Mueller matrix retinal imager with optimized polarization conditions.

A new Mueller matrix polarimeter was used to image the retinas of normal subjects. Light from a linearly polarized 780 nm laser was passed through a system of variable retarders and scanned across the retina. Light returned from the eye passed through a second system of retarders and a polarizing beamsplitter to two confocal detection channels. Optimization of the polarimetric data reduction matrix was via a condition number metric. The accuracy and repeatability of polarization parameter measurements were within +/- 5%. The magnitudes and orientations of retardance and diattenuation, plus depolarization, were measured over 15 degrees of retina for 15 normal eyes.

Relationship between foveal birefringence and visual acuity in neovascular age-related macular degeneration.

PURPOSE: To investigate the relationship between visual acuity and foveal birefringence in patients with neovascular age-related macular degeneration. METHODS: In total, 40 patients with choroidal neovascularization underwent macular imaging with scanning laser polarimetry. Bowtie patterns, typically seen in birefringence images of the macula, were evaluated and classified into three categories: (1) regular bowtie present; (2) bowtie present, but disrupted; and (3) no bowtie present. The relation of the bowtie appearance to the best-corrected logMAR visual acuities was tested (ANOVA). RESULTS: Mean visual acuity was best for the group that had regular bowties (mean logMAR=0.34) and differed statistically significantly from the disrupted bowtie group and no bowtie group (P=0.01 and 0.0007). Ages for the three groups did not differ (P=0.31). CONCLUSIONS: Appearance of a regular bowtie indicates a substantially intact Henle fibre layer with the potential for good visual function, despite the presence of underlying pathology. Conversely, disruption or absence of a bowtie may indicate severe damage to the photoreceptors, consistent with the finding of poorer visual acuity.

Blue on yellow perimetry with scanning laser ophthalmoscopy in patients with age related macular disease.

BACKGROUND/AIM: The loss of short wavelength sensitive (SWS) cone mechanism sensitivity is related to severe vision loss in patients with age related maculopathy (ARM). A case-control study of patients with ARM and age matched controls was performed, using blue on yellow static perimetry. METHODS: A bright yellow background at 594 nm isolated the responses of short wavelength cone mechanisms to 458 nm targets. A scanning laser ophthalmoscope produced stimuli and provided real time, simultaneous fundus illumination. The macula was probed with 16 Goldmann IV targets, 1-10 degrees from fixation, using a staircase method. RESULTS: 24 patients with non-exudative ARM were matched to 24 subjects with normal fundus appearance. SWS cone pathway sensitivity for macular targets was significantly reduced in the patients with ARM compared to normals--15.45 (SD 4.56) dB v 17.22 (0.28) dB, respectively (p<0.0005). There was not only a diffuse loss of sensitivity in ARM patients, but also a localised loss of sensitivity over drusen (p<0.025). Neither the mean age, 69 (8) years, nor the mean visual acuity differed between groups, logMAR 0.09 (0.10) v 0.05 (0.06) for ARM patients v normals, respectively. Patients with soft drusen had lower sensitivity than those with hard drusen (p<0.05). CONCLUSION: A loss of SWS cone pathway sensitivity occurred in most patients with early ARM, despite good visual acuity, demonstrating a loss of visual function that cannot be attributed to ageing changes. The loss of sensitivity, despite good visual acuity, included both a diffuse loss and localised losses.

Short wavelength automated perimetry in age related maculopathy.

BACKGROUND/AIMS: Previous studies reported the predictive value of the short wavelength sensitive (SWS) cone mediated sensitivity for visual outcome in age related macular degeneration. In this study SWS sensitivity was measured by commercially available blue on yellow perimetry in patients with non-exudative age related maculopathy (ARM) and compared with the presence of morphological risk factors and the status of the fellow eye. METHODS: In a prospective cross sectional study, 126 patients (57 males, 69 females, mean age 71 (SD 6) years) with ARM (visual acuity >20/50) were tested. Central visual fields (blue on yellow) were obtained with a conventional perimeter. Fundus slides were graded by two independent observers for soft drusen and presence of focal hyperpigmentation. RESULTS: Mean sensitivity and standard deviation of all patients exhibited a significant reduction with age. Patients with soft drusen had significantly lower sensitivity than those without, whereas there were no differences in visual acuity (log MAR). Sensitivity was also reduced in those eyes with fellow eyes having a sight threatening complication of age related macular degeneration (AMD). Eyes with focal hyperpigmentation compared with those without had no loss of sensitivity, but did have a significant decrease in the central part of the field compared with the more eccentric. CONCLUSION: SWS sensitivity loss is associated with common risk factors for progression to AMD. Short wavelength automated perimetry is moderately rapid and readily available. It may serve as a tool in future ARM trials.

