Psychophysical Contrast Sensitivity Research Articles

Spatial–temporal contrast sensitivity of the eye alignment reflex

The binocular alignment of the eyes involves both voluntary and reflexive mechanisms, but little is known about the visual input and neurological pathway of the reflex component. Our studies examined the role of spatiotemporal frequency and contrast in the control of reflex eye alignment, and compared the contrast sensitivity of the alignment reflex with psychophysical contrast sensitivity. We measured the contrast sensitivity of vertical disparity-driven vergence eye movements in response to bandwidth filtered static or 6 Hz counterphase flickering noise and measured psychophysical detection sensitivity for the same stimuli. Contrast thresholds for producing a detectable vertical alignment change (measured with nonius lines) were determined using a staircase method for 7 spatial frequencies [0.25–16 cycles per degree] and 3 vertical disparities [5, 10, and 30 arcmin] in 7 adults with normal or corrected to normal vision. The main findings of this study are, (1) the vertical alignment reflex had overall relatively high contrast sensitivity, comparable to but somewhat less than visual detection thresholds, (2) the most effective stimulus spatial frequency scaled in inverse proportion to the disparity being stimulated, and (3) unlike psychophysical contrast sensitivity, the eye alignment reflex contrast sensitivity was not improved by flickering low spatial frequencies.

Retinal-image quality and contrast sensitivity function in eyes with epiretinal membrane: a cross-sectional observational clinical study

BackgroundTo investigate the effect of idiopathic epiretinal membrane (ERM) on the retinal-image quality and psychophysical contrast sensitivity function (CSF).MethodsForty-four subjects with diagnosis of idiopathic unilateral ERM were enrolled in this cross-sectional observational clinical study. The fellow unaffected eyes were set as the control group. For retinal-image quality assessment, an Optical Quality Analysis System (OQAS) based on double-pass technique was used to evaluate objective scatter index (OSI) and Strehl ratio. For visual performance, the CSF under photopic condition was measured.ResultsFor retinal-image quality, the result of double-pass device revealed a significant lower Strehl ratio and larger OSI in the ERM eyes compared to the fellow eyes (all P < 0.05). For visual performance, the CSF at all spatial frequencies under photopic condition were also significantly degraded in the ERM eyes compared to the fellow eyes (all P < 0.05). For the ERM eyes, the reduction of Strehl ratio and CSF was 29.41 and 54.39%, respectively, and the increase of OSI was 164.10% compared to the fellow eyes. Besides, BCVA significantly correlated to the total CSF (ERM eyes, r = − 0.53, P < 0.001; the fellow eyes, r = − 0.467, P = 0.002) and Strehl ratio (ERM eyes, r = − 0.485, P = 0.001; the fellow eyes, r = − 0.311, P = 0.043) in both of the ERM and the fellow eyes.ConclusionEyes affected with ERM showed poorer retinal-image quality and visual performance than the normal eyes. Retinal-image quality measured by OQAS based on double-pass technique could be useful for assessing the retinal-image quality for ERM-affected eyes, in which retinal scattering was significantly increased.

Contrast and spatial frequency modulation for diagnosis of amblyopia: An electrophysiological approach

PurposeTo evaluate the diagnostic value of visual evoked potentials (VEPs) and to find out which test setting has the most sensitivity and specificity for amblyopia diagnosis. MethodsThirty-three adult anisometropic amblyopes were intended in this study and were tested for visual evoked potentials with different stimulus conditions including three spatial frequencies [1, 2, and 4-cycles-per-degree (cpd)] at four contrast levels (100, 50, 25, and 5%). We also tested psychophysical contrast sensitivity and compared the results with electrophysiological ones. We plotted Receiver Operating Characteristic (ROC) curve for each VEP recording and psychophysical contrast sensitivity to evaluate the area under the curve, sensitivity, specificity, and cut-point value of each test stimulus for detecting amblyopic eyes. ResultsThirty-three amblyopic and 33 non-amblyopic eyes were examined for psychophysical contrast sensitivity and VEPs. Area under the ROC curve (AURC) findings showed that VEP with different stimulus settings can significantly detect amblyopic eyes, as well as psychophysical contrast sensitivity test. We found that P100 amplitudes had the largest AURC in response to stimuli of 2-cpd spatial frequency at 50 (P < 0.001) and 25% (P < 0.001) contrast levels, respectively. Cut-off amplitudes for these stimuli were 8.65 and 4.50 μV, which had a sensitivity of 0.758 and 0.697 and a specificity of 0.788 and 0.848, respectively. The sensitivity and specificity of VEP P100 amplitude in response to the stimuli with 2 cpd spatial frequency and 50 and 25% contrast were greater than the findings obtained from psychophysical contrast sensitivity test. ConclusionAccording to our findings, assessment of VEP amplitudes in response to stimuli of 2-cpd spatial frequency at 50 and 25% contrast levels can best detect amblyopia with highest sensitivity and specificity and thus, are the protocols of choice for detection of amblyopic eyes.

