Don't look now! Parietal activity when the saccade goal ...
The oculomotor system Or Fear and Loathing at the Orbit Michael E. Goldberg, M.D. First you tell them what your gonna tell them The phenomenology of eye movements. The anatomy and physiology of the extraocular muscles and nerves. The supranuclear control of eye
movements: motor control and cognitive plans. The purposes of eye movements Keep an object on the fovea Fixation Smooth pursuit Keep the eyes still when the head moves Vestibulocular reflex Optokinetic reflex
Change what you are looking at ( move the fovea from one object to another) Saccade Change the depth plane of the foveal object Vergence eyes move in different directions The vestibuloocular reflex. The semicircular canals provide a head velocity signal. The vestibuloocular reflex (VOR) provides an equal and opposite
eye velocity signal to keep the eyes still in space when the head moves. The vestibular signal habituates, and is supplemented by vision the optokinetic response Smooth pursuit matches eye velocity to target velocity Saccades move the fovea to a new position
6 Muscles move the eyes Levator Palpebrae Superior Rectus Lateral Rectus Medial Rectus Superior Oblique Inferior Oblique Inferior Rectus How the single eye moves Horizontal: Abduction (away from the nose)
Adduction (toward the nose). Vertical: Elevation (the pupil moves up) Depression (the pupil moves down) Torsional: Intorsion: the top of the eye moves towards the nose Extorsion: the top of the eye moves towards the ear.
The obliques are counterintuitive Each oblique inserts behind the equator of the eye. The superior oblique rotates the eye downward and intorts it! The inferior oblique rotates the eye upward and extorts it. Vertical recti tort the
eye as well as elevate or depress it. Oblique action depends on orbital position The superior oblique depresses the eye when it is adducted (looking at the nose). The superior oblique intorts the eye when it is abducted
(looking towards the ear) 3 Cranial Nerves Control the Eye Levator Palpebrae Superior Rectus Inferior Rectus Medial Rectus Nerve III: Oculomotor
Inferior Oblique Superior Oblique Lateral Rectus Nerve IV: Trochlear Nerve VI: Abducens Left fourth nerve palsy Hyperopia in central gaze. Worse on right gaze.
Better on left gaze. Worse looking down to right Better looking up to right. Head tilt to right improves gaze. Head tilt to left worsens gaze. Listings Law Torsion must be constrained or else vertical lines would not remain vertical. Listings law accomplishes this: the axes
of rotation of the eye from any position to any other position lie in a single plane, Listings plane. This is accomplished by moving the axis of rotation half the angle of the eye movement The pulleys: something new in orbital anatomy and physiology. How is Listings law accomplished? Extraocular muscles have two layers A global layer that inserts on the sclera
An orbital layer that inserts on a collagenelastin structure between the orbit and globe. This structure serves as a PULLEY through which the global layer moves the eye. Moving the pulleys accomplish listings law. (Demer). Pulley Anatomy The pulleys Horizontal rectus pulleys change their position with
horizontal gaze. Eye muscle nuclei Mesencephalic Thalamus Reticular Formation Superior Colliculus Inferior Colliculus III IV
Cerebellum VI Pontine Nuclei Vestibular Nuclei Oculomotor neurons describe eye position and velocity. Lateral Eye position the step
Medial Eye velocity the pulse Medial - Lateral Eye Position Abducens neuron Sp/s Pulse
Step Neuron Abducens neuron The transformation from muscle activation to gaze The pulse of velocity and the step of position are generated independently. For horizontal saccades the pulse is generated in the paramedian pontine reticular formation.