Blue-on-yellow perimetry with a scanning laser ophthalmoscope: small alterations in the central macula with aging.

The sensitivity of short-wavelength-sensitive (SWS) cone pathways was measured in the central fields of 74 normal subjects, aged 17-86 yr, with healthy maculas. The new fundus perimetry technique used a research scanning laser ophthalmoscope with a small entrance pupil to present blue static perimetry targets on a bright yellow background. Simultaneous infrared imaging aided target positioning and rapid assessment of potential pathology in elderly subjects. Targets were positioned peripheral to fixation, avoiding both the SWS-cone-free area and the peak macular pigment, determined in 11 subjects. Sensitivity declined 0.019 log unit per decade, while intraindividual variability across loci increased. The nasal-temporal asymmetry remained constant. Sensitivity of older subjects was relatively less for the most central targets but was unrelated to transmission through macular pigment. Retinal changes with age occur to differing extents or at differing rates and are readily detectable in the central macula.

Detecting AMD with multiply scattered light tomography.

PURPOSE: to use a novel technique, Multiply Scattered Light Tomography (MSLT), to provide a comfortable, rapid, and noninvasive method for detection and management of Age-related Macualar Degeneration. METHODS: two patient groups were studied in clinical settings with MSLT and confocal scanning laser tomography. In Poway, CA, 21 retinal patients underwent tomography, and the 17 patients with suspicion of exudation also had ICG. An Angio-Scan (Laser Diagnostic Technologies, Inc.) was used to provide simultaneous fundus reflectance and ICG imaging. In Methuen, MA, 20 retinal patients underwent tomography with fluorescein angiography for suspicion of exudation. The MSLT was based on the TopSS (Laser Diagnostic Technologies, Inc.), with a Vertical Cavity Surface Emitting Laser array at 850 mm as the illumination source. The central laser produced confocal images. The surrounding lasers produced multiply scattered light images. RESULTS: MSLT emphasized structures beneath the retina such as drusen, choroidal new vessel membranes, and pigment epithelial detachments. Exudation seen on angiography was visualized by MSLT as topographical structures with distinct borders. Superficial structures, e.g., cysts and epiretinal membranes, were visualized in 850 nm images. DISCUSSION: confocal tomography and MSLT provided a rapid, noninvasive method to detect and localize macular degeneration and pathological structures found in eyes of older patients.

Infrared scanning laser tomography of macular cysts.

OBJECTIVE: To perform three-dimensional, noninvasive, quantitative analysis of cystoid macular edema and macular cysts using infrared scanning laser tomography and to correlate findings with visual acuity (VA) as a basis for interventional studies. DESIGN: Cross-sectional, nonrandomized study. PARTICIPANTS: Seventeen patients (29-86 years of age) with macular cysts associated with a broad spectrum of diseases. INTERVENTION: Confocal infrared imaging with scanning laser tomography with the TopSS (790 nm) (Laser Diagnostic Technologies, San Diego, CA) with digitized images was used to perform three-dimensional, quantitative analysis of cysts in the central 5 degrees of the macula. MAIN OUTCOME MEASURES: Measurements of macular cyst number, area, volume, depth, slope, height of the surrounding macular elevation, and correlation with VA. RESULTS: Scanning laser tomography detected macular cysts in all patients. The number per patient ranged from 1 through 15. Cysts were accompanied by surrounding macular elevation in 16 patients (mean macular height, 216 microm). The area covered by cysts in the central 5 degrees was 0.087 to 0.969 mm2, and volume was 0.007 to 0.549 mm3. Visual acuity was significantly poorer in patients with greater cyst area (P = 0.0007), cyst volume (P = 0.0009), macular thickening (P = 0.0002), and cyst depth (P = 0.0013). Cyst number, average slope, and maximum slope, however, did not correlate significantly with VA. Grouping of macular cysts according to macular height and average cyst depth revealed that cysts in a more thickened retina were significantly deeper, had steeper slopes, and corresponded to worse VA. Macular height and average cyst depth were highly associated with each other, suggesting that in eyes with surrounding macular edema, cysts were deeper and may reflect more widespread tissue destruction. Individual confocal tomographic images provided additional information. Neither ophthalmoscopy nor fluorescein angiography delineated features such as retinal folds that suggested vitreous traction or changes in deeper layers that suggested occult choroidal new vessels. CONCLUSIONS: Infrared scanning laser tomography is a rapid and noninvasive imaging method that provides quantitative analysis of macular cysts in addition to qualitative information not seen clinically. Because poor VA is related to severe involvement of the central retina, scanning laser tomography could provide an objective outcome measure for interventional studies.

