Eye Movement and Visual Stability

• Images need to remain stationary on the retina to be seen clearly due to the slow transduction process of photoreceptors.
• The eye acts like a camera with a shutter speed of about 100 ms.
• Eye movements are categorized into those that get images onto the fovea and those that keep the images there.
• Animals use a strategy of short, fast movements that change the direction of gaze, interrupted by periods of images moving little on the retina.
• Depth perception is made possible by keeping the eyes pointed in such a way that both foveae are directed at the same object of interest.
• Diplopia (double vision) results if the images on the two foveae do not correspond.
• Two kinds of movement are required to keep the eyes lined up: conjugate movements and vergence movements.
• Conjugate movements move both eyes the same amount in the same direction.
• Vergence movements converge or diverge the eyes.
• Precisely controlled patterns of contraction of extraocular muscles, using mechanisms parallel to lower motor neurons, central pattern generators, and upper motor neurons, plus modulation by the basal ganglia and cerebellum, are needed for all movements.

Extraocular Muscles

• Each eye is rotated in the orbit by the combined action of six extraocular muscles.
• The medial, lateral, superior, and inferior rectus muscles originate from the back of the orbit and insert in the sclera of the anterior half of the eye.
• The superior oblique muscle originates from the back of the orbit, but its tendon passes through a fibrous pulley (the trochlea) before turning posteriorly and inserting in the sclera of the posterior half of the eye.
• The inferior oblique muscle originates from the floor of the orbit and passes diagonally backward to insert in the sclera of the posterior half of the eye.
• The medial rectus rotates the eye toward the nose (adduction), and the lateral rectus rotates it away from the nose (abduction).
• The actions of the other four extraocular muscles are more complex because the direction in which each pulls is not usually in line with the optical axis of the eye.
• The primary actions of the superior and inferior rectus are to rotate the eye upward and downward (elevation and depression, respectively), but each rotates the eye a little bit around other axes.
• The superior and inferior obliques primarily rotate the top of the eye toward or away from the nose (intorsion and extorsion, respectively), although this can change depending on the direction in which the eye is pointed.
• Contractions and relaxations of all six extraocular muscles contribute to most eye movements.
• The lateral rectus and the obliques are antagonists in this movement, so their motor neurons are inhibited during adduction.

Lower Motor Neurons

• The lower motor neurons for the six extraocular muscles are located in a series of three nuclei in the pons and midbrain.
• The abducens nucleus innervates the lateral rectus.
• The trochlear nucleus innervates the superior oblique.
• The oculomotor nucleus innervates the other four muscles.
• Looking to one side or the other requires simultaneous contraction of one lateral rectus and the contralateral medial rectus, which is coordinated by interneurons whose axons cross the midline and ascend through the medial longitudinal fasciculus (MLF) to contralateral medial rectus motor neurons.
• Damage to one abducens nucleus causes inability of both eyes to rotate toward the side of the damage.
• Damage to one MLF causes a selective inability to use the medial rectus on that side during conjugate gaze, even though the same muscle may still be able to contract during convergence.

Conjugate Movements

• Two different kinds of conjugate movements are needed: fast movements (called saccades) and slow movements.
• Saccades are used to get an image onto each fovea.
• Slow movements are used to keep the image there.
• Cortical areas can initiate or prevent these movements.

Fast Conjugate Movements

• Saccades are extremely fast, almost step-like movements during which the eyes can rotate as much as 700 degrees per second.
• Getting an eye moving this quickly requires a very rapid burst of action potentials in the appropriate motor neurons, followed by a slower maintained rate suitable for keeping the eye in its new position.
• The required timing signals are set up by networks of neurons in the brainstem, the central pattern generators for saccades.
• The motor neurons for all the muscles needed for vertical saccades are located in the midbrain, and the pattern generator is located nearby in the reticular formation of the rostral midbrain.
• Each side of the midbrain projects to the oculomotor nuclei of both sides, so vertical saccades are not lost unless there is bilateral damage.
• The interneurons and half of the motor neurons required for movement toward the ipsilateral side live in the pons, in the abducens nucleus, and the pattern generator is located nearby in the pontine reticular formation, or PPRF.
• Simultaneous inputs to both vertical and horizontal pattern generators can elicit saccades in any direction.
• The superior colliculus of the rostral midbrain is a prominent source that forms a retinotopic map in its superficial layers.
• Collicular neurons project to the saccade pattern generators and to neck muscle motor neurons in the cervical spinal cord, triggering gaze shifts directed upward, downward, or toward the contralateral side.
• The frontal eye field, located mostly in the walls of the precentral sulcus near the hand area in motor cortex, is involved in most voluntary eye movements.
• The supplementary motor area also projects to both the saccade pattern generators and the superior colliculus and probably is involved when sequences of saccades or combinations of saccades and body movements are required.
• The parietal eye field, in the banks of the intraparietal sulcus, projects to the superior colliculus and initiates automatic saccades to interesting things that appear in peripheral parts of the contralateral visual field.
• Damage to the frontal or parietal eye field on one side causes a reduced ability to produce contralaterally directed saccades, which resolves quickly.
• Vertically directed saccades are not affected because both hemispheres participate in both upgaze and downgaze.

