Central Visual Pathways and the Perceived Visual Field (Week 2, Mod 8)

What are 4 ways we can assess vision in the veterinary world? Keep in mind the limitations that we have.

1) Observing the patient in an unfamiliar environment

• If blind, may bump into things

2) Ability to track

• Cotton ball test - drop a cotton ball on either side of the animal just at its peripheral… animal should turn to look at the object

3) Visual placing

• Dog being picked up and brought forward to a table… should reach out to step onto the table on its own 

4) Menace response 

What are the 7 general steps of the visual pathway? KNOW THIS

Retina →

Optic nerve →

Optic chiasm →

Optic TRACT →

Lateral geniculate nucleus (of the thalamus) →

Optic radiation →

Occipital cortex 

What is the “visual field”? How does it differ between predators and prey?

Visual field: The total area in which objects can be seen in the peripheral vision as you focus your eyes on a central point 

Predators: 

• Eyes forward, towards front of the skull

• Shorter snout 

• = BINOCULAR vision 

Prey:

• Eyes on the sides of the face; have their own visual fields, only become binocular directly in front of the snout

• Long snout

• Allows for better peripheral vision

Describe the variation of the level of CROSSOVER at the optic chiasm between species… is there more crossover in animals with increased binocular vision?

In predatory animals, the degree of crossover of visual information from the eyes to the lateral geniculate nucleus is LESS than prey animals:

• Humans: 50%

• Cats: 65%

• Dogs: 57%

• Horse: 80%

• Birds and fish: 100%

Is INVERSELY correlated to the size of the binocular visual field

Describe the pupillary light reflex test… what parts of the visual pathway is this testing?

PLR = shining a bright light in ONE eye… should see BOTH pupils contract 

• Tests the function of the Optic Nerve (CN II) → optic chiasm → optic tract → Pretectal nucleus → Parasympathetic nucleus of Oculomotor nerve (CN III) pathway 

** CN III = oculomotor nerve = constriction of the pupil

What is the menace response? What parts of the visual pathway is this testing?

The menace response is a reaction where a hand is moved in front of the eye suddenly, and the animal blinks in response

• Is a RESPONSE, not a reflex, so will have cortical involvement and will therefore be more complicated 

Pathway it testsL

• Optic nerve (CN II) → optic chiasm → optic tract → lateral geniculate nucleus → optic cortex → motor cortex → pontine nucleus → cerebellum → FACIAL nerve nuclei (CN VII)

What is the major difference between the menace response and the pupillary right reflex?  Why might we use the both of these during a visual assessment?

Both evaluate the retina, the optic nerve, and the optic chiasm

• BUT the PLR test doesn’t asses the cortex portion of the visual pathway… simply tests the REFLEX itself 

Combining the two tests in a visual assessment can tell us what kind of blindness a patient may have (essentially, whether the damage is localized to the eye or brain)

What is central (cortical) blindness?  Where would you find a lesion in the visual pathway with this kind of blindness?  What are the clinical signs?

Is blindness without apparent lesions of the eyes

• Involves the PROCESSING system, rather than the hardware 

Will see a lesion in:

• Lateral geniculate nucleus 

• Optic radiation 

• Occipital cortex

Clinical signs:

• NORMAL PLR

• ABSENT menace response

• Blind

What is peripheral (subcortical) blindness?  Where would you find a lesion in the visual pathway with this kind of blindness?  What are the clinical signs?

Involved the collection / distribution system

Will see lesions in the:

• Eye

• Optic nerves

• Optic chiasm

• Optic tract

Clinical signs:

• ABSENT PLR

• ABSENT menace response

• Blind

What is the dazzle reflex? What parts of the visual pathway is this testing?

Is very similar to the PLR test, but triggers FACIAL NERVE CN VII to produce a BLINK REFLEX

• Instead of Oculomotor nerve CN III, which is used for pupil constriction

Should see BILATERAL blinking

• Light should be very bright for the reflex

Associate the extraocular muscles with the nerve that innervates them… might be easier to remember them this way 

Oculomotor nerve (CN III)

• Dorsal Rectus 

• Medial Rectus 

• Ventral OBLIQUE 

• Ventral Rectus 

  • Think of it this way: oculomotor = eye movement… so the oculomotor nerve innervates the MOST extraocular muscles

  • But doesn’t move laterally because ABDUCTING is the NEXT nerve’s job…

Abducens nerve (CN VI)

• LATERAL Rectus 

• Retractor bulbi muscle 

Trochlear nerve (CN IV)

• Dorsal Oblique 

  • Think of it as the muscle that turns the bottom portion of the eye towards the trochlea…

What is strabismus clinically? What would happen to the eye if there was a lesion at CN III?  CN VI?  CN IV?

Strabismus is ABNORMAL positioning of the eye 

Lesion at CN III → eye moves ventrolaterally because the lateral rectus and other ventral muscles are the ONLY ones working still

Lesion at CN IV → eye moves dorsomedially… dorsal oblique muscles have stopped working, so no longer turning the bottom portion of the eye toward the trochlea 

• Makes pupil tilt

• Visible in the cat, NOT noticeable in the dog 

Lesion at CN VI → eye moves medially, lateral rectus no longer functioning