Spatial Interactions in Interocular and Monocular "Blur Suppression".

SIGNIFICANCE: The suppression of blurred images in one eye by clear images in the other eye is thought to contribute to the success of monovision correction. We show that interocular suppression occurs also for low-contrast targets that are not blurred and, to a lesser extent, when clear and low-contrast targets are presented to the same eye. PURPOSE: A blurred target presented to one eye may be suppressed when a clear target is presented to the other eye. We sought to determine how this interocular suppression varies according to the separation between the blurred and clear targets and the magnitude of imposed blur. In addition, we examined whether a similar suppression occurs when the clear and blurred targets are imaged in the same eye. METHODS: Subjects (N = 4) viewed a clear 20/40 Sloan letter surrounded by four 2 × 10 min-arc flanking bars. In different blocks of trials, the gap between the letter and flanking bars varied from 0.5 to 4 bar widths. In addition, the flanking bars were either clear or spatially filtered to simulate 0.5 to 2 D of blur. The contrast required to detect the flanking bars was determined when the letter and flanking bars were presented either dichoptically or monoptically and compared with the thresholds for the bar targets presented alone. RESULTS: In both dichoptic and monoptic viewing conditions, detection thresholds for the blurred flanking bars are highest for the smallest spatial gap and decrease systematically as the gap increases. Thresholds are uniformly higher during dichoptic than monocular viewing, but the proportional change with the bar-to-letter separation is similar in both conditions. Surprisingly, the magnitude of imposed blur has very little influence on the magnitude of threshold elevation in either the dichoptic or monoptic viewing conditions. CONCLUSIONS: Because threshold elevation is nearly the same in the presence of 0 to 2 D of blur, we prefer to designate the phenomenon we studied as "contrast suppression." The similar spatial characteristics of suppression during dichoptic and monoptic viewing are consistent with contributions from a common neural mechanism.

Stereothresholds during Voluntary Head Movement and Disconjugate Image Motion.

SIGNIFICANCE: Stereothresholds increase in the presence of disconjugate image motion, whether this motion results from vergence errors that occur during active head movements or is imposed externally. PURPOSE: During rapid voluntary oscillations of the head, vergence eye position has been reported to vary with a peak-to-peak amplitude of about 0.5°-a considerably greater amplitude than when the head is still. Concurrently, stereopsis was reported to be unaffected by voluntary head motion. In the present study, we measured stereothresholds during voluntary side-to-side head movements and during imposed disconjugate image motion with the head stationary, to simulate that produced during active head movement. METHODS: Stereothresholds were measured for a pair of 30-arcmin bright vertical lines presented on an oscilloscope and viewed through a custom mirror haploscope. Data were obtained from four normal observers during voluntary side-to-side head movements at temporal frequencies up to 1.5 Hz and also while the head remained still. In addition, stereothresholds were measured with the head stationary when opposite rotations of the galvanometer-driven mirrors in each channel of the haploscope created disconjugate image motion to simulate vergence variability during active head movement. RESULTS: During head motion, average stereothresholds increased from about 10 to about 14 arcsec. With imposed disconjugate image motion, stereothresholds rose systematically to about 35 arcsec when the peak-to-peak motion amplitude was 0.5°. Stereothresholds depend primarily on the amplitude of imposed motion and only marginally on variations of the disjunctive-motion wave form. CONCLUSIONS: Stereothresholds are elevated modestly during active head movements. The results obtained with imposed disjunctive image motion are consistent with a previously proposal that stereothresholds vary according to the unsigned, time-averaged deviation of the stereotarget from the plane of the horopter.

High- and Low-contrast Letter Acuity during Image Motion in Normal Observers and Observers with Infantile Nystagmus Syndrome.

