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Biomedical subjects

W H Swanson

Publications and source records attributed to W H Swanson.

At least 19 recordsLinked to original sources
PubMed2001

Chromatic and achromatic defects in patients with progressing glaucoma.

To evaluate the pattern of losses associated with glaucomatous injury in patients with progressing glaucoma, functional losses were examined in 14 patients with progressing glaucoma using tests for which detection should be selectively mediated by one of three psychophysical mechanisms. Red-on-white increments, blue-on-white increments and critical flicker frequency were used to isolate the responses of the red-green chromatic mechanism, the blue-on chromatic mechanism, and the high-frequency flicker achromatic mechanism. For our 3.1 degrees circular stimuli, chromatic defects were found in a greater number of the patients with glaucoma than were achromatic defects. We evaluated these defects in terms of two existing hypotheses: preferential loss and reduced redundancy. The greater sensitivity to glaucomatous injury of chromatic tests, compared to achromatic tests, found in this and other studies and the apparent discrepancy between anatomical and psychophysical studies can be parsimoniously explained by differences in cortical summation of ganglion cell responses for the chromatic and achromatic pathways.

Aged↗
PubMed2000

Hyperacuity deficits in anisometropic and strabismic amblyopes with known ages of onset.

In order to evaluate the influence of etiology of amblyopia and of age at onset of amblyopia on the resulting constellation of spatial vision deficits, resolution/vernier and recognition/resolution acuity ratios were measured in groups of children with either strabismic amblyopia or anisometropic amblyopia with known ages of onset. Strabismic amblyopia with infantile onset (<9 months) and strabismic amblyopia with late onset (18-30 months) were both associated with abnormally low resolution/vernier and abnormally high recognition/resolution acuity ratios. Among amblyopes with infantile onset (<9 months), moderate amblyopia was associated with different resolution/vernier and recognition/resolution acuity ratios in anisometropic and strabismic groups. Infantile amblyopes with poor acuity outcomes included children who initially presented with anisometropia but later developed strabismus and children who initially presented with esotropia but later developed anisometropia; both subgroups with mixed amblyopia had poor resolution/vernier acuity ratios. Data from moderate amblyopes support the hypothesis that anisometropia and strabismus disrupt visual maturation in fundamentally different ways rather than simply at different stages in visual development.

Age of Onset↗
PubMed2000

Effect of stimulus size on static visual fields in patients with retinitis pigmentosa.

PURPOSE: To determine the effect of stimulus size on sensitivity of patients with retinitis pigmentosa (RP) as measured by automated static perimetry. DESIGN: Comparative case series. PARTICIPANTS: Thirty-nine patients with RP and a control group of 10 healthy volunteers. METHODS: Automated static perimetry (full threshold programs 24-2 or 30-2) was performed twice on one eye of each participant using stimulus sizes III (0.43 degrees diameter) and V (1.72 degrees diameter). Data from the same 50 test locations were used from each field. MAIN OUTCOME MEASURES: At each location, for each participant, the size effect was computed as the difference (in decibels) in sensitivities for sizes V and III, and the average sensitivity was computed as the mean of sensitivities for the two sizes. RESULTS: For both patient and control groups, the size effect was negatively correlated with average sensitivity (r(2) > 0.124; P: < 0.001). The mean size effect was significantly greater for the patient group than for the control group: 8.6 (+/- 3.6) dB versus 5. 4 (+/- 2.2) dB (t = 18.0; P: < 0.001). The percentage of abnormal locations (more than 8 dB below mean normal) tended to be lower for size V than for size III, with a mean of 67% for size V versus 95% for size III. The percentage of absolute defects was also lower for size V than for size III, with a mean of 35% for size V versus 54% for size III. CONCLUSIONS: In damaged regions of the visual fields of patients with RP, increase in stimulus size from III to V can produce abnormally large increases in perimetric sensitivity. Size III may be more useful than size V for detection of field abnormality, whereas size V may be more useful than size III for observing progression of advanced RP.

Humans↗
PubMed2000

Chromatic contrast sensitivity: the role of absolute threshold and gain constant in differences between the fovea and the periphery.

A model of foveal achromatic and chromatic sensitivity [Vision Res. 36, 1597 (1996)] was extended to the peripheral visual field. Threshold-versus-illuminance functions were analyzed to determine effects of eccentricity on absolute thresholds and gain constants of chromatic and luminance mechanisms. The resulting peripheral model successfully predicted peripheral contrast sensitivity as a function of wavelength, for both white and 500-nm backgrounds. We conclude that the short-wavelength-sensitive cone opponent mechanism may mediate thresholds in Sloan's notch in the normal periphery and that interpretation of reduced chromatic sensitivity in the periphery requires an explicit model of how eccentricity affects both the gain constant and the absolute threshold.