Infrared imaging of cystoid macular edema.

BACKGROUND: Cystoid macular edema (CME), a cause of central visual loss, is described in various pathologies. Typically, the fluorescein angiogram confirms the diagnosis and provides qualitative information as to the extent of leakage. This study was performed to investigate the features of cysts and quantify the extent of CME using non-invasive infrared imaging. METHODS: Eighteen eyes of 16 successive patients with CME in association with a broad spectrum of diseases were included in the study. The diagnosis of CME was established clinically and confirmed by fluorescein angiography. Digital infrared imaging was performed with a research scanning laser ophthalmoscope with different apertures, providing direct confocal and indirect imaging modes, to discriminate superficial features from deeper ones and to emphasize sources of multiple laterally scattered light. RESULTS: CME was easily detected with infrared imaging in all eyes. Confocal mode visualized the cysts themselves, while indirect mode emphasized borders. Large central cysts were detected as distinct, non-confluent structures. In addition, folds detected with infrared imaging in the macula in 12 of the 18 eyes were not always observed clinically. CONCLUSION: Infrared imaging provides a quick and safe diagnostic tool for patients with CME. The cystoid structures are readily localized and quantified, useful for monitoring CME. Despite differences in the pathophysiology, cysts did not differ qualitatively in a variety of diseases with infrared imaging.

Spatial extent of pigment epithelial detachments in age-related macular degeneration.

OBJECTIVE: To quantify the spatial extent of pigment epithelial detachment (PED) associated with age-related macular degeneration (AMD) using a rapid, noninvasive method. DESIGN: Prospective, cross-sectional study. PARTICIPANTS: The authors tested 32 eyes of 21 patients (13 women and 8 men) with AMD 56 to 91 years of age (mean = 72.8 years). The authors retested seven eyes of six patients. INTERVENTION: Three-dimensional imaging and quantification of PED were performed in each subject using confocal infrared imaging (790 nm) with a Topographic Scanning System (TopSS). The data consisted of a series of 32 images within a 3-mm-depth range requiring 0.9 second. Three-dimensional calculations were made from the series. MAIN OUTCOME MEASURE: Height, diameter, area, volume, and slope of each PED were obtained with two calculation methods. The Ellipse Method used a graphics tool to draw elliptical borders circumscribing the region of interest in the image. The software automatically calculated the values for all parameters for the region inside the ellipse, with the retinal reference plane adjusted to match the height of the surrounding retina. The User-Defined Region Method differed in that the region of interest was drawn manually. RESULTS: PEDs were easily detected in all patients using the TopSS. Maximum height of all PEDs above the reference plane ranged from 0.204 to 1.818 mm (mean = 0.57 mm). The diameter was 0.501 to 5.151 mm (mean = 2.711 mm), area was 0.179 to 20.402 mm2 (mean = 6.585 mm2), and volume was 0.012 to 13.981 mm3 (mean = 2.173 mm3). Intraobserver variability was low, with correlations between first and second measurements for the first visit ranging from r = 0.906 to 0.997 for slope and diameter, respectively. CONCLUSIONS: Three-dimensional analysis with the TopSS provides objective outcome measures not obtainable with typical clinical methods such as fundus photography and angiography. Height and volume are crucial in determining whether neovascularization is worsening or persistent after photocoagulation. More longitudinal data are needed to determine whether tomographic data reduce the need for angiography. Unique to scanning laser tomography, exudative features were imaged at different depths.

Small incision cataract surgery and placement of posterior chamber intraocular lenses in patients with diabetic retinopathy.

BACKGROUND AND OBJECTIVE: Diabetic patients undergoing cataract surgery have been demonstrated to have a guarded prognosis. Small incision cataract surgery has been postulated to cause fewer complications than other techniques of cataract extraction. We looked at small incision cataract surgery to see how diabetics fared. PATIENTS AND METHODS: One hundred fifty-four eyes of diabetic patients were evaluated preoperatively through dilated fundus examination and Snellen visual acuity. The eyes then underwent small incision cataract surgery and were followed, undergoing periodic ophthalmoscopy, Snellen visual acuity measurement and additional postoperative therapeutic and surgical intervention. RESULTS: Final visual acuity improved by two Snellen lines or more in 61 out of 154 (40%) eyes. Final visual acuity worsened in 38 out of 154 (25%) eyes. CONCLUSIONS: Our results are similar to those reported for extracapsular surgery and phacoemulsification, suggesting that the prognosis is guarded for diabetics, even when undergoing small incision cataract surgery.

