A saccade is a small, rapid adjustment of the eye in order to bring an object into better focus. These movements are so rapid and small that they are hardly ever noticed. Typically, another person needs to watch in order to observe saccades.
Generally, these movements are necessary due to the small area of detailed vision in the eye named the fovea. This area contains the largest concentration of high-resolution retinal cells and allows for detailed vision. A saccade relies on complicated neurological connections between the frontal cortex, superior colliculus, and three pairs of ocular muscles. Ultimately, this quick motion helps build a rich, dynamic interpretation of the visual world.
Anatomy and Physiology
A typical scenario of saccade initiation would be a new stimulus in the visual field. Imagine a balloon became loosened on a windy day. This mobile, bright object would enter the visual field and information from the retina would be transmitted to different parts of the brain. Once this new piece of information was recognized, both the frontal eye field and superior colliculus would coordinate eye movement toward the object.
Prior to the fovea centering on an object, peripheral retinal cells become activated. These neurons are especially sensitive to movement and make up peripheral vision. In this part of the retina, a higher concentration of rod cells exists. Interestingly, the information gathered from these cells is only black and white. New information (such as the balloon) in the visual field is then sent to the frontal eye field.
It is believed that in this region of the cortex, attention is given to new information. This information—requiring further processing—is relayed to other portions of the visual pathways. One important connection from the frontal eye field is the superior colliculus, a region in the midbrain responsible for initiating saccades.
A detailed map of the visual world exists in this part of the brain. Information from the retina is linked to specific portions of the superior colliculus, with the front portion dedicated to the fovea. This part of the brain exists in the rostral and posterior parts of the midbrain. Neuron bodies in this region directly control the muscles that move the eye.
Within the detailed visual map of the superior colliculus, the direction and speed of a saccade are dictated. In the case of the balloon, imagine this image is toward the left side of the visual field. The superior colliculus neurons are able to discern where the fovea is currently pointed and where the new stimulus exists. Based on the distance, a saccade is initiated at speeds usually around 400 degrees/second, but can go as fast as 700 degrees/second. The total right-to-left visual field, for reference, is typically 160 degrees.
The signal to initiate a saccade is a highly coordinated movement of ocular muscles. A leftward saccade is managed by the paramedian pontine reticular formation (PPRF). Here, the signal to activate leftward movement of the left eye is controlled by the abducens nerve and lateral rectus muscle. Simultaneously, the right eye is moved left through the median longitudinal fasciculus. Both the oculomotor nerve and the medial rectus muscle facilitate this movement.
Once the saccade has completed, the image should now be directly placed on the fovea. This portion of the retina contains a higher proportion of cone cells than rod cells. This information is transmitted to the occipital cortex in the back part of the brain and the image is interpreted.
Interestingly, saccades also occur in the dark. This unique feature highlights the various functions of the superior colliculus. Not only does this part of the brain respond to signals from the frontal eye field, but it also receives somatic (peripheral sensory) stimulation. Touching an arm could also be enough to trigger a saccade toward the stimulus.
Diseases and Drugs
An exaggerated, biphasic (fast and slow) saccade is known as nystagmus. There are instances where this type of movement is normal, such as with the rotation of the head, following a repetitively moving object with your eyes, and so forth. Brain lesions or the presence of drugs and/or alcohol can bring on this type of saccade.
Active substance usage or abuse can be detected with the aid of observing for nystagmus. Alcohol, for instance, specifically contributes to horizontal or lateral gaze nystagmus. An intoxicated person following a finger or pen light from left to right will reveal quick lateral saccades with a slow correction opposite of the fast movement. Phencyclidine (PCP) is an illegal drug known to induce vertical nystagmus.
Lesion or tumors within the brain, brainstem, or cerebellum can also be associated with nystagmus. Downbeat nystagmus is associated with lesions of the brainstem at the level of the foramen magnum. A person afflicted with this malady will experience a fast downbeat with a slow upbeat saccade when focusing on an object in the center of the visual field.
See also: Nystagmus; Superior Colliculus; Visual Motor System; Visual System
American Optometric Association. (2014). Nystagmus. Retrieved from http://www.aoa.org/patients-and-public/eye-and-vision-problems/glossary-of-eye-and-vision-conditions/nystagmus
Purves, Dale, George J. Augustine, David Fitzpatrick, William C. Hall, Anthony-Samuel LaMantia, James O. McNamara, & S. Mark Williams (Eds.). (2001). Neural control and saccadic eye movements. In Neuroscience (2nd ed.). Sunderland, MA: Sinauer Associates. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK10992/
Wisconsin University. (2006). Unit No. 2, brain stem: Superior colliculus. Retrieved from http://www.neuroanatomy.wisc.edu/virtualbrain/BrainStem/23Colliculus.html