SIGNIFICANCE: High-contrast acuity in individuals with infantile nystagmus syndrome (INS) is poorer than expected from their ongoing retinal image motion, indicating a sensory loss. Conversely, acuity for larger low-contrast letters in these observers may be limited by image motion alone. PURPOSE: The aim of this study was to assess visual acuity for letters of different contrast in normal observers and individuals with idiopathic INS under conditions of comparable retinal image motion. METHODS: Visual acuity was measured using projected Landolt C charts in 3 normal observers and 11 observers with presumed idiopathic INS. Normal observers viewed each chart after reflection from a front-surface mirror that underwent continuous 4-Hz ramp motion with amplitudes ranging from 4 to 9.6° and simulated foveation durations of 20 to 80 milliseconds. Observers with INS viewed the charts directly. By reciprocally varying the luminance of the projected charts and a superimposed veiling source, Landolt C's were presented on a background luminance of 43 cd/m2 with Weber contrasts between -12 and -89%. RESULTS: Whereas normal observers' high-contrast acuity during imposed image motion depends only on the duration of the simulated foveation periods, acuity for low-contrast optotypes also worsens systematically as motion intensity (frequency × amplitude) increases. For comparable parameters of retinal image motion, high-contrast acuity in all but one of the observers with INS was poorer than in normal observers. On the other hand, low-contrast acuity in the two groups of observers was similar when the retinal image motion was comparable. CONCLUSIONS: Reduced high-contrast acuity in observers with INS appears to be attributable primarily to a sensory deficit. On the other hand, the reduction of low-contrast acuity in observers with INS may be accounted for on the basis of retinal image motion.

A Comparison of Foveal and Peripheral Contour Interaction and Crowding.

SIGNIFICANCE: Performance on clinical tests of visual acuity can be influenced by the presence of nearby targets. This study compared the influence of neighboring flanking bars and letters on foveal and peripheral letter identification. PURPOSE: Contour interaction and crowding refer to an impairment of visual resolution or discrimination produced by different types of flanking stimuli. This study compared the impairment of percent correct letter identification that is produced in normal observers when a target letter is surrounded by an array of four flanking bars (contour interaction) or four flanking letters (crowding). METHODS: Performance was measured at the fovea and at eccentricities of 1.25, 2.5, and 5° for photopic (200 cd/m2) and mesopic stimuli (0.5 cd/m2) and a range of target-to-flanker separations. RESULTS: Consistent with previous reports, foveal contour interaction and crowding were more pronounced for photopic than mesopic targets. However, no statistically significant difference existed between foveal contour-interaction and crowding functions at either luminance level. On the other hand, flanking bars produced much less impairment of letter identification than letter flankers at all three peripheral locations, indicating that crowding is more severe than contour interaction in peripheral vision. In contrast to the fovea, peripheral crowding and contour-interaction functions did not differ systematically for targets of photopic and mesopic luminance. CONCLUSION: The similarity between foveal contour interaction and crowding and the dissimilarity between peripheral contour interaction and crowding suggest the involvement of different mechanisms at different retinal locations.

Technical Report: The Mechanism of Contour Interaction Differs in the Fovea and Periphery.

SIGNIFICANCE: Both foveal and peripheral contour interactions are based on, as yet, unexplained neural mechanisms. Our results show that, unlike foveal contour interaction, peripheral contour interaction cannot be explained on the basis of the antagonistic structure of neural receptive fields. PURPOSE: Foveal contour interaction is markedly reduced for mesopic compared with photopic targets. This finding is consistent with an explanation based on the antagonistic structure of neural receptive fields. However, no reduction was found for low-luminance targets in the periphery, possibly because the luminances used previously remained substantially above peripheral scotopic detection thresholds. In this study, we compared foveal and peripheral contour interactions for long-wavelength photopic and mesopic targets, which would be expected to significantly elevate the peripheral retinal detection threshold. METHODS: Five normal observers viewed a randomly selected Sloan letter surrounded by four flanking bars at several edge-to-edge separations (min arc). Photopic and mesopic stimuli were viewed foveally and at 6° peripherally through a selective red filter that ensured that mesopic targets were within 1 log unit of detection threshold at both retinal locations. RESULTS: Whereas the magnitude of foveal contour interaction was substantially less at mesopic compared with photopic luminance (20 vs. 46% reduction of percent correct, on average), no significant difference was observed in peripheral contour interaction, which had average mesopic and photopic magnitudes of 38 and 40%. Moreover, confusion matrices representing photopic and mesopic contour interaction differed in the fovea but not in the periphery. The extent of contour interaction did not change with luminance at either retinal location. CONCLUSIONS: Our results indicate that, although the characteristics of foveal contour interaction can be accounted for by the antagonistic structure of neural receptive fields, the same mechanism is not compatible with the characteristics of peripheral contour interaction.