Comparison of two methods for measuring contrast sensitivity in anisometropic amblyopia

PurposeThis study aimed to evaluate two psychophysical contrast sensitivity testing methods in amblyopic patients. MethodsThirty-three adults with anisometropic amblyopia participated in this study. Psychophysical contrast sensitivity was measured for both amblyopic and fellow eyes of the participants at 1, 3, and 5 cycles per degree (cpd) spatial frequencies by Freiburg visual acuity and contrast test (FrACT) and Metrovision contrast sensitivity test, which employ sine-wave gratings for measurement of contrast sensitivity. We evaluated the correlation between the two tests and used Bland–Altman analysis to measure the agreement between the two methods. ResultsExcept for 1 cpd in amblyopic eyes, FrACT showed significantly higher contrast sensitivity measurements than Metrovision at all spatial frequencies both in normal and amblyopic eyes (P < 0.01). The difference between the two methods increased with an increase in spatial frequency. There was a significant correlation between the two tests at most of the spatial frequencies. While the difference between the results of the two tests increased with an increase in contrast sensitivity in amblyopic eyes, we found an inter-test agreement in normal eyes. ConclusionAlthough both FrACT and Metrovision employ sine-wave gratings to measure contrast sensitivity, there are some differences between them, and their results can not be used interchangeably.

The relationship between contrast detection and saccadic reaction times with attention.

Visual spatial attention can increase contrast sensitivity and decrease saccadic or manual reaction times (RTs) in attended compared to unattended locations. As stimulus contrast also influences saccadic RT, we asked whether the effect of attention on saccadic RT can be accounted for by the change in contrast sensitivity. We used a dual task paradigm that recorded saccadic RTs and psychophysical responses to targets of various contrast. On each trial, subjects were presented with a central fixation stimulus, which also carried an endogenous cue (an arrow) to the target's radial direction but not its eccentricity. Across trials, the cue was 75% valid, 12.5 % invalid and 12.5 % neutral (no arrow). Circular gratings m-scaled for eccentricity were flashed for 8 ms in one of 8 radial directions around fixation, at either 3 or 6 deg eccentricity, with one of 9 contrasts. The subjects' tasks were to (1) saccade to the location where the target flashed and (2) respond via button press if the target flashed at the nearer (3 deg) or the farther (6 deg) eccentricity. Saccadic RTs and psychophysical responses to targets at the different contrast levels were analyzed to assess the effects of cueing. Valid cueing produced a factor of 1.2 increase in contrast sensitivity, relative to invalid-cue trials. Valid cueing also reduced saccadic RTs for contrast levels at and above the detection threshold. However, the effect of cueing on saccadic RT is accounted for only partly by the increase in psychophysical contrast sensitivity. A comparison of RT vs. contrast for valid and invalid cueing shows a combined effect: a shift consistent with the observed change in contrast sensitivity, and an overall change in latency of roughly 20 msec. Attention appears to influence saccadic RTs in more than one way. Meeting abstract presented at VSS 2014

Chromatic spatial contrast sensitivity estimated by visual evoked cortical potential and psychophysics

