VT and Eye Movements
Eye movements can be broken down into three of the following areas: fixations, pursuits, and saccades.
Fixations, also known as a zero pursuit (or a pursuit standing still), are defined as the process, condition, or act of directing the eyes toward an object of regard, causing the object to be centered on the fovea. Both pursuits and saccades depend on foveal fixation of the object and where it is in space. Occurring primarily in the Occipital Lobe of the brain, the ability to accurately fixate is not innate to all human beings and is learned. A baby’s fovea fully develops between 4 to 6 months of age, which is when the first true fixation is possible. It is in the area of directing fixation, maintaining fixation, and controlling one’s fixation that Vision Therapy can be most powerful. Strong fixation skills are but one pillar in the foundation of visual acuity, but also sound visual perception. In Skeffington’s Model of Vision, fixations are part of the “What is it”, or Identification circle. Since the mechanical portion of fixations is controlled by the Occipital Lobe, and the perceptual portion of fixations occurs primarily in the Temporal Lobe, the act of fixating on an object and deriving meaning from that object can be very complex. Important in Identification is accommodation. Without accurate fixation, stimulus for accommodation is compromised. Without accurate foveal fixation, i.e. eccentric fixation, decreased visual acuity occurs. Training visual skills needed for object localization and object identification are just the beginning of how Vision Therapy can help improve the “What is it?” process.
Pursuits, or tracking, are defined as any eye movement intended to follow a moving target. The Occipital Lobe of the brain is responsible for smooth pursuits. In Skeffington’s model of vision pursuits is mainly part of the “where is it” or localization circle. It is the movement of the target that stimulates the eyes to follow the target to figure out “where is it” and thereby fixate on the target to determine “what is it.”
Saccades are defined as a series of eye movements designed to fixate on an object quickly and accurately, such as moving the eyes from one group of words to the next in reading. Primarily controlled by the Frontal Eye Fields of the brain, saccades are intricately involved with motor planning and peripheral awareness. The goal of a saccade is to accurately land the fovea on the intended target. It judges and plans how far it is going to move the eyes, executes its plan while suppressing vision, then allows fixation to occur on the object of regard. Central-Peripheral integration is important to this process because peripheral vision is responsible for determining “where is it” and thereby giving the information needed of where the eyes will saccade to, then determining “what is it” after it executes the eye movement.
Assuming the patient is non-strabismic, Vision Therapy designed to improve all three eye movements generally should start on a monocular basis, move to monocular fixation in a binocular field (MFBF), then be mastered in binocular conditions.
In regards to fixations, monocular activities require fixation on a target one eye at a time, thus lowering the demand and providing the opportunity to equalize abilities between both eyes. The demand of a monocular fixation can be lowered or elevated by increasing the power of the stimulus. For example, a six year old child with only mild difficulty with fixation may be able to practice fixations with a simple target, such as a pencil. However, a different six year old child with a more severe fixation difficulty may require a stronger stimulus such as a puppet or a flashing light. Once monocular fixations are accomplished, the next steps in treatment should be MFBF. The purpose of MFBF when working on fixation is to provide stimulus to both eyes while emphasizing the use of the weaker eye. When the patient is introduced to binocular fixations, suppression controls should be used by the Vision Therapist to ensure that true binocularity is occurring during the fixation process. An example of this would be red/green filters with a BAR reader. In this activity, the patient is asked to have simultaneous perception in order to view the entire field through the red/green filters.
When working on pursuits, the activities should include a target which is moving slowly at first, increasing eye speed as a patient’s abilities improve. When working on saccades, the activities should begin with two targets which are spaced fairly close together which can be spaced further and further apart as the patient’s abilities improve. During all activities, the patient should be encouraged to open periphery and be aware of his or her surroundings, while continually working on equalizing the eyes monocularly, then arranging MFBF conditions before working binocularly.
As therapy progresses, the activities should be designed to improve stamina, ease and automaticity of the eye movements, as well as integrate these skills with other visual skills and systems, (i.e. vestibular and proprioception) thereby generalizing the use of eye movements.