Ebook: Low Vision

Research and Development

Today society places more demands on seeing and processing of visual information than ever before. Information is given increasingly in a visual way and with a great diversity like traffic signs, high definition computer monitors, newspapers, books, television, LCD control panels, etcetera. The expanding flow of visual information places a considerable demand on visual processing in the occupational as well as in daily life conditions. As a consequence people with a decreased visual performance will be confronted sooner than before with their disabilities, which can result in a handicap. More than in the past rehabilitation is aimed at using the visual potential of the visually impaired person and the visual function has to be restored to a higher degree of performance in order to cope with the higher visual demands in daily life. It requires a thorough knowledge of the visual system and visual rehabilitation. This knowledge can only be extended by interdisciplinary cooperation: rehabilitation therapist, optometrist, occupational therapist, ophthalmologist, educationist, psychologist, clinical physicist; all of them have professional knowledge in the field. Some possess a large amount of empirical knowledge, other are more educated in the theoretical aspects of vision and visual impairment.

Visio, the National Foundation for the Visually Impaired and Blind, and the University of Groningen organized the international conference on low vision “VISION 1993” in Groningen and edited this book to provide a forum for exchange of information, experiences and results from research and the different disciplines in the field of rehabilitation of visually impaired people. Scientists have to listen to and to learn from the workers in the field and reverse. Rehabilitation protocols have to be evaluated on their beneficial effects and new scientific knowledge and technical developments have to be implemented in rehabilitation. Capability and mutual confidence are essential for such processes. This book reflects the process of integration of rehabilitation and research: it contains descriptions of practical experiences as well as manuscripts reporting in a scientific format the results of research projects.

Integral rehabilitation and ICIDH

Another aim of the conference was to stimulate the integration of different parts of the rehabilitation process. It have to result in a rehabilitation model for visually impaired people in which rehabilitation is an interdisciplinary process. Results of assessment and the rehabilitation activities by the different disciplines have to be integrated in one rehabilitation process of the visually impaired person. Several contributions deal with the relation between impairments, disabilities and handicaps and scaling of assessment data.

The specific problems of developing countries concerning the rehabilitation of visually impaired people obtained attention in a workshop preceding the start of the conference and in presentations by rehabilitation workers.

International Society

Scientific research on the visual system of people with low vision and the implementation of the results in the rehabilitation programmes of the visually impaired people will become of growing importance with the increasing knowledge of normal signal processing in the visual system and the brains and the increasing number of rehabilitation services. During the conference scientists presented assessment methods which are especially adapted to be used with visually impaired people, which enable a better understanding of the possibilities of the visually impaired person, and which improve the individual adaptation of the rehabilitation process. This development of assessment protocols, called ‘Videology’ in the organisation of Visio, is one of the expressions of the impact of new knowledge and new technological possibilities on rehabilitation.

We are very glad that during the conference the initiative has been taken to establish an international society for research in low vision and development of visual rehabilitation. We are confident that this society will stimulate and assure the exchange between the fields of research and rehabilitation of visual impairment people.

VISION 1993, The international conference on low vision, Groningen, July 5-9, 1993

J.A. Welling

chairman organizing committee

The Low Vision Committee of the EBU is of the opinion that definition must cover a larger number of parameters to better define visual problems: As a first step to better assessment of visual abilities a list of parameters has been worked out which should be taken into account in assessing partial sight. In order to help low vision professionals to get better information from ophthalmologists the low vision committee has designed a new questionnaire wich will be sent out to the national organisations of the EBU for discussion. The need of magnification and the visual acuity are recorded on a thermometer like scale based on logarithmic steps. Most other questions are multiple choice

The Visual Advice Centre Eindhoven (VAC–E) provides the partially sighted with advice and prescriptions for low vision aids and illumination. As a first step an assessment interview is held. The visual problems of the patient are inquired and the demands for help defined. The assessment interview provides information on the desirability for intervention by the social worker and the need for ADL and mobility training, advice on illumination at home and information about the visual handicap. An inventarisation of the demands for help, present low vision aids and other than visual restrictions is made during this interview.

Determining the incidence of visual impairment in elderly persons (>60 years) is one of the objectives of the NESTOR subprogram GLAS: “Functional Status and Need for Care”. For this purpose we designed a self-report questionnaire with 13 questions concerning day-to-day life such as “Can you see approaching bicycles and cars in the street?”, “Can you read numbers in a phone book?”, etc., and a set of screening techniques to measure visual acuity, edge contrast sensitivity, disability glare, near reading acuity, macular light sensitivity and quadrant visual field defects to test different aspects of visual functioning.