The purpose of the present study was to measure contrast sensitivity to equiluminant gratings using steady-state visual evoked cortical potential (ssVECP) and psychophysics. Six healthy volunteers were evaluated with ssVECPs and psychophysics. The visual stimuli were red-green or blue-yellow horizontal sinusoidal gratings, 5° × 5°, 34.3 cd/m2 mean luminance, presented at 6 Hz. Eight spatial frequencies from 0.2 to 8 cpd were used, each presented at 8 contrast levels. Contrast threshold was obtained by extrapolating second harmonic amplitude values to zero. Psychophysical contrast thresholds were measured using stimuli at 6 Hz and static presentation. Contrast sensitivity was calculated as the inverse function of the pooled cone contrast threshold. ssVECP and both psychophysical contrast sensitivity functions (CSFs) were low-pass functions for red-green gratings. For electrophysiology, the highest contrast sensitivity values were found at 0.4 cpd (1.95 ± 0.15). ssVECP CSF was similar to dynamic psychophysical CSF, while static CSF had higher values ranging from 0.4 to 6 cpd (P < 0.05, ANOVA). Blue-yellow chromatic functions showed no specific tuning shape; however, at high spatial frequencies the evoked potentials showed higher contrast sensitivity than the psychophysical methods (P < 0.05, ANOVA). Evoked potentials can be used reliably to evaluate chromatic red-green CSFs in agreement with psychophysical thresholds, mainly if the same temporal properties are applied to the stimulus. For blue-yellow CSF, correlation between electrophysiology and psychophysics was poor at high spatial frequency, possibly due to a greater effect of chromatic aberration on this kind of stimulus.

Specific impairment of visual spatial covert attention mechanisms in Parkinson's disease

Visual deficits in early and high level processing nodes have been documented in Parkinson's disease (PD). Non-motor high level visual integration deficits in PD seem to have a cortical basis independently of a low level retinal contribution. It is however an open question whether sensory and visual attention deficits can be separated in PD. Here, we have explicitly separated visual and attentional disease related patterns of performance, by using bias free staircase procedures measuring psychophysical contrast sensitivity across visual space under covert attention conditions with distinct types of cues (valid, neutral and invalid). This further enabled the analysis of patterns of dorsal–ventral (up–down) and physiological inter-hemispheric asymmetries. We have found that under these carefully controlled covert attention conditions PD subjects show impaired psychophysical performance enhancement by valid attentional cues. Interestingly, PD patients also show paradoxically increased visual homogeneity of spatial performance profiles, suggesting flattening of high level modulation of spatial attention. Finally we have found impaired higher level attentional modulation of contrast sensitivity in the visual periphery, where mechanisms of covert attention are at higher demands. These findings demonstrate a specific loss of attentional mechanisms in PD and a pathological redistribution of spatial mechanisms of covert attention.

Responses to chromatic and luminance contrast in glaucoma: a psychophysical and electrophysiological study

Increasing anatomical evidence indicates that large retinal ganglion cells (M-cells) are preferentially damaged in primary open angle glaucoma (OAG), while the smaller ganglion cells (P-cells) are relatively spared. In 13 patients with defined OAG and modest visual field defects, we evaluated the responses to stimuli that are expected to involve primarily the function of the P-pathway and compared them with those of control subjects. The psychophysical contrast sensitivity (CS), the PERG and VEPs were measured for red-green gratings of pure chromatic contrast, as well as yellow-black gratings of pure luminance contrast. As compared with controls, OAG patients had reduced CS for both luminance and chromatic contrast stimuli by about 6 dB. PERGs and VEPs to luminance stimuli were little affected, whereas those to chromatic stimuli were both reduced in amplitude and delayed. These results indicate that visual dysfunction in glaucoma is not selective for the M-pathway, and that responses to equiluminant colour-contrast stimuli may be of diagnostic value.

Optical Modulation Transfer and Contrast Sensitivity with Decentered Small Pupils in the Human Eye

Human observers experience a large decrement in visual acuity when a small artificial pupil is displaced from the center to the edge of the dilated natural pupil. This decrement in visual resolution, called the Campbell effect, has been attributed to the retina, the ocular optics, or a combination of the two. Given the uncertainty about the relative magnitudes of these two components over the range of spatial frequencies used in normal vision, we have obtained objective measurements of the retinal image quality and psychophysical measurements of visual performance, with decentered pupils. The contributions of monochromatic aberrations were determined by using double pass measurements of the modulus of the optical transfer function (MTF). For all of the observers, there was a substantial decrement in the MTF with decentering, showing that even when using a 1.5 mm pupil and appropriate spherical/cylindrical refractive corrections, there is a considerable contribution of monochromatic aberrations to the effect. We have compared these optical MTFs with the psychophysical contrast sensitivity functions (CSFs) measured under exactly the same conditions using green gratings generated on the screen of a color monitor. At the low and intermediate spatial frequencies considered (2–16 c/deg), we find the fall in the CSF is much greater than the fall in the monochromatic MTF, with the difference becoming greater as the spatial frequency increases. We show that this discrepancy can be mostly attributed to the effect of transverse chromatic aberration due to the bandwidth of the green stimulus used for the CSF measurements. In conclusion, the combination of the ocular transverse chromatic aberration and monochromatic aberrations accounts for the loss in visual sensitivity found with a decentered small pupil at low and intermediate spatial frequencies. Copyright © 1996 Elsevier Science Ltd.

