D-CHART: A Novel Method of Measuring Metamorphopsia in Epiretinal Membrane and Macular Hole.

BACKGROUND: Metamorphopsia is common in macular disease. Current techniques for measuring metamorphopsia require good vision or costly equipment. The authors report a method that uses printed cards. METHODS: The cards have a grid of squares arranged in a ring around fixation. There are four rings, at different distances from fixation, divided into eight sectors. The separation of the grid elements ranges from 0.4° to 1.8°. Subjects indicate in which sector lines of squares are distorted. The sum of the maximum separation perceived as distorted in each sector gives the total metamorphopsia score. Thirty-three eyes with epiretinal membrane and 29 eyes with macular hole were tested. Twenty-four eyes were tested again after surgery. In 18 subjects, the preoperative test was performed twice to assess repeatability. RESULTS: The median preoperative total metamorphopsia score was 10.2 for macular hole and 5.2 for epiretinal membrane. After surgery, the median total metamorphopsia score was 0.5 for macular hole and 0.45 for epiretinal membrane. Test-retest results showed good correlation. Improvement in metamorphopsia did not correlate with change in visual acuity. CONCLUSION: Measurement of metamorphopsia may be useful in the management of macular hole and epiretinal membrane. D-charts are a simple and inexpensive method of quantifying metamorphopsia that can be used in a clinical setting.

Contact lens in vitro wettability by interferometry measures of drying dynamics.

PURPOSE: To develop with a thin film interferometer new parameters to describe the drying properties of contact lenses; these are: time to first break-up (onset latency), duration of lens surface drying (drying duration), maximum speed of increase in the drying area (maximum speed), and the time to reach maximum drying speed (peak latency). These new parameters were compared with the contact angle (CA) measurement of contact lenses by the captive bubble (CB) technique. METHODS: A thin film interferometer was connected to a digital camera, which captured images of the pre-lens liquid film of seven soft contact lenses in vitro: 1-Day Acuvue Moist, ACUVUE Oasys (Johnson & Johnson Vision Care); SofLens 38, PureVision (Bausch & Lomb); AirOptix Night & Day, AirOptix Aqua (CIBA Vision, Corp); and Proclear (Cooper Vision). The images were obtained from the lens surface when it was wet until it became dry (7 images per second) to generate video clips. A program was created in MATLAB to analyze the results. The CAs of the lens surfaces were measured by the CB technique with an OCA-20 contact angle analyzer (Data Physics Instruments). RESULTS: There were no significant relationships found between the CA and the new drying parameters (P > 0.05). Analysis of the results using analysis of variance and post hoc tests showed that the surface drying dynamics measures effectively differentiated between more of the soft lenses than the CA. CONCLUSIONS: A new technique is described for assessing the drying characteristics of contact lenses by observations of the pattern, size, and speed of pre-lens liquid film break-up observed by a thin film interferometry. The interferometry method allowed evaluation and differentiation between lens materials. This new technique has the major advantage that it can be applied to evaluate contact lens wettability in vivo and in vitro.

Spatial frequency selectivity of visual suppression during convergence eye movements.

Visual suppression of low-spatial frequency information during eye movements is believed to contribute to a stable perception of our visual environment. While visual perception has been studied extensively during saccades, vergence has been somewhat neglected. Here, we show that convergence eye movements reduce contrast sensitivity to low spatial frequency information around the onset of the eye movements, but do not affect sensitivity to higher spatial frequencies. This suggests that visual suppression elicited by convergence eye movements may have the same temporal and spatial characteristics as saccadic suppression.

Global shape processing deficits are amplified by temporal masking in migraine.

