Research paper for PSY 506 Fall 2004 (Return to class web page)
Perception plays a significant role in our ability to live life. It aids in providing information about the properties in our environment, along with letting us act in relation to those properties. In other words, our perceptions let us experience our environment and live within it. The question is how are we actually able to perceive things? We perceive what is in our environment by a filtered process that occurs through our visual system. Although all senses play a significant role, the visual system is one of the most important. Without vision, we are no longer able to see our environment, which puts a huge damper on the perceptual process itself. Vision loss is extremely debilitating to many people in our world today. Impairments range from loss in the visual field, visual acuity, to even a loss in the ability to recognize faces. There are numerous ways one can acquire visual deficits, but a leading cause is injury to the brain. Damaging various parts of the brain can lead to specific visual deficits. Numerous cases have reported spontaneous recovery, however complete recovery is unlikely. Current research is being examined in an attempt to improve the likelihood of recovery. The training of certain areas has been found to improve vision deficits in some disorders, but again, the extent of recovery is limited.
The brain is the most intricate organ in the human body, and the visual pathways within the brain are also very complex. Due to the interconnections between the brain and visual system, damage to the brain can bring about various cerebral visual disorders. The visual cortex has its own specialized organization, causing the likelihood of specific visual disorders if damaged. The occipitotemporal area is connected with the "what" pathway. Thus, injury to this ventral pathway leading to the temporal area of the brain is expected to affect the processing of shape and color. This can make perceiving and identifying objects difficult. The occipitoparietal area is relative to the "where" or "action" pathway. Injury to this dorsal pathway leading to the parietal lobe will increase the likelihood of difficulties in position and spatial relationships. In cases of injury, one will find it hard to determine an object's location and may also discover impaired visual navigation. The most frequent causes for brain injury have been found to be strokes, trauma, and tumors (Zihl, 2003). It is highly unlikely that a person with brain injury will only have one visual deficit, but usually a combination of them due to the complexity of the organization between the visual pathway and the brain.
The most common cerebral visual disorder after brain injury involves the visual field loss. A large loss of vision in the visual field is called anopia, whereas smaller visual field deficits are called scotomas. The anopias range from hemianopia, the loss of one hemifield, to quadranoapia, the loss of one quadrant. The scotomas vary from a paracentral scotoma, loss of a small part outside the central field, or a central scotoma, loss of a small part within the central field (Zihl, 2003). When an injury is unilateral, it affects the contralateral visual pathway. If a bilateral injury occurs, the corresponding portions of either visual field can be affected. The most common forms are bilateral hemianopia (tunnel vision), bilateral upper/lower quadranopia, or a bilateral paracentral scotoma. If one acquires a central scotoma, there was a bilateral injury to the part of the visual pathway containing foveal connections to the striate cortex. In addition to visual field deficits, many other cerebral visual disorders can also be extremely damaging.
The following cerebral visual deficits have been classified as elementary cases, or having clearly defined visual functions, including visual field loss (Zihl, 2003). Particular deficits involving spatial contrast sensitivity, visual acuity, and visual adaptation are usually impaired when one acquires a retrogeniculate injury, either unilateral or bilateral. A person with a contrast sensitivity disorder will have a hard time determining different contrasts of objects, have a hard time seeing fine details, and may have reduced light and dark adaptation. Colour vision is affected when right or left fusiform gyri are injured, creating difficulty detecting colors. Spatial vision deficiencies occur most commonly after the occipitoparietal and posterior parietal areas are injured. A deficit in spatial vision can cause difficulty in accurate visual locations and depth perception. Furthermore, visual agnosic disorders are a result of injury to the temporooccipital cortex, either posterior or medial, and generally are more typical of right-sided injuries (Zihl, 2003). These impairments can range from the inability to recognize objects to not being able to identify or recognize faces.
The more complex cerebral visual disorders show impairment on a collection of functions, thus creating their complexity. As reported by Zihl (2003) visual spatial neglect is associated with right hemisphere injuries, usually involving the occipitoparietotemporal area. Individuals that acquire this disorder have a tendency not to respond to stimuli that are contralateral to the side of the brain injured. For example, if one was injured on the right side of the brain, one would not explore or respond to stimuli in the information being processed in the left visual field. Finally, Balint's syndrome is a disorder accompanied with the inability to process information coming from the binocular field. A person with Balint's syndrome may have a hard time shifting their gaze or directing their movement with visual guidance, which will make reading and other hand-motor activities extremely difficult. Many of these simple activities performed throughout our daily lives are a result of seeing, and over half of the activities require some aspect of the visual process. Having visual deficits can be extremely debilitating, however reports of spontaneous recovery have been noted in numerous cases. Complete recovery is uncommon, but minimal return of visual functioning is encouraging.
