Visual System Flashcards
What 3 things does the visual system provide?
- sight for the recognition and location of objects
- eye movement control
- information used in postural and limb movement control
Where do visual pathways begin?
with cells in the retina that convert light into neural signals
Describe the pathways neural signals travel
1) optic nerve
2) optic chiasm
3) optic tract
4) synapses in the lateral geniculate nucleus of the thalamus
What type of information does the primary visual cortex process?
It discriminate the shape, size, or texture of objects
What is information conveyed to the visual association cortex/secondary visual cortex is analyzed for?
colors and motion
What is the action stream used for?
to direct movement
What is the perception stream used to recognize?
visual objects
the action stream is a stream of visual information that flows ______, whereas the perception stream flows ______.
dorsally
ventrally
Normal eye movements require synthesis of information concerning what 4 things?
- Head movements (vestibular information)
- Visual objects (vision information)
- Eye movement and position (proprioceptive information)
- Selection of a visual target (brainstem and cortical areas)
The best visual acuity is available on a small region of the retina called the _____.
fovea
The exquisite control of eye position that is required to view something as a single object is achieved by what 3 things?
- MLF
- reflexes
- cerebral centers
What are the 2 types of eye movements?
- Conjugate
- Vergence
Describe conjugate eye movement
both eyes move in the same direction
Describe vergence eye movement
eyes move toward the midline or away from the midline (convergence vs. divergence)
What are the 2 objectives of eye movements?
- Gaze stabilization
- Direction of gaze
Gaze stabilization during head movements is achieved by what 2 reflexes?
- Vestibulo-Ocular Reflex (VOR)
- Optokinetic reflex
When is the vestibulo-ocular reflex initiated?
During fast movements of the head
When is the optokinetic reflex initiated?
during slow movements of the head
Direction of gaze is achieved by what 3 things?
- Saccades
- Smooth pursuits
- Vergence movements
What are saccades?
fast eye movements to switch gaze from one object to another
Describe an example of a saccadic eye movement
If a person is reading and someone comes into the room, a saccadic eye movement shifts the reader’s gaze from the text to the person.
What are smooth pursuit eye movements?
eye movements that follow a moving object
What are vergence eye movements?
movement of the eyes toward or away from midline to adjust for different distances between the eyes and the visual target
What do VORs stabilize?
visual images during head movements
Give an example of how a VOR reflex occurs?
A person attempts to maintain visual fixation of a target while their head turns to the right. The movement of the eyes is matched with the same magnitude and velocity, but in the opposite direction.
Describe the pathway a VOR reflex takes starting with rotational acceleration or deceleration of the head
1) rotational acceleration or deceleration of the head
2) receptors in semicircular canals are activated
3) send information to the vestibular nuclei
4) which send info to the oculomotor nuclei
5) which tell the extraocular muscles to respond accordingly
The optokinetic reflex adjusts eye position during ____ head movements.
slow
Give an example of when the optokinetic reflex is initiated
When a person is walking, the head moves relative to objects in the environment.
What elicits an optokinetic reflex?
moving visual stimuli
What does the optokinetic reflex allow for?
the eyes to follow large objects in the visual field
Cortical centers that influence eye movements include what 3 fields?
- frontal
- occipital
- temporal
What does the frontal eye field provide?
voluntary control of eye movements
What do the occipital and temporal eye fields provide?
information for pursuit eye movements
What 2 areas of the cortex influence eye movements?
- Posterior parietal cortex
- Dorsolateral prefrontal cortex
What does the Posterior parietal cortex provide?
cortical inputs for smooth pursuit movements
What does the dorsolateral prefrontal cortex inhibit?
reflexive eye movements as appropriate
What 5 things may influence eye movements?
- Auditory information (via the superior colliculus)
- VOR
- Visual stimuli
- Sensory information from extraocular muscles
- Limbic system and voluntary control
What controls horizontal saccades?
The paramedian pontine reticular formation (PPRF)
The paramedian pontine reticular formation (PPRF) controls horizontal saccades via which cranial nerves?
CN III and VI
What controls vertical saccades?
The midbrain reticular formation
The midbrain reticular formation controls vertical saccades via which cranial nerves?
CN III and IV
What controls diagonal saccades?
Adjusting the relative levels of activity of both the PPRF and midbrain reticular formation
What kind of eye movements are essential for following a moving object?
Smooth pursuit eye movements
What is vection?
the illusion of motion
During vection what is inhibited?
activity in the vestibular cortex
Activation of the vestibular cortex _____ visual areas of the cortex.
inhibits
