Stimulus contrast, pursuit mode, and age strongly influence tracking performance on a continuous visual tracking task.

Human observers tend to naturally track moving stimuli. This tendency may be exploited towards an intuitive means of screening visual function as an impairment induced reduction in stimulus visibility will decrease tracking performance. Yet, to be able to detect subtle impairments, stimulus contrast is critical. If too high, the decrease in performance may remain undetected. Therefore, for this approach to become reliable and sensitive, we need a detailed understanding of how age, stimulus contrast, and the type of stimulus movement affect continuous tracking performance. To do so, we evaluated how well twenty younger and twenty older participants tracked a semi-randomly moving stimulus (Goldmann size III, 0.43 degrees of visual angle), presented at five contrast levels (5%-10%-20%-40%-80%). The stimulus could move smoothly only (smooth pursuit mode) or in alternation with displacements (saccadic pursuit mode). Additionally, we assessed static foveal and peripheral contrast thresholds. For all participants, tracking performance improved with increasing contrast in both pursuit modes. To reach threshold performance levels, older participants required about twice as much contrast (20% vs. 10% and 40% vs. 20% in smooth and saccadic modes respectively). Saccadic pursuit detection thresholds correlated significantly with static peripheral contrast thresholds (rho = 0.64). Smooth pursuit detection thresholds were uncorrelated with static foveal contrast thresholds (rho = 0.29). We conclude that continuous visual stimulus tracking is strongly affected by stimulus contrast, pursuit mode, and age. This provides essential insights that can be applied towards new and intuitive approaches of screening visual function.

Microstructural Visual Pathway White Matter Alterations in Primary Open-Angle Glaucoma: A Neurite Orientation Dispersion and Density Imaging Study.

BACKGROUND AND PURPOSE: DTI studies of patients with primary open-angle glaucoma have demonstrated that glaucomatous degeneration is not confined to the retina but involves the entire visual pathway. Due to the lack of direct biologic interpretation of DTI parameters, the structural nature of this degeneration is still poorly understood. We used neurite orientation dispersion and density imaging (NODDI) to characterize the microstructural changes in the pregeniculate optic tracts and the postgeniculate optic radiations of patients with primary open-angle glaucoma, to better understand the mechanisms underlying these changes. MATERIALS AND METHODS: T1- and multishell diffusion-weighted scans were obtained from 23 patients with primary open-angle glaucoma and 29 controls. NODDI parametric maps were produced from the diffusion-weighted scans, and probabilistic tractography was used to track the optic tracts and optic radiations. NODDI parameters were computed for the tracked pathways, and the measures were compared between both groups. The retinal nerve fiber layer thickness and visual field loss were assessed for the patients with glaucoma. RESULTS: The optic tracts of the patients with glaucoma showed a higher orientation dispersion index and a lower neurite density index compared with the controls (P < .001 and P = .001, respectively), while their optic radiations showed a higher orientation dispersion index only (P = .003). CONCLUSIONS: The pregeniculate visual pathways of the patients with primary open-angle glaucoma exhibited a loss of both axonal coherence and density, while the postgeniculate pathways exhibited a loss of axonal coherence only. Further longitudinal studies are needed to assess the progression of NODDI alterations in the visual pathways of patients with primary open-angle glaucoma across time.

Spatial contrast sensitivity from star- to sunlight in healthy subjects and patients with glaucoma.

Glaucoma is traditionally considered an asymptomatic disease until later stages. However, questionnaire studies revealed visual complaints related to various tasks, especially under extreme luminance conditions (such as outdoor at night on an unlit road or outside in the sun). We measured contrast sensitivity (CS) over a luminance range of 6 log units spanning the scotopic to photopic range and we aimed (1) to determine whether Weber's law also holds under extremely high luminance conditions and (2) to compare CS as a function of spatial frequency and luminance between glaucoma patients and healthy subjects. We included 22 glaucoma patients and 51 controls, all with normal visual acuity. For the second aim, we used a subgroup of 22 age-similar controls. Vertically oriented sine-wave gratings were generated with a projector-based setup (stimulus size 8x5 degrees). CS was measured monocularly at 1, 3, and 10 cycles per degree (cpd); mean luminance ranged from 0.0085 to 8500 cd/m2. ANOVA was used to analyze the effect of glaucoma, luminance, and spatial frequency on logCS. In controls, Weber's law held for 3 and 10 cpd; for 1 cpd, CS dropped above 1000 cd/m2 (P = 0.003). The logCS versus log luminance curves did not differ grossly between patients and controls (P = 0.14; typically 0-0.2 log units); the difference became larger with decreasing luminance (P = 0.003) but did not depend clearly on spatial frequency (P = 0.27). We conclude that differences between glaucoma and healthy were relatively modest for the spatially redundant, static stimulus as used in the current study.

