Understanding Blindness

An Integrative Approach

Mark Hollins

• Publisher: Routledge
• Year: 2022
• ISBN:
  9781000549492
• Edition: 1
• Topic: Psychology
• Subtopic: History & Theory in Psychology
• Index: Psychology

Originally published in 1989, much was known about blindness, but the field was divided into specialties. Experts in the different areas were widely dispersed among university departments, rehabilitation agencies, and school systems, with the result that people in one specialty area often knew little about developments in other areas. It was hoped that this work would be useful in reducing that isolation, by presenting, within a single volume, basic information derived from different approaches to the subject of blindness. Individuals already familiar with material in some of the chapters could gain added perspective on the field as a whole by reading about other aspects of blindness outside their specialty area.

1 BLINDNESS AND THE EYE

Definitions of Blindness

This book is about blindness. It might seem that there is little to say about this subject, without delving into medical details, that the average person does not know. Blind people are, after all, just like the rest of us except that they cannot see. What more is there to tell? The answer is, a lot. Recent scientific investigations have begun to answer questions about blindness that have been asked for centuries, such as “Do congenitally blind people have mental images? How are they sometimes able to detect an object without touching it? Is the visual part of the brain doing anything in a blind person?“ On a more practical level, recent technological advances offer great promise to blind people, for example, by making it possible for them to read print, not just braille, without assistance. The role that public attitudes can play in helping blind people to fulfill their aspirations is better understood now than in the past. And new studies of blind infants and children have turned up important new principles regarding their development. Many of these recent developments rest on a foundation of earlier knowledge, and one goal of the book is to explain how new findings have both changed and added to our previous level of understanding. Before we begin, however, we must define blindness, or rather explain that there are multiple definitions, for much confusion has been caused in the past by the fact that this term is interpreted differently by different people.

The confusion arises because there are degrees of blindness, with some blind people able to see nothing, while others can see a little, and still others can see quite a bit—enough to recognize another person by his or her bearing or gait, for example. The 52,000 individuals in this country who are totally insensitive to light are in fact outnumbered about nine to one by others who, although they are ordinarily considered blind, have some residual vision (National Society to Prevent Blindness, 1980). About equal in number to the totally blind are those who can tell whether they are in a bright or dark environment, but cannot say, without turning their head, from which direction the light comes; they have light perception. Still others are able to point out the source of light, say a window in an otherwise dark room, but cannot make out any details or shapes; this basic visual ability is called projection.

Although individuals with light perception and projection are not totally blind, they are blind in the sense that they can make little use of their vision. And even persons who have still more vision, so that they can see faint or hazy shapes, but can make out no details, may be considered blind for some purposes. But how blurred should vision have to be before this term is applied? Any answer to this question must be arbitrary, for all degrees of visual impairment exist, from total blindness to just a slight blurring of vision. For statistical and legal purposes, however, it is useful to have a cutoff point which separates those who are legally blind from those with a lesser visual impairment, or none at all.

The definition of legal blindness involves measurements of acuity, that is, the visual system's ability to perceive detail. This is typically measured with an eyechart, placed 20 feet from the person being tested. The average person with unimpaired vision can just read letters of a certain size at this distance, and is said to have 20/20 vision. A person who cannot read these letters, but is just able to make out others of twice that size, has 20/40 vision, which is to say that he or she can just read at 20 feet letters that someone with unimpaired vision could distinguish at a distance of 40 feet from the chart. The worse the person's acuity, the larger the denominator. Someone who, even when wearing glasses, has an acuity of 20/200 or worse—that is, 10 times poorer than that of a person with good vision—is legally blind.

It is possible, however, to have good acuity and still not get much practical use from vision, if the field of vision is very restricted. In certain conditions, such as retinitis pigmentosa, to be discussed later, an individual may be able to see straight ahead, but not off to the sides. This failure of peripheral vision can be quantified by a diagnostic procedure called perimetry, in which the patient stares fixedly at a small mark, while the person administering the test moves a spot of light from place to place in the visual field. This examination is usually carried out using a half-sphere, about 60 cm in diameter, into which the patient looks. The inside of this perimeter is painted white and is diffusely illuminated; the additional moving spot of light can be shined onto the inner surface of the sphere from a small projector located just above and behind the patient's shoulder. An overhead view of a perimeter is shown in Fig 1.1.

FIGURE 1.1. A perimeter. The patient stares at the point of fixation within the diffusely illuminated bowl, while a small spot of light is moved about to detect blind regions within the visual field. (Reprinted by permission from D. R. Anderson, Perimetry With and Without Automation, 2nd edition, St. Louis, 1987, The C. V. Mosby Co.)

A person who cannot see the moving light in a certain region has a scotoma, or blind spot, in that region of the visual field. The size of a scotoma is expressed in terms of degrees of arc subtended at the center of the sphere, where the patient's eye is located. The same units of measurement are used to specify the size of a small sighted area that is surrounded by blind regions. Normally the eye's visual field is almost 180 degrees from side to side, but in some cases of tunnel vision only an island of sight a few degrees in diameter may exist (see Fig 1.2). Anyone whose visual field is less than 20 degrees in diameter qualifies as being legally blind, regardless of the acuity in the small region of the field that remains.

FIGURE 1.2. Visual fields determined with a perimeter. In both cases the large circle represents the edge of the perimeter. Throughout testing, the patient looks fixedly at a dark spot (F) in the center of the bowl. The white area is the visual field, that is, the region within which the subject can detect visual stimuli. The upper drawing shows the visual field of a normal right eye; the lower figure shows the small visual field of someone with tunnel vision.

