Does visual impairment lead to additional disability in adults with intellectual disabilities?

BACKGROUND: This study addresses the question to what extent visual impairment leads to additional disability in adults with intellectual disabilities (ID). METHOD: In a multi-centre cross-sectional study of 269 adults with mild to profound ID, social and behavioural functioning was assessed with observant-based questionnaires, prior to expert assessment of visual function. With linear regression analysis the percentage of variance, explained by levels of visual function, was calculated for the total population and per ID level. RESULTS: A total of 107/269 participants were visually impaired or blind (WHO criteria). On top of the decrease by ID visual impairment significantly decreased daily living skills, communication & language, recognition/communication. Visual impairment did not cause more self-absorbed and withdrawn behaviour or anxiety. Peculiar looking habits correlated with visual impairment and not with ID. In the groups with moderate and severe ID this effect seems stronger than in the group with profound ID. CONCLUSION: Although ID alone impairs daily functioning, visual impairment diminishes the daily functioning even more. Timely detection and treatment or rehabilitation of visual impairment may positively influence daily functioning, language development, initiative and persistence, social skills, communication skills and insecure movement.

Night-vision goggles for night-blind subjects: subjective evaluation after 2 years of use.

PURPOSE: To evaluate the usefulness of night-vision goggles (NVG) for night-blind subjects after 1 and 2 years of use. METHODS: Eleven night-blind subjects with retinitis pigmentosa used NVG for a 2-year period. At the end of each year, they were requested to fill-in two questionnaires regarding their use of the instrument. RESULTS: At both the 1- and 2-year evaluations, the subjects reported fewer problems with mobility in the dark when they used the goggles. At the 2-year evaluation, two individuals stated that they had stopped using the instrument, while seven used it at least twice a week. The nine subjects still using the instrument after 2 years reported an increase in their sense of independence and an increase in their potential for activities. The instrument was used most often during the dark winter season and for purposes such as visiting friends and family, travelling to work or sports, attending meetings and strolling. CONCLUSION: At the 2-year follow-up, NVG were still being used by most of the night-blind subjects. Moreover, the device had a positive effect on the subjects' experienced opportunities and sense of independence. The instrument was considered useful in the daily lives of our subjects.

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.

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.

Hemianopic visual field defects elicit hemianopic scanning.

Previous explanations for the variability in success of compensating for homonymous hemianopia (HH) has been in terms of extent of the brain injury. In using on-line eye movement registrations, we simulated HH in 16 healthy subjects and compared their scanning performance on a dot counting task to their own "normal" condition and to real HH patients' performance. We evidenced clear parallels between simulated and real HH, suggesting that hemianopic scanning behaviour is primarily visually elicited, namely by the visual field defect, and not by the additional brain damage. We further observed age-related processes in compensating for the HH.

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.

Grey scales uncover similar attentional effects in homonymous hemianopia and visual hemi-neglect.

Multi-component models of visual hemi-neglect have postulated that visual hemi-neglect is characterised by various attentional deficits. A grey scales task has been developed to quantify the early, automatic, (perhaps obligatory) ipsilesional orienting of visual attention, frequently assumed as the first of these attentional deficits. Explanations for this attentional imbalance are up until now mainly formulated in terms of right hemisphere activation. This lateral attentional bias has also been demonstrated in controls, in whom it is expressed as a leftward perceptual asymmetry. We reproduced previous literature findings on a grey scales task, considering controls and neglect patients. Three patients with neglect showed an extreme ipsilesional lateral bias. This bias did not change during or after cognitive rehabilitation. Additionally, we presented this grey scale task to 32 patients with left- and right-sided homonymous hemianopia (HP). HP is the loss of sight in one visual hemi-field. The HH patients had no clinical signs of impaired lateralised attention. Results revealed that HH patients showed a similar ipsilesional bias, albeit to a lesser degree than in neglect. Left-sided HH patients presented a quantitatively similar, but qualitatively opposite bias than the right-sided HH patients. We suggest that sensory effects can be an alternative source of attentional imbalance, which can interact with the previously proposed (right) hemispheric effects. This suggests that the perceptual asymmetry in the grey scales task is not necessarily an indicator of impaired right hemisphere attention. It rather suggests a pattern of functional cerebral asymmetry, which can also be caused by asymmetric sensory input.