Glaucoma screening during regular optician visits: can the population at risk of developing glaucoma be reached?

AIM: To determine the percentage of the population at risk of developing glaucoma, which can potentially be reached by conducting glaucoma screening during regular optician visits. METHODS: 1,200 inhabitants aged >40 years were randomly selected from Dutch community population databases. A questionnaire was mailed to these inhabitants with questions on their latest optician visit and risk factors for glaucoma. A second questionnaire was sent to their opticians, who were asked about their willingness to conduct an additional glaucoma screening programme in the future. RESULTS: The questionnaire was returned by 959 of 1,200 inhabitants and 37 of 50 opticians. The percentage of inhabitants who visited an optician during a 5-year period was 83% (95% confidence interval (CI) 80% to 85%). This percentage was adjusted for the presence of risk factors for glaucoma to obtain the percentage of the population at risk of developing glaucoma. The percentage of opticians willing to cooperate in a glaucoma screening programme extended beyond a non-contact tonometry measurement alone was 91% (95% CI 77% to 98%). CONCLUSION: By conducting glaucoma screening during regular optician visits, a large section of the population at risk of developing glaucoma can be reached.

Evidence-based guidelines on the referral of visually impaired persons to low vision services.

PURPOSE: One to two percent of the population in the Western world is visually impaired or blind. For most of these people there is no curative therapy. Therefore, the Dutch Ophthalmic Society has taken the initiative to develop an evidence-based guideline for the referral of visually impaired persons to low vision services. METHODS: A systematic literature search was performed in the Embase (1991-2001) and Medline (1966-2003) databases. Literature was searched for definitions of visual impairment, for physician-patient communication, and for outcome of interventions for visually impaired persons. Results of the articles that were selected were summarized and rated according to the level of evidence. Other considerations such as the current organization of rehabilitation for visually impaired persons in the Netherlands were also taken into account. RESULTS: The World Health Organization criteria were slightly adapted in order to include all people who experience problems with reading and other daily life activities due to visual impairment. A large number of recommendations were devised. Among these is that the complete diagnosis should be communicated to the patient and that a second appointment should be offered in which the diagnosis and potential treatment options are discussed again. Another recommendation is that in general visually impaired adults eligible for referral should be referred for the provision of low vision aids and that patients with complex problems or extensive rehabilitative demand should be referred to a rehabilitation center. CONCLUSIONS: This article presents a summary of the first European evidence-based guideline for the referral of visually impaired persons.

[Referral for rehabilitation in case of permanent visual handicap; guideline of the Dutch Society of Ophthalmology]. / Verwijzing naar revalidatie bij blijvende visuele beperkingen; richtlijn van het Nederlands Oogheelkundig Gezelschap.

The Dutch Society of Ophthalmology (NOG) has developed an evidence-based guideline for the referral of visually impaired people for rehabilitation and support. Referral for rehabilitation and support must be preceded by diagnosis and treatment. Consultation of an ophthalmologist is essential. Information about the disease should be given to the patient orally as well as in writing. The ophthalmologist brings up the possibility of rehabilitation in the presence of a visual acuity < 0.5 and/or visual field of < 30 degrees in the better eye and a well-defined request for help. Visually impaired patients with a relatively simple request for help are referred to a specialised optometrist whenever possible. Visually impaired patients with more complex requests for help are referred to a multidisciplinary (regional or national) rehabilitation centre for people with a visual handicap. Visually impaired and blind patients are informed about the existence of patient organisations. Referral for rehabilitation is done by means of a structured letter with all relevant information. A copy of this letter should be sent to the family physician and all other attending physicians.

Incorporating life expectancy in glaucoma care.

AIM: To calculate for which combinations of age and perimetric disease stage glaucoma patients are unlikely to become visually impaired during their lifetime. METHODS: We used residual life expectancy data (life expectancy adjusted for the age already reached) as provided by Statistics Netherlands and rates of progression as derived from published studies. We calculated the baseline mean deviation (MD) for which an individual would reach a MD of -20 dB at the end of life as a function of age and rate of progression. For situations in which the individual rate of progression is unknown, we used the 90th percentiles of rate of progression and residual life expectancy. For situations in which the individual rate of progression is known, we used the 95th percentile of the residual life expectancy. RESULTS: An easily applicable graphical tool was developed that enables an accurate estimate of the probability of becoming visually impaired during lifetime, given age, current glaucomatous damage, and--if available--the individual rate of progression. CONCLUSIONS: This novel tool enables the clinician to incorporate life expectancy in glaucoma care in a well-founded manner and may serve as a starting point for personalized decision making.

