Vision is a complex processing system that involves a lot more than just visual acuity, and being a complex system, it is useful to have a systematic approach to evaluate and treat visual deficits. This systematic approach is helpful in answering the question āwhat do we treat firstā as sequencing does matter. If we view the visual system as a hierarchy where deficits in basic or foundational visual processing can adversely affect complex visual processing, we can easily see why it would be beneficial to address the foundational deficits first (see Figure 1.1).
VISUAL ACUITY AND VISUAL FIELD
The first aspect of vision is the simplest aspect of āseeing.ā Without having adequate visual acuity and visual field, there is a limit to what can be achieved with vision rehabilitation. This can be a major problem in patients who have traumatic optic neuropathy, hydrocephalous, posterior cerebral artery stroke resulting in hemianopia, and optic tract lesions, just to name a few. Rehabilitation is possible, but more challenging, and low vision will play a significant part in this patientās rehabilitation. Thankfully, this is by far the smaller percentage of patients who come in for evaluation. Significant decreased vision or visual field is uncommon in the mildāmoderate traumatic brain injury (TBI) population. It is worth mentioning that when we are speaking of normal visual fields we are referring to peripheral visual sensitivity as testing with a static visual field instrument such as a Humphrey. Functional visual fields are frequently affected in mildāmoderate TBI, and we will review that topic later on. With the mildāmoderate TBI population, we are concerned with refractive error. Small amounts of refractive error can make a huge difference in this populationāthe brain is injured, and with visually symptomatic patients, we know that the visual pathway connections are slowed (Ciuffreda et al. 2012), blur interpretation is decreased, and so small amounts of refractive errors are much more disabling than in a non-brain-injured individual. This is very important to keep in mind. Refractive error is also important in bottom-up visual processing as a correction at the ābottomā (see Figure 1.1)āvisual acuity will affect everything else in the hierarchy; it will improve accommodation, fixation, pursuits, saccades, and binocularity. This is an important point to keep in mind.
Accommodation
Much like visual acuity, accurate and adequate accommodation is necessary for maintaining clear and comfortable vision (Leigh and Zee 2006, 345). Accommodation is dynamic and therefore is much more complex. The neuropathways are not completely understood, but it is important to know that accommodation is controlled by higher-order visual processes, in addition to the parasympathetic pathway.
Fixation
In the simplest form, it is the ability for the eye to maintain steady foveation of an object in space. The frontal eye field and rostral pole of the superior colliculus are the cortical and subcortical regions that control fixation (Leigh and Zee 2006, 291). Fixation is also a measurement of global attention; therefore, if a patient has poor attention, they will oftentimes have difficulty sustaining fixation on a visual task (Thompson and Amedee 2009, 20ā26). Nystagmus is an obvious defect of fixation, where a steady fixation is absent and replaced with a back-and-forth movement. In patients with TBI, the nystagmus can be peripheral vestibular in nature such as in benign paroxysmal positional vertigo or central vestibular in nature arising from cerebellar or brainstem lesions (Leigh and Zee 2006, 673). It is important to address deficits of fixation early on in vision rehabilitation as pursuit and saccadic eye movements depend on accurate fixation.
Figure 1.1 Visual hierarchy and therapy sequencing.
Pursuit
Smooth-pursuit eye movements are utilized when tracking moving objects in an environment. The ability to execute accurate pursuits relies on a complex scheme where a movement of an object in space is perceived by the retina, which then travels from the lateral geniculate nucleus to the primary visual cortex (V1). From V1, the signal travels to the extra striate cortex and frontal eye fields where it then travels down to the cerebellum and projects to the brainstem premotor areas. Finally, signal arrives at the ocular motor neurons (cranial nerves 3, 4, and 6), where it results in a smooth-pursuit eye movement (Leigh and Zee 2006, 203). In the TBI patient, addressing pursuit deficits can improve symptoms of sensitivity to visual motion. It is important to note that severe convergence disorders can also affect smooth pursuits and that if this is found upon examination, pursuit therapy should be performed monocularly to ensure that both eyes are able to perform the task before pursuing binocular therapy.