Color Perception↗
PubMed2000

Infant hyperacuity for radial deformation.

PURPOSE: Poor response rates and/or the confounding of motion and offset responses make it difficult to interpret results of previous studies of infant hyperacuity. The aim of the present study was to design a protocol that overcomes these limitations and to investigate the normal maturation of hyperacuity. METHODS: Hyperacuity of 31 healthy term infants aged 4 to 12 months was measured using radial deformation of static circular D4 patterns with a two-alternative, forced-choice, preferential-looking (FPL) protocol and maximum likelihood threshold estimation. FPL grating resolution acuity was assessed on the same visit. RESULTS: Both hyperacuity and resolution acuity were 1.1 to 1.2 logMAR (12-16 minutes arc) at 4 months of age. Hyperacuity improved rapidly to approximately 0.3 logMAR (2.0 minutes arc) by 9 to 12 months of age. This 0.9 log unit improvement in the hyperacuity still leaves the 12-month-old infant at a level 0.4 log unit poorer than adults' thresholds. Resolution acuity improved more gradually to approximately 0.7 logMAR (5 minutes arc) by 9 to 12 months of age. This 0.4 log unit improvement leaves the 12-month-old infant at a level 0.6 log unit poorer than adults' resolution acuity. CONCLUSIONS: Hyperacuity measured via radial deformation thresholds matures very rapidly between 4 and 6 months of age and continues to mature more slowly throughout infancy and into early childhood. The radial deformation protocol may provide a sensitive index for detecting and monitoring abnormalities in spatial vision in cases of infantile esotropia.

Adult↗
PubMed1999

Photopic temporal processing in retinitis pigmentosa.

PURPOSE: The relation between early changes in the photopic flicker electroretinogram (ERG) and photopic psychophysical changes in retinitis pigmentosa (RP) is poorly understood. Here, abnormalities in foveal and extrafoveal temporal contrast sensitivity functions (TCSFs) were studied in a group of carefully selected patients with RP who had relatively preserved macular function. The psychophysical results were compared with changes in the timing of the multifocal ERG. METHODS: Subjects were patients with RP who had acuity > or =20/32 and no visual field defects within 6 degrees from the fovea. Maxwellian-view and direct-view optical systems were used to obtain foveal and extrafoveal TCSFs under a range of test conditions, including high retinal illuminances that yielded temporal contrast sensitivity independent of mean retinal illuminance. TCSFs were described using log sensitivity and corner frequency parameters. RESULTS: Foveal TCSFs in these patients showed overall reductions in sensitivity but no frequency-dependent defects. Also, no macular defects were found in the timing of the multifocal ERG. TCSFs from extrafoveal locations in moderate field defects, obtained at retinal illuminances that were sufficient to render flicker sensitivity independent of effective mean luminance, showed reductions in overall sensitivity as well as changes in temporal tuning. The multifocal ERGs from these extrafoveal locations showed signs of temporal slowing. CONCLUSIONS: Changes in temporal tuning (both psychophysical and electroretinographic) were found only within visual field scotomas, whereas changes of the log sensitivity parameter were found also in the relatively preserved foveas of this group of patients with early stage RP.

Adaptation, Ocular↗
PubMed1996

Age-related changes in the color-match-area effect.

Color matches were measured as a function of field diameter (1, 2, 4 and 8 deg) for 53 normal observers aged 13-80 yr. The difference between match midpoints for the 1 deg diameter and the other diameters decreased throughout adulthood, indicating an age-related change in optical density of cone outer segments in the central 1 deg. For all ages, there were large interobserver differences in the magnitude of the difference scores. These results provide an explanation for contradictions in the literature on the effects of age on cone optical density and on the magnitude of the color-match-area effect.

Adolescent↗
PubMed1996

S-cone spatial contrast sensitivity can be independent of pre-receptoral factors.

Short-wavelength-sensitive (S-) cone-mediated thresholds have been used to study the early stages of visual loss, but due to the effects of non-neural factors (pupil size, lenticular density, macular pigment density) S-cone thresholds are often of limited clinical utility. The current study evaluates four possible effects of non-neural factors on S-cone contrast sensitivity, and shows how these can be minimized by measuring sensitivity for 1-5 c/deg with a range of retinal illuminances for blue test gratings on yellow backgrounds. The data are fit well with a simple four-parameter model which indicates that S-cone contrast sensitivity can be relatively independent on non-neural factors. A simple control experiment is described for evaluating the independence in individual patients.

Adaptation, Ocular↗
PubMed1996

Visual field defects after macular hole surgery. A new finding.