PURPOSE: Individuals with migraine show subtle defects in a range of visual tasks compared to nonmigraineurs. Increased neuronal noise can account for some of these deficits. To examine the generality of increased noise in migraine, masking effects were compared in migraineurs and headache-free controls using a shape discrimination task, thought to involve processing in extrastriate cortical areas. METHODS: Nine migraineurs with aura, nine migraineurs without aura, and nine headache-free controls participated. observers had to detect deviations in circular shapes with or without a larger contour mask. The nonoverlapping mask was presented at five temporal intervals (stimulus onset asynchronies, SOA): 0 (simultaneous), 66, 100, 133, and 250 ms. RESULTS: Migraineurs with aura performed worse in all tests than migraineurs without aura and controls. Both migraine groups performed poorer than controls at discriminating shapes without masks. Typical masking functions were obtained from all groups, but they were steeper for migraineurs than controls with thresholds raised most dramatically (2.1 and 4.4 times for migraineurs without and with aura relative to controls, respectively) at SOAs where masks had their most detrimental effect (66-100 ms). Modeling the effect of masking showed that raised internal noise alone is insufficient to explain these deficits. Rather, an abnormal nonlinear transducer function (e.g., as part of gain-control) together with increased multiplicative noise is required to capture the data. CONCLUSIONS: The findings are consistent with an extrastriate deficit in migraine that cannot be explained completely by defective inhibition.

Age-related deficits in attentional control of perceptual rivalry.

Some aspects of attentional processing are known to decline with normal aging. To understand how age affects the attentional control of perceptual stability, we investigated age-related changes in voluntarily controlled perceptual rivalry. Durations of the dominant percept, produced by an ambiguous Rubin vase-faces figure, were measured in conditions that required passive viewing and attentional control: holding and switching the dominant percept. During passive viewing, mean dominance duration in the older group was significantly longer (63%) than the dominance duration found in the young group. This age-related deficit could be due to a decline in the apparent strength of the alternating percepts as a result of higher contrast gain of visual cortical activity and a reduction in the amount of attentional resources allocated to the ambiguous stimulus in older people compared to young adults. In comparison to passive viewing, holding the dominant percept did not significantly alter the dominance durations in the older group, while the dominance durations in the young group were increased (∼100%). The dominance durations for both age groups in switch conditions were reduced compared to their passive viewing durations (∼40%). The inability of older people to voluntarily prolong the duration of the dominant percept suggests that they may have abnormal attentional mechanisms, which are inefficient at enhancing the effective strength of the dominant percept. Results suggest that older adults have difficulty holding attended visual objects in focus, a problem that could affect their ability to carry out everyday tasks.

Image jitter enhances visual performance when spatial resolution is impaired.

PURPOSE: Visibility of low-spatial frequency stimuli improves when their contrast is modulated at 5 to 10 Hz compared with stationary stimuli. Therefore, temporal modulations of visual objects could enhance the performance of low vision patients who primarily perceive images of low-spatial frequency content. We investigated the effect of retinal-image jitter on word recognition speed and facial emotion recognition in subjects with central visual impairment. METHODS: Word recognition speed and accuracy of facial emotion discrimination were measured in volunteers with AMD under stationary and jittering conditions. Computer-driven and optoelectronic approaches were used to induce retinal-image jitter with duration of 100 or 166 ms and amplitude within the range of 0.5 to 2.6° visual angle. Word recognition speed was also measured for participants with simulated (Bangerter filters) visual impairment. RESULTS: Text jittering markedly enhanced word recognition speed for people with severe visual loss (101 ± 25%), while for those with moderate visual impairment, this effect was weaker (19 ± 9%). The ability of low vision patients to discriminate the facial emotions of jittering images improved by a factor of 2. A prototype of optoelectronic jitter goggles produced similar improvement in facial emotion discrimination. Word recognition speed in participants with simulated visual impairment was enhanced for interjitter intervals over 100 ms and reduced for shorter intervals. CONCLUSIONS: Results suggest that retinal-image jitter with optimal frequency and amplitude is an effective strategy for enhancing visual information processing in the absence of spatial detail. These findings will enable the development of novel tools to improve the quality of life of low vision patients.

Long-range inhibitory mechanisms in the visual system are impaired in migraine sufferers.

BACKGROUND: After viewing dynamic noise surrounding a homogeneous grey patch (artificial scotoma), observers perceive a prolonged twinkling-noise after-image within the unstimulated area. It has been suggested that noise-stimulated neurons induce a long-range inhibition in neurons within the artificial scotoma, which generates a rebound signal perceived as twinkling noise following noise termination. We used this paradigm to test whether migraineurs have enhanced excitability or weakened inhibition. METHODS: Twinkling-noise duration was measured in 13 headache-free volunteers, 13 migraineurs with aura and 13 migraineurs without aura. RESULTS: The durations of the after-image were significantly shorter for both migraine groups compared to controls. DISCUSSION: Enhanced excitation of noise-activated neurons in migraineurs would produce stronger rebound activity and longer after-image durations, while weakened inhibitory mechanisms would diminish the rebound activity and shorten the after-image durations compared to control subjects. The results suggest that cortical inhibitory mechanisms might be impaired in migraineurs with and without aura.

