The human visual system is a complex relationship of sensory-motor functions that are controlled and organized in the brain. People who have suffered a stroke or a brain injury often experience problems with vision afterwards. This is called Post Trauma Vision Syndrome. It is the result of either eye misalignment, or damaged or partially damaged brain cells where the brain processes vision. Many stroke or brain injury patients suffer from double vision, causing confusion and disorientation, and sometimes difficulty in reading.
Just as common is losing half of one's field of vision. Either the right or left side can be lost. The affected individual has a built-in blind spot, unable to see anything to the left or right, depending on where the loss occurs. A similar effect can happen when damage occurs in the parietal lobe, the part of the brain that controls sensory perception. Such people are not only unable to see objects or people on the left side of their field of vision, but they may show a marked preference for take right instead of left turns. They may also be unaware of the left side of their own body, forgetting to shave or apply makeup to the left side of their face. If asked to draw a daisy or a clock they may place all the petals on the right side of the daisy, or all 12 numbers of the clock dial onto the right side.
But some neurons in the brain have a unique ability. If they are partially damaged, they can compensate for the injury and adjust their activity in response to stimulation from the environment. Even after a stroke or brain injury causes visual problems, there is still a zone of residual vision between regions within the brain's vision-processing areas. These areas can be strengthened to compensate for the vision loss by using precise patters of visual stimulation. The patient is instructed to fix his or her gaze on a screen or other stable point, and bright lights are flashed on and off in precise patterns in the affected area. This can increase awareness and sensitivity of vision in that region.
Patients who lose the left part of their field of vision can compensate by situating themselves so that most of the objects of interest -- other people, TV screen -- are on the right. They can also scan the left side of the room when they enter to note objects and people that might otherwise not be seen.