Biomedical subjects

William H Swanson

Publications and source records attributed to William H Swanson.

14 recordsLinked to original sources

PubMed2006

A cortical pooling model of spatial summation for perimetric stimuli.

Contemporary models of perimetric sensitivity assume probability summation of retinal ganglion cell sensitivities, ignoring cortical processing. To assess the role of cortical processing in perimetric spatial summation, we used a common form of multiple-mechanism spatial vision model in which the stimulus is sampled by receptive fields analogous to those of simple cells in primary visual cortex. Psychophysical threshold was computed by probability summation across the receptive fields. When the receptive fields were nonoriented (like ganglion cells), the spatial summation function had a large nonmonotonic transitional region that was inconsistent with perimetric spatial summation data. When the receptive fields were orientation tuned (like cortical cells), the model was able to give good fits to perimetric spatial summation data. The predictions of the model were evaluated with a masking study, in which noise masks either enlarged the critical area or changed the shape of the spatial summation functions. We conclude that cortical pooling by multiple spatial mechanisms can account for perimetric spatial summation, whereas probability summation across ganglion cells cannot.

Artifacts↗

PubMed2006

Development and evaluation of a linear staircase strategy for the measurement of perimetric sensitivity.

Perimetric sensitivity of patients with glaucoma has traditionally been measured in logarithmic (dB) units, but linear sensitivity correlates better with conventional structural measures of glaucomatous damage. Monte Carlo simulations of perimetric algorithms were used to assess potential effects of logarithmic steps on bias and variability when perimetric sensitivity was represented in linear units, and to assess the potential benefits of algorithms using linear steps. Simulations predicted that linear staircases could reduce the sensitivity-dependence of bias, variability and efficiency. These predictions were supported by a perimetric study of 21 patients with glaucoma and 20 age-similar controls who made repeat visits over several weeks.

Aged↗

PubMed2006

Ganglion cell loss and age-related visual loss: a cortical pooling analysis.

PURPOSE: To evaluate the ability of the cortical pooling model to predict the effects of random, mild ganglion cell loss, we compared the predictions of the model with the age-related loss and variability in achromatic and chromatic contrast sensitivity. METHODS: The relative sensitivity to small (0.5 degrees ) and large (3.0 degrees ) stimuli was compared in older (mean = 67 years, n = 27) and younger (mean = 23 years, n = 32) adults. Contrast sensitivity for modulations along the luminance, equiluminant L-cone, and equiluminant S-cone axes was assessed at the fovea and at four peripheral locations (12 degrees ). RESULTS: When the stimuli were large, threshold measurements obtained from all participants were reliable and well within the range of modulations along the chromatic axes that could be produced by the phosphors of the CRT. For the large stimuli, neither long- nor short-term variability increased as a function of age. Increasing the size of the stimulus did not decrease the magnitude of the age-related losses when the stimulus was chromatic, and visual losses observed with large chromatic stimuli were not different from those obtained with small achromatic stimuli. Moreover, chromatic contrast sensitivity assessments identified significant visual losses in four individuals who were not identified by achromatic contrast sensitivity assessments and only missed identifying one individual with significant losses in achromatic contrast sensitivity. CONCLUSIONS: The declines in achromatic and chromatic sensitivity as a function of age (0.4-0.7 dB per decade) were similar to those obtained in previous studies of achromatic and chromatic perimetry and are consistent with the loss of retinal ganglion cells reported in histologic studies. The results of this study are consistent with the predictions the cortical pooling model makes for both variability and contrast sensitivity. These findings emphasize that selective visual impairments do not necessarily reflect preferential damage to a single ganglion cell class and that it is important to include the influence of higher cortical processing when quantifying the relation between ganglion cells and visual function.

Adult↗

PubMed2006

Evaluation of a two-stage neural model of glaucomatous defect: an approach to reduce test-retest variability.