Visual Ageing: Unspecific Decline of the Responses to Luminance and Colour

We have investigated whether ageing affects selectively the responses to equiluminant patterns of pure colour contrast. In two groups of subjects (mean ages 29 and 72 yr) contrast thresholds were measured psychophysically for the detection and for the discrimination of the direction of motion of drifting gratings. The gratings were modulated either in pure luminance contrast (and uniform colour), or pure chromatic contrast (red-green equiluminant gratings). In subjects of the same age groups, visual evoked potentials (VEP) were recorded in response to gratings with either pure luminance contrast or pure colour contrast sinusoidally reversed in contrast at various temporal frequencies. It was shown that psychophysical contrast sensitivity for equiluminant patterns deteriorates significantly with age, and VEP latency increases. However, these effects of ageing on the responses to patterns of pure colour contrast are substantially the same as those observed in the same subjects for stimuli with pure luminance contrast. The results suggest that ageing causes a small and unspecific decline of the response of the visual system to luminance and colour contrast. Copyright © 1996 Elsevier Science Ltd.

Retinal and cortical evoked responses to chromatic contrast stimuli. Specific losses in both eyes of patients with multiple sclerosis and unilateral optic neuritis.

It is known that colour vision may be altered in optic neuritis. Our aim was to establish whether chromatic and achromatic vision are differentially impaired using stimuli designed to favour the activity of either the magnocellular or the parvocellular stream of the visual pathway. Fourteen patients with a past history of unilateral optic neuritis in the course of multiple sclerosis and 10 age-matched control subjects were included in the study. Patients had relatively good visual acuity in the affected eyes and no gross colour deficits (Ishihara). Stimuli were alternating gratings of low spatial frequency and of different chromaticity along the red-green axis. The psychophysical contrast sensitivity (CS) was measured at 5 Hz as a function of colour ratio [red/(red + green)] to evaluate both the equiluminant point (the colour ratio corresponding to the lowest CS) and the CS for isochromatic, luminance gratings (red-black and green-black). Steady-state (2-24 Hz) and transient pattern electroretinograms (PERGs) and visually evoked potentials (VEPs) were recorded in response to high contrast (90%) stimuli of low spatial frequency (0.3 cycles deg-1) modulated in either pure chromatic contrast (equiluminant red-green) or pure luminance contrast (yellow-black). On average, CSs were reduced (10 dB) in optic neuritis eyes compared with controls for both luminance and chromatic gratings. In the VEPs (both transient and steady-state) amplitude losses and latency delays were far larger for the chromatic VEPs than for the luminance VEPs. Chromatic VEP latency delays were remarkable also in the fellow, clinically normal, eyes. Significant losses were apparent in both the luminance and chromatic PERG. However, the chromatic PERG was comparatively more altered. In agreement with previous reports, selective losses were not apparent at threshold. By contrast, suprathreshold electrophysiological responses displayed a clear dissociation between luminance and colour, suggesting that the parvocellular stream, compared with the magnocellular stream is more impaired in optic neuritis.

Low luminance contrast sensitivity: Effects of training on psychophysical and optokinetic nystagmus thresholds in man

We compared psychophysical contrast sensitivity function (Ψ-CSF) and optokinetic contrast sensitivity function (OKN-CSF) in man, for the combination of three spatial and three temporal frequencies. Ψ-CSF was defined as the inverse of the contrast threshold, that is the contrast value of a sinusoidal grating for which a subject was able to identify the width of a drifting grating. OKN-CSF was defined as the inverse of the contrast value of the grating which triggered an involuntary optokinetic nystagmus. In highly experienced subjects, OKN-CSF was overall higher than Ψ-CSF. More precisely, differences between both contrast sensitivity functions occurred mainly in the low spatio-temporal frequency range (below 4 c/deg and 9 Hz). In naive subjects, Ψ-CSF reached the level of OKN-CSF after two consecutive test sessions. OKN-CSF did not change with training. Similarly, high spatio-temporal frequency psychophysical thresholds did not change with training and, moreover, approximated OKN-CSF thresholds. Low spatio-temporal frequency psychophysical sensitivity was initially lower than corresponding OKN-CSF sensitivity; however, after only two training sessions, the two functions were indistinguishable due to a selective increase in psychophysical low spatio-temporal frequency sensitivity.