Surface regions of illusory images are detected with a slower processing speed than those of luminance-defined images.

Research has shown that the processing time for discriminating illusory contours is longer than for real contours. We know, however, little whether the visual processes, associated with detecting regions of illusory surfaces, are also slower as those responsible for detecting luminance-defined images. Using a speed-accuracy trade-off (SAT) procedure, we measured accuracy as a function of processing time for detecting illusory Kanizsa-type and luminance-defined squares embedded in 2D static luminance noise. The data revealed that the illusory images were detected at slower processing speed than the real images, while the points in time, when accuracy departed from chance, were not significantly different for both stimuli. The classification images for detecting illusory and real squares showed that observers employed similar detection strategies using surface regions of the real and illusory squares. The lack of significant differences between the x-intercepts of the SAT functions for illusory and luminance-modulated stimuli suggests that the detection of surface regions of both images could be based on activation of a single mechanism (the dorsal magnocellular visual pathway). The slower speed for detecting illusory images as compared to luminance-defined images could be attributed to slower processes of filling-in of regions of illusory images within the dorsal pathway.

Investigating the mechanisms that may underlie the reduction in contrast sensitivity during dynamic accommodation.

Head and eye movements, together with ocular accommodation enable us to explore our visual environment. The stability of this environment is maintained during saccadic and vergence eye movements due to reduced contrast sensitivity to low spatial frequency information. Our recent work has revealed a new type of selective reduction of contrast sensitivity to high spatial frequency patterns during the fast phase of dynamic accommodation responses compared with steady-state accommodation. Here were report data which show a strong correlation between the effects of reduced contrast sensitivity during dynamic accommodation and velocity of accommodation responses, elicited by ramp changes in accommodative demand. The results were accounted for by a contrast gain control model of a cortical mechanism for contrast detection during dynamic ocular accommodation. Sensitivity, however, was not altered during attempted accommodation responses in the absence of crystalline-lens changes due to cycloplegia. These findings suggest that contrast sensitivity reduction during dynamic accommodation may be a consequence of cortical inhibition driven by proprioceptive-like signals originating within the ciliary muscle, rather than by corollary discharge signals elicited simultaneously with the motor command to the ciliary muscle.

Visual noise selectively degrades vision in migraine.

Purpose. Migraine is a disabling condition with underlying neuronal mechanisms that remain elusive. Migraineurs experience hyperresponsivity to visual stimuli and frequently experience visual disturbances. In the present study, the equivalent input noise approach was used to reveal abnormalities of visual processing and to isolate factors responsible for any such deficits. This approach partitions visual sensitivity into components that represent the efficiency of using the available stimulus information, the background internal noise due to irregular neuronal fluctuations, and the neuronal noise induced by the external stimulation. Methods. Ten migraine with aura, ten migraine without aura, and ten age-matched headache-free subjects participated. Performance in detecting luminance targets embedded in visual noise, resembling grainy photographs, was measured at various noise levels. Results. Contrast thresholds of the three subject groups were similar in the absence of noise, but both migraine groups performed worse in the presence of high noise levels, with performance of migraineurs with aura significantly poorer (P < 0.05) than that of control subjects. Data were fitted with a perceptual template model that showed that the model parameter determining the internal (neuronal) noise triggered by the external (stimulus) noise was significantly higher (P < 0.001) in both migraine groups than in the non-migraineur group. Migraineurs without aura also showed a significant (P < 0.05) though weak reduction of sampling efficiency (0.12 +/- 0.02) compared with control subjects (0.17 +/- 0.02). Conclusions. The results revealed substantial external noise-exclusion deficits in migraine with aura and a minor impairment of noise exclusion in migraine without aura. Migraineurs appeared prone to abnormally high variability of neuronal activity. This result provides a promising explanation of observed visual deficits in migraine.

