Oculomotor System

Question 1

Describe the Function of the Oculomotor System:

Answer 1

To work efficiently, both eyes must focus and remain focused on an object so the information can reach cortex in a form that we perceive as vision. While we can detect objects over a large rate extending into the periphery, we “see” objects only when they are focused on the fovea. The oculomotor system, controlling the movement of our eyes, is designed to 1) bring targets onto the fovea, and 2) keep them there. Historically, 5 movement systems have been identified that put the fovea on a target (both eyes in a conjugate manner; foveation) and keep it there.

Question 2

What are the 5 movement systems that have been identified that put the fovea on a target?

Answer 2

• Movements that stabilize the eye when the head moves: 1) Vestibulo-ocular, 2)Optokinetics
• Movements that keep the fovea on a visual target: 3) Saccade, 4) Smooth Pursuit, and 5) Vergence

Question 3

Describe the 2 movement systems that help stabilize the eye when the head moves:

Answer 3

1) Vestibulo-ocular: Vestibular input holds images stable on retina during head and body movement; gaze stabilization.
2) Optokinetics: Visual input holds images stable on retina during sustained or slow head rotation

Question 4

Describe the 3 movement systems that keep the fovea on a visual target:

Answer 4

3) Saccade: brings new objects of interest onto fovea.
4) smooth pursuit: holds image of a moving target on the fovea
5) Vergence: adjusts the eyes for viewing different distances in depth.

Question 5

Describe a normally functioning oculomotor system:

Answer 5

The two eyes are “locked” together. This allows both eyes to be focused on an object. If the system becomes “unlocked” and the eyes focus on different targets (retinal disparity), diplopia or double vision occurs. While the first four systems allow for the conjugate movements of the eyes for foveation, the vergence system allows for convergence or divergence (disconjugate movements)

Question 6

What is retinal disparity?

Answer 6

when the oculomotor system becomes “unlocked” and the eyes focus on different targets

Question 7

Describe the Saccade system:

Answer 7

It is used to quickly reposition the eyes (fovea) to focus on an image that has suddenly been moved from the fovea. Saccades can be horizontal or vertical, wit the horizontal being the best understood from a neuronal pathway point of view. The rapid eye movement required to bring the image back into focus is a saccade. Example: Reading a handout is done by a series of conjugate saccadic eye movements. Saccadic eye movements can be elicited by visual input, as well as other stimuli, such as auditory stimuli, memories of locations, or verbal commands. Both eyes move in conjugate manner in terms of velocity, amplitude, and direction for reestablishment of an image on the fovea. This movement is similar to the quick or saccade phase of vestibular nystagmus. This conjugate system is directed through the Horizontal Gaze Center located in the pontine reticular formation (paramedic pontine reticular formation; PPRF), adjacent to the abducens nucleus. The upper motor neuron control is a complicated system of interneurons, that involves a part of the middle frontal gyrus termed the Frontal Eye Fields (FEF; Area 8) and the Superior Colliculus.