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Questions and Answers
The study of eye movements is relatively straightforward due to the complex interplay of numerous muscles.
The study of eye movements is relatively straightforward due to the complex interplay of numerous muscles.
False (B)
The visual acuity of the retina is uniform across its entire surface.
The visual acuity of the retina is uniform across its entire surface.
False (B)
The fovea has a high degree of convergence from photoreceptors to ganglion cells.
The fovea has a high degree of convergence from photoreceptors to ganglion cells.
False (B)
To achieve the best visual information of an object, it is necessary for the light points composing the object to be focused on the peripheral regions of the retina.
To achieve the best visual information of an object, it is necessary for the light points composing the object to be focused on the peripheral regions of the retina.
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The size of the fovea poses no limitations to the ability to perceive the entire image of very large objects.
The size of the fovea poses no limitations to the ability to perceive the entire image of very large objects.
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Among the mechanisms to correct for limitations on visual acuity, there are retinal stabilization reflexes that occur voluntarily.
Among the mechanisms to correct for limitations on visual acuity, there are retinal stabilization reflexes that occur voluntarily.
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Gaze stabilization movements are unique to humans.
Gaze stabilization movements are unique to humans.
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Gaze stabilization movements aim to compensate for self-motion in order to avoid a blurred vision.
Gaze stabilization movements aim to compensate for self-motion in order to avoid a blurred vision.
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The optokinetic system induces compensatory eye movements in the same direction as a visual image slipping across the retina.
The optokinetic system induces compensatory eye movements in the same direction as a visual image slipping across the retina.
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Optokinetic nystagmus involves an alteration of slow and fast eye movements.
Optokinetic nystagmus involves an alteration of slow and fast eye movements.
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Pathological nystagmus always results from damage to the visual system.
Pathological nystagmus always results from damage to the visual system.
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Optic flow is tuned for very high-speed movements exceeding human capabilities.
Optic flow is tuned for very high-speed movements exceeding human capabilities.
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Gaze shifting is a visual feature common to all animals.
Gaze shifting is a visual feature common to all animals.
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The fovea limits the velocity of retinal stimuli.
The fovea limits the velocity of retinal stimuli.
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Pursuit movements are slow and abrupt.
Pursuit movements are slow and abrupt.
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Saccadic movements are the slow, smooth tracking movements of the eyes.
Saccadic movements are the slow, smooth tracking movements of the eyes.
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People are consciously aware of performing saccadic movements.
People are consciously aware of performing saccadic movements.
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During gaze shifting both eyes move independently to explore an object.
During gaze shifting both eyes move independently to explore an object.
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Saccadic movements are involuntary eye movements used to track a moving object.
Saccadic movements are involuntary eye movements used to track a moving object.
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Pursuit movements keep a moving stimulus on the fovea once foveation is achieved and are always under conscious control of the observer.
Pursuit movements keep a moving stimulus on the fovea once foveation is achieved and are always under conscious control of the observer.
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Vergence movements are unique to animals with only retinal specialization.
Vergence movements are unique to animals with only retinal specialization.
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The medial rectus and lateral rectus are an example of an antagonistic pair of extraocular muscles.
The medial rectus and lateral rectus are an example of an antagonistic pair of extraocular muscles.
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The abducens nerve (IV) is responsible for controlling eye movement muscles.
The abducens nerve (IV) is responsible for controlling eye movement muscles.
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The resting centric position of the eye is maintained through active contraction of eye muscles.
The resting centric position of the eye is maintained through active contraction of eye muscles.
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Moving the eyes from the resting centric position always requires a dynamic force, but not necessarily a static force.
Moving the eyes from the resting centric position always requires a dynamic force, but not necessarily a static force.
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The rate of motoneuronal discharge is directly proportional to the absolute distance from the resting eye position, not the degree of distance.
The rate of motoneuronal discharge is directly proportional to the absolute distance from the resting eye position, not the degree of distance.
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The vestibulo-ocular reflex (VOR) uses the semicircular canals to detect head rotations and produce eye movements in a parallel direction to stabilize the line of sight.
The vestibulo-ocular reflex (VOR) uses the semicircular canals to detect head rotations and produce eye movements in a parallel direction to stabilize the line of sight.
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The VOR is typically suppressed and non-functional in an awake, healthy person who is actively turning their head.
The VOR is typically suppressed and non-functional in an awake, healthy person who is actively turning their head.
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Impairment of cranial nerves can be evaluated by assessing the integrity of the brainstem using the VOR.
Impairment of cranial nerves can be evaluated by assessing the integrity of the brainstem using the VOR.
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During a VOR assessment, having the patient focus on a point while the head is moved quickly primarily causes saccadic eye movements.
During a VOR assessment, having the patient focus on a point while the head is moved quickly primarily causes saccadic eye movements.
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The medial longitudinal fasciculus (MLF) and ocular movements terminate within the cerebral cortex.
The medial longitudinal fasciculus (MLF) and ocular movements terminate within the cerebral cortex.
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The VOR compensates only for rotational movements, not for tilting of the head.
The VOR compensates only for rotational movements, not for tilting of the head.
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The oculomotor nuclei receive information from the labyrinth that promotes consensual eye movements by sending different information to each eye.
The oculomotor nuclei receive information from the labyrinth that promotes consensual eye movements by sending different information to each eye.
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Omnipause neurons must be activated to initiate a saccade.
Omnipause neurons must be activated to initiate a saccade.
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The VOR can effectively counteract head rotations up to 360 degrees.