Two eyes: square root 2 better than one?

Classical data on the detection of simple patterns show that two eyes are more sensitive than one eye. The degree of binocular summation is important for inferences about the underlying combination mechanism. In a signal detection theory framework, sensitivity is limited by internal noise. If noise is added centrally after binocular combination, binocular sensitivity is expected to be twice as good as monocular. If the noise is added peripherally at each eye prior to combination, binocular sensitivity will be sqrt[2] higher than monocular. In a large sample of observers (51), we measured contrast sensitivity for detection of gratings at several spatial frequencies using left, right, or both eyes. Estimates of binocular summation using both binocular summation ratios and Minkowski coefficients show a summation ratio with means in the range of 1.5-1.6. The 95% confidence interval overlaps with the value of sqrt[2] predicted by the peripheral noise model and does not overlap with the value of 2 predicted by the central noise model.

Induced internal noise in perceptual artificial scotomas created by surrounding dynamic noise.

Research has shown that exposure to a homogeneous gray patch surrounded by a dynamic noise background causes filling-in of the artificial scotoma by the twinkling noise from the surround. When the background is switched off, observers report perception of a prolonged patch of twinkling noise in the unstimulated area. We studied the effects of exposure to a centrally presented artificial scotoma and the twinkling aftereffect on the threshold for detecting a foveal Gabor patch embedded in external scotoma noise. The detection thresholds were mainly elevated in the absence of scotoma noise and less affected at higher levels of scotoma noise. The analysis of the experimental data using the equivalent input noise approach revealed that the reduced contrast sensitivity is due to induced internal noise whose variance is proportional to the strength of the surrounding noise. We did not find significant effects on the internal noise in a control experiment using flickering Gaussian noise samples of 1.6 Hz which did not cause filling-in and dynamic afterimage. These findings suggest that the perceptual phenomena caused by artificial scotomas may reflect increased variability of neural activity due to long-range interactions between the surrounding noise and unstimulated region of the artificial scotoma.

The two-pulse experiment and cross-correlation.

Rashbass [Rashbass, C. (1970). The visibility of transient changes of luminance. Journal of Physiology, 210, 165-186] presented pairs of flashes having various contrasts separated by a delay, and found that the thresholds for detecting the pairs fell on an ellipse. He fit the data using a model that computed the filtered energy of the pulses. Although this Rashbass model is phase-insensitive, many other experimental results show that humans can perform phase-sensitive detection consistent with a template-matching mechanism. We show that an observer who uses a form of template-matching produces thresholds that fall on an ellipse, just like the Rashbass model. The results from two-pulse experiments are consistent with the idea that humans cross-correlate the stimulus (signal or noise) with a filtered version of the expected signal rather than the signal itself. In symbols, we propose that observers compute integral r(t)[s(t) *h(t)]dt where r(t) is the received stimulus on a given trial [s(t)+n(t) or n(t)], s(t) is the signal, h(t) is the visual filter, and * is convolution.

Why is second-order vision less efficient than first-order vision?

Research has shown that the sensitivity to second-order modulations of carrier contrast is lower than that to first-order luminance modulations stimuli. We sought to compare the efficiency of processing first- and second-order information. Employing a phase-discrimination paradigm we found that when humans were given sufficient a priori information of signal parameters they detected both luminance and contrast modulations of 0.6 and 2c/deg by a phase-sensitive algorithm. The overall detection efficiency for second-order patterns, however, was lower that that for first-order stimuli. To study the factors which limit the efficiency of first- and second-order vision, we measured detection performance for luminance and contrast modulations of 0.6 and 2c/deg embedded in Gaussian noise. The results showed that the detection of second-order patterns had lower sampling efficiency and higher additive internal noise as compared to the detection of first-order stimuli. Classification images for detecting contrast modulations of 2c/deg resembled the side-band component of the contrast modulations which suggests that human observers may detect contrast modulations of a sinusoidal carrier using first-order luminance channels. The lower sensitivity of the mechanism detecting second-order patterns might be due to higher levels of additive internal noise and lower sampling efficiency than those of the mechanism analysing first-order patterns.

Human cortical responses to contrast modulations of visual noise.