On the role of X and simple cells in human contrast processing

We investigated the potential role of retinal X and cortical simple cells in determining human psychophysical detection performance under contrast masking conditions. Since both X and simple cells exhibit a null phase, the phase of a background mask should affect the visibility of a test grating processed by such cells. Sinusoidal test gratings of either 1 or 7 c/deg were presented as a sustained or transient increment against a background mask of the same size and spatial frequency at either 0 or 90 deg phase. For background contrasts from 0.5% up to 40%, psychophysical contrast sensitivity was phase-independent for all conditions. Therefore, either (1) contrast threshold is mediated by cells with non-linear spatial summation properties, such as F or complex cells, or (2) the masking effect of the background occurs after a phase-insensitive combination or pooling of simple cell responses in the cortex.

Psychophysical and electrofunctional contrast sensitivity in cataractous patients treated with bendazac-lysine salt

The clinical progression of the cataract may be influenced by drugs which reduce the denaturation of lens proteins. One of the most promising drugs is the bendazac-lysine salt. The drug was used in a double-blind study of a group of patients with initial cortical cataract in order to evaluate the changes in visual acuity and contrast sensitivity by means of a psychophysical and an electrophysiological method. After 6 months of treatment with bendazac the mean values of visual acuity showed a statistically significant increase in respect to baseline values, as well as an improvement of the threshold of contrast for most spatial frequencies. In the eyes treated with placebo there was no statistical difference between the visual acuity at baseline and after the treatment, but an increase of the contrast threshold for many spatial frequencies. The treatment with bendazac, when compared to the administration of placebo, leads to a statistically significant improvement of the contrast threshold and induces a global improvement on the visual conditions.

Variability of the steady-state visually evoked potential: Interindividual variance and intraindividual reproducibility of spatial frequency tuning

At low contrast levels there is good agreement between the psychophysical contrast sensitivity function and the tuning curve of the visually evoked potential (i.e., VEP amplitude vs spatial frequency). At high contrast, however, some researchers have found bimodal VEP tuning curves whereas others have not. We studied the VEP in 22 subjects in a short-term cross-sectional study and in 13 subjects in a longitudinal study over 8 sessions covering 28 days. Grating stimuli with 60% contrast were square-wave modulated in time (7.8 reversals/s) and space (0.06-16 cycles/degree). We found large interindividual variance in the shape of the tuning curves; about half of the subjects showed a unimodal shape, while the other half showed a bimodal one (with a 'notch' between 1 and 2 cycles/degree). These features turned out to be stable in the longitudinal study, where variability could mainly be ascribed to a multiplicative influence common to all spatial frequencies. The marked interindividual differences in the shape of the tuning curve, which seem to be intraindividually stable, may explain previous discrepancies. It is not yet clear why the notch exists in about half of our subjects.

Fluctuations of visual evoked potential amplitudes and of contrast sensitivity in Uhthoff's symptom.

Visual evoked potentials (VEPs) to pattern reversal and psychophysical contrast sensitivity (CS) were tested in five patients with Uhthoff's symptom before and after exercise. Four of the cases were related to a demyelinating disease and one patient had a severe brain injury. Uhthoff's symptom occurred also in neuropathies without previous clinical neuritis and in eyes with normal latency time of the VEPs. A depression of CS was observed by all five patients after exercise; a transient "overshoot" of CS following the depression was present in three of the patients. VEP amplitudes were reduced by exercise in three cases. Responses by stimulation with small checks were more affected. Paradoxical increased VEP amplitudes after exercise in the presence of a loss of psychophysical CS were found in one case.

Contrast in night vision

Simultaneous contrast has been ascribed so far to spatial interactions in retinal receptive fields. The antagonistic surround of retinal receptive fields disappears after dark adaptation. The present work shows that simultaneous contrast for large fields and border contrast effects are preceived at low scotopic luminances. Moreover, both the spatial and the temporal psychophysical contrast sensitivity functions show a low-frequency attenuation at scotopic mean luminances. These findings support the conclusion drawn from recent electrophysiological results ( Maffei and Fiorentini, 1972 ) that the analysis of contrast is not performed in the retina, but at a higher level of the visual system.