Saccades
Saccades are rapid eye movements that shift the center focus between successive points of fixation. Saccades are broken into many different types and can be classified into voluntary and involuntary saccades. Reflexive saccades are involuntary; they are generated to something unexpected in the environment, such as a loud sound or bright light. Reading is an example of voluntary saccades as they are made as part of purposeful behavior. Saccades are also very important for visual scan, which is necessary for seeing the gestalt when looking at a picture or taking in a new environment. The neurological pathway for saccades involves both cortical and subcortical areas. Excitatory burst neurons in the midbrain and pons generate the premotor commands for saccades. The cerebral cortex can trigger saccades via parallel descending pathway to the superior colliculus. Voluntary saccades depend on the frontal eye fields, and involuntary saccades depend on the parietal lobe to the superior colliculus. Finally, the cerebellum calibrates the saccade for accurate visual ocular motor movement. Injury or degeneration of the cerebellum will result in consistent hypermetric/hypometric saccades (Leigh and Zee 2006, 108ā167). It is worth mentioning that adequate binocular vision is important for accurate saccades as well. If the patient has difficulty with coordinating their eyes and are not able to compute the three-dimensional location of the object, they are also going to have a lot of difficulty making an accurate saccade to the object and will likely make an over- or undershoot. Again if you are going to be performing a saccadic therapy technique in the setting of very poor binocularity, you would want to approach this therapy monocularly at first.
Binocular vision
Binocular vision is the ability for the two eyes to maintain bifoveal fixation of a single object of interest. If the eyes cannot perform this task, one will experience diplopia or suppression. Diplopia or double vision is usually very disturbing, so if the diplopia is not resolved within a certain period of time, the brain will suppress the vision of one eye. It is important to note that suppression is only under binocular viewing conditions. If either eye is occluded, there will not be suppression. Correcting for diplopia and suppression is an important step in neurovision rehabilitation as vergence eye movements and stereopsis cannot reliably occur with either of these conditions. Suppression is oftentimes difficult to recognize and is frequently missed in therapists who are not formally trained in neurovision rehabilitation. Red/green-based tests are best to use to detect suppression.
In addition to suppression, physiological diplopia is important as well. Physiological diplopia is a normal phenomenon where objects not within the area of fixation are seen as double. It is not just the absence of diplopia that indicates strong binocularity but the presence of physiological diplopia (pdipl). The general trend is that esotropes/esophores will tend to suppress pdipl proximal to the object being viewed, while exotropes/exophores will do the opposite. Without the proper pdipl feedback, the patient does not have the neuronal circuits to sustain vergence; if pdipl is not normalized upon completion of therapy, vergence deficits are more likely to regress.
BINOCULAR FUSION AND STEREOPSIS
Binocular fusion is the ability for the image of the object of interest to fall on corresponding retinal points of each eye and be perceived as a single image. Stereopsis is the three-dimensional perception of an object that occurs because each eye receives a slightly different image of an object.
VERGENCE
Vergence is a disconjugate eye movement, meaning the eyes are rotated in opposite directions, that is necessary to maintain binocular vision in the three-dimensional world. There are two main stimuli for vergence eye movements, the first is disparity-induced vergence, and this is when the new object of interest is at a different focal distance than the previous one. The new object falls onto noncorresponding retinal points, which causes diplopia; this drives the vergence system to make a movement to bring the new object onto corresponding retinal points and resolve the diplopia. The second stimuli for vergence is blur-induced vergence; the accommodative system is linked with the vergence system. For each diopter of accommodation, there is a set degree change of vergence. This relationship is referred to as the accommodative convergence to accommodation (AC/A) ratio. The normal AC/A ratio is 6, in which 6 diopters of convergence is exerted for every diopter increase in accommodation. This is an important concept as many binocular vision disorders arise from abnormally high or low AC/A ratios (Leigh and Zee 2006, 343ā358).
The neurological pathway for convergence involves the midbrain, which houses the neurons involved specifically in the control of vergence; this then projects to the ocular motor neurons. There are also a number of cortical areas that contribute to vergence movements. The primary visual cortex contains neurons for stereopsis and vergence responses for small disparities. The middle temporal lobe and, important for perception of depth, the parietal lobe contribute to transforming visual signals from retinal to body-centered coordinates so that objects can be located in three-dimensional space. The frontal eye fields contain neurons that discharge for objects moving in depth. The cerebellum also plays a role in control of convergence eye movements as impaired convergence is often seen in patients with cerebellar lesions.