The results obtained with the screening tests are compared with the results of the self-report questionnaire. A validated self-report questionnaire will be a simple but adequate instrument in future studies and we believe that it will be helpful in the intake of visually impaired persons for rehabilitation programmes.

When assessing vision and practising visual rehabilitation in the very young and/or handicapped child we need an instrument by which we might be able to use behavioural observation in appraisal of vision.

An ideal checking list should be both reliable and handy, supplying relevant information on visual ability. It should help in identifying children with visual impairments, particularly those difficult-to-test by other methods, and in assessing the degree and character of impairments of specific visual functions. It should also assist us when assessing the visual development during the course of visual rehabilitation.

In the paper general scientific criteria of behavioural methods are discuss as well as the actual checking list developed by the author together with the visual habilitation team at TRC.

Abstract at Visio visual assessment and visual training has been done since 1991. The framework that is used, is the international classification of impairment disabilities and handicaps of the World Health Organisation. The awareness to the visual impairment and (dis)abilities is a great issue for the client. By carrying out visual assessment the client and the therapist will obtain a clear insight into the client’s awareness of his or her disabilities. When this step to awareness has been made, supplementary assessment will be done. The goal of the assessment and the training is proving a condition which enables the client to develop and train skills. We will give an explanation about observation and training by discussing one case: a child. In this lecture an explanation will be given about perception and especially about observation and training of visual skills.

The paper outlines a procedure to assess for visual functioning related to classroom-tasks. As an example perception of detail, one of the twelve selected assessment items, is subdivided according to a task analysis.

How the procedure can be used in a school for the visually impaired is demonstrated by describing a case of an eleven year old boy with damage tot the occipital, visual, cortex.

A fair number of physical and biological limits of human vision will be reviewed and related to recent insights in the mechanisms of visual behaviour and perception.

(a) Physical limits and laws determine the optical and mechanical properties of our eyes and the kind of visual information available in our environment (ecological physics). (b) The information formats to which the visual system became tuned during evolution determine a functional architecture that is arbitrary from a physical or engineering perspective (many altemative solutions imaginable). However, the resulting system is meaningful biologically and based on the limits set by biological material (proteins) and coding (DNA, neural coding), phylogenetic incidents and the like. Of particular interest are the resulting neuronal processing principles and plasticity. (c) The actual (principled and incidental) mechanisms and limits of vision are nowadays studied more intensively than ever and with interesting results. It appears that our visual system sorts dynamical visual image information according to categories that are not a priori evident and it processes these ‘pieces’ of information in separate nuclei and cortical areas. The consequences of this fractionation principle are noticeable if lesions in one or more of these specialised brain regions lead to curious syndromes like face blindness (prosopagnosia), motion blindness, blindsight, visual agnosia, achromatopsia, a loss of local sign information (certain forms of amblyopia), etc. The mechanisms and limits of normal vision should be known in detail to understand pathologies, play with nervous plasticity or offer alternative visual cues and thus contribute to rehabilitation.

Despite the elemental relationship of letters to written words, letter acuity is well-known to be a poor predictor of several indices of reading performance. This is why direct measurement of reading acuity is necessary in prescription of effective reading aids in low vision rehabilitation.

This paper presents a) a general discussion of differences between reading and letter acuity stimuli, and cognitive demands in the two acuity tasks, that may account for the poor power of acuity for single letters to predict reading performance, and b) an outline of a sensor model of text processing that can account for text and optotype crowding phenomena. The model has a linear filtering stage consisting of an array of sensors localized in space and spatial frequency that produce parallel bandpass representations of the text stimulus, and a decision stage consisting of a letter template-matcher and an attentional mechanism that selects the bandpass representations on which to perform the match. It is suggested that crowding effects at the acuity limit occur because neighboring letters or contour elements effectively add noise that falls within the the receptive fields and pass bands of sensors processing the letter.

The problems faced by developing countries for low vision assessment and training include the size and distribution of the population in relation to available resources, the lack of appropriate assessment materials, and lack of skills and knowledge in the assessment and rehabilitation of low vision. This paper reports on tests and assessment procedures which have been designed utilising technology appropriate to developing countries. Manuals include information on low vision and suggestions for a vision training program. Together these form a multi-purpose kit. Relevant parts can be selected for screening, identification of children with low vision, a comprehensive assessment of functional vision, or a short assessment for a particular purpose such as mobility. The kit has been used and evaluated in many parts of the developing world to ensure its appropriateness.