Foveal cone photopigment distribution: small alterations associated with macular pigment distribution.

PURPOSE: To map the photopigment distribution of central foveal cones in healthy adult subjects before potential onset of age-related macular degeneration. To compare alterations in cone photopigment distribution to those of macular pigment and examine those loci for subretinal changes. METHODS: Eleven healthy subjects (age range, 31-59 years) underwent reflectometry with a scanning laser ophthalmoscope. The difference in cone photopigment density in the fovea was mapped for the long-wavelength- and middle-wavelength-sensitive cones, using 594-nm light. Macular pigment was mapped with 488-nm and 514-nm light. Subretinal changes were investigated with infrared imaging (830-860 nm). RESULTS: Most subjects had small alterations in the regularity of their foveal cone photopigment distribution. Alterations were spatially related to macular pigment alterations but not to the presence of subretinal defects. Subjects were classified into three groups according to the type of alterations in the regularity of pigment distributions: central peak of photopigment and macular pigment, small foveal alterations, and broad distribution with missing central peak of photopigment or macular pigment. The resultant groups differed significantly in age, 43, 46, and 59 years, for groups 1, 2, and 3, respectively (P < 0.05). CONCLUSIONS: Small alterations in the distributions of foveal cone photopigment or macular pigment were found that varied among the subjects. Larger alterations in older subjects may indicate changes in foveal architecture with age, including potential vulnerability of central cones before the onset of clinically significant changes in the retinal pigment epithelium.

Entoptic image quality of the retinal vasculature.

Spatial details of entoptically visible retinal vessels were investigated using transcleral and Maxwellian-view stimulators. Nine normal subjects provided detailed drawings of the entoptic images which were digitized and superimposed onto digitized fundus photographs and fluorescein angiograms from the same eyes. Subjects also used a tracing method to locate visible entoptic features. The trans-scleral method provided images similar in detail to standard fundus photography (lacking capillary detail, but capturing larger arteries, veins, arterioles and venules) in the macula and around the disk. The Maxwellian-view method illuminated the fovea (7.7 degree field) and provided foveola capillary detail (capillaries traversing the foveola, the capillary arcade forming the FAZ) as well as the larger foveal vessels supplying the foveola, and often contained more foveal detail that available with fluorescein angiography.

Quantitative analysis of macular holes with scanning laser tomography.

OBJECTIVE: The purpose of the study is to establish objective, quantitative, and reproducible three-dimensional analysis for macular holes with scanning laser tomography and to correlate measurements with visual acuity. DESIGN: The authors performed a cross-sectional, nonrandomized study. PARTICIPANTS: The authors examined 28 full-thickness macular holes of 23 patients aged 61 to 84 years. INTERVENTION: Confocal infrared imaging with scanning laser tomography using the TopSS (790 nm, 10 degrees field) three-dimensional measurements and macular hole analysis were performed. MAIN OUTCOME MEASURES: Area, depth, and volume parameters for both macular holes and rims were obtained in two ways: (1) reference plane for analysis equal to height of the retina (offset surface distance [OSD] 0) or (2) equal to height of the surrounding edematous rim of the hole (OSD user). Correlation of measurements with visual acuity and groups of macular holes was performed. Reproducibility from three image series per subject and intraobserver variability from ten measurements in four subjects were computed. RESULTS: Scanning laser tomography could detect macular hole and rim features in all subjects. Infrared images provided clinically useful information that may help explain macular hole pathophysiology. Based on quantitative, three-dimensional measurements, holes were assigned to four groups: large, small, shallow, and average. Groups varied significantly (P < 0.05) for the majority of measurements. Visual acuity correlated significantly (P < 0.05) with macular hole volume, depth, slope, and rim height with OSD user only, but not with hole area. Holes computed with OSD user were deeper and of greater volume. Reproducibility was excellent for hole area, slope, and rim area; good for hole volume and depth; variable for rim volume; and improved with OSD user. Intraobserver variability was low in each group. CONCLUSIONS: Scanning laser tomography is a reproducible three-dimensional imaging technique providing objective and quantitative clinical information in assessing, grouping, and managing macular holes. By setting the OSD to rim height, additional information of rim height and hole volume was provided and correlated with visual acuity. In addition, more reliable differences among macular hole groups were found. Axial measurements such as macular hole depth, volume, and rim height may be more important for visual acuity than hole area indicating their possible predictive value for outcome measures.