The VOR can effectively counteract head rotations up to 360 degrees.
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Voluntary, dexterous movements are organized by a command center in the brainstem.
Voluntary, dexterous movements are organized by a command center in the brainstem.
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The posterior parietal cortex is responsible for executing saccadic eye movements.
The posterior parietal cortex is responsible for executing saccadic eye movements.
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Smooth pursuit eye movements help stabilize the retinal image of a large, rapidly moving background object.
Smooth pursuit eye movements help stabilize the retinal image of a large, rapidly moving background object.
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A lesion of the third cranial nerve typically causes converging strabismus.
A lesion of the third cranial nerve typically causes converging strabismus.
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Damage to the oculomotor nerve always results in complete ptosis and a fully closed eye.
Damage to the oculomotor nerve always results in complete ptosis and a fully closed eye.
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An oculomotor nerve lesion leads to a constricted pupil because the parasympathetic nerve supply is hyperactive.
An oculomotor nerve lesion leads to a constricted pupil because the parasympathetic nerve supply is hyperactive.
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In the case of an oculomotor nerve lesion, the consensual pupillary light reflex in the unaffected eye is also absent.
In the case of an oculomotor nerve lesion, the consensual pupillary light reflex in the unaffected eye is also absent.
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An oculomotor nerve lesion impairs the eyes’ ability to accommodate, preventing focus on close objects.
An oculomotor nerve lesion impairs the eyes’ ability to accommodate, preventing focus on close objects.
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Study Notes
Ocular Movements
- Eye movements are easier to study than movements of other body parts due to the relative simplicity of muscle actions on the eyeball.
- There are 6 extraocular muscles, each with a specific role in adjusting eye position.
- Eye movements are stereotyped, with partially distinct central circuits governing each.
Retinal Stabilization
- Spatial resolution of the retinal transduction system is limited by retinal photoreceptor mosaic properties.
- Retina is not homogeneous, with visual acuity varying across its parts.
- Fovea is the high-acuity region of the retina, containing cones with small receptive fields.
- Fovea has a 1:1 receptor-to-ganglion cell correspondence in some areas.
- The fovea's high acuity allows for precise object details focusing on it.
- Fovea size limits how much of a large object can be focused.
- Eye movement degrees of freedom need adjusting to maintain visual acuity with body movements.
- Retinal stabilization reflexes (involuntary) compensate for these limitations by adjusting eye movement in response to body movement to keep images steady.
- Different types of stabilization movements exist based on aim; - Gaze stabilization compensates for self-motion to avoid blurry vision. - Gaze shifting accurately positions the fovea for targeted stimuli. - Vergence is part of binocular vision, adjusting eye angles for clear focus of one object.
Gaze Stabilization (all animals)
- These movements counteract self-motion effects on visual acuity, keeping images stable during head movement.
- Adjusted eye movements compensate for motion.
Vestibulo-ocular Reflex (VOR)
- This involuntary reflex uses vestibular information to counteract head movements.
- The eyes move in the opposite direction to the head to maintain visual stability.
- Works rapidly during brief, transient head movements.
- Less effective for slow or sustained head rotations.
- VOR is active from the lowest to the highest velocity of head movement.
- VOR is used to examine or assess the integrity of the brainstem.
Optokinetic Reflex (OKR)
- This reflex uses visual cues (the visual field) to counteract visual movement by moving the eyes in the opposite direction.
- Especially sensitive to slow visual field movements.
- Response builds up slowly.
- Works well when head movements are slow.
- OKR is complementary to the VOR.
Gaze Shifting
- This specific type of movement (only in animals with retinal specializations) accurately points the foveal regions of the retina at a target in order to maintain visual stability.
- Two types of movement exist: saccades and pursuit movements - Saccades: swift, jerky movements rapidly changing eye direction. - Pursuit movements: smooth, tracking eye movement necessary to follow a moving stimulus to maintain it focused on the fovea.
Vergence
- Unique to animals with binocular vision.
- Adjusting eye angles for the focus of an object at varying distances.
- Crucial for maintaining a unified view of the scene.
Eye Movements overview
- All eye movement mechanisms rely on a set of common motor neurons and muscles organized in antagonistic pairs.
- Three cranial nerves (III, IV, and VI) control the eye muscles.
Nystagmus
- A reflexive eye movement that involves quick ("fast") and slow movements.
- Occurs when trying to rotate the eye past its maximum ability to counteract motion.
- A visual cue is used to reset and adjust the eyes location from other movements
Optokinetic Reflex overview
- Extracting visual information to determine relative visual field motion and responding with a compensating eye movement.
- Compensating for slow visual movements using inputs from the retina
- Damage to the AOS/pretectum can disrupt this visual feedback response.
Saccades
- Rapid, precise eye movements.
- Typically high velocity (as fast as 800°/sec)
- Used to shift focus from one location to another.
- Evoked by a variety of stimuli (visual, somatosensory, and auditory).
Smooth Pursuit
- Continuous tracking movements following a moving image.
- Eye and image movement are roughly the same speed.
- A dedicated neural circuit processes information for these subtle eye movements.
Clinical Aspects (III cranial nerve lesion)
- Complete ptosis (drooping eyelid)
- Divergent strabismus ("down and out") eye position
- Dilated pupil, unresponsive to light or accommodation.
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Description
This quiz focuses on the intricate mechanisms involved in eye movements, visual acuity, and gaze stabilization. It covers concepts such as the role of the fovea, retinal stabilization reflexes, and optokinetic nystagmus. Test your understanding of how these systems work together to enhance vision.