This research addresses the problems faced by patients with macular vision loss who are trying to adopt new viewing strategies. The hypothesis is that these problems are tightly connected with the detailed topography of residual vision in both eyes. In order to assess this topography in a quick and cost-effective way, a computerized test was developed and is here compared to already established methods. The results show that existing methods of analyzing visual field properties do not yield the information necessary to guide patients with macular vision loss through the difficult period of adopting new viewing habits. In contrast, our test provides the necessary data and does so in much less time and with less fatigue for the patient.

A novel approach to the assessment of residual vision in patients with maculopathies is described. It stresses finding intact areas of the retina that might be useful for re-learning to read using eccentric viewing. The test is implemented on an affordable off-the-shelf microcomputer and can yield results within minutes without discomfort to the patient. It uses a set of peripheral landmarks to stabilize gaze without foveation. Local responses of the visual field are tested using brief (234 msec) displays of letters or groups of letters. Preliminary results show that the test can be used with good reliability on a variety of patients with macular vision loss. Individual test data enable the low vision specialist to make clear and goal-directed recommendations to the patient regarding new viewing strategies that can help to make the best of residual vision.

In two experiments we investigated visual functioning of visually impaired persons at different illumination levels. Often, substantial improvements in visual performance can occur at light levels where normal subjects show no or only very little improvement. Although there is reasonable agreement on the preference for a certain light level and the improvement in visual performance that can be obtained by adapted lighting, not all subjects adjust light levels to such an extent that they achieve maximal performance. Subjective measures, therefore, do not always suffice for optimizing illumination. We further found, that subjects appear to choose preferred levels in such a way that they optimize their vision of detail, that optimal light levels for near and distance vision should be determined separately, and that the light tolerance of visually impaired subjects is often reduced.

In this study we investigated the relation between the level of illumination and the perception of objects in a realistic visual environment by visually impaired subjects. Not fully surprising, we found that object perception improved when we increased light levels. It should be noticed that substantial improvements could often be observed at light levels that were considerably higher than normal. Furthermore, we found that integrated contrast sensitivity is a suitable summary measure for the contrast sensitivity function. It correlates very well with the number of objects that were detected and recognized at different light levels. It also proved to be a much better predictor of visual performance than visual acuity. We infer from this that when we try to create the best possible illumination for orientation and day-to-day purposes we should optimize a person’s contrast sensitivity. In order to predict perceptual abilities in real life, it is of paramount importance to have a clear description of someone’s contrast sensitivity.

Color testing procedures are not well adapted to visually impaired person. A new testing method has been developped on a color Cathode Ray Tube. This special software program can easily evaluate the “functional” color vision of a patient whatever the deficiency is. The first results are described.

Several visual diseases not only reduce visual acuity, they also may impair a client’s ability to detect lower contrast objects that are well above acuity threshold. A letter contrast threshold function describes the lowest contrast letters that a client can recognize, over a range of sizes. Contrast sensitivity is the reciprocal of contrast threshold; higher sensitivity is better. Poor overall contrast sensitivity would increase a client’s problems with glare, and decrease reading performance with low contrast materials like newspapers, or currency. Very poor overall contrast sensitivity would slow reading of even the highest contrast magnified print and indicate the need for a CCTV or non-visual devices.

We have reviewed 4 experiments that related contrast sensitivity to reading performance in normally sighted and low vision observers. Our conclusions are that 1) letter contrast sensitivity charts are easier to administer and more predictive of reading performance than grating charts. Letter size may be varied by varying chart distance. 2) Letter contrast less than 10 times contrast threshold will reduce reading rate: Letter contrast less than 4 times contrast threshold will significantly impair reading rate and accuracy.

Thus, letter contrast thresholds higher than 9 % are considered poor and indicate a need for lighting assessment, extra magnification or a CCTV. Contrast threshold higher than 25 % are very poor and indicate the need for CCTV or non-visual interventions for fluent reading. These numbers are based on controlled experiments not clinical studies and should be used as guidelines not firm criteria.

In order to gain a better understanding of the visual capabilities of the visually impaired and in order to learn to use them optimally in the rehabilitation, it is essential to have adequate (preferably quantitative) information about the residual visual functions of the visually impaired individual.