Impact of simulated micro-scotomas on reading performance in central and peripheral retina.

Observers with central field loss typically fixate within a non-foveal region called the preferred retinal locus, which can include localized sensitivity losses, or micro-scotomas (Krishnan and Bedell, 2018). In this study, we simulated micro-scotomas at the fovea and in the peripheral retina to assess their impact on reading speed. Ten younger (<36 years old) and 8 older (>50 years old) naïve observers with normal vision monocularly read high and/or low contrast sentences, presented at or above the critical print size for young observers at the fovea and at 5 and 10 deg in the inferior visual field. Reading material comprised MNREAD sentences and sentences taken from novels that were presented in rapid serial visual presentation (RSVP) format. Randomly distributed 13 × 13 arc min blocks corresponding to 0-78% of the text area (corresponding to ∼0-17 micro-scotomas/deg2) were set to the background luminance to simulate micro-scotomas. A staircase algorithm estimated maximum reading speed from the threshold exposure duration for each combination of retinal eccentricity, contrast and micro-scotoma density in both age groups. Log10(RSVP reading speed) decreased significantly with simulated micro-scotoma density and eccentricity. Across conditions, reading speed was slower with low-compared to high-contrast text and was faster in younger than older normal observers. For a given eccentricity and contrast, a higher density of random element losses maximally affected older observers with normal vision. These outcomes may explain some of the reading deficits observed in older observers with central field loss.

Contrast Energy and Contour Interaction.

SIGNIFICANCE: Contour interaction describes an impairment of visual acuity produced by nearby flanking features, which exerts a significant impact in many clinical tests of visual acuity. Our results indicate that the magnitude of interaction depends either on the flanker contrast energy (i.e., the product of flanker contrast and width) or the flanker contrast alone, depending on the contrast energy of the flankers. PURPOSE: The discrimination of acuity targets is impaired by the presence of nearby flanking contours, a phenomenon known as contour interaction. METHODS: In this study, we measured percent correct identification for threshold size, high-contrast Sloan letters at the fovea and at 5° in the inferior visual field for different combinations of flanking-bar width, and Weber contrast corresponding to specific fixed values of contrast energy (width × contrast, in %-min arc). RESULTS: For flanking bars with low-contrast energy, contour interaction exhibited no systematic dependence on the flanking-bar width. However, when the flanking bars had higher contrast energy, narrower high-contrast bars produced significantly greater contour interaction than wider bars of lower contrast. CONCLUSIONS: The results are consistent with the interpretation that contour interaction depends primarily on the contrast energy of flanking contours when their contrast energy is low. As the contrast energy of the flanking contours increases, the magnitude of contour interaction depends on the flanker contrast. For high-contrast flanking contours, the magnitude of contour interaction saturates when the width of the flanking contours is approximately 20% of letter size.

Functional changes at the preferred retinal locus in subjects with bilateral central vision loss.