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.

Age-related changes of defocus-specific contrast sensitivity in healthy subjects.

To investigate the effect of defocus on contrast sensitivity as a function of age in healthy subjects, the through focus contrast sensitivity was measured in 100 healthy subjects aged 20-69. Defocus-specific changes in contrast sensitivity reflect age-related changes in the optics of the eye. Tests were performed in cycloplegic eyes varying artificial pupil size (2, 4 and 6 mm), defocus (-1 to +2 D), and spatial frequency (1-16 cpd). Integrated contrast sensitivity was taken as a measure for the total amount of visual information transferred by the optical media. At optimal focus, integrated contrast sensitivity and log contrast sensitivity at 8 cpd showed a significant age-related decline. The log contrast sensitivity at 1 cpd appeared to be independent of age. The depth of focus for a 4-mm pupil increased significantly with age, even though contrast sensitivity at +2 D defocus decreases with age too, but not as much as the contrast sensitivity at optimal focus. Our study indicates that the effect of defocus on contrast sensitivity decreases with age; this was attributed to age-related changes in the optical media.

Does age change the distribution of visual attention? A comment on McCalley, Bouwhuis, and Juola (1995).

A paper by McCalley, Bouwhuis, and Juola (1995) suggested differences between younger and older adults in the use of visual cues. Furthermore, they reported these differences could largely be attributed to diminished (peripheral) visual processing capacities of elderly adults. Here, we reanalyze the data of McCalley and colleagues emphasizing relative rather than absolute differences. We find that when doing so, the data do not reveal differences in the way older and younger adults transiently allocate attention during visual search. Contrary to the conclusions of McCalley and colleagues, the similarity between the younger and older observers is therefore independent of the characteristics of the visual information. Furthermore, in our view the data suggest that older adults have foveal rather than peripheral visual processing difficulties. The results reemphasize the importance of the analytical approach taken in aging research. We discuss the difficulties and relevance of controlling and separating visual and attentional factors in age-related studies.

The relationship of functional limitations to disability and the moderating effects of psychological attributes in community-dwelling older persons.

This paper examines the moderating effects of three psychological attributes (neuroticism, self-efficacy expectancies and mastery) on the association between functional limitations (motor and cognitive limitation, vision and hearing loss) and disability ((instrumental) activities of daily living, role function and social function) in a sample of 624 community-dwelling older persons. In contrast to our hypothesis, we did not find any evidence for interaction effects. This means that low levels of psychological resources do not exacerbate the effect of functional limitation on disability in community-dwelling older persons. We found significant unique contributions of the psychological attributes to disability. Even when all three psychological attributes were taken into account, neuroticism and mastery had unique effects on social and role function, and self-efficacy expectancies had unique effects on (instrumental) activities of daily living. We conclude that the effects of functional limitation and psychological attributes on disability can be considered as additive. Older persons with less psychological resources are particularly at risk in developing disability.

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.

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.

Factors affecting contrasting results between self-reported and performance-based levels of physical limitation.

We have examined the role of socio-demographic variables, cognitive and affective functioning, and personality in discrepancies between performance-based and self-report measures within three domains of physical limitations: motor functioning, hearing and vision. Data are drawn from a community-based sample of 624 persons of 57 years of age and older. The strength of the association between self-reported and performance-based levels of physical limitations is moderate. Socio-demographic variables and levels of cognitive functioning explained some of the discrepancies between self-reported and performance-based vision. Within the domains of motor functioning and hearing, discrepancies were substantially influenced by affective functioning and personality. The discrepancies may reflect bias in perception or true variation in the effect of limitations on daily functioning. Both self-report and performance-based measures seem to complement each other in providing useful information about physical limitations.

