Ophthalmic and visual profile of guide dog owners in Scotland.
(57/4118)
BACKGROUND/AIMS: Out of an estimated 90,000 visually impaired people in Scotland, 509 make use of a guide dog. Initial research in Northern Ireland suggests that the ophthalmic profile of guide dog owners (GDOs) is highly specific. The aim of this study was to compare the ophthalmic and visual characteristics of Scottish GDOs with other groups of visually impaired people. METHODS: A random sample of GDOs from central and northern Scotland (n = 82) underwent a detailed assessment of residual vision (distance and near acuity, visual fields, contrast, and glare sensitivity). Comparative data were obtained from two populations of visually impaired non-GDOs-one group attending hospital ophthalmic and low vision clinics (n = 50) the other social services rehabilitation clients (n = 35). All participants completed a questionnaire to elicit ophthalmic history, age, and registration details. RESULTS: GDOs were found to be significantly younger and more profoundly visually impaired than non-GDOs. The main causes of visual impairment were retinitis pigmentosa (23%), optic atrophy (15%), and retinopathy of prematurity (7%). Ninety nine per cent of GDOs were registered blind and had been visually impaired for an average of 39 years. Only 31% were totally blind. CONCLUSION: GDOs represent a unique minority of the visually impaired population. Epidemiological registration trends would suggest that the numbers of young profoundly visually impaired people are unlikely to increase relative to their elderly counterparts. This has implications on the future demand for guide dog ownership. (+info)
The subregion correspondence model of binocular simple cells.
(58/4118)
We explore the hypothesis that binocular simple cells in cat areas 17 and 18 show subregion correspondence, defined as follows: within the region of overlap of the two eye's receptive fields, their ON subregions lie in corresponding locations, as do their OFF subregions. This hypothesis is motivated by a developmental model (Erwin and Miller, 1998) that suggested that simple cells could develop binocularly matched preferred orientations and spatial frequencies by developing subregion correspondence. Binocular organization of simple cell receptive fields is commonly characterized by two quantities: interocular position shift, the distance in visual space between the center positions of the two eye's receptive fields; and interocular phase shift, the difference in the spatial phases of those receptive fields, each measured relative to its center position. The subregion correspondence hypothesis implies that interocular position and phase shifts are linearly related. We compare this hypothesis with the null hypothesis, assumed by most previous models of binocular organization, that the two types of shift are uncorrelated. We demonstrate that the subregion correspondence and null hypotheses are equally consistent with previous measurements of binocular response properties of individual simple cells in the cat and other species and with measurements of the distribution of interocular phase shifts versus preferred orientations or versus interocular position shifts. However, the observed tendency of binocular simple cells in the cat to have "tuned excitatory" disparity tuning curves with preferred disparities tightly clustered around zero (Fischer and Kruger, 1979; Ferster, 1981; LeVay and Voigt, 1988) follows naturally from the subregion correspondence hypothesis but is inconsistent with the null hypothesis. We describe tests that could more conclusively differentiate between the hypotheses. The most straightforward test requires simultaneous determination of the receptive fields of groups of three or more binocular simple cells. (+info)
The SITA perimetric threshold algorithms in glaucoma.
(59/4118)
PURPOSE: To determine the within-visit between-algorithm and the within-algorithm between-visit differences in sensitivity for the SITA Standard, SITA Fast, FASTPAC, and Full Threshold algorithms in stable primary open angle glaucoma. METHODS: One designated eye from each of 29 patients (age 67.3 +/- 10.2 years; mean +/- SD) experienced in automated perimetry was examined with the four algorithms on each of three visits, using the Humphrey Field Analyzer 750 and Program 30-2. RESULTS: The group mean Mean Sensitivity was 1.0 dB greater for SITA Standard than Full Threshold (P < 0.001), 0.7 dB greater for SITA Standard than FASTPAC (P < 0.001), 1.6 dB greater for SITA Fast than FASTPAC (P < 0.001), and 0.9 dB greater for SITA Fast than SITA Standard (P < 0.001). The higher pointwise sensitivity for SITA Fast compared to Full Threshold, FASTPAC, and SITA Standard increased with increase in defect depth. The examination duration for SITA Standard was 53% of that for Full Threshold and 50% shorter for SITA Fast compared to FASTPAC (P < 0.001), regardless of age (P = 0.932). The examination duration increased with increase in severity of field loss (P < 0.001), and this increase was proportionately greater for both SITA algorithms (P < 0.001), particularly SITA Fast. The Total and Pattern Deviation probability analyses of both SITA algorithms yielded a statistically greater defect than Full Threshold or FASTPAC (P < 0.001). The within-algorithm between-visit differences were similar between SITA Standard and Full Threshold and between SITA Fast and FASTPAC. CONCLUSIONS: Both SITA algorithms produce a marginally higher differential light sensitivity compared to existing algorithms but with a statistically deeper defect and a marked reduction in examination duration. (+info)
Tubby-like protein 1 homozygous splice-site mutation causes early-onset severe retinal degeneration.
