporal frequency (f), such that log A_c increased and log η decreased linearly with log f. Remarkably, the increase in spatial integration area and the decrease in efficiency were just balanced, so A_c(f)η(f) was invariant against f. Thus the bandpass characteristics of S_max(f) directly reflected the composite effect of the distal filters R(f) and P(f). The temporal equivalent (N_it) of internal neural noise (N_i) decreased in inverse proportion to spot area up to A_c and then stayed constant indicating that spatially homogeneous signals and noise are integrated over the same area.