Object perception by visually impaired people at different light levels.

We investigated the relationship between the illumination level and the ability of visually impaired subjects to detect and recognize objects in a realistic visual environment. Subjects often continued to show substantial improvement at light levels where normal subjects have reached maximum performance. Integrated contrast sensitivity, a summary measure for the contrast sensitivity function, was better at predicting performance than either visual acuity or peak contrast sensitivity. However, when combined, the latter two predicted performance as well as the former. We conclude that when we try to find the best illumination for orientation and day-to-day activities we should optimize it for both visual acuity and contrast sensitivity.

Measurement of visual acuity with two different charts; a comparison of results and repeatability in patients with cataract.

PURPOSE: To compare two different optotypes to measure visual acuity. METHODS: Experiment 1: Fifty patients with moderate cataracts were asked to read a chart consisting of letters of the alphabet (Sloan letters) first and a chart comprising Landolt's broken rings afterwards. Experiment 2: Half of patients were instructed to repeat the reading with a second letter chart, the other half was instructed to read the chart with the broken rings again. RESULTS AND CONCLUSIONS: Experiment 1: It was found that with the letter chart more optotypes (two to four) were recognized than with the broken ring chart. The different result of visual acuity measurement with the two optotypes is irrespective of the visual acuity. Experiment 2: The re-read instruction revealed that the measurements were reproduced equally for both charts.

Interactions between modulated luminance patterns and random-dot patterns.

It has been suggested that density modulated random-dot patterns can be used to study higher order pattern vision [Van Meeteren and Barlow (1981) Vision Research, 21, 765-777]. The high contrast dots of which the pattern is composed, are assumed to be reliably transduced and transmitted by the lower levels of the visual system. Therefore, such dot patterns could offer a way of by-passing the limits set by these earlier steps in the visual system. So, detection performance should reflect the capacity of more central visual mechanisms to combine and compare groups of dots. We test this assumption by selectively desensitizing the spatial frequency channels which are involved in detecting luminance contrast patterns. The results show a selective decrease in sensitivity for modulations in dot density at the adapting spatial frequency. We conclude that detection of differences in dot density is mediated by the same channels that detect luminance contrast. The conclusion by Van Meeteren and Barlow that dot patterns can be used to study higher-order processing in the visual system appears not to be valid. In addition, we present a new type of modulated dot pattern of which the density modulations are shown to be invisible for the spatial frequency channels. This pattern may therefore be used to study higher order visual processing.

Towards a classification of visual impairment.

For the rehabilitation of people with impaired vision, it is essential to have adequate (preferably quantitative) information about their residual visual functions. Special attention is given to the extra information provided by the results of measurement of the contrast sensitivity, especially in combination with the results of other measurements, such as the visual field, the amount of intraocular straylight and the visual acuity. The value of the contrast sensitivity function as a predictor of the extent of dysfunctioning in the visual activities of everyday life, such as outdoor vision, reading and recognition of faces, will be discussed. As far as this is concerned, a comparison is made between the value of visual acuity and contrast sensitivity as measures of the extent of vision and visual dysfunctioning.

Illumination research as part of a visual assessment of visually impaired individuals.

In this paper we describe a method for the preliminary assessment of the illumination needs of partially sighted individuals. The method is based on determining the smallest readable lettersize under different levels of illumination. Half of the partially sighted in our population had a performance that was dependent upon the level of illumination. At the same time, most of those for whom no improvement in performance was found, did express a preference for a certain level of illumination. Therefore, there remain discrepancies between the objectively determined optimal level and the preference for a certain level. We conclude that the current method is a useful but insufficient method of determining the illumination needs of partially sighted individuals.

Lens reflectometry with a CCD video camera and computer image analysis.

Lens reflectometry is a useful method of determining the back scatter of light from the cataractous lens. Making use of normal programmes, the personal computer can be used for the quantitative determination of the lens reflex, as seen in the slit-lamp image. If three different images of the same eye are compared, it appears that the mean variation in the back-scatter readings made by our method is 8.3%.