Depth perception development in people who gain sight after congenital blindness

¶ … Perception

How does depth perception occur in a person who gains sight after being congenital blind?

Depth perception is necessary for the ability to perform many tasks including driving, and many 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 depths. Their world is flat compared to that experienced by the rest of the world. When that person undergoes surgery or other procedures to restore sight to the blind eye many of these patients are able to perceive depth. The ability to do this defies commonly held views on the connection between visual acuity, depth perception and motor development.

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

2.0 Binocular Vision and Stereoscopic Vision

How the Brain Processes the Information

Stereoscopic vision, the ability to see three the using two eyes is the result of the brain being able to measure the relative distances of images in reconciling the differences. There are several different theories on how this is accomplished. The first theory suggests that this processes occurs in a series of steps. First, the brain defines simple shapes within the images such as the orientation of lines and edges. Then it adds information as to the direction of any movement, and colors that are present. The information then moves to a different part of the brain where it is further processed according to a hierarchy where the details become evident (Ramachandran and Ramachandran). This process is similar to refining the information from general to specific in a stepwise fashion.

Infant motor behavior was found to contain unique patterns of organization and control. These new developments have sparked renewed interest in this area. Motor development may play a role in determining these development sequences or what has been referred to as "timetables" in other domains of development. Developmental milestones are often similar in all human beings, with some room for individuality. The argument has been presented that certain motor developmental milestones are integral to the elements in the domains of haptic perception and depth perception. At present, this theory has not been proven and is still 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 of this is the ability to reach and grab for an object. Depth perception is important in this ability. The infant must master this task before they can move on to manipulate the object.

The two visual processing centers of the brain send the image back and for fourth several times in a process similar to a game of 20 questions to arrive at a solution. At some point in the process, a comparison between the two images from each eye is made. If the brain is unable to compare the two images properly, then the differences cannot be measured and stereoscopic vision cannot be possible (Ramachandran and Ramachandran). The most amazing thing about this process is to speed at which it occurs. Our 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 form of the image first and then compared the two pictures. However, more recent studies have shown that sometimes stereoscopic vision occurs first and then the brain finds the forms afterwards. At times, the brain defines the forms and then compares the two forms almost as if it is comparing them 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 even more difficult, but also makes it possible for them to see stereoscopically after sight is been restored. The ability of the brain to arrive at the same conclusion using several different methods makes stereoscopic vision highly adaptable. It also explains how people with only one eye can function in a three dimensional world. Their brain has different ways of processing the information, making the problem of explaining stereoscopic vision even more difficult.

Binocular neurons in the visual cortex are responsible for combining the signals from both the right and left eyes. A more in-depth study of these neurons found that models are inadequate to explain the ability of the brain to compute the perception in both transparent and opaque surfaces. For instance, the ability to look out a window and determine that the window was closed and that cars outside of it are moving closer or farther away is an example of this problem. An experiment where scenes were rendered on transparent slanted planes separated a certain distance. The subjects successfully segregated the planes in depth once the disparity between them reached a small threshold. It processes opaque surfaces more easily than transparent ones. The estimates of disparities between transparent surfaces are filled in by the positions that are 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 the input is compact and efficient. Parallel input signals are integrated into the cortex into uniform sets of perception. Studies on the primate visual cortex have contributed to our understanding of this process. It was found that just as the processes involved in stereoscopic vision are complex and can be achieved by multiple strategies, so does the brain achieve detailed information from the five senses. It appears at the brain has many different strategies for achieving the same goal. Information is defined spatially and by cell type-specific connections that are used to provide detailed information of our visual surroundings (Nassi and Callaway).

3.0 Biology vs. Environment

Thus far, perception and stereoscopic vision has been discussed using only neurological terms. However, an environmental side also exists for the problem. It was found that familial deprivation of pictorial stimulus significantly impacts the acquisition of skills required for pictorial depth perception. The study took place in Indian nurseries and orphanages using children ranging from 3 to 6 1/2 years old. The children were asked to judge distance by interpretation of six common pictorial clues in a set of pictures. The pictorial clues were provided one of the time, rather than in combination. Several significant findings were found in the study. For instance, intelligence had a strong correlation with the ability to distinguish depth perception cues in pictures. The effects of deprivation were higher in older children but none were found in the younger years. A lack of stimulation in orphanages was found to have a retarding affect on the developmental skill associated with the pictorial depth perception.