PURPOSE: The purpose of this study is to model perimetric defect and variability and identify stimulus conditions that can reduce variability while retaining good ability to detect glaucomatous defects. METHODS: The two-stage neural model of Swanson et al. was extended to explore relations among perimetric defect, response variability, and heterogeneous glaucomatous ganglion cell damage. Predictions of the model were evaluated by testing patients with glaucoma using a standard luminance increment 0.43 degrees in diameter and two innovative stimuli designed to tap cortical mechanisms tuned to low spatial frequencies. The innovative stimuli were a luminance-modulated Gabor stimulus (0.5 c/deg) and circular equiluminant red-green chromatic stimuli whose sizes were close to normal Ricco's areas for the chromatic mechanism. Seventeen patients with glaucoma were each tested twice within a 2-week period. Sensitivities were measured at eight locations at eccentricities from 10 degrees to 21 degrees selected in terms of the retinal nerve fiber bundle patterns. Defect depth and response (test-retest) variability were compared for the innovative stimuli and the standard stimulus. RESULTS: The model predicted that response variability in defective areas would be lower for our innovative stimuli than for the conventional perimetric stimulus with similar defect depths if detection of the chromatic and Gabor stimuli was mediated by spatial mechanisms tuned to low spatial frequencies. Experimental data were consistent with these predictions. Depth of defect was similar for all three stimuli (F = 1.67, p > 0.19). Mean response variability was lower for the chromatic stimulus than for the other stimuli (F = 5.58, p < 0.005) and was lower for the Gabor stimulus than for the standard stimulus in areas with more severe defects (t = 2.68, p < 0.005). Variability increased with defect depth for the standard and Gabor stimuli (p < 0.005) but not for the chromatic stimulus (slope less than zero). CONCLUSIONS: Use of large perimetric stimuli detected by cortical mechanisms tuned to low spatial frequencies can make it possible to lower response variability without comprising the ability to detect glaucomatous defect.

Color Perception↗

PubMed2006

Linearity can account for the similarity among conventional, frequency-doubling, and gabor-based perimetric tests in the glaucomatous macula.

PURPOSES: The purposes of this study are to compare macular perimetric sensitivities for conventional size III, frequency-doubling, and Gabor stimuli in terms of Weber contrast and to provide a theoretical interpretation of the results. METHODS: Twenty-two patients with glaucoma performed four perimetric tests: a conventional Swedish Interactive Threshold Algorithm (SITA) 10-2 test with Goldmann size III stimuli, two frequency-doubling tests (FDT 10-2, FDT Macula) with counterphase-modulated grating stimuli, and a laboratory-designed test with Gabor stimuli. Perimetric sensitivities were converted to the reciprocal of Weber contrast and sensitivities from different tests were compared using the Bland-Altman method. Effects of ganglion cell loss on perimetric sensitivities were then simulated with a two-stage neural model. RESULTS: The average perimetric loss was similar for all stimuli until advanced stages of ganglion cell loss, in which perimetric loss tended to be greater for size III stimuli than for frequency-doubling and Gabor stimuli. Comparison of the experimental data and model simulation suggests that, in the macula, linear relations between ganglion cell loss and perimetric sensitivity loss hold for all three stimuli. CONCLUSIONS: Linear relations between perimetric loss and ganglion cell loss for all three stimuli can account for the similarity in perimetric loss until advanced stages. The results do not support the hypothesis that redundancy for frequency-doubling stimuli is lower than redundancy for size III stimuli.

Adult↗

PubMed2006

A clinical evaluation of proview pressure phosphene tonometry in children.

PURPOSE: The Proview tonometer measures intraocular pressure by inducing a pressure phosphene through the eyelid and, if reliable and valid, may offer a quick, nonthreatening and noninvasive alternative method of obtaining intraocular pressures (IOPs) without the use of eye drops. This study compares the IOP measurements obtained in children using Proview pressure phosphene tonometry (PPPT) and Goldmann tonometry (GT). METHODS: One hundred four 5- to 12-year-old patients of the University Optometric Center/SUNY College of Optometry participated in the study. Subjects were randomized to receive, by different investigators, either PPPT or GT first. Two measurements with each instrument were attempted on each eye of all subjects. A subgroup of 41 subjects was asked which of the two methods was preferred. RESULTS: Seven percent of the subjects did not report a pressure phosphene response compared with 12% of the subjects on whom the investigators were unable to perform GT. The remaining 85 subjects completed the subject protocol. Of the 41 subjects asked, 56% preferred PPPT, 24% had no preference, and 20% preferred GT. The coefficient of repeatability between the two readings was higher for PPPT (3-4 mm Hg) than for GT (1 mm Hg). Mean IOP was 4 mm Hg higher for PPPT than GT with the difference in readings between the two instruments increasing with higher IOPs (r >19%, p < 0.005). CONCLUSIONS: In our study of healthy young subjects, PPPT measurements of IOP appear to be repeatable within a few millimeters of mercury in most children, but for some children, variability in repeat measurements can be substantial. Our data showed a mean difference in readings of 4 mm Hg with a 95% confidence interval that the PPPT reading was between 12 mm Hg above GT and 4 mm Hg below GT. This wide range of values indicates that PPPT is not comparable to GT. However, because our study found that children can appreciate pressure phosphenes and most prefer PPPT over GT, the Proview monitor may have value as a noninvasive, portable screener in pediatric patients. To fully evaluate this potential, further studies are needed that include patients with high IOPs.

