Updated: Jun 27, 2020
By Christina Min
Stereopsis, from the Greek stereo meaning “solid” and opsos meaning “appearance or sight,” is a term most often used to refer to the perception of depth and three-dimensional structures obtained on the basis of visual information processed by individuals with normally developed binocular vision (vision through two eyes with overlapping fields of view).
Stereoscopic depth perception is an intricate process requiring complex neural processes. Because the eyes of humans and many animals are located at different positions on the head, binocular vision results in two slightly different images projected onto the retinas of the eye. Fusing the images requires matching up features in the two eyes by identifying common features in the left and right retinas at the same point in the visual scene. The fusion process begins in the a region of the brain called the visual cortex, then integrates with other cues to determine depth and distance.
The most common cause for the loss of stereoscopic vision is amblyopia, in which one eye fails to form an adequate input to the visual cortex, usually due to strabismus (which results in crossed eyes). Individuals with strabismus find it difficult to direct both eyes at any object because the images on the two retinas are too far apart and do not fall within the field of view that the brain can process.
For decades, scientists have reasoned that the loss of stereoscopic vision is irreversible once the individual has passed the critical stage within the first few years since birth. In fact, this notion of the “critical period” in vision development seemed to be a concrete and unambiguous fact until studies in 1976. Additionally, all earlier studies supported the view that there had to be specific stimulation during the first 3 months of development. However, in the 1970s, Hubel and Wiesel began to find evidence that partially disproved this theory — there are processes that regulate periods of increased/decreased connections between neurons of the brain. Therefore, although it is true that the brain may be more responsive and plastic to treatments in infancy, there exists a possibility for improvement later in life.
In fact, twenty-first-century research indicates that the adult nervous system is not ‘hardwired.’ This is because the brain contains neurons that send their impulses all over the central nervous system. So, when a strabismic individual learns to aim both eyes simultaneously at the same point in space, the junction at which the impulses from one neuron meets the others are strengthened. This may help to reinforce ineffective connections, allowing individuals to revive the synapses of weak visual neurons. Essentially, there are connections in the brain that remain untapped unless circumstances, such as learning a new skill, require their participation. Thus, the important step looking forward is to design visual therapy treatments that unmask these neural connections.
Therefore, the plasticity of the brain which allows it to form new connections between neurons provides hope for patients with amblyopia. The brain has a limitless potential and the strengthening of synaptic connections expands the possibility for new treatments. Now, people with amblyopia are able to take comfort in the fact that they are not the victims of a permanent fate. They can rehabilitate their own vision and liberate their own visual problems.
Fixing my Gaze by Sue Barry