Spontaneous recovery of visual impairments is not yet completely understood. Although it has been frequently reported, the amount appears to be limited and is different for each individual. Zihl (2000) reported that approximately 15% of patients with anopia showed recovery in the visual field by about 3-24 degrees, with an average of 5 degrees. In the case of contrast sensitivity, acuity, and colour vision, spontaneous recovery has not been reported as frequently. There have been single cases of recovery noted, but the amount and time tend to vary. Meerwaldt reported spatial vision is another with minimal recovery rates (as cited by Zihl, 2003). The evidence that has been found tends to show improvement no earlier than six months after injury. Visual agnosic disorders have revealed some improvement, but in many cases years go by before even the slightest sign of recovery. Bruyer found prosopagnosia to have the least cases of spontaneous recovery (as cited by Zihl, 2003). A study looking at DN, a patient with prosopagnosia concluded, "DN's most severe and persistent impairment, however, is reportedly her inability to recognize familiar faces, despite recovered visual perception of other objects." (Mattson, Levin, & Grafman, 2000, p.134). In DN's case, her ability to recognize other objects had improved, but her struggle with faces remained.
On the other hand, Denes and Zoccolotti found that visual spatial neglect has a spontaneous recovery rate of about 66% (as cited by Zihl, 2003). Researchers have found the amount of recovery is highly contingent with the extent of the brain injury. The amount of spontaneous recovery for Balint's syndrome is still undecided. Some evidence suggests a couple of years before reported recovery and others suggest the impairment continues to persist. The fact that spontaneous recovery is even an option is astounding. Initially, a patient with a gunshot lesion had been completely blind, but showed spontaneous recovery after about six months (Poggel, Kasten, Muller-Oehring, Sabel, & Brandt, 2001). Further improvement was achieved after visual restitution training, which is a computer-based training program that includes thousands of stimuli presented between the intact and blind visual sector (Kasten, Muller-Oehring, & Sabel, 2001). Fortunately, research has been currently directed to visual deficits and many types of specific training have been developed for further recovery.
There have been major improvements in the area of training to help compensate for cerebral visual disorders. Anderson (2003) stated, "Impairments of visual perception resulting from brain damage can have serious implications for many aspects of patients' lives, and these impairments should be the target for intervention." A wide range of training has been developed for the different types of disorders, however, elementary cases are more easily studied then complex capacities. There are many more components with complex capacities, therefore determining which one(s) is (are) affected is extremely challenging.
Visual field deficits have received the most attention because of the reliable and available methods to the tests the deficits. The most utilized training in this specific area is the regaining of effective oculomotor gaze shifts into the visual field area that was lost. Kasten, Muller-Oehring, and Sable (2001) found a way to compensate for a hemianopic visual defect. A simple way to compensate for a visual field deficit is to train eye movements in the direction of the blind field. This automated training method for saccadic eye movements was developed by Zihl and called the Electronic Reading and Exploration Device (as cited by Kasten et al 2001). In addition, Perez and Jose (2003) reported, "The use of prisms incorporated into a spectacle correction has been a successfully clinical option for managing visual field loss." Furthermore, systematic training of spatial resolution, in which the patient would be asked to discriminate between different spatial frequencies, can help improve spatial contrast sensitivity. Systematic training has been found to improve contrast sensitivity, acuity, and reading. Colour vision has been improved by making a patient practice distinguishing colour hues. Spatial vision has been recovered by practice with distance estimation and discriminating between line lengths and orientations.
In looking at complex functions, training and practice are based on relatively recent findings. Visual agnosic patients have been trained to recognize letters, faces, and other objects by using context information, specifically the processing and selection of relevant features, to help with identification. The training developed for patients with visual neglect is to direct attention to the neglected contralesional space. Optical aids such as prism systems have been used to increase use and attention of contralesional space. Mattingley (2002) states, "The discovery of a long-lasting and effective treatment would significantly reduce the burden of patients, along with caregivers and the wider community. The prism adaptation technique potentially alleviates the problem, as shown thus far" (Mattingley, 2002, p.278). However, prisms can cause blurred vision and can be very expensive, causing many people not to proceed with the treatment. Specific training with Balint's syndrome included focusing attention and gaze shifts to stimuli outside the actual field of attention. Once doing this, patients must reach for the stimuli. This has shown to enlarge the field of attention and has improved reading and scanning of objects in the environment. However, further research is needed to understand the complexity and improve cerebral visual deficits.
There are many things in life we take for granted, however, vision, along with its complexity, are probably the least recognized. Without vision and visual perception, the environment would be nothing but feeling, sounds, and smells. Despite the fact that studies have shown that spontaneous recovery of cerebral visual impairments, it is not likely to be complete. Specific training and rehabilitation have been focused on individual functions, allowing for more recovery, but much more research and help is needed. The Journal of Impairment and Blindness (2004) recognized a new rehabilitation association. The National Vision Rehabilitation Association (NVRA) was recently established in April 2004. The NVRA will help to advance the quality of life for those blind or visually impaired. Other goals include the expansion of rehabilitative services and public awareness. Vision loss is an extremely important issue, and hopefully with our advanced technology and research, we will one day be able to provide effective, long-lasting rehabilitative services to those who are visually impaired.
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