Variations in photoreceptor directionally across the central retina.

Cones show a differential sensitivity to light coming from different portions of the pupil, typically being most sensitive to light from the center of the pupil. We measured the directional properties of the cones across the central 6 deg of the retina, using an optical imaging technique. We find that the cones in the center of the fovea have the broadest tuning. The width of the angular tuning changes rapidly from 0 deg to 1 deg retinal eccentricity, with cones at 1 deg being much more narrowly tuned that the cones in the center of the fovea. Directional tuning of the cones remains relatively constant from 1 deg to 3 deg retinal eccentricity. Receptoral disarray contributes minimally to the measured directional properties of the foveal cones, and there is no evidence of asymmetry between horizontal and vertical retinal locations. There are only small differences among the five subjects in the change in angular tuning of the cones with retinal location. We find that at the foveal center the directional tuning of the cones is limited by the diameter of the cone apertures.

Rapid sensitivity changes on flickering backgrounds: tests of models of light adaptation.

To investigate mechanisms underlying sensitivity changes that are capable of following rapid variations in intensity of the background field, we measured the threshold radiance needed to detect a 2-ms probe flash presented at various phases relative to a sinusoidally flickering background. The temporal frequency, mean luminance, and modulation of the background were systematically varied. The sensitivity change consisted of two components: a phase-insensitive increase in threshold that occurs at all the phases of the background field (a change in the dc level of the threshold), and a phase-dependent variation in threshold. Both components can reliably be measured at temporal frequencies up to approximately 50 Hz. On a 30-Hz background, the threshold varied with phase over roughly 0.5 log unit within a half-cycle (17 ms). For background flicker rates of 20-40 Hz the probe threshold increased with increasing instantaneous background radiance, following a typical threshold-versus-radiance template, and approaching Weber-law behavior during the peak of the background flicker. This pattern of threshold elevation was measured at mean background illuminances from 580 to 9100 Td (trolands), with the dimmer backgrounds being slightly less effective in producing threshold elevations. The measured increase in the dc level commenced as soon as the modulation of the background flicker began, and the amount of threshold elevation followed the envelope of the background flicker, ruling out modulation gain control explanations for the change in sensitivity on flickering backgrounds. The threshold elevations measured on a 30-Hz, 25% modulation background were lower than those measured on a 30-Hz, 100% modulation background at all phases. The measured changes in threshold with changes in background modulation rule out all adaptation models consisting of a multiplicative and a subtractive adaptation processes followed by a single, late, static nonlinearity.

Foveal cone spacing and cone photopigment density difference: objective measurements in the same subjects.

Foveal cone spacing was measured in vivo using an objective technique: ocular speckle interferometry. Cone packing density was computed from cone spacing data. Foveal cone photopigment density difference was measured in the same subjects using retinal densitometry with a scanning laser ophthalmoscope. Both the cone packing density and cone photopigment density difference decreased sharply with increasing retinal eccentricity. From the comparison of both sets of measurements, the computed amounts of photopigment per cone increased slightly with increasing retinal eccentricity. Consistent with previous results, decreases in cone outer segment length are over-compensated by an increase in the outer segment area, at least in retinal eccentricities up to 1 deg.

Deep retinal vascular anomalous complexes in advanced age-related macular degeneration.

PURPOSE: The authors describe the clinical characteristics of a group of patients with age-related macular degeneration (AMD), deep retinal vascular anomalous complexes (RVACs), advanced Bruch membrane changes, and severe visual loss. Based on clinical evaluation and imaging studies, the authors hypothesize the cause of such retinal vascular formations. PATIENTS AND METHODS: The authors quantified an initial case series of 6 patients and expanded it to 11 patients (14 eyes) with AMD and RVACs diagnosed by fluorescein angiography or slit-lamp examination. Associated pigment epithelial detachments (PEDs) of 13 eyes are described. In addition to the clinical and fluorescein angiography descriptions, infrared imaging and indocyanine green angiography were used to characterize more recently described RVACs and fellow eyes. RESULTS: Each study eye had a clearly defined anastomosis connecting the retinal circulation to a vascular complex in the deep retina. The RVACs associated with PEDs assumed a more central location than did typical choroidal neovascularization associated with PEDs. In seven eyes with RVACs, there were clinically recognizable retinovascular findings: intraretinal hemorrhages, telangiectasia, or microaneurysms. Legal blindness occurred in 9 of 11 patients. CONCLUSION: These results indicate that retinovascular changes can be associated with nondisciform AMD. The authors speculate that neurodegenerative changes and hypoxia may lead to such changes, the RVAC being a more advanced finding. Closure of an RVAC with photocoagulation is difficult, perhaps because of its higher blood flow. The visual outcome is poor, not only because of the advanced state of the underlying AMD, but also because of the exudative nature of the RVAC.