VISUAL PERCEPTION
Visual perception is its own category, but many of the concepts in visual perception are closely intertwined with lower visual processes. An example is visual attention and visual fixation. Visual fixation is a very basic process in which the subject has to stabilize their gaze on a target, while visual attention is a perceptual process that the mind has to focus on the object, which is challenged in a busy environment. Keeping in mind the parallels, let us delve into hierarchy of visual perception. At the basic level, we have visual attention. It came to be thought of as a three-step process. The first is disengaging the first object, the second is shifting of gaze to the new object location, and the third is to actively engage on the new object. Visual fixation and saccades are essential in visual attention. The next step is pattern recognition, which is the ability to identify the salient features of an object, or the gestalt. Aspects such as shape contour and color are all aspects of pattern recognition. And as with all hierarchies, visual attention is crucial in pattern recognition; inefficient attention will make identifying details challenging. Visual memory is the next component that involves recalling the image immediately after seeing it, as well is storing it in the memory to retrieve it later. The last component is visual cognition, which is the ability to mentally manipulate visual information and integrate it with other sense organs. This includes activities of everyday life such as making decisions and solving problems both in and out of the classroom (Warren 1993, 42).
TOP-DOWN VERSUS BOTTOM-UP APPROACH TO VISION REHABILITATION
Now that we have reviewed the visual hierarchy, it is time to discuss the two different pathways for treatment. In treating TBI patients, I have found that there are really two different groups of patients with one group responding best to the top-down approach and the second group responding best to the bottom-up approach. This distinction has been essential in effectively treating patients with TBI.
We will define top-down vision therapy as therapy that primarily uses the frontal cortex and visual attention to rehabilitate visual deficits. Bottom-up vision therapy is primarily sensory and subcortical based; this therapy utilizes the visual, vestibular, auditory, and somatosensory system to rehabilitate visual deficits.
Patients who benefit most from the top-down vision therapy approach are usually the less symptomatic group. There is not as much of an overload on their visual system. They typically are less light sensitive and many times are less aware that their symptoms are related to vision problems. These patients typically have normal distance binocular findings, accommodation and convergence are only mildly reduced, and oculomotor skills are very close to normal. For these patients, they need to improve the accuracy, latency, and endurance of their binocular, accommodative, and oculomotor skills. It is recommended to follow the sequencing of therapy as described in the therapy section.
Patients who benefit from bottom-up vision therapy are usually very symptomatic; they are easily overstimulated by light and sound. They also usually have symptoms of disequilibrium and imbalance. They are usually very disturbed by visual motion, such as watching cars pass on the road, or even a hand moving in front of them. Their visual ability fluctuates and is greatly influenced by outside factors such as fatigue, headaches, sleep, and stress. These patients often have reduced convergence and divergence ranges. These patients do not seem to respond as well to the top-down therapy approach as they usually feel a significant increase in symptoms with accommodation and convergence therapy. For these patients, tinted lenses, binasal occlusion, and yoked prism glasses are helpful treatment modalities. In neurovision rehabilitation, these patients benefit from sensory integrationātype therapy, integrating their balance, vision, and auditory systems. Peripheral awareness therapy is also very helpful for these patients.
COMPONENTS FOR EFFECTIVE VISION THERAPY*
To be effective when treating patients with TBI, vision therapy techniques must incorporate the newly understood mechanisms of top-down visual processing and neuroplasticity. Cohen refers to five components of effective vision therapy: motivation, feedback, repetition, sensorimotor mismatch, and intermodal integration. Each component involves some degree of top-down processing. By incorporating these components into a vision therapy program, neuroplastic changes can be enhanced, resulting in a more effective treatment program. Each component is discussed in detail along with the associated neuroscience foundation.
Motivation/active participation
This is a conscious, goal-directed effort, which results in the activation of the prefrontal cortex to effect neuroplastic changes in the complex processing streams involved in visual perception. Motivation drives the patient to be an active participant in therapy, and understanding the goals and process of each procedure h...
