Ocular Movements - An Overview

Introduction:

The eyes are the body's most essential organ, and eye movements are an essential part of the vision system. People see things through their eyes. The eyes capture things through specific movements applicable to the scenario and create an image for vision. Eye movements act as a window to our minds. These eye movements are controlled by the ocular muscles (eyes muscles).

What Are the Types of Eye Movements?

Eye movements are commonly grouped into two categories.

• Saccades.

• Vestibulo ocular reflex.

• Vergence.

• Smooth pursuit.

1. Saccades:

Human eyes are made to scan things around them and create an imaginary scene. Saccades are movements of observing a location where the eyes will focus on an object for some time and move on to the next.

The superior colliculus and frontal eye fields are involved in the saccades. This is very similar to rapid eye movement (REM) during sleep (during the fourth stage of sleep, the brain activity increases and causes dreams, followed by rapid eye movements where the eyes wander).

While reading a book or a newspaper, the saccade movements show two eyes looking at different things and not one. Though it was believed that the eyes always look at the same thing at once, and many studies were based on that, it is certainly not valid.

Saccades are considered the fastest eye movement, and their peak speed is found to be 700°/s. Saccades do not follow torque (they will never rotate in the clockwise or anti-clockwise direction along the line of vision).

What Are the Types of Saccade Movements?

Saccades of the eyes are classified into the following:

• Visually Guided Saccades: Here, the eyes will move towards a stimulus.

• Memory Guided Saccade: The eyes will move towards a memory of a place or a thing rather than a visual stimulus.

• Anti Saccade: This is the opposite of visually guided saccades. Here, the eyes move away from the visual stimulus and in the opposite direction.

• Predictive Saccade: This saccade aligns with an object's movement prediction.

• Saccadic Reading: Saccadic reading comes with the disadvantage of skipping words due to gaze movement.

What Are the Disorders That Can Affect Saccade Eye Movement?

• Parkinson's disease.

• Multiple system atrophy (neurological disorders affecting involuntary functions).

• Oculomotor apraxia (defective control over eye movement).

• Huntington's disease (breakdown of nerve cells in the brain) - primary eye defect is the absence of saccades.

• Autism.

• Alzheimer's disease.

2. Vestibulo-Ocular Reflex:

Our heads tend to move in all directions during a walk, and our eyes gaze around. The vestibular-ocular reflex focuses on an image with all these walk and head movements. In this case, the vestibular system (an apparatus in the ears that detects and balances activities) gets activated.

The vestibular canals detect the head rotations, and the ganglion reaches the vestibular nuclei in the brainstem through the vestibular nerve, causing the reflex.

Surgical Considerations:

The most common conditions of impaired vestibulo ocular reflex are nystagmus (eyes will make uncontrolled repetitive movements) and oscillopsia (illusion of an unstable vision).

If the vestibulocochlear reflex gets impaired, it restores its function quickly due to its neuroplasticity (the brain's ability to adapt and change its structure and function all life).

It will need rehabilitation therapy only when it does not restore at all.

Vestibular rehabilitation therapy includes training patients to practice head and eye movements in different directions. It is advised to begin with slow-paced activities while sitting to avoid dizziness and anxiety and gradually move on to high-velocity exercises.

3. Smooth Pursuit Movements:

Smooth pursuit movements focus on objects in line and move slowly.

It can focus on a speed stimulus up to 30°/s, after which the saccades will take over. Smooth eye pursuit movements involve two phases.

1. Open-loop condition - the target is stabilized on the retinal layer. This is the first response to the moving stimulus, and it lasts for a shorter time in which the velocity cannot be altered.

2. Closed-loop condition - the target is fixed in the space. The eye angular velocity and target angular velocity are equal in this phase.

The performance of pursuit movements is usually measured by pursuit gain. It is the ratio of the eye's velocity to the target's.

What Are the Disorders That Can Affect Smooth Pursuit Eye Movements?

• Parkinson's disease.

• Huntington's disease.

• Alzheimer's disease.

• Autism.

• Drugs and alcohol.

• Preterm birth.

4. Vergence:

This involves two eyes moving in different directions to obtain single vision (binocular fusion).

When a person with binocular vision focuses on an object closer, the eyes rotate towards each other, called convergence. Vergence works closely with the accommodation of the eye (the physiological process of adjusting the eyes to focus sharply on objects closer to the eyes). When concentrating on an object farther away, the eyes rotate away from each other, called divergence.

What Are the Types of Vergence Eye Movements?

• Tonic Vergence: The vergence acts due to the working of extraocular muscles, and no accommodation of the eye or stimulus is involved.

• Accommodative Vergence: The vergence is driven by blur. When focusing on an object closer, a signal will be sent to the extraocular muscles, which causes convergence, thus having a focused vision during reading (example).

• Fusional Vergence: The vergence caused by the fusion of monocular vision to form binocular vision.

• Proximal Vergence: It happens when someone tries to focus on something in a dark space. The object could be present nearby or farther away.

What Are Conditions That Cause Vergence Dysfunction?

• Esophoria (eye deviated inwards).

• Exophoria (eyes deviated outwards).

• Heterophoria.

• Micropsia.

• Strabismus.

Conclusion:

An American naturalist, Henry David Thoreau, said, 'The eye is the jewel of the body.’ What we see through our eyes determines the overall functioning of the brain system. As we discussed, the eye movements in each situation are beyond amusing and are a lot of work. Dysfunction in any of them can affect the quality of life considerably. But with advancing medicine and technology, it has undoubtedly found its cure.