The most obvious time to see if this visual process is learned is through the observation of the visual processes of infants. The earliest studies of depth perception and babies used an apparatus called the visual cliff (Berk 151). This device consists of a glass covered table and a central platform, from which babies are encouraged to crawl. Patterns are placed beneath the glass to create the appearance of a shallow side and a deep side. When trying to get the babies to crawl across the glass, the babies readily crossed the shallow side and all but a few reacted with fear to the deep side. The researchers concluded that about the time infants crawl, most of them have an understanding of depth perception.
Another study carried out to examine the development of depth perception in infants tested the ability of babies to detect particular depth cues. Kinetic depth cues are depth cues that are created by movements of the body or of objects in the environment. Researchers have found that babies as young as 3 to 4 weeks of age blink their eyes defensively to surface moving towards their face that looks as if it is going to hit them (Berk 152). Motion is not the only important depth cue. Researchers have used methods to find out if infants are sensitive to binocular cues. In one experiment, infants were presented with two overlapping images and they wore special glasses to ensure that each eye receives one of the images. If babies use binocular cues, they perceived and organized a form rather than random dots. Results show that this binocular sensitivity emerges between 2 to 3 months and gradually improves within 6 months (Berk 152). Infants make use of binocular cues when they reach for objects because they are adjusting their hand and arm movements to match the distance of the objects perceived from the eye. Another important depth cue that was observed in infants is their use of pictorial depth cues. These depth cues are used by artists to make paintings look three-dimensional. In one experiment infants (with one eye covered) were presented with stimuli containing certain pictorial cues and were then observed to see what they would reach out for. Results show that 7 month olds respond to a variety of pictorial cues (Berk 153). By examining infants sensitivity to depth cues, it can be concluded that infants have the ability to perceive depth from a very early age. From this conclusion it appears as if infants dont get much of a chance to learn depth perception. Is it something that we are born with? There is now a mystery of where the brain gets this ability of converting two-dimensional images into three-dimensional images.
From the above observations, I strongly believed that depth perception is an ability that we are born with and cannot be learned. If one examines those with impairments in their binocular vision, the brain doesnt try to compensate and learn to create and a three-dimensional image. This led me to assume that we have the visual abilities that we are born with and that they are never learned. I maintained this assumption for a while until I came across some web pages that spoke of something called vision therapy for those with binocular vision impairments. Upon further investigation of vision therapy, it led me to believe that the brain does have the capabilities of learning depth perception and that even though the brain doesnt compensate for impairments, we can through our behaviors. A binocular vision impairment is defined as any visual condition that results in partial or total loss of stereoscopic vision and binocular depth perception (1). This impairment is very common and affects about 12 percent of the population. Two of the most prevalent impairments are amblyopia and strabismus. Amblyopia, also known as lazy eye, is a condition of reduced visual acuity, which is not correctable by refractive means and is not attributable to any obvious structural anomalies or ocular disease (4). Even if the brain receives a clearly focused image, it cannot process that information. Strabismus, also known as cross eye, is defined as the misalignment of the eyes, and failure of the eyes to look simultaneously at the same point in space (4). The visual system needs to see equally and simultaneously from both eyes. If this does not occur then the sensory input that is received is distorted, resulting in distorted images in the brain, leading to behaviors that are based on that distorted image. The fact that these vision impairments involve deficits in the processing of visual information, it can be assumed that there is more of a brain problem than a physiological eye problem, thus surgery is not considered a reliable option.
Vision therapy seems like more of a logical form of treatment compared to surgery. Vision therapy is defined as a process of retraining the visual perceptual system so it functions with optimal efficiency (3). The training involves the improvement of visual skills, such as eye teaming, depth perception, tracking and vision-body coordination (1). It can be considered as a form of physical therapy for the brain and eyes. Vision therapy works on the assumption that vision must follow a progression of development, just as language and motor skill are achieved through developmental stages. Since vision development follows these developmental stages, it is a process that can respond to training if disruptions occur (3). The basic premise is that vision is learned and if this learning is hindered, it can be relearned. Results show that vision therapy does have positive effects on those with binocular vision impairments (according to the web pages). Those patients that were told by optometrists that their lack of binocular depth perception was permanent and untreatable went on to develop normal binocular depth perception by going under visual therapy (1). One particular success story was documented on one of the web pages. A woman was diagnosed at twelve years of age as having amblyopia, which basically meant that she didnt have the abilities to sense depth perception. Doctors told her that it was too late for her to have any types of treatment or surgery because she was too old. At 33 years of age, she decided to undergo vision therapy. What is remarkable is that after two years of therapy, she acquired binocular depth perception and stereo vision. She described her previous two-dimensional world as looking flat where she felt as if she was here and everything that she looked at was over there (1). She couldnt visually perceive the space between her and other objects.
Since vision therapy is based on the ideas that behavior is learned, these positive outcomes for vision therapy can support a theory that vision is a learned process and that we arent just born with our visual abilities. If we were born with our visual abilities then, the success story woman would not have been able to acquire depth perception at such a late age. Perhaps learning does take place at a very rapid rate during the first 3 to 4 weeks of life. This learning could be similar to vision therapy, where the infant trains itself to process depth perception. More studies are needed that observe the visual development during these crucial first weeks of life. The efficacy of vision therapy is something that should also be studied. Not much has been heard about this therapy and if it is effective as it says to be, then the visual system can be better understood. It would also be interesting to note if any changes in brain structure take place as a result of vision therapy. Further investigations (like those mentioned above) can help us better understand whether vision is learned or not. At the beginning of researching this paper, I was ready to say that we were born with our visual abilities and were stuck with them due to genetics, however I am now willing to support a theory that vision is learned and can be re-learned, due to the findings of vision therapy.
1. The Stereo Vision Project
2. Acquired Brain Injury and Hidden Visual Problems
3. Vision Therapy: A Beneficial Intervention for Developmental Disabilities
4. The Eye Care Reports
5. Berk, Laura E. Child Development 4th Ed. Allyn and Bacon, Boston, 1997.
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This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.