Child↗

PubMed2005

Pupillary evaluation of retinal asymmetry: development and initial testing of a technique.

Glaucomatous damage to upper and lower retina is often unequal. We have developed a rapid, objective, quantitative measure of asymmetry of retinal sensitivity, using infrared pupillometry and pairs of large stimuli that are symmetric about the horizontal meridian. Results for a group of 11 young subjects free of eye disease indicate that the distribution of asymmetry is close to a normal distribution centered near upper/lower symmetry. Some subjects showed modest amounts of asymmetry, which was relatively uniform within each eye, and between the two eyes, of the subject. This approach to determination of asymmetry within an eye is potentially applicable to testing patients with glaucoma. The narrowness of the distribution should make it possible to detect asymmetries caused by disease.

Adult↗

PubMed2005

Empiric determination of corrected visual acuity standards for train crews.

PURPOSE: Probably the most common visual standard for employment in the transportation industry is best-corrected, high-contrast visual acuity. Because such standards were often established absent empiric linkage to job performance, it is possible that a job applicant or employee who has visual acuity less than the standard may be able to satisfactorily perform the required job activities. For the transportation system that we examined, the train crew is required to inspect visually the length of the train before and during the time it leaves the station. The purpose of the inspection is to determine if an individual is in a hazardous position with respect to the train. In this article, we determine the extent to which high-contrast visual acuity can predict performance on a simulated task. METHODS: Performance at discriminating hazardous from safe conditions, as depicted in projected photographic slides, was determined as a function of visual acuity. For different levels of visual acuity, which was varied through the use of optical defocus, a subject was required to label scenes as hazardous or safe. RESULTS: Task performance was highly correlated with visual acuity as measured under conditions normally used for vision screenings (high-illumination and high-contrast): as the acuity decreases, performance at discriminating hazardous from safe scenes worsens. CONCLUSIONS: This empirically based methodology can be used to establish a corrected high-contrast visual acuity standard for safety-sensitive work in transportation that is linked to the performance of a job-critical task.

Adult↗

PubMed2005

Quantifying effects of retinal illuminance on frequency doubling perimetry.

PURPOSE: To measure and quantify effects of variation in retinal illuminance on frequency doubling technology (FDT) perimetry. METHODS: A Zeiss-Humphrey/Welch Allyn FDT perimeter was used with the threshold N-30 strategy. Study 1, quantifying adaptation: 11 eyes of 11 subjects (24-46 years old) were tested with natural pupils, and then retested after stable pupillary dilation with neutral density filters of 0.0, 0.6, 1.2, and 1.6 log unit in front of the subject's eye. Study 2, predicting effect of reduced illuminance: 17 eyes of 17 subjects (26-61 years old) were tested with natural pupils, and then retested after stable pupillary miosis (assessed with an infrared camera). A quantitative adaptation model was fit to results of Study 1; the mean adaptation parameter was used to predict change in Study 2. RESULTS: Study 1: Mean defect (MD) decreased by 10 dB over a 1.6 log unit range of retinal illuminances; model fits for all subjects had r2> 95%. Study 2: Change in MD (DeltaMD) ranged from -7.3 dB to +0.8 dB. The mean adaptation parameter from Study 1 accounted for 69% of the variance in DeltaMD (P <0.0005), and accuracy of the model was independent of the magnitude of DeltaMD (r2< 1%, P >0.75). CONCLUSIONS: The results confirmed previous findings that FDT perimetry can be dramatically affected by variations in retinal illuminance. Application of a quantitative adaptation model provided guidelines for estimating effects of pupil diameter and lens density on FDT perimetry.