This represents a significant finding for this study 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. This literature review indicates that there is both a neurological and environmental side to this ability. The brain's ability to use different processes for the same function further complicates the ability to predict the ability of a person to see depth perception once sight is restored for those who were blind in one eye since birth. Many factors could affect the ability to predict the outcome in such a person. For instance, if a person was provided sufficient visual stimulation in the early years they have a better chance of developing coping skills, therefore reducing the effects of a lack of depth perception. The effects ion such a person may be minimal compared to someone who has not been able to train their brain to develop coping mechanisms and has spent their entire life in a two dimensional world. In a person whose world has been essentially two dimensional since birth their brain must be able to process information from two eyes instead of one.

A study conducted on people who have been blind all their life in one eye and whose site is restored must consider many variables. In addition, the person's brain must be given time to make the adjustment. It cannot be expected that a person would develop full depth perception the moment site is restored. It may take some time and practice for them to be able to develop the ability to distinguish distance. Pictorial therapy may help the person to improve their skills and ability to distinguish depth. The person's ability to learn would depend on many factors both physical and environmental. In theory, there are many ways that a person could develop depth perception when sight to one eye is restored. It is possible that a person could train themselves to see depth perception 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 on pictures. The test was administered to school age boys in Central Africa and domestic servants. The study found that a significant portion of the subjects that were initially judged to be two dimensional perceivers built three-dimensional models. Some of them were distorted and oddly shaped. One of the more interesting findings of this test was that domestic servants were found to be two-dimensional perceivers more often than schoolboys. The explanation is that the schoolboys have been exposed to pictorial material more than domestic servants (Deregowski). Therefore, passive exposure to pictures plays a minor role in determining the ability to perceive distances in a picture. The study suggests that two-dimensional responses on Hudson's test may be due to the subjects inability to organize test picture of material in such a manner that allows them to understand and reproduce it. This may have to do with organizational skills more than visual skills.

One study manipulated pictorial depth cues for children between ages of 4 and 10. The pictures were reduced to represent the walking or standing posture of two figures. One condition of the study presented a training picture that reminded subjects of the actual size. This also helped to test the effects of memory. In the other group the training picture was absent. Children were to construct three-dimensional models that represented the size and spatial relationships between two pictures of figures. It was found that elevation alone can serve as an effective depth cue even though it is a relatively weak one. Moderate memory effect was demonstrated and posture was found to be insignificant. All of the children had no problem with size but spatial responses were found to be very low in the group of four-year-olds. By the age of six these responses increased significantly. The most predominant error at both age levels was producing the figures with horizontal rather than diagonal orientation (McGurk and Jahoda).

These studies suggest that depth perception and stereoscopic vision have both a biological and an environmental component. They must have the physiology developed to be able to receive and interpret their visual world, but they must also be exposed to pictures so that they can hone their skills at distinguishing distance as represented on a flat surface. These studies indicate that depth perception is a skill that can be improved with practice.

4.0 Importance of Vision Therapy

Three dimensional imaging on display screens utilizes conventional stereoscopic principle. At first, this was attempted by physically displaying two views of different perspectives on a screen and then delivering them to the right and to the left eye. These methods were found to be inadequate because the user must override the physiologically coupled oculomotor processes of convergence and focus. The result was that the viewer experienced visual discomfort and fatigue after only a short time (Reichelt, Aussler, Futterer and Leister). Understanding depth cues is the key to developing holographic displays that do not cause visual strain for the viewer. Overcoming these physical limitations involved and stereoscopic vision and will be one of the key problems that it's still under study in this field. Understanding how to build better stereoscopic displays will help to further research into how those with one eye process information and perceive their world.

Binocular vision has been studied more extensively in children with special-needs than any other population. Binocular vision, requires that both eyes work together simultaneously as a team. Binocular vision is necessary to achieve Stereopsis, otherwise known as stereoscopic vision. Stereopsis is where the images from two eyes are combined into one image by the brain. When monocular vision is impaired stereopsis cannot be achieved in many cases (Optomistrists Network)

Binocular vision involves several different separate skills. The first is tracking, which is the ability to move the eyes across a sheet of paper. The second is fusion, which is the ability to use both eyes together at the same time. The third is stereopsis, which is binocular depth perception. The fourth is convergence which is the ability of the eyes to move and work as a team. The fifth is visual motor integration, which is the ability to transform images from a vertical to a horizontal plane. Impairments that interfere with the ability to perform any one of these five tasks can interfere with a person's ability to translate the three dimensional world (Optomistrists Network).