PURPOSE: The purpose of the study is to report the problem of a temporal visual field defect occurring after macular hole surgery. METHODS: The authors reviewed the records of 13 patients found to have visual field defects after vitrectomy for macular holes. Fluorescein angiograms (13 patients), optic nerve photographs (13 patients), focal electroretinograms (3 patients), and nerve fiber analyses (8 patients) were performed in patients with visual field defects. RESULTS: An absolute, temporal, usually inferior field defect was noted in 13 patients. In eight patients, the defect was detected because of specific reports or retrospective field examination results. Five patients examined in a prospective manner were found to have field defects. No history of abnormal intraocular pressure or direct trauma to the optic nerve or retinal vessels was identified. Four patients showed optic nerve pallor and three had an anomalous-appearing disc. Focal electroretinograms were of similar amplitude in the involved retina compared to corresponding areas in the healthy fellow eye. Nerve fiber analysis showed a reduction in nerve fiber layer thickness correlating to the visual field defect in those eight patients in which this test was used. CONCLUSION: A significant temporal field defect may occur in patients after otherwise uncomplicated surgery for macular holes. The cause is unclear; however, reductions in nerve fiber layer thickness from the superior and nasal peripapillary area suggest that acute surgical release of the posterior hyaloid and the use of long-acting intraocular gas may in certain patients result in visual field defects.

Aged↗
PubMed1995

Color matches in diseased eyes with good acuity: detection of deficits in cone optical density and in chromatic discrimination.

Reduced foveal cone optical density in diseased eyes with normal acuity can affect color matches. Using field diameters of 1 degree, 2 degrees, 4 degrees, and 8 degrees, we measured mean color-match midpoints and match widths in patients who had good acuity and who exhibited three categories of eye disease: hereditary macular degeneration (n = 12), retinitis pigmentosa (n = 19), and glaucoma (n = 18). Results were compared with those for normal observers of comparable ages. Mean color-match midpoints were abnormal only for the population with hereditary macular degeneration, indicating a reduction in cone optical density in the central 4 degrees. Mean color-match widths were enlarged for both hereditary macular degeneration and retinitis pigmentosa, a result consistent with a reduction in the number of foveal cones.

Adult↗
PubMed1993

Chromatic adaptation alters spectral sensitivity at high temporal frequencies.

To evaluate the effects of chromatic adaptation on spectral sensitivity at temporal frequencies within the region of high-frequency linearity, critical flicker frequency was measured as a function of red-green luminance ratio for counterphase flicker of 649- and 555-nm light. For eight observers, the relative weight of the contribution of the long-wavelength-sensitive cones to flicker detection was smaller on long-wavelength adapting fields than on middle-wavelength adapting fields even though long-wavelength-sensitive-cone modulations were high. These data indicate that chromatic adaptation can confound the interpretation of flicker-sensitivity data that are gathered with long-wavelength test lights or with equiluminant heterochromatic flicker and that there can be considerable interobserver variability in the effects of chromatic adaptation.

Adaptation, Ocular↗
PubMed1993

Analysis of Rayleigh match data with psychometric functions.

Color matches have been used for a variety of purposes, yet the psychometric properties of color-matching data have not been thoroughly investigated. A method is given for generating psychometric functions for the two ends of the color-matching range by use of a perceptual dimension for stimulus magnitude based on ratios of cone quantal catches. The analysis was applied to Rayleigh match data gathered from 250 naïve observers with an automated protocol. Slopes of the psychometric functions were significantly shallower for anomalous trichromats than for normal trichromats, consistent with the assumption that stimulus magnitude is based on ratios of cone quantal catches. These results indicate that the tester's criterion for response consistency can strongly affect Rayleigh match widths. The analysis may also be useful for other perceptual tasks, such as contrast matching and spatial alignment.

Adolescent↗
PubMed1993

Outcome after very early treatment of dense congenital unilateral cataract.

PURPOSE: To evaluate whether very early treatment for congenital unilateral cataract results in better long-term functional outcomes, grating acuity, contrast sensitivity, recognition acuity, and random-dot stereoacuity were evaluated in two groups of children. METHODS: Grating acuity and contrast sensitivity data were obtained with standard forced-choice protocols. Stereoacuity data were obtained both in a forced-choice laboratory protocol and by the Randot test. RESULTS: Immediately after treatment, both the very early (1 to 6 weeks; n = 8) and early (2 to 8 months; n = 6) treatment groups showed a 0.3 log unit grating acuity deficit in the aphakic eye. Grating acuity in the early group showed some improvement with age but reached a plateau of 0.75 logMAR at 18 to 24 months; the very early group showed more improvement and reached a plateau of 0.22 logMAR at 37 to 48 months. At 5 to 8 years of age, aphakic eyes of the very early group had significantly better contrast sensitivity and recognition acuity than the aphakic eyes of the early group. Overall, grating acuity deficits during years 2 through 5 were significantly correlated with contrast sensitivity and recognition acuity outcomes measured at 5 to 8 years of age. No deficits in grating acuity, contrast sensitivity, or recognition acuity were found for phakic fellow eyes in either group. Three children in the very early treatment group (37.5%) were orthotropic and demonstrated gross random-dot stereopsis; one child in the early group was orthotropic but none of these children demonstrated random-dot stereopsis. CONCLUSIONS: These results suggest that treatment initiated at 1 to 6 weeks of age maximizes the opportunity for normal or near-normal visual development of a congenitally cataractous eye with little or no risk to the phakic fellow eye.