We studied visual evoked potentials (VEPs) elicited by second-order contrast modulations of binary dynamic noise and first-order luminance modulations. Using a 3-point Laplacian operator centred on Oz, we found that contrast modulations of both low and higher spatial frequencies elicited a negative component whose latency was about 200 ms. The latency of this component was significantly longer than that of the early Laplacian components to first-order luminance modulations. These findings could be due to slower first-stage linear filters and additional processing stages of the second-order pathway. The topographical analysis of scalp recorded VEPs to central and half-field stimulation has suggested that the responses to second-order patterns are likely to be generated by neuronal structures within the primary visual cortex which may have inputs from extrastriate neurons via feedback connections.

Illusory percepts of moving patterns due to discrete temporal sampling.

Continuously, moving objects under continuous illumination can be seen to move in a direction opposite to their actual motion. This illusory reversed motion can be explained as due to discrete temporal sampling of the moving stimulus by the visual system. If temporal sampling lies behind the illusory motion, then the probability of illusory motion should depend on the temporal frequency of the motion stimulus. By presenting contracting bull's-eye gratings of various spatial frequencies we were able to tease apart the drift speed and temporal frequency. The prevalence of illusory percepts depended on the temporal frequency, not the speed. The data suggest that the human visual system samples the incoming stimulation at a rate near 16 Hz.

Factors limiting suprathreshold vision measured by a flash-sound simultaneity paradigm.

Internal noise and sampling efficiency are the main factors which limit visual performance. In a previous study [Vis. Res. 43 (2003) 1103] we compared the variance of human reaction time to that of an ideal observer and found that the sampling efficiency to suprathreshold stimuli was much lower than that obtained in detection experiments. In order to bypass the effects of the motor system on visual performance, we used a flash-sound simultaneity paradigm. We found that the sampling efficiency for 0.4- and 4-c/deg near-threshold Gabor patches is higher only by a factor of 2.5 than that to above-threshold patterns. The signal-dependent multiplicative internal noise was similar to the additive internal noise at lower signal contrast levels and exceeded it at higher signal contrast levels. The results show that real observers' performance for detecting suprathreshold stimuli can be accounted for by a model taking into account the non-linear visual-signal transduction and multiplicative components of the internal noise induced by the signal and external noise. In addition, this model assumes that performance depends on the response duration, rather than signal duration. The results imply that the multiplicative internal noise induced by high contrast visual signals determines performance for suprathreshold visual detection.

Sampling efficiency and internal noise for motion detection, discrimination, and summation.

By comparing real observers to an ideal observer, previous studies have found that the detection of static patterns is limited by internal noise and by imperfect sampling efficiency. We developed and applied ideal observer models for the detection, discrimination, and summation of oppositely drifting gratings in Gaussian white noise. The three tasks share a common source of internal noise. The sampling efficiencies were on the order of 1-2% except for much lower efficiency in direction discrimination for faster moving gratings. The efficiency of direction discrimination relative to detection systematically declines as the speed is increased from 1 to 6 Hz. These results suggest that observers use mismatched filters tuned to slow speeds regardless of the signal speed. Human visual motion sensing appears to use distorted representations of the incoming signals, and this distortion is a major limitation to visual performance.

Temporal properties of the visual responses to luminance and contrast modulated noise.

Vision is sensitive to first-order luminance modulations and second-order modulations of carrier contrast. Our knowledge of the temporal properties of second-order vision is insufficient and contradictory. Using temporal summation and reaction time paradigms, we found that the type of visual noise (static or dynamic) determines the temporal properties of the responses to luminance and contrast modulations. In the presence of static noise, the temporal responses to both types of modulation of low and higher spatial frequencies were transient. When dynamic noise was used, the temporal responses to luminance and contrast modulations of higher spatial frequencies were sustained. At low spatial frequency, however, luminance modulations elicited transient responses, while contrast modulated dynamic noise produced sustained responses. The reaction times to near-threshold contrast modulations of low spatial frequency were slower than those to first-order patterns and they did not significantly differ at modulations of higher spatial frequency. The results suggest that the temporal characteristics of first-stage linear filters which feed the second-order pathway may determine the temporal responses to contrast modulated noise.