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Voluntary and involuntary eye movements
In order to acquire, fixate, and track the visual stimuli, human eyes move both voluntarily and involuntarily. Most of the eye movements are involuntary reflexes except for saccades, vergence shifts, and smooth pursuits.
Voluntary eye movements
- Saccades: Voluntary eye movements occur in small jumps called saccades. Horizontal and vertical saccades use different neuronal circuitry. Horizontal saccades are initiated by neurons in the frontal eye fields of the cerebral cortex. Activation of the right frontal eye field will cause the eyes to look to the left and activation of the left frontal eye field will cause the eyes to look to the right. Vertical saccades are activated by diffuse areas of the cortex.
In terms of eye movement types, we can classify them as follows
- voluntary motion
- tracking (both voluntary and involuntary)
- pupillary reactions
- control of the lens
They are mostly reflexes rather than voluntary movement.
Voluntary eye movements occur in saccades. Saccade movement indicates very fast jump from one eye position to another whereas smooth pursuit indicates slow and smooth eye movement. Saccades serve as fixation, rapid eye movement, and the fast-phase of optokinetic nystagmus. We call small jumps as microsaccades and they occur even when the eye is still.
Voluntary horizontal and vertical gaze conjugate different neuronal circuitry. The voluntary horizontal gaze is initiated by neurons in the frontal eye field of the cerebral cortex. Activation of the right frontal eye field will cause the eyes to look to the left and activation of the left frontal eye field will cause the eye to look to the right. The voluntary vertical gaze follows a different pathway. First of all, there is no single cortical center responsible for the vertical gaze. Instead, diffuse areas of the cortex project to the rostral interstitial nucleus of the Medial Longitudinal Fasciculus (MLF).
Most of our normal voluntary eye movements are not smooth, but rather occur in saccades. However, we can move our eye smoothly when tracking a moving object. This smooth pursuit utilizes a part of the vestibulo-ocular reflex (VOR) pathways and require a visual input to the occipital cortex in order to lock the eyes onto the target.
The fixation reflexes and optokinetic reflexes use the same path as smooth pursuit movements. Fixation reflex refers to the ability to fixate eyes on a moving target. It compensates the VOR to stabilize the eyes when the head tracks the moving target. The optokinetic reflex (or optokinetic nystagmus; OKN) is an involuntary fixation on moving objects.
 Optokinetic nystagmus:
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