Slower Conjugate Movements

• The vestibular system addresses most movements of the fovea, and the visual system addresses movement of objects in space.
• The required compensatory eye movements are produced by the vestibulo-ocular reflex, or VOR.
• The VOR has a latency of only 10–20 ms and can move the eyes at up to 300 degrees per second.
• The VOR is less effective in dealing with very slow or prolonged movements.
• The VOR, like other reflexes, can be modified to suit different circumstances.
• VOR suppression is accomplished by projections from the flocculus to the vestibular nuclei, enabling the eyes to remain stationary as the head turns.
• If an object whose image is on the fovea begins to move, the visual system can detect this and initiate smooth pursuit eye movements to prevent the image from leaving the fovea.
• Smooth pursuit can only keep up with objects that move relatively slowly-no more than about 100 degrees per second.
• Smooth pursuit is like a reversed version of the VOR in that it is a smooth eye movement produced in the absence of vestibular stimulation and probably evolved from the same circuitry used to cancel the VOR.
• Pursuit is initiated by the same cortical network that initiates saccades and is implemented by the same flocculus-vestibular nuclei system that controls VOR cancellation. Signals from the cerebral cortex reach the flocculus via the pontine nuclei.
• Projections from the cerebral cortex to the flocculus are mostly crossed, just like other corticocerebellar pathways.
• Smooth pursuit in both directions is impaired after cortical damage, but the deficit is greater in the direction ipsilateral to the damage.

Vergence Movements

• The vergence movements required to keep both foveae pointed at objects moving closer or farther away are accompanied by changes in the shape of the lens and changes in pupil diameter.
• The lower motor neurons and the preganglionic parasympathetic neurons required for this three-part response are located in the oculomotor nucleus, and the pattern generator for vergence and accommodation is located nearby in the midbrain reticular formation.
• The cortical areas involved in initiating these movements include the frontal eye fields and more posterior parietal and occipital areas that analyze the blur and retinal disparity signals generated by objects moving in depth.

Basal Ganglia and Eye Movements

• Most basal ganglia disorders are characterized by various combinations of involuntary movements and movements of decreased velocity and amplitude, which extends to eye movements as well.
• Patients with Parkinson's disease make voluntary saccades and smooth pursuit movements that are smaller and slower than normal.
• Patients with Huntington's disease often have involuntary saccades as they try to look at something.
• The loop starts with eye movement-related cortical areas that project to part of the caudate nucleus, which projects to the reticular part of the substantia nigra.
• Increased inhibition of the superior colliculus by the reticular part of the substantia nigra in Parkinson's disease is thought to result in smaller and slower saccades.

Cerebellum and Eye Movements

• Two parts of the cerebellum play a role in eye movements, the flocculonodular lobe and an additional part of the vermis dorsal to the nodulus, the latter sometimes referred to as the oculomotor vermis.
• Eye movement-related areas of cerebral cortex project to the oculomotor vermis, which in turn projects via the fastigial nucleus to all the brainstem pattern generators for eye movements.
• Damage to this part of the vermis, or to the fastigial nucleus, or even to specific pontine nuclei, can cause saccades that undershoot or overshoot, or impaired smooth pursuit or vergence movements.
• The flocculonodular lobe receives inputs from comparable areas of cerebral cortex, as well as extensive vestibular inputs, and projects back to the vestibular nuclei.
• Damage here, especially to the flocculus, causes impairment of smooth pursuit movements and loss of the ability to change the gain of the VOR.
• Combined damage to the flocculonodular lobe and the oculomotor vermis causes a loss of smooth pursuit movements.