The groningen longitudinal glaucoma study III. The predictive value of frequency-doubling perimetry and GDx nerve fibre analyser test results for the development of glaucomatous visual field loss.

PURPOSE: To investigate whether frequency-doubling perimetry (FDT) and nerve fibre analyser (GDx) test results are able to predict glaucomatous visual field loss in glaucoma suspect patients. METHODS: A large cohort of glaucoma suspect patients (patients with ocular hypertension or a positive family history of glaucoma without visual field abnormalities at baseline) was followed prospectively for 4 years with SAP (HFA 30-2 SITA fast), FDT (C-20 full threshold), and GDx (version 2.010) in a clinical setting. After the follow-up period, baseline FDT and GDx test results of converters (glaucoma suspect patients who had converted to a reproducible abnormal SAP test result during follow-up) were compared to that of non-converters (suspects with normal SAP test results at the end of the follow-up) by calculating relative risks. Cutoff point for FDT was >1 depressed test point P<0.01 in the total deviation probability plot; cutoff point for GDx was the number >29. RESULTS: Of 174 glaucoma suspect patients, 26 had developed reproducible glaucomatous visual field loss (conversion rate: 3.7% per year). Relative risk was 1.8 (95% confidence interval: 0.9-3.7; P=0.10) for FDT and 2.7 (95% confidence interval: 1.2-6.3; P=0.01) for GDx. Positive predictive value was 0.22 for both FDT and GDx; negative predictive value was 0.88 for FDT and 0.92 for GDx. CONCLUSIONS: In a clinical setting, especially GDx may be helpful for identifying glaucoma suspect patients at risk of developing glaucomatous visual field loss as assessed by SAP.

A mathematical description of nerve fiber bundle trajectories and their variability in the human retina.

We developed a mathematical model wherein retinal nerve fiber trajectories can be described and the corresponding inter-subject variability analyzed. The model was based on traced nerve fiber bundle trajectories extracted from 55 fundus photographs of 55 human subjects. The model resembled the typical retinal nerve fiber layer course within 20 degrees eccentricity. Depending on the location of the visual field test point, the standard deviation of the calculated corresponding angular location at the optic nerve head circumference ranged from less than 1 degrees to 18 degrees , with an average of 8.8 degrees .

The additional yield of a periodic screening programme for open-angle glaucoma: a population-based comparison of incident glaucoma cases detected in regular ophthalmic care with cases detected during screening.

AIM: To study the additional yield of a periodic screening programme for open-angle glaucoma (OAG) by comparing, in a population-based setting, incident OAG (iOAG) cases detected in regular ophthalmic care with those detected during screening. METHODS: Participants aged 55 and over from the population-based Rotterdam Study underwent the same ophthalmic examination at baseline (1991-3) and follow-up (1997-9), including visual field testing and simultaneous stereo optic disc photography. Of 3842 participants, 87 (2.3%) developed iOAG during a mean follow-up time of 6.5 years. Of these 87 iOAG cases, 78 (90%) were included in this study. RESULTS: Of the 78 iOAG cases detected at follow-up, 23 (29%) had already been detected before during regular ophthalmic care. The remaining 55 (71%) undetected iOAG cases more often showed glaucomatous optic neuropathy without glaucomatous visual field loss (29 of 55 (53%)) as compared with the detected cases (four of 23 (17%); p = 0.009). Of the undetected iOAG cases, only four had developed significant visual field loss in their better eye. CONCLUSION: The additional yield of a periodic OAG screening programme is lower than expected from published prevalence data. In the discussion, the authors estimate that-in a white population with a low prevalence of pseudoexfoliation-about one in 1000 screened persons could be saved from bilateral end-stage OAG.

Influence of the rate of contrast change on the quality of contrast sensitivity assessment: a comparison of three psychophysical methods.

Criterion-free forced-choice procedures for measuring contrast sensitivity with a cathode ray tube (CRT) have low within-subject, intersubject, and test-retest variabilities, but a long test time compared with psychophysical methods that rely on the subject's criterion to determine threshold. Test time and variability of criterion-dependent methods depend on the rate at which the contrast changes on the CRT display. This study compared two criterion-dependent psychophysical methods for measuring contrast sensitivity (the method of increasing contrast and the von Békésy tracking method) with a criterion-free two-alternative forced-choice procedure. A range of rates of contrast change was studied: 0.1, 0.3, 0.5, 0.7 and 1.0 log unit s-1. Contrast sensitivity, within-subject variability, intersubject variability, test-retest variability, and test time of the three methods were compared. The 2-AFC procedure performed best with regard to within-subject, intersubject, and test-retest variabilities. A time-efficient alternative was the von Békésy tracking method at rates between 0.1 and 0.5 log unit s-1.