PURPOSE: Subjects with bilateral central vision loss (CVL) use a retinal region called the preferred retinal locus (PRL) for performing various visual tasks. We probed the fixation PRL in individuals with bilateral macular disease, including age-related macular degeneration (AMD) and Stargardt disease (STGD), for localized sensitivity deficits. METHODS: Three letter words at the critical print size were presented in the NIDEK MP-1 microperimeter to determine the fixation PRL and its radial retinal eccentricity from the residual fovea in 29 subjects with bilateral CVL. Fixation stability was defined as the median bivariate contour ellipse area (BCEA) from 3 fixation assessments. A standard 10-2 grid (68 locations, 2° apart) was used to determine central retinal sensitivity for Goldmann size II test spots. Baseline and follow-up supra-threshold screening of the fixation PRL for localized sensitivity deficits was performed using high density (0.2° or 0.3° apart) 0 dB Goldmann size II test spots. Custom MATLAB code and a dual bootstrapping algorithm were used to register test-spot locations from the baseline and follow-up tests. Locations where the 0 dB test spots were not seen on either test were labeled as micro-scotomas (MSs). RESULTS: Median BCEA correlated poorly with the radial eccentricity of the fixation PRL. Mean (±SD) sensitivity around the PRL from 10-2 testing was 4.93 ± 4.73 dB. The average percentage of MSs was similar for patients with AMD (25.4%), STGD (20.3%), and other etiologies of CVL (27.1%). CONCLUSIONS: The fixation PRL in subjects with bilateral CVL frequently includes local regions of sensitivity loss.

Contour interaction under photopic and scotopic conditions.

In the present study, we asked whether contour interaction undergoes significant changes for different luminance levels in the central and peripheral visual field. This study included nine normal observers at two laboratories (five at Palacky University Olomouc, Czech Republic and four at the University of Houston, USA). Observers viewed a randomly selected Sloan letter surrounded by four equally spaced bars for several separations measured edge-to-edge in min arc. Stimuli were viewed foveally under photopic and mesopic luminances and between 5° and 12° peripherally for four different background luminances of the display monitors, corresponding to photopic, mesopic, scotopic, and dim scotopic levels. The extent of the contour interaction in the fovea is approximately 20 times smaller than in the periphery. Whereas the magnitude of foveal contour interaction markedly decreases with decreasing luminance, no consistent luminance-induced change occurs in peripheral contour interaction. The extent of contour interaction does not scale with the size of the target letter, either in the fovea or peripherally. The results support a neural origin of contour interaction consistent with the properties of center-surround antagonism.

Scotoma Visibility and Reading Rate with Bilateral Central Scotomas.

PURPOSE: In this experiment, we tested whether perceptually delineating the scotoma location and border with a gaze contingent polygon overlay improves reading speed and reading eye movements in patients with bilateral central scotomas. METHODS: Eight patients with age-related macular degeneration and bilateral central scotomas read aloud MNRead style sentences with their preferred eye. Eye movement signals from an EyeLink II eyetracker were used to create a gaze contingent display in which a polygon overlay delineating the area of the patient's scotoma was superimposed on the text during 18 of the 42 trials. Blocks of six trials with the superimposed polygon were alternated with blocks of six trials without the polygon. Reading speed and reading eye movements were assessed before and after the subjects practiced reading with the polygon overlay. RESULTS: All of the subjects but one showed an increase in reading speed. A paired-samples t-test for the group as a whole revealed a statistically significant increase in reading speed of 0.075 ± 0.060 (SD) log wpm after reading with the superimposed polygon. Individual subjects demonstrated significant changes in reading eye movements, with the greatest number of subjects demonstrating a shift in the average vertical fixation locus. Across subjects, there was no significant difference between the initial and final reading eye movements in terms of saccades per second, average fixation duration, average amplitude of saccades, or proportion of non-horizontal saccades. CONCLUSIONS: The improvement in reading speed (0.075 log wpm or 19%) over the short experimental session for the majority of subjects indicates that making the scotoma location more visible is potentially beneficial for improving reading speed in patients with bilateral central scotomas. Additional research to examine the efficacy of more extended training with this paradigm is warranted.

Monocular microsaccades are visual-task related.

