Visual Pathways

Michael Davidson

Professor, Ophthalmology

Diplomate, American College of

Veterinary Ophthalmologists

Department of Clinical Sciences

College of Veterinary Medicine

North Carolina State University

Raleigh, North