The effect of defocus on edge contrast sensitivity.

The effect of optical blur (defocus) on edge contrast sensitivity was studied. Edge contrast sensitivity detoriates with fairly small amounts of blur (approximately 0.5 D) and is roughly reduced by half for each dioptre of blur. The effect of blur on edge contrast sensitivity equals the effect of blur on sine wave contrast sensitivity for a spatial frequency of approximately 3 cpd.

The effect of spherical and other aberrations upon the modulation transfer of the defocussed human eye.

Relative modulation transfer is defined as contrast sensitivity under blur normalised to contrast sensitivity at optimum focus. Measured relative modulation transfer exceeds relative modulation transfer as calculated for aberration free optics at higher spatial frequencies (> 2 cpd). The contribution of spherical aberration to this discrepancy was studied. In the case of positive blur, spherical aberration increases relative modulation transfer for spatial frequencies of approximately 4 cpd and over. However, spherical aberration does not explain the entire difference between measured and calculated relative modulation transfer. Chromatic aberration does not provide a noticeable additional explanation. With negative blur, spherical aberration increases the relative modulation transfer much more. The effect of other monochromatic aberrations on relative modulation transfer is discussed.

Fast temporal adaptation of on-off units in the first optic chiasm of the blowfly.

1. We recorded from spiking units in the first optic chiasm between lamina and medulla in the brain of the blowfly (Calliphora vicina). Both previously characterized neuron types, on-off units and sustaining units, were encountered. On-off units had a temporal frequency response with a lower cut-off frequency than blowfly photoreceptors. This low cut-off frequency is related to a fast temporal adaptation of the on-off units to trains of short light pulses. Temporal adaptation occurred independently for short on- and off-pulses. 2. On-off units only responded to stimuli of relatively large contrast. Contrasts of less than 10% gave little or no response.

Chiasmal coefficient of flash and pattern visual evoked potentials for detection of chiasmal misrouting in albinism.

The diagnosis of albinism can be confirmed by electrophysiological examination, when chiasmal misrouting can be demonstrated. The present study describes a quantitative analysis method for this purpose. A chiasmal coefficient (CC) was calculated by correlating the differential potential over left and right hemisphere, when stimulating left versus right eye. This CC will be negative in albinism and positive in normal individuals. VEPs were recorded in 20 control subjects, four children with congenital motor nystagmus and six children with albinism. In up to 25% of the controls the CC was negative, when using flash VEP. However, with pattern VEP all had a positive CC. All children with albinism had a negative CC. Three of the four patients with congenital motor nystagmus had a positive CC, and one child had a small negative value with flash stimulation. In conclusion, determination of CC is a valuable and objective analysis method for electrophysiological determination of chiasmal misrouting. The method is relatively simple and only needs two electrode tracings. One should be aware of false-positive results when using flash stimulation. Whenever possible pattern stimulation should be used.

Spherical and irregular aberrations are important for the optimal performance of the human eye.

Contrast sensitivity measured psychophysically at different levels of defocus can be used to evaluate the eye optics. Possible parameters of spherical and irregular aberrations, e.g. relative modulation transfer (RMT), myopic shift, and depth of focus, can be determined from these measurements. The present paper compares measured results of RMT, myopic shift, and depth of focus with the theoretical results found in the two eye models described by Jansonius and Kooijman (1998). The RMT data in the present study agree with those found in other studies, e.g. Campbell and Green (1965) and Jansonius and Kooijman (1997). A new theoretical eye model using a spherical aberration intermediate between those of the eye models described by Jansonius and Kooijman (1998) and an irregular aberration with a typical S.D. of 0.3-0.5 D could adequately explain the measured RMT, myopic shift, and depth of focus data. Both spherical and irregular aberrations increased the depth of focus, but decreased the modulation transfer (MT) at high spatial frequencies at optimum focus. These aberrations, therefore, play an important role in the balance between acuity and depth of focus.

A girl without a chiasm: electrophysiologic and MRI evidence for the absence of crossing optic nerve fibers in a girl with a congenital nystagmus.

An otherwise healthy 15-year-old girl with a congenital nystagmus was evaluated at our department using visual evoked potential recording and magnetic resonance imaging. She appears to have the unique isolated inborn absence of the optic chiasm, described only once before in two unrelated girls. Unlike these previously described cases, our patient does not seem to display a see-saw nystagmus.