The estimates from the research literature states that 70-90% of sensory input received by an athlete/client’s nervous system/brain comes from the visual system. Thus, it should behoove any movement coach or fitness professional who are seeking to guide their athlete/client to enhanced performance in life/sport that the visual system must be addressed.
“Vision is a dynamic interactive process of motor and sensory function, mediated by the eyes for the purpose of simultaneous organization of posture, movement and spatial orientation, for manipulation of the environment and to its highest degree, of perception and thoughts.” – Dr. William Padulla
This quote further encapsulates the contribution of vision to human movement.
Basic Neuroanatomy of the Vision System
Each eye has approximately 1,000,000 axons (nerve fibers) that make up the optic nerve. In comparison to what is known about corticospinal tract which is responsible for about 90% of the brain’s output to control voluntary movement. The corticospinal tract also contains about 1,000,000 fibers. Thus, this gives us an idea about the vast importance of the visual system on human function.
Next let’s look at the primary visual structures;
Eye/Retina – The retina consists of roughly 130 million photoreceptors that absorbs light.
Rods – These are found in the periphery of the retina and are used to see in low levels of light and to sense motion.
Cones – found primarily in the center (foveal) of the retina. These photoreceptors are used to distinguish color and fine details. There are 3 different types of cones that are sensitive to different wavelengths of light.
- Short – Blue
- Middle – Green
- Long – Red
Then we have five types of cells that transmit visual information to the brain;
- M Cells – Magnocellular cells are sensitive to depth
- P Cells – Parvocellular cells are sensitive to color
- K Cells – Koniocellular cells are sensitive to color
- IpRGC’s – Intrinsically Photosensitive Retinal Ganglion Cells are sensitive to light and contain melanopsin
- Ganglion Cells – project to the superior colliculus for eye movements
The signals of the above cells travel to three primary locations;
- Lateral Geniculate Nucleus is a sensory relay nucleus that is in the thalamus. After processing input it sends signals on to the primary visual cortex in the occipital lobe. It also has crosstalk communication with the brainstem, cortex and primary visual cortex.
- Pretectal Nucleus is found in the midbrain, the pretectal nucleus utilizes incoming input from the retina to help control pupillary constriction.
- Superior Colliculus receives input from the optic tract, occipital cortex and spinotectal tract. Because of its link to posture, movement and balance, the superior colliculus sends fiber tracts to all other areas of higher cortical function. It is currently believed that the Ambient visual system becomes part of the sensorimotor feedback loop in the midbrain, thus the superior colliculus acts as an integrator of ambient vision, proprioception, vestibular and tactile systems to allow for accurate higher cortical functions.
Finally, visual information is processed in the visual cortex, which is the largest system in the human brain. The visual data moves into the Visual Association Area where it divides into two streams:
- Dorsal Stream is known as the where/how stream. It directs spatial attention and connects with areas of the brain involved in motor control of the eyes, hands, and body that rely on movement and location information.
- Ventral stream is known as the what stream. It is involved in recognizing and identifying visual information such as shape and purpose.
Human Vision has two distinct parts:
- Foveal or Focal Vision – This is the portion of the visual field dedicated to object recognition, high resolution and depth perception.
- Ambient or Peripheral Vision – This is the part of the visual field dedicated to visual function in low light and movement recognition.
Now that we understand the visual system’s impact on brain function and that relationship to human movement we now move onto to what is called visual needs hierarchy. This model of visual needs hierarchy is the model that is used to develop drills/exercises for training various aspects of vision.
Visual Acuity – The clarity or sharpness of vision, which includes both static and dynamic components.
Visual Field – The human visual field represents the total area of viewing space both monocularly and binocularly in normal athletes.
Accommodation – The process that allows the lens of the eye to change shape to alter the optical power of the eye to maintain clear focus on objects at different distances.
Gaze Stabilization and Attention – This the ability of each eye to maintain a steady fixation on a point in space. The physical act of fixation involves the frontal eye fields, superior colliculus and subcortical brain regions.
Smooth Pursuits – These are eye movements that are required when tracking a moving object through an environment. The neurology of smooth pursuits is complex and involves the ipsilateral parietal, temporal, occipital and frontal lobes, superior colliculus, pons and cerebellum.
Saccades – A saccade is a quick, simultaneous movement of both eyes between two points of fixation. Neurologically, saccades involve the frontal lobe, superior colliculus and cerebellum.
Binocular Vision – This is the ability for both eyes to maintain bifoveal fixation on a single object of interest. If this ability is impaired, athletes will suffer from visual suppression or diplopia (double vision}.
Stereopsis – This refers to the ability to see an object three dimensionally and to perceive depth.
Vergence – Both convergence and divergence fit into this category and they represent disconjugate eye movements in which eyes are moving in different directions. Vergence movements involved the midbrain, pons, frontal lobe, parietal lobe, temporal lobe, occipital lobe and cerebellum.
Visual Perception – This is its own category because it requires the combination of multiple skills and abilities. There is believed to be a hierarchy within visual perception that includes:
- Visual Attention
- Pattern Recognition
- Visual Memory
- Visual Cognition – ZHealth 9S Speed Certification Manual
Poor movement or less than optimal movement and pain can be a result of one or more of these qualities on the vision hierarchy not functioning well or not at all. Visual acuity is most important for all the other qualities to function and for optimal movement and force production. For example, midline stability (spinal stability) is immediately improved with clearing up suppression which allows for optimal visual acuity. Midline stability will allow better and more powerful movement of the extremities (arms and legs), such as throwing a ball harder, shooting a basketball better, running faster, doing more pushups or pull ups. There is a tract called the tectospinal tract that is directly connected to the eyes via the optical nerve and plays an important role in cervical spine stabilization, so cervical spine stability is directly affected by eye function. The eyes are important in the activation and function of the vestibular system. The vestibular system is the sensory system that tells the brain where you are in space and head position. So, if you want to improve your performance in life/sport your eyes must be addressed.
ZHealth Performance Solutions; 9S Speed Certification Manual and Class notes.
ZHealth Performance Solutions; 9S Next Evolution Certification Manual and Class notes.
ZHealth Performance Solutions; S-Phase Certification Manual and Class Notes.
See to Play; by Michael Peters, OD
Neurovision Rehabilitation Guide; by Chang, Ritter, Xi Yu.