(60/4118)
PURPOSE: To characterize the disease expression of an autosomal recessive human retinal degeneration associated with a mutation in TULP1 (tubby-like protein 1), a gene with currently unknown function. METHODS: Homozygotes and heterozygotes from an extended Dominican kindred with a TULP1 splice-site gene mutation (IVS14+1,G-->A) were studied clinically and with visual function tests. Sequence analysis of TULP1 was also performed in unrelated patients with severe retinal degeneration from a North American clinic population. RESULTS: Homozygotes had nystagmus, visual acuity of 20/200 or worse, color vision disturbances, bull's eye maculopathy, and peripheral pigmentary retinopathy. Younger patients had a relatively wide extent of kinetic visual fields; older patients had only peripheral islands. No rod function was measurable by psychophysics in any of the patients; markedly reduced cone function was detectable across the visual field of younger patients and in the remaining peripheral islands of older patients. Rod and cone electroretinograms (ERGs) were not detectable using standard methods; microvolt-level cone ERGs were present in some patients. Heterozygotes had normal visual function. No putative pathogenic sequence changes in TULP1 were observed in North American patients with comparably severe retinal phenotypes, mainly in the diagnostic category of Leber congenital amaurosis. CONCLUSIONS: This TULP1 splice-site mutation in homozygotes causes early-onset, severe retinal degeneration involving macular and peripheral cones and rods. The constellation of phenotypic findings suggests that the TULP1 gene product is critically important for normal photoreceptor function and may play a role in retinal development. (+info)
Neural mechanisms for encoding binocular disparity: receptive field position versus phase.
(61/4118)
The visual system uses binocular disparity to discriminate the relative depth of objects in space. Because the striate cortex is the first site along the central visual pathways at which signals from the left and right eyes converge onto a single neuron, encoding of binocular disparity is thought to begin in this region. There are two possible mechanisms for encoding binocular disparity through simple cells in the striate cortex: a difference in receptive field (RF) position between the two eyes (RF position disparity) and a difference in RF profiles between the two eyes (RF phase disparity). Although there is evidence that supports each of these schemes, both mechanisms have not been examined in a single study to determine their relative roles. In this study, we have measured RF position and phase disparities of individual simple cells in the cat's striate cortex to address this issue. Using a sophisticated RF mapping technique that employs binary m-sequences, we have obtained left and right eye RF profiles of two or more cells recorded simultaneously. A version of the reference-cell method was used to estimate RF position disparity. We find that RF position disparities generally are limited to values that are not sufficient to encode large binocular disparities. In contrast, RF phase disparities cover a wide range of binocular disparities and exhibit dependencies on RF orientation and spatial frequency in a manner expected for a mechanism that encodes binocular disparity. These results suggest that binocular disparity is encoded mainly through RF phase disparity. However, RF position disparity may play a significant role for cells with high spatial frequency selectivity that are constrained to have only small RF phase disparities. (+info)
Errors of memory-guided saccades in humans with lesions of the frontal eye field and the dorsolateral prefrontal cortex.
(62/4118)
Behavioral studies in monkeys and humans suggest that systematic and variable errors of memory-guided saccades reflect distinct neuronal computations in primate spatial memory. We recorded memory-guided saccades with a 2-s delay in three patients with unilateral ischemic lesions of the frontal eye field and in three patients with unilateral ischemic lesions of the frontal eye field and the dorsolateral prefrontal cortex. Results suggest that systematic errors of memory-guided saccades originate in the frontal eye field and variable errors in the dorsolateral prefrontal cortex. These data are the first human lesion data to support the hypothesis that these regions provide functionally distinct contributions to spatial short-term memory. (+info)
Hyperintensities of the optic radiation on T2-weighted MR images of elderly subjects.