Thus a person may be legally blind either because of low acuity or because of a restricted visual field. The two eyes may of course differ in acuity, or in size of the visual field, and legal blindness exists only when both eyes are sufficiently impaired to meet the criterion.

There are some who say that the legal definition of blindness is too encompassing, because it includes many people who can still get a lot of use from sight: It would be a disservice to categorize these people as blind if that caused some of them to give up on trying to use their remaining vision. An alternative term, low vision, is now sometimes used to describe individuals who have a serious visual impairment, but nevertheless still have some useful vision (Colenbrander, 1976). Thus some people who are legally blind, and some who are not, would be considered to fall into this broad category. Perhaps the best course to follow is to use both terms, but in different ways: “legal blindness“ for statistical and governmental purposes, where an unambiguous method of categorization is needed, and “low vision“ for medical and optometric purposes, and in everyday life, where the emphasis should be on making the best of residual vision. This book is concerned primarily with people who are totally blind or have light perception only, rather than with those who have low vision. In this initial chapter, however, information on the prevalence of blindness will be presented in terms of legal blindness, for that is the form in which the most systematic data are available.

Prevalence of Blindness

How many people in the United States are legally blind? This is difficult to determine, for there is no nationwide registry of blind people. Some data do exist at the federal level, but they are very incomplete. For example, the Social Security Administration has records indicating the visual status of Social Security recipients. What fraction these may represent of the total blind population of the country, however, is unknown. In 1962, a group of nine states decided to cooperate, with federal financial support, in an attempt to standardize, and improve, the collection of statistical information about blindness. This undertaking became known as the Model Reporting Area project, the idea being that its methods would serve as a model for the rest of the country. In fact, seven other states did join the MRA within a decade, and a systematic record of blind people in those states, using uniform criteria not just to define blindness, but to specify its cause, came into being (Kahn &. Moorhead, 1973). In 1971, however, the MRA project was terminated by the federal government as a cost-cutting measure.

In short, there is currently no way to determine with certainty the number of cases of blindness in the country. However, the National Society to Prevent Blindness, a private, non-profit organization, has for many years combined information from several sources in order to arrive at overall statistical estimates. For its most recent comprehensive estimates, published in 1980 (National Society to Prevent Blindness, 1980), the Society used the last available data from the Model Reporting Area to ascertain the rate of blindness per 100,000 persons, for each MRA state. Since the MRA data indicate how the rate varies as a function of the age, sex, and other demographic characteristics of the population, it was possible to estimate the rates of blindness in non-MRA states as well, based on their demographic makeup. (For example, the rate of blindness is higher in the elderly than in young people. Therefore, a state with a large elderly population, such as Florida, would be expected to have a higher rate of blindness than a state with a small elderly population.) The Society then combined these rates with 1978 data on the size of the population, obtained from the Bureau of the Census, to arrive at estimates of the prevalence of blindness as of that year. The numbers given here are drawn from their report.

There are roughly half a million legally blind people in the United States. For those under the age of twenty, 55 out of every 100,000 are blind; but for those 65 and over, the rate is more than 1,100 per 100,000, so that people in this age group make up more than half the blind population. In between those two extremes there is a gradual increase with age in the prevalence of blindness. This is a result of three factors, one of which is purely statistical: An elderly person has been exposed to life's vicissitudes longer, and the chances of his or her having suffered an irreversible loss of vision at some time are therefore greater than they are for a young person. A second factor is that even a healthy eye undergoes changes with advancing age, such as a yellowing of the lens and constriction of the pupil, which gradually reduce visual ability; any additional loss of sight through trauma or disease is thus more likely to push the person over the threshold of severe visual impairment. Third and most importantly, however, some blinding diseases, such as macular degeneration, strike much more often in elderly people than in the young. Later in this chapter, the most common causes of blindness will be discussed, and information about their frequency will be given.

Such statistics on the prevalence and causes of blindness are useful in that they help to guide long-term planning, both in and out of government. For example, the National Eye Institute, the agency that oversees federally sponsored research on medical aspects of blindness, takes the prevalence of various blinding diseases into account, along with other factors, in setting research priorities. Data on the prevalence of blindness are also important in the private sector, where research on the development of sensory aids for blind people, such as computer terminals that "talk" or use braille, requires a knowledge of the potential market for such devices.

So far only blindness in the United States has been discussed. Estimates of the prevalence of blindness worldwide are much more difficult to make. While many countries collect data on the prevalence of blindness, they often use different definitions. Thus legal blindness in some countries (as in the United States) is acuity of 20/200 or worse or a severe restriction of the visual field, while in other countries it is defined in a more everyday context, such as the inability to count the examiner's fingers at a certain distance, or even as the inability to earn a living using vision. It is seldom possible to convert statistics obtained using one criterion to what they would have been if another criterion had been employed. Another difficulty is that some countries have a well-organized system of reporting, while others do not. Even if they use the same definition of blindness, it is not certain that their data are comparable. Despite these difficulties, the World Health Organization (1979/1980) has been able to piece together data from many sources to reach an overall estimate: Approximately 40 million persons worldwide are, by the U.S. definition, leg...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Original Title Page
• Original Copyright Page
• Table of Contents
• Preface
• 1 Blindness and the Eye
• 2 Blindness and the Brain
• 3 Blindness and Perception
• 4 Blindness and Cognition
• 5 Attitudes and Emotional Reactions to Blindness
• 6 The Rehabilitation Process
• 7 The Blind Child
• Concluding Remarks
• References
• Author Index
• Subject Index