One of the most common impairments that results in the inability to perform one of these tasks is where the eye is obviously turned or crossed. This impairment is easy to see because both eyes do not aim in the same direction at the same time. There are many different forms of misalignments, but for the purposes of this study all of them have the same affect on the ability of the person to perceive depth perception. Cerebral palsy and cataracts can also affect the ability of the eyes to track properly and to form images properly.

Poor binocular vision can often not be corrected by corrective lenses alone. Vision therapy has been demonstrated to be ineffective corrective action for tracking problems that interfere with depth perception. It would stand to reason that vision therapy would also be helpful for a person was blind in one eye since birth and his vision was restored. The therapy would work in a similar way to those who had less severe problems that interfered with the ability of both eyes to work together. Vision therapy has a long history of effectiveness and the ability to restore binocular sight and depth perception almost completely. Many children with special needs have been given special attention to binocular vision and correcting impairments. Vision therapy has a much higher success rate than eye muscle surgery for correcting these problems.

In terms of this study, it is important to understand the importance of vision therapy and the ability of the person to regain binocular vision and depth perception. Surgery alone and restoring a person's sight alone would not necessarily make them able to use their eyes together and develop depth perception. If they have been blind in one eye since birth then their eyes never learned to track together. The first step will be to train their eyes to track together as a team. It is also likely that the eye to which sight was restored would have some muscular development to undergo as well. If the blind eye is not used to track, then it will have to develop the muscles so that it can be strong enough to track with the other one. Recovery and restoring binocular vision is likely to be a long process for one who is not used to using both eyes. It could be expected that the formerly blind eye would be much weaker than the one that has been used to working its entire life. Treatment options available to special needs children would also be helpful for an adult who has undergone surgery to restore eyesight in their one blind eye. Enough is known about vision therapy and its impact on children that are not necessarily blind in one eye, but who have reduced vision and the inability to track. Vision therapy is a standard treatment to help restore binocular vision and help children develop the necessary depth perception.

All of this information would apply to the person who has had their vision restored in one eye. A similar program would be likely to be helpful to this person, as they would have to be trained in a similar way to use both eyes together. It would be unlikely for the person to have the motor skill necessary in the blind eye to produce binocular vision and achieve stereopsis. The brain will also have to undergo some training as the eye learns to track and it begins to learn to process signals differently than it has all of its life to that point. Without therapy, it would be a reasonable assumption that without therapy, practice, and adjustment, they would be no better off in their ability to develop depth perception than if they were still blind. However, work with special needs children demonstrates a high success rate with vision therapy and the ability to learn to use binocular vision. Vision therapy will be an essential component of the person's ability to regain depth perception.

Getting the optic nerve to regenerate after it has been injured has been a goal of many researchers. Recently, researchers at a Harvard-affiliated children's hospital have reported success using a three-pronged intervention to get the optic nerve fibers to regrow. Not only did they get them to regrow, they got them to regrow to the full length of the visual pathway from retina to all visual areas of the brain. The study utilized mice as participants and researchers were able to restore some basic elements of vision in the mice. The mice regained some depth perception, the ability to detect overall movement of the visual field, the ability to perceive light, and the ability to synchronize their sleep and wake cycles (Weber)

The mice were blinded by optic nerve damage from trauma or glaucoma. These conditions affect more than four million Americans (Weber). This research indicates that they may be able to be regain some or all of the visual function. This research is important to the study because it demonstrates that some depth perception returns when damage to the optic nerve is corrected.

Vision therapy was found to be an important element of the recovery process for a person who was blind in one eye and whose vision has been restored. However, this research indicates that some natural that perception may return simply by restoring the site. Taking this into consideration, the person's best chances for regaining depth perception and binocular vision would be to add vision therapy to the corrective surgery. This combination gives the person the greatest chance for a full recovery and the ability to gain depth perception. The difference between mice in the study and those that have been blind since birth in one eye is that the mice once had the ability to see and lost their vision due to trauma. The neural pathways were already in place and the mice already knew how to process binocular vision information. This may be different from the person that is been blind in one eye since births as they may never have developed the ability to process information from two eyes. This is an important difference between those that are blind because they lost vision in one eye and those that were blind in one eye since birth.