Adaptation, Ocular↗

PubMed2004

Perimetric defects and ganglion cell damage: interpreting linear relations using a two-stage neural model.

PURPOSE: To better understand the relations between glaucomatous perimetric defects and ganglion cell damage, a neural model was developed to interpret empiric findings on linear relations between perimetric defects and measures of ganglion cell loss. METHODS: A two-stage model computed responses of ganglion cell mosaics (first stage), then computed perimetric sensitivity in terms of processing by spatial filters (second stage) that pool the ganglion cell responses. Cell death and dysfunction were introduced in a local patch of the first-stage ganglion cell mosaic, and perimetric defect depth was computed for the corresponding region of the visual field. Calculations were performed for both sparse and dense ganglion cell mosaics and for spatial filters with peak frequencies from 0.5 to 4.0 cyc/deg. RESULTS: The model yielded nonlinear functions for perimetric defect depth in decibel versus the percentage of ganglion cell damage, but functions for lower spatial frequencies became linear when perimetric defect was expressed as a percentage of normal. The relations between perimetric defects and percentage of ganglion cell loss were determined primarily by spatial tuning of the second-stage spatial filters. For averaging sensitivities across different visual field locations, linear units (arithmetic mean) can more closely approximate mean ganglion cell loss than decibel units (geometric mean). Fits to data from experimental glaucoma required ganglion cell dysfunction in addition to ganglion cell loss. CONCLUSIONS: Pooling by second-stage spatial filters can account for empiric findings of linear relations between perimetric defects and measures of ganglion cell loss.

Glaucoma↗

PubMed2003

Color vision.

Many visual disorders produce acquired color vision defects. Color vision theory emphasizes several stages of visual processing: prereceptoral filters (lens, macular pigment, pupil), cone photopigments (L-, M-, and S-cones), and postreceptoral processes (red-green, S-cone, and luminance channels). Congenital color defects, which affect 8% to 10% of males and 0.4% to 0.5% of females, result from alterations in the photopigment absorption spectra or the absence of one or more photopigments. The most common defects are color vision deficiencies (protan and deutan defects), which are milder than the rarer achromatopsias (complete loss of color vision). Acquired color vision defects can be attributed to a number of different causes: alteration of prereceptoral filters, reduced cone photopigment optical density, greater loss of one cone type than the others, and disruption of postreceptoral processes. Acquired color vision defects have been divided into three classes: type 1, red-green defect with scotopization; type 2, red-green defect without scotopization; and type 3, blue defects (with or without pseudoprotanomaly). Blue defects are usually type 3 acquired defects because congenital tritan defects have an incidence of one in several tens of thousands. Red-green defects can be acquired or congenital, and ruling out acquired defects can require a battery of tests (plates and arrangement tests, anomaloscopy, perhaps genetic analysis). Color vision tests must be administered carefully (with a standard illuminant and protocol), and pupillary miosis or high lens density should be noted and their possible effects considered when interpreting test results. Plate tests provide a simple screening method but do not provide a diagnosis. Arrangement tests and anomaloscope testing take more time and make greater demands on the tester, but they provide a more thorough evaluation. When standard protocols are followed and results are interpreted in terms of prereceptoral filters, photopigment optical density, cone loss, and disruption of postreceptoral processes, a battery of color vision tests can be useful in the differential diagnosis, after progression of the disease, and for evaluating the effectiveness of treatment.

Color Perception↗

PubMed2003

Effects of cone adaptation on variability in S-cone increment thresholds.