Special attention is given to the extra information that is 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 intra-ocular straylight and the visual acuity.

The value of the contrast sensitivity function will be discussed, as a predictor for the extent of dysfunctioning in visual activities of everyday life. As far as this is concerned a comparison is made between the value of the visual acuity and the contrast sensitivity as a measure of the extent of vision and visual dysfunctioning.

A new edge contrast chart (60x60 cm) has been designed to use in a population study and for contrast sensitivity assessment in visually impaired people. The chart contains 16 circular test targets (diam. 8 cm), one half of each target has a lower reflectance than the other half and a sharp transition edge in between. Edge contrasts vary between 40 and 0.7 % in steps of about 0.15 log units. Normal population data are collected at viewing distances of 6, 3, and 1 meter and at illumination levels of 65, 500, and 2000 lux.

The GECKO contrast sensitivity scores of normal subjects are 0.25 log units lower than the maximal contrast sensitivity scores obtained with the Vistech VCTS 6500 wall chart (3 meter viewing distance); in visually impaired people with widely varying visual acuities and contrast sensitivities this difference is smaller (1 meter viewing distance).

Disability glare has been measured as the loss of log (contrast sensitivity) caused by using the Mentor Brightness Acuity Tester (BAT) at the medium and high luminance settings in normal subjects.

The GECKO chart in combination with the BAT is used in a population screening study and in a rehabilitation institute. Its use is easy to explain, both to the test leader and to the subjects. In visually impaired persons assessment of contrast sensitivity and disability glare is less tiring and much quicker to perform than a sine-wave modulated contrast sensitivity test and an intra-ocular straylight test. Subjects who could not see the targets of the Vistech chart (1 meter) or of the straylight test, succeeded in doing the GECKO-BAT test.

Human vision evolved as a means of gathering information from the environment. Efficient use of this sense requires not only a clear image of an object on the retina, but also suitable transmission of the neural representation of the object to the visual cortex and the addition of memory associations to attach meaning to the image. Conditions that cause visual impairments degrade the optical image. They also alter the transmission of the neural representation and decrease the ability to attach meaning to the image, thus affecting the perception of the object. The assessment of visual impairment should include an assessment of the perceptual capacity of the individual, as well as assessments of visual acuity, visual field, and contrast sensitivity. This paper explores how a better understanding of human visual perception can improve the assessment of visual impairment and provide greater specificity in the provision of rehabilitation services. The possble application of one visual perceptual test, the Frostig Figure- Ground, is described.

This research report concerns the adaptation of the dutch version of the Bayley Scales of Infant Development for use with the low vision child. Testing conditions, materials and instructions of this instrument are adapted for children with low vision. Furthermore an observation scale for visual abilities is developed and added to the instrument. Testresults of a group children with low vision will be surveyed. Results suggest that the adapted Bayley Developmental Scales will be a welcome addition to early guidance teams.

In this contribution a retrospective study is presented with respect to the Reynell-Zinkin Scales [1] for visually handicapped children. The children of the Bartiméus Centre, assessed with this instrument, seemed to be overestimated by the reference-values of the scales. The main aim of this study was to find out if this could be proved. In order to examine this aim, the reference-values were evaluated. Futherniore, the internal reliabiltity of the scales was determined. It was concluded that the Reynell-Zinkin Scales need new norm-references for the sample involved in this study. These norms are now established and some preliminary results are discussed in this contribution.

An intelligence test for blind and low vision children -aged 6 to 15 years- had been constructed and published in 1991. Based on a short vision test, two groups of braille educated children were distinguished: children without usable vision and children with usable vision. No blindfold is used in this test. The results of different types of statistical analyses have been published recently. Within this framework, more detailed quantitative research can be interpreted. The subtests appear to be highly reliable, and achievement in school can be predicted with some accuracy. Factor analysis indicates four interpretable factors in both vision groups. The vision groups -for which separate age-norms were establisheddiffered mainly on tests measuring spatial ability. A Manual in English is available.

As part of a study recently completed at BCCH the cognitive profiles of 65 children with partial sight were analyzed with the Wechsler batteries. A consistent profile emerged, related to the degree of residual vision. Subsequently the profiles of children between the ages of 6 and 17 with unequal vision of at least one octave, were analyzed. Contrary to expectations the profile of children with unequal vision did not correspond to the vision in the better eye, but was indicative of a more severe impairment.