The step is generated in the medial vestibular nucleus and the prepositus hypoglossi by a neural network that integrates the velocity signal to derive the position signal. Horizontal saccades are generated in the pons and medulla Thalamus Superior Colliculus Inferior Colliculus Medial longitudinal
fasciculus III IV Cerebellum Paramedian Pontine Reticular Formation Pontine Nuclei VI
Vestibular Nuclei and Nucleus Prepositus Hypoglossi Digression on Neural Integration Intuitively, you move your eyes from position to position (the step). Higher centers describe a saccadic position error. The pontine reticular formation changes the position error to a desired velocity (the
pulse). The vestibulo-ocular reflex also provides the desired velocity. In order to maintain eye position after the velocity signal has ended, this signal must be mathematically integrated. Neurons involved in the generation of a saccade ` Generating the horizontal gaze
signal The medial rectus of one eye and the lateral rectus of the other eye must be coordinated. This coordination arises from interneurons in the abducens nucleus that project to the contralateral medial rectus nucleus via the medial longitudinal fasciculus. Left lateral rectus Abducens nerve Paramedian
pontine reticular formation (saccade velocity) Abducens nucleus: motor neurons and interneurons. Medial vestibular nucleus: eye position, VOR and smooth
pursuit velocity . Right medial rectus Oculomotor nucleus and nerve: motor neurons only Medial longitudinal fasciculus
Nucleus prepositus hypoglossi (eye position) To reiterate Ocular motor neurons describe eye position and velocity. For smooth pursuit and the VOR the major signal is the velocity signal, which comes from the contralateral medial vestibular nucleus. The neural integrator in the medial vestibular nucleus and nucleus prepositus hypoglossi converts the velocity signal into a position signal which holds eye
position. For horizontal saccades the paramedian pontine reticular formation converts the position signal from supranuclear centers into a velocity signal. This signal is also integrated by the medial vestibular nucleus and the nucleus prepositus hypoglossi. Abducens interneurons send the position and velocity signals to the oculomotor nucleus via the medial longitudinal fasciculus. Vertical movements and vergence are organized in the midbrain Mesencephalic
Thalamus Reticular Formation Posterior commissure Superior Colliculus Inferior Colliculus rIMLF III Medial Longitudinal Fasciculus
Paramedian Pontine Reticular Formation Pontine Nuclei IV Cerebellum VI Vestibular Nuclei
Internuclear ophthalmoplegia The medial longitudinal fasciculus is a vulnerable fiber tract. It is often damaged in multiple sclerosis and strokes. The resultant deficit is internuclear ophthalmoplegia The horizontal version signal cannot reach the medial rectus nucleus, but the convergence signal can. Supranuclear control of saccades
The brainstem can make a rapid eye movement all by itself (the quick phase of nystagmus). The supranuclear control of saccades requires controlling the rapid eye movement for cognitive reasons. In most cases saccades are driven by attention Humans look at where they attend
Supranuclear control of saccades Supplementary Eye Field Posterior Parietal Cortex Frontal Eye Field Caudate Nucleus Superior Colliculus Substantia Nigra Pars Reticulata Reticular Formation
Supranuclear Control of Saccades Superior colliculus drives the reticular formation to make contralateral saccades. The frontal eye fields and the parietal cortex drive the colliculus. The parietal cortex provides an attentional signal and the frontal eye fields a motor signal. The substantia nigra inhibits the colliculus unless It is inhibited by the caudate nucleus Which is, in turn, excited by the frontal eye field.
The effect of lesions Monkeys with collicular or frontal eye field lesions make saccades with a slightly longer reaction time. Monkeys with combined lesions cannot make saccades at all. Humans with parietal lesions neglect visual stimuli, and make slightly hypometric saccades with longer reaction times. Often their saccades are normal: if they can see it they can make saccades to it. Humans with frontal lesions cannot make
antisaccades. The Antisaccade Task The Antisaccade Task Look away from a stimulus. The parietal cortex has a powerful signal describing the attended stimulus. The colliculus does not respond to this signal. The frontal motor signal drives the eyes away from the stimulus. Patients with frontal lesions cannot
ignore the stimulus, but must respond to the parietal signal Antisaccades Supplementary Eye Field Posterior Parietal Cortex Frontal Eye Field Caudate Nucleus Superior Colliculus Substantia Nigra Pars Reticulata
Reticular Formation Supranuclear control of pursuit: pursuit matches eye velocity to target velocity Middle temporal and middle Frontal Eye Field provides the trigger to start the pursuit. superior temporal (MT and MST) provide the velocity signal Striate Cortex
Dorsolateral pontine nuclei Vestibular nucleus Cerebellum vermis and flocculus Smooth pursuit Requires cortical areas that compute target velocity, the
dorsolateral pontine nuclei, and the cerebellum. Utilizes many of the brainstem structures for the vestibuloocular reflex Requires attention to the target. Clinical deficits of smooth pursuit Cerebellar and brainstem disease Specific parietotemporal or frontal lesions Any clinical disease with an
attentional deficit Alzheimers or any frontal dementia, schizophrenia
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