During visual fixation, we constantly move our eyes. These microscopic eye movements are composed of tremor, drift, and microsaccades. Early studies concluded that microsaccades, like larger saccades, are binocular and conjugate, as expected from Hering's law of equal innervation. Here, we document the existence of monocular microsaccades during both fixation and a discrimination task, reporting the location of the gap in a foveal, low-contrast letter C. Monocular microsaccades differ in frequency, amplitude, and peak velocity from binocular microsaccades. Our analyses show that these differences are robust to different velocity and duration criteria that have been used previously to identify microsaccades. Also, the frequency of monocular microsaccades differs systematically according to the task: monocular microsaccades occur more frequently during fixation than discrimination, the opposite of their binocular equivalents. However, during discrimination, monocular microsaccades occur more often around the discrimination threshold, particularly for each subject's dominant eye and in case of successful discrimination. We suggest that monocular microsaccades play a functional role in the production of fine corrections of eye position and vergence during demanding visual tasks.

Evidence for an eye-movement contribution to normal foveal crowding.

PURPOSE: Along with contour interaction, inaccurate and imprecise eye movements and attention have been suggested to contribute to poorer acuity for "crowded" versus uncrowded targets. To investigate the role of eye movements in foveal crowding, we compared percent correct letter identification for short and long lines of near-threshold letters with different separations. METHODS: Five normal observers read short (4 to 6 letters) and long (10 to 12 letters) lines of near-threshold, Sloan letters with edge-to-edge letter separations of 0.5, 1, and 2 letter spaces. Percent correct letter identification for the 2 to 4 interior letters in short strings and the 8 to 10 interior letters in long strings was compared with a no-crowding condition. RESULTS: Letter identification was significantly worse than the no-crowding condition for long letter strings with a separation of 1 letter space and for both long and short letter strings with a separation of 0.5 letter spaces. Observers more often reported the incorrect number of letters in long than in short letter strings, even for a separation of 2 letter spaces. Similar results were obtained during straight-ahead gaze and while viewing in 30 to 40 degrees left gaze, where two of the five observers exhibited an increase in horizontal fixational instability. CONCLUSIONS: We argue that lower percent correct letter identification and more frequent errors in reporting the number of letters in long compared with short letter strings reflect an eye-movement contribution to foveal crowding.

Stream specificity and asymmetries in feature binding and content-addressable access in visual encoding and memory.

Human memory is content addressable-i.e., contents of the memory can be accessed using partial information about the bound features of a stored item. In this study, we used a cross-feature cuing technique to examine how the human visual system encodes, binds, and retains information about multiple stimulus features within a set of moving objects. We sought to characterize the roles of three different features (position, color, and direction of motion, the latter two of which are processed preferentially within the ventral and dorsal visual streams, respectively) in the construction and maintenance of object representations. We investigated the extent to which these features are bound together across the following processing stages: during stimulus encoding, sensory (iconic) memory, and visual short-term memory. Whereas all features examined here can serve as cues for addressing content, their effectiveness shows asymmetries and varies according to cue-report pairings and the stage of information processing and storage. Position-based indexing theories predict that position should be more effective as a cue compared to other features. While we found a privileged role for position as a cue at the stimulus-encoding stage, position was not the privileged cue at the sensory and visual short-term memory stages. Instead, the pattern that emerged from our findings is one that mirrors the parallel processing streams in the visual system. This stream-specific binding and cuing effectiveness manifests itself in all three stages of information processing examined here. Finally, we find that the Leaky Flask model proposed in our previous study is applicable to all three features.

Visual-function tests for self-monitoring of age-related macular degeneration.

Age-related macular degeneration (AMD) is one of the leading causes of severe visual impairment in the United States. Changes in lifestyle can slow the progression of AMD, and new therapies that arrest choroidal neovascularization can preserve vision in patients who progress to the neovascular form of advanced AMD. Appropriate timing is required for these interventions to be optimally effective, which, in turn, depends critically on early diagnosis. Because annual or semiannual eye examinations may not be sufficient to ensure an early diagnosis, the preferred practice for AMD management must include self-monitoring by patients for disease onset or progression. In this review, we discuss a number of visual functions that have been shown to be impaired in eyes with AMD and specify desirable characteristics of visual-function tests that can be used for self-monitoring by populations at risk for AMD.