Aphakia, Postcataract↗
PubMed1993

S-cone function in patients with retinitis pigmentosa.

PURPOSE: To determine whether short-wavelength-sensitive (S-) cones are more severely damaged in patients with retinitis pigmentosa than long-wavelength-sensitive (L-) and middle-wavelength-sensitive (M-) cones. To determine whether there are differences in the amount of S-cone damage in patients with dominant versus nondominant inheritance patterns. To accomplish these goals with methods that provide information not furnished by previous studies with two-color increment thresholds. METHODS: Acuity mediated by the S-cones was measured in 56 patients with retinitis pigmentosa, and the electroretinogram (ERG) generated by the S-cones was measured in 11 of these patients. Mixed L- and M-cone acuity, mixed L- and M-cone ERGs, and clinical full-field rod and cone ERGs were obtained for all patients. Data for both dominant and nondominant patient groups were compared with data from age-matched normal subjects. RESULTS: Only the nondominant group had reduced S-cone acuity, and 43% of patients in this group had selective reduction of S-cone acuity. In this particular sample the dominant and nondominant groups were comparable in clinical full-field ERG parameters and mixed L- and M-cone acuity, so the difference in S-cone acuities is not due to the dominant group having less advanced retinal degeneration. All 11 patients tested had reduced S-cone ERGs, 6 with significantly greater loss in the S-cone ERG than in the mixed L- and M-cone ERG. CONCLUSIONS: These data provide evidence that retinitis pigmentosa can produce greater loss of S-cones than L- and M-cones, and that this selective loss is primarily seen in patients with nondominant forms of retinitis pigmentosa.

Adolescent↗
PubMed1992

Probability summation of acuity in the human infant.

Raw data from two studies of monocular and binocular acuity development were used to determine whether a binocular acuity advantage as a function of age is predicted by probability summation and whether these predictions accurately describe the course of binocular acuity development. Two decision rules for the combination of the outputs of right eye and left eye "channels" were evaluated, the decision-threshold rule and the integration rule. Both decision rules predicted a binocular acuity advantage for infants and children aged 0-60 months. However, both rules failed to adequately describe the normal course of monocular and binocular acuity development. No binocular acuity advantage was found prior to 6 months of age while, after 6 months of age, binocular acuity was superior to monocular acuity by 0.12 log unit (0.4 octave). The absence of binocular acuity superiority prior to 6 months of age is consistent with suggestions by other authors that the immature human visual system combines information from the two eyes nonselectively.

Aging↗
PubMed1992

Extracting thresholds from noisy psychophysical data.

Psychophysical studies with infants or with patients often are unable to use pilot data, training, or large numbers of trials. To evaluate threshold estimates under these conditions, computer simulations of experiments with small numbers of trials were performed by using psychometric functions based on a model of two types of noise: stimulus-related noise (affecting slope) and extraneous noise (affecting upper asymptote). Threshold estimates were biased and imprecise when extraneous noise was high, as were the estimates of extraneous noise. Strategies were developed for rejecting data sets as too noisy for unbiased and precise threshold estimation; these strategies were most successful when extraneous noise was low for most of the data sets. An analysis of 1,026 data sets from visual function tests of infants and toddlers showed that extraneous noise is often considerable, that experimental paradigms can be developed that minimize extraneous noise, and that data analysis that does not consider the effects of extraneous noise may underestimate test-retest reliability and overestimate interocular differences.

Attention↗
PubMed1990

Infant spatiotemporal vision: dependence of spatial contrast sensitivity on temporal frequency.

Effects of temporal frequency on infant spatial contrast sensitivity were studied with forced-choice preferential looking, using localized 1.0 and 0.35 c/deg grating patches. Between 4 and 8 months, contrast sensitivity at 1.0 c/deg increased significantly at 8-17 Hz but showed little change at 2-4 Hz, so temporal tuning became increasingly bandpass with age. At 4 months, temporal tuning was lowpass at 1.0 c/deg and bandpass at 0.35 c/deg. Control data on adults indicate that the differences between the infant and adult data cannot be accounted for solely by reduced photoreceptor density and quantal catch, and suggest developmental changes in tuning and/or relative sensitivities of spatiotemporal mechanisms.

Adult↗