Angioscotometry with the scanning laser ophthalmoscope. Comparison of the effect of different wavelengths.

PURPOSE: Angioscotomas are scotomata caused by vessel shadows. Their extent may be influenced by physiological and pharmacologic conditions and disease. In this study, the authors quantified angioscotomas in normal subjects using a fundus perimetry technique with a scanning laser ophthalmoscope. They further investigated the influence of two different wavelengths on scotoma depth. METHODS: For blue-on-yellow perimetry, the authors used two different lasers--an argon laser (lambda = 458 nm) for stimuli and a low background and a HeNE (lambda = 594 nm) for a superimposed yellow background. For red-on-red perimetry, the authors used another HeNe laser (lambda = 633 nm). Fundus illumination was provided by an infrared light. Five healthy subjects were examined. Twenty-one to 24 stimuli (200 msec duration, 0.4 degree x 0.4 degree) were presented at different intensities in randomized order in a 5 degrees x 2.5 degrees retinal test field, directly inferior and adjacent to the disk. RESULTS: The depth of scotomas caused by major vessels varied in all subjects and depended on perimetry condition. To quantify the influence of vessels on sensitivity, the authors analyzed psychometric functions for stimuli projected on the vessels and for those far from the vessels. The authors found a significant difference for targets on the vessel compared to those far, which was more pronounced for the blue-on-yellow condition. CONCLUSIONS: Angioscotomas are detected better with blue targets on a yellow background than with red-on-red perimetry. The greater light absorption by hemoglobin and oxyhemoglobin at short wavelengths compared to longer wavelengths is not compensated for by visual mechanisms.

[Retinal vessels before and after photocoagulation in diabetic retinopathy. Determining the diameter using digitized color fundus slides]. / Retinale Gefässe vor und nach Photokoagulation bei diabetischer Retinopathie. Durchmesserbestimmungen mittels digitalisierter Farbdiapositive.

BACKGROUND: Retinal vessel diameter is an important parameter in blood flow analysis. Despite modern digital image technology, most clinical studies investigate diameters subjectively using projected fundus slides or negatives. In the present study we used a technique to examine vessel diameters by digital image analysis of color fundus slides. We investigated in a retrospective manner diameter changes in twenty diabetic patients before and after panretinal laser coagulation. MATERIAL AND METHODS: Color fundus slides were digitized by a new high resolution scanning device. The resulting images consisted in three channels (red, green, blue). Since vessel contrast was the highest in the green channel, we assessed grey value profiles perpendicular to the vessels in the green channel. Diameters were measured at the half-height of the profile. RESULTS: After panretinal laser coagulation, average venous diameter was decreased, whereas arterial diameter remained unchanged. There was no significant relation between the diameter change and the number of laser burns or the presence of neovascularization. CONCLUSIONS: Splitting digitized images into color planes enables objective measurements of retinal diameters in conventional color slides.

Flicker brightness enhancement and visual nonlinearity.

The purpose of this study was to investigate the nonlinear mechanism underlying brightness enhancement, in which a flickering stimulus appears brighter than a steady stimulus of equal mean luminance. The flickering and matching stimuli were temporally alternated. Both were cosine windowed to minimize the potential effects of temporal transients. Subjects adjusted the amplitude of the matching stimulus to match it in brightness to the flickering stimulus. The temporal frequency, modulation, and waveform of the flickering stimulus were varied. With sinusoidal flicker, brightness enhancement increased with increasing modulation at all frequencies, peaking at about 16 Hz at full modulation. The results were modeled by a broad temporal filter followed by a single accelerating nonlinearity. The derived temporal sensitivity of the early filter inferred from brightness enhancement decreased more slowly at high frequencies than the filter(s) inferred from flicker modulation thresholds. With low frequency sawtooth flicker, brightness enhancement was phase-dependent at low, but not at high modulations, suggesting that multiple neural mechanisms may also be involved in addition to an early nonlinearity.