PURPOSE: Short-wavelength automated perimetry (SWAP) has gained popularity as a clinical tool for the assessment of short-wavelength-sensitive (S)-cone visual function, but has also been shown to have higher threshold variability than conventional achromatic perimetry, possibly due to an imbalance between S-cone adaptation and long (L)- and medium (M)-wavelength-sensitive cone adaptation. To investigate potential causes for this relatively high variability, we studied the effects of luminance and S-cone adaptation on variability in S-cone increment thresholds. METHODS: Foveal S-cone increment thresholds were measured on adapting backgrounds ranging from 1.17 to 4.17 log troland (Td) and from -0.16 to 3.66 log S-cone trolands (Td(S)). Within-session variability (slope of the psychometric function) was evaluated in 2 trained and 15 inexperienced normal observers. Test-retest variability was evaluated in the 2 trained observers, and interobserver variability in the group of 15 observers. Multiple linear regression was used to model the effects of log luminance, log S-cone adaptation, and second-site polarization (ratio of luminance and S-cone adaptation; log [Td/Td(S)]). RESULTS: Test-retest variability was lower for conditions with higher levels of S-cone adaptation (F = 9.04, P = 0.013, for the trained observers). Adaptation conditions with lower levels of polarization were associated with lower within-session variability (F = 6.9, P = 0.011; trained observers) and interobserver variability (F = 33.7, P = 0.004; group of 15 observers). CONCLUSIONS: Variability of S-cone increment thresholds can be reduced by using adaptation conditions with a higher level of S-cone adaptation and/or a more balanced ratio between luminance and S-cone adaptation than is used for SWAP.

Adaptation, Ocular↗

PubMed2002

A double-blind placebo-controlled evaluation of the acute effects of sildenafil citrate (Viagra) on visual function in subjects with early-stage age-related macular degeneration.

PURPOSE: To assess the effects of a single 100-mg dose of sildenafil citrate on visual function in men with early-stage age-related macular degeneration. DESIGN: Randomized double-blind placebo-controlled clinical trial. METHODS: Nine men (mean age 71 years, range 59-85 years) with early-stage (minimal visual impairment and large drusen in the macula) age-related macular degeneration and 20/40 or better-corrected visual acuity in at least one eye were prospectively randomized to receive either placebo or sildenafil citrate (Viagra; Pfizer Inc, New York, New York) 100 mg as a single oral dose. After 7-14 days, they received the alternate treatment. Subjects underwent visual acuity, Amsler grid, color discrimination (D15), traffic light, Humphrey perimetry, and photo-stress testing in each eye before and at specific intervals within 8 hours after dosing. RESULTS: Compared with placebo, no pattern of errors were evident in any visual function test following sildenafil administration. No statistically or clinically relevant changes from baseline were observed in visual acuity, Humphrey perimetry (corrected pattern standard deviation), D15 color discrimination, or photo-stress tests. No clinically relevant changes were observed in the Amsler grid or traffic light tests. Sildenafil treatment was associated with transient mild or moderate headache, flushing, and rhinitis. There were no visual adverse events spontaneously reported to the investigator. CONCLUSION: A single 100-mg dose of sildenafil was well tolerated and produced no acute visual effects or exacerbation of preexisting visual impairment in nine men with early-stage age-related macular degeneration.

3',5'-Cyclic-GMP Phosphodiesterases↗

PubMed2002

An examination of physiological mechanisms underlying the frequency-doubling illusion.

PURPOSE: The frequency-doubling illusion is an apparent doubling of spatial frequency when a sinusoidal grating is modulated rapidly in temporal counterphase. It has been proposed that the illusion arises from a spatially nonlinear ganglion cell class. The current study reexamines this possibility and investigates other mechanisms that may underlie the illusion. METHODS: Responses of macaque magnocellular (MC) retinal ganglion cells were recorded to counterphase-modulated sinusoidal gratings of various spatial frequencies, and linearity of spatial summation was assessed. Human psychophysical thresholds were measured for a variety of phase discrimination and matching tasks. RESULTS: Consistent with lateral geniculate recordings reported by other authors, no evidence was found of a separate nonlinear (M(y)) MC cell class. The small, spatially nonlinear responses found were least at the low spatial frequencies used in clinical testing. Further analysis showed that no spatially modulated signal can be expected from the nonlinear response of a ganglion cell; the nonlinearity of spatial summation gives a doubled response in time but not across space. Psychophysical performance was consistent with an inability to distinguish the temporal phase of counterphase-modulated gratings when the illusion occurs. From 4 to 40 Hz, the zero-crossings of the modulated sinusoidal grating provided a spatial cue and were matched to comparison patterns at twice the stimulus spatial frequency. CONCLUSIONS: These results are inconsistent with the hypothesis that spatially nonlinear (M(y)) retinal ganglion cells are the physiological substrate of the frequency-doubling illusion. A cortical loss of temporal phase discrimination may be the principle cause of the illusion.

Animals↗