Depth Perception After Sight Restoration in the Congenitally Blind

Introduction

Depth perception is necessary for the ability to perform many tasks, including driving and numerous other activities. The ability to perceive the distance of objects is a complex process. When people are born blind in one eye, regardless of the reason, they do not develop the ability to perceive depth. Their world is flat compared to that experienced by those with normal binocular vision. When such a person undergoes surgery or other procedures to restore sight to the blind eye, many of these patients become able to perceive depth. This ability defies commonly held views on the connection between visual acuity, depth perception, and motor development.

This paper explores current research on depth perception and its development. Studies in this area are limited to animal studies and those involving persons who were blind in both eyes but had their sight restored. No cases could be found involving a patient who was blind in one eye and then had sight restored to that eye. Due to this lack of case study evidence, the research depends on academic evidence related to the study problem. This is clearly an area that needs further study in the future. For the time being, however, conclusions must rely on the information that is available.

Binocular Vision and Stereoscopic Vision

Stereoscopic vision — the ability to see in three dimensions using two eyes — results from the brain measuring the relative distances of images and reconciling the differences between them. Several different theories explain how this is accomplished. The first suggests that the process occurs in a series of steps. First, the brain defines simple shapes within the images, such as the orientation of lines and edges. It then adds information about the direction of any movement and the colors present. The information moves to a different part of the brain where it is further processed in a hierarchical fashion, from general to specific, making finer details evident (Ramachandran and Ramachandran).

Infant motor behavior has been found to contain unique patterns of organization and control. These findings have sparked renewed interest in this area. Motor development may play a role in determining developmental sequences, sometimes referred to as "timetables," in other domains of development. Developmental milestones are often similar across all human beings, with some room for individual variation. It has been argued that certain motor developmental milestones are integral to elements in the domains of haptic perception and depth perception. At present, this theory has not been proven and remains under study. Motor development plays a significant role in the ability to reach milestones in other perceptual areas during infancy (Bushnell and Boudreau). One example is the ability to reach and grab for an object, a task that requires depth perception. The infant must master this task before they can move on to manipulate the object.

The two visual processing centers of the brain send images back and forth several times in a process similar to a game of twenty questions, arriving at a solution through repeated comparison. At some point in the process, a comparison between the two images — one from each eye — is made. If the brain is unable to compare the two images properly, the differences cannot be measured and stereoscopic vision cannot occur (Ramachandran and Ramachandran). The most remarkable aspect of this process is the speed at which it operates; the brain continually analyzes images at lightning-fast speed using the incredibly complex hierarchical strategy described.

Stereoscopic vision works by the brain comparing separate images from both eyes. In the past, it was thought that the brain perceived the form of an image first and then compared the two pictures. More recent studies have shown, however, that sometimes stereoscopic vision occurs first and the brain finds the forms afterward. At other times, the brain defines the forms and then compares the two almost pixel by pixel. The brain has several different ways of processing images to arrive at the same goal of stereoscopic vision (Ramachandran and Ramachandran). This makes understanding vision in a person who was blind in one eye all their life more difficult, but it also makes it possible for such a person to see stereoscopically once sight is restored. The brain's ability to reach the same conclusion through several different methods makes stereoscopic vision highly adaptable and also explains how people with only one functional eye can navigate a three-dimensional world.

Binocular neurons in the visual cortex are responsible for combining signals from both the right and left eyes. More in-depth study of these neurons has found that existing models are inadequate to explain the brain's ability to compute perception across both transparent and opaque surfaces. For instance, the ability to look through a window and determine that the window is closed while cars outside are moving closer or farther away illustrates this problem. In one experiment, scenes were rendered on transparent slanted planes separated by a set distance. Subjects successfully segregated the planes in depth once the disparity between them reached a small threshold. The brain processes opaque surfaces more easily than transparent ones, and estimates of disparities between transparent surfaces are filled in by the positions visible to both eyes via a feedback loop between two different visual processing areas (Parker, Raudes, Mingolla, and Neuman).

The five senses perceive information and send it to the brain via specific channels according to the type of information. These channels parse the signals into parallel streams so that input is compact and efficient. Parallel input signals are then integrated in the cortex into uniform sets of perception. Studies on the primate visual cortex have contributed significantly to our understanding of this process. It was found that, just as the processes involved in stereoscopic vision are complex and can be achieved through multiple strategies, the brain also achieves detailed information from the five senses through multiple pathways. Information is defined spatially and by cell-type-specific connections that provide detailed information about our visual surroundings (Nassi and Callaway).

