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…