(63/4118)
BACKGROUND AND PURPOSE: Although abnormal hyperintensities are frequently observed at or around the optic radiation in elderly subjects, no previous reports have mentioned the clinical significance and pathologic changes of these hyperintensities. We evaluated the hyperintensity patterns of the optic radiation and its surrounding structures on T2-weighted MR images and compared these findings with pathologic observations and visual field measurements. METHODS: High-resolution coronal T2-weighted MR images of 102 consecutive patients (51-84 years old) were evaluated retrospectively for the presence and morphology of hyperintensities of the optic radiation (204 sides) and its surrounding structures. Pathologic specimens were obtained from 25 other patients (60-91 years old) who had died of nonneurologic causes. The histopathologic changes of the optic radiation and its surrounding structures were evaluated and correlated with the MR findings. Finally, MR findings and visual field measurements were correlated in 46 elderly volunteers (70-91 years old). RESULTS: Hyperintensities of the optic radiation or its surrounding structures or both were observed on 125 sides (93%) of 67 patients (61%), and linear/laminar hyperintensity of the optic radiation and the tapetum was the characteristic finding. Eleven (44%) of 25 pathologic specimens exhibited pallor of three anatomic layers (the external sagittal stratum or the optic radiation, the internal sagittal stratum, and the tapetum). No subjects with hyperintensity of the optic radiation had visual field abnormalities. CONCLUSION: Linear/laminar hyperintensity of the optic radiation and tapetum on T2-weighted images is common in elderly subjects, and may reflect differences in the internal structures and in the water content of three anatomic structures. Hyperintensities of this region did not cause visual field abnormalities in a group of elderly volunteers. (+info)
Clinical factors influencing the visual prognosis of the fellow eyes of normal tension glaucoma patients with unilateral field loss.
(64/4118)
AIM: To investigate the influence of several clinical variables on the development of visual field loss in the "second eye" of patients with normal tension glaucoma (NTG) presenting with unilateral field loss. METHODS: Patients with NTG and unilateral field loss at presentation were selected from a cohort of 403 consecutive diagnoses of NTG. The state of the visual field "normal" or with a visual field defect was defined using the Advanced Glaucoma Intervention Study (AGIS) template. Where available, optic disc planimetry was carried out on stereo photographs taken at presentation. Measurements of the topography of each of these optic discs were compared with morphometric values from a group of normal subjects, allowing for differences in age and disc size. For each patient the percentage of the relative neuroretinal rim (NRR) area was calculated. The time taken to develop a visual field defect was related to clinical factors including age, sex, peak and mean diurnal intraocular pressure (IOP), refraction, relative NRR area, and the AGIS score of the fellow eye at presentation RESULTS: 54 patients were included in the study. The median (range) follow up time was 49.2 (11.1-116.7) months. 14 (26%) patients developed field loss in the eyes with an initially normal field. The estimate of the median time to field loss onset was 95.1 months. Field damage developed more rapidly in women and in patients with greater AGIS score in the contralateral eye at the beginning of follow up ((adjusted hazard ratio, HR (95% confidence interval, CI) 0.20 (0. 04; 0.93); 1.19 (1.02; 1.41) respectively)). Little evidence of any association was found between time to onset of field loss and each of age, refraction, and peak or mean diurnal IOP. Planimetric disc analysis was carried out in 33 (61%) patients. Of these 10 (30%) developed field loss in the eyes with initial normal field at a median follow up of 95.1 months. After adjustment for sex and AGIS, relative NRR area was found to be significantly related to the time of onset of field damage, the greater the reduction in relative NRR area, the shorter the time to visual field loss (HR 0.93 (0.89; 0. 99)). CONCLUSIONS: NTG patients with unilateral field loss are at high risk of developing field damage in the eyes with an initially normal visual field. In this study, the visual prognosis of the eye with the normal visual field at presentation was found to be influenced by the extent of the reduction in relative NRR area together with the severity of field damage in the contralateral eye at presentation. (+info)