Biology vs. Environment

Thus far, perception and stereoscopic vision have been discussed primarily in neurological terms. However, an environmental dimension also exists. Research found that familial deprivation of pictorial stimulation significantly impacts the acquisition of skills required for pictorial depth perception. One study took place in Indian nurseries and orphanages using children ranging from three to six and a half years old. The children were asked to judge distance by interpreting six common pictorial depth cues in a set of pictures, each cue presented individually rather than in combination. Several significant findings emerged. For instance, intelligence correlated strongly with the ability to distinguish depth perception cues in pictures. The effects of deprivation were stronger in older children, but none were found in the youngest age group. A lack of stimulation in orphanages was found to have a retarding effect on the developmental skills associated with pictorial depth perception (Sinha and Shukla).

This represents a significant finding because it demonstrates that the ability of a person who has been blind since birth to develop depth perception when sight is restored is complex. There is both a neurological and an environmental side to this ability. The brain's capacity to use different processes for the same function further complicates any attempt to predict outcomes for a person who was blind in one eye since birth and then has sight restored. Many factors could affect such a prediction. For instance, if a person was provided sufficient visual stimulation in the early years, they have a better chance of developing coping skills, thereby reducing the effects of a lack of depth perception. Such a person may be minimally affected compared to someone whose entire life has been essentially two-dimensional and who has not had the opportunity to train the brain to develop compensatory mechanisms. In a person whose visual world has been two-dimensional since birth, the brain must learn to process information from two eyes rather than one.

A study of people blind in one eye from birth whose sight is restored must therefore account for many variables. Moreover, the person's brain must be given adequate time to adjust. It cannot be expected that a person would develop full depth perception the moment sight is restored. It may take time and practice to develop the ability to distinguish distance. Pictorial therapy may help the person improve their skills. The person's ability to learn would depend on many factors, both physical and environmental. There are, in theory, many ways that a person could develop depth perception once sight to one eye is restored, and it is possible that a person could train themselves through a number of exercises (Sinha and Shukla). The degree of sight restored to the previously blind eye would also have an impact on their ability to adjust to a two-eyed world.

Hudson's pictorial perception test and construction test asks participants to construct geometric models shown in pictures. The test was administered to school-age boys in Central Africa and to domestic servants. A significant portion of the subjects initially judged to be two-dimensional perceivers went on to build three-dimensional models, though some were distorted and oddly shaped. One notable finding was that domestic servants were identified as two-dimensional perceivers more often than schoolboys. The explanation offered is that schoolboys had been exposed to pictorial material more frequently than domestic servants (Deregowski). Passive exposure to pictures therefore plays a role — though a modest one — in determining the ability to perceive distances in a picture. The study also suggests that two-dimensional responses on Hudson's test may reflect an inability to organize pictorial material in a way that allows understanding and reproduction, which may relate more to organizational skills than to visual acuity alone.

One study manipulated pictorial depth cues for children between the ages of four and ten. Pictures were reduced to represent the walking or standing posture of two figures. In one condition, a training picture reminding subjects of actual size was provided; in the other condition, the training picture was absent. Children were asked to construct three-dimensional models representing the size and spatial relationships between the two figures. It was found that elevation alone can serve as an effective depth cue even though it is a relatively weak one. A moderate memory effect was demonstrated, and posture was found to be insignificant. All children had no difficulty with size, but spatial responses were very low in the four-year-old group. By age six, these responses increased significantly. The most common error at both age levels was producing the figures in horizontal rather than diagonal orientation (McGurk and Jahoda).

These studies suggest that depth perception and stereoscopic vision have both biological and environmental components. Individuals must have the physiology developed to receive and interpret visual information, but they must also be exposed to pictorial material in order to hone their skills at distinguishing distance as represented on a flat surface. Taken together, these studies indicate that depth perception is a skill that can be improved with practice.

Conclusion

At present, research concerning the ability of a person who was blind in one eye since birth to regain depth perception when vision is restored is inconclusive. The area contains well-developed theory, but academic evidence is limited to animal studies. Human case studies were unable to be located. Conclusions about how — and to what degree — a person who has been blind since birth could develop depth perception are therefore limited. This is an area that requires further research before conclusive results can be obtained.

The research reviewed here highlights the importance of experience with pictorial vision as a key component in the development of binocular and stereoscopic vision in infants, suggesting that experience must supplement the physiological processes involved. Much work has been done in improving binocular vision and depth perception in the general population, and vision therapy is now an accepted intervention to help children develop these abilities. The most significant finding of this review is that a person who has sight restored in one eye will need to train themselves to see with two eyes. It is possible, but it will require time for the necessary skills to be learned.