Fixation locus in patients with bilateral central scotomas for targets that perceptually fill in.

PURPOSE: In this experiment, we investigated whether target type affects the retinal fixation location and stability in patients with bilateral central scotomas and, specifically, whether targets expected to perceptually fill in are imaged at or near the vestigial fovea. METHODS: The retinal location and stability of fixation were measured using the Nidek MP-1 microperimeter in 12 patients with bilateral central scotomas for six types of fixation target, three expected to fill in and three that included letters. The approximate position of the vestigial fovea was delineated in 10 of the patients either by using residual retinal landmarks or by locating the residual foveal pit in a dense macular scan obtained with a Spectralis optical coherence tomographer. Fixation location and stability were compared for the different target types and referenced to the position of the vestigial fovea. RESULTS: All of the subjects except one fixated consistently on targets that included a letter using peripheral retinal locations outside of the central scotoma. Eleven of the 12 subjects used a retinal location closer to the vestigial fovea to fixate targets expected to fill in compared with letters. Although four of the subjects imaged the filled-in targets at or within a half degree of the vestigial fovea, six other subjects imaged the filled-in targets at a retinal locus removed from the vestigial fovea. Target type produced no overall significant difference in fixation stability, specified in terms of bivariate contour ellipse area. However, in some individual subjects, fixation tended to be more stable on letter targets than on filled-in targets. CONCLUSIONS: In patients with central field loss, letter targets generate more consistent fixation behavior than filled-in targets and should be used for eccentric viewing training and perimetry.

Large crowding zones in peripheral vision for briefly presented stimuli.

When a target is flanked by distractors, it becomes more difficult to identify. In the periphery, this crowding effect extends over a wide range of target-flanker separations, called the spatial extent of interaction (EoI). A recent study showed that the EoI dramatically increases in size for short presentation durations (Chung & Mansfield, 2009). Here we investigate this duration-EoI relation in greater detail and show that (a) it holds even when visibility of the unflanked target is equated for different durations, (b) the function saturates for durations shorter than 30 to 80 ms, and (c) the largest EoIs represent a critical spacing greater than 50% of eccentricity. We also investigated the effect of same or different polarity for targets and flankers across different presentation durations. We found that EoIs for target and flankers having opposite polarity (one white, the other black) show the same temporal pattern as for same polarity stimuli, but are smaller at all durations by 29% to 44%. The observed saturation of the EoI for short-duration stimuli suggests that crowding follows the locus of temporal integration. Overall, the results constrain theories that map crowding zones to fixed spatial extents or to lateral connections of fixed length in the cortex.

Registration tolerance of a custom correction to maintain visual acuity.

PURPOSE: To present a predictive model of the registration tolerance for wavefront-guided correction to maintain acuity within fixed limits and demonstrate the potential utility using two typical keratoconic eyes. METHODS: Change in log visual Strehl was plotted as a function of translation error for a series of rotations of a wavefront-guided correction. Contour lines were added at Δlog visual Strehl levels predicted to induce one- and two-line losses of logMAR visual acuity. The model was validated by regressing measured acuity loss from subjects viewing acuity charts that were degraded by the residual wavefront error resulting from the movement of wavefront-guided correction against the model's predicted acuity. RESULTS: The model's predicted change in acuity can be substituted for measured change in acuity (R² = 0.91) within measurement error (±0.1 logMAR). Translation and/or rotation of a wavefront-guided correction induced asymmetric optical tolerance to movement. Induced errors depended on the wavefront error being corrected, the wavefront-guided correction design, and the amount of registration error. CONCLUSIONS: Change in log visual Strehl can be used to determine the registration tolerance necessary to keep the variation in acuity within user-defined limits. This tolerance is unique for each wavefront error and wavefront-guided correction design.

Change in visual acuity is well correlated with change in image-quality metrics for both normal and keratoconic wavefront errors.

We determined the degree to which change in visual acuity (VA) correlates with change in optical quality using image-quality (IQ) metrics for both normal and keratoconic wavefront errors (WFEs). VA was recorded for five normal subjects reading simulated, logMAR acuity charts generated from the scaled WFEs of 15 normal and seven keratoconic eyes. We examined the correlations over a large range of acuity loss (up to 11 lines) and a smaller, more clinically relevant range (up to four lines). Nine IQ metrics were well correlated for both ranges. Over the smaller range of primary interest, eight were also accurate and precise in estimating the variations in logMAR acuity in both normal and keratoconic WFEs. The accuracy for these eight best metrics in estimating the mean change in logMAR acuity ranged between ±0.0065 to ±0.017 logMAR (all less than one letter), and the precision ranged between ±0.10 to ±0.14 logMAR (all less than seven letters).

Optimizing wavefront-guided corrections for highly aberrated eyes in the presence of registration uncertainty.

Dynamic registration uncertainty of a wavefront-guided correction with respect to underlying wavefront error (WFE) inevitably decreases retinal image quality. A partial correction may improve average retinal image quality and visual acuity in the presence of registration uncertainties. The purpose of this paper is to (a) develop an algorithm to optimize wavefront-guided correction that improves visual acuity given registration uncertainty and (b) test the hypothesis that these corrections provide improved visual performance in the presence of these uncertainties as compared to a full-magnitude correction or a correction by Guirao, Cox, and Williams (2002). A stochastic parallel gradient descent (SPGD) algorithm was used to optimize the partial-magnitude correction for three keratoconic eyes based on measured scleral contact lens movement. Given its high correlation with logMAR acuity, the retinal image quality metric log visual Strehl was used as a predictor of visual acuity. Predicted values of visual acuity with the optimized corrections were validated by regressing measured acuity loss against predicted loss. Measured loss was obtained from normal subjects viewing acuity charts that were degraded by the residual aberrations generated by the movement of the full-magnitude correction, the correction by Guirao, and optimized SPGD correction. Partial-magnitude corrections optimized with an SPGD algorithm provide at least one line improvement of average visual acuity over the full magnitude and the correction by Guirao given the registration uncertainty. This study demonstrates that it is possible to improve the average visual acuity by optimizing wavefront-guided correction in the presence of registration uncertainty.

Perceived suprathreshold depth under conditions that elevate the stereothreshold.

PURPOSE: Previous studies considered the possibility that individuals with impaired stereoacuity can be identified by estimating the perceived depth of a target with a suprathreshold retinal image disparity. These studies showed that perceived suprathreshold depth is reduced when the image presented to one eye is blurred, but they did not address whether a similar reduction of perceived depth occurs when the stereothreshold is elevated using other manipulations. METHODS: Stereothresholds were measured in six adult observers for a pair of bright 1-degree vertical lines during normal viewing and under five conditions that elevated the stereothreshold: monocular dioptric blur, monocular glare, binocular luminance reduction, monocular luminance reduction, and imposed disjunctive image motion. The observers subsequently matched the perceived depth of degraded targets presented with crossed or uncrossed disparities corresponding to two, four, and six times the elevated stereothreshold for each stimulus condition. RESULTS: The image manipulations used elevated the stereothreshold by a factor of 3.7 to 5.5 times. For targets with suprathreshold disparities, monocular blur, monocular luminance reduction, and disjunctive image motion resulted in a significant decrease in perceived depth. However, the magnitude of perceived suprathreshold depth was unaffected when monocular glare was introduced or the binocular luminance of the stereotargets was reduced. CONCLUSIONS: Not all conditions that increase the stereothreshold reduce the perceived depth of targets with suprathreshold disparities. Observers who have poor stereopsis therefore may or may not exhibit an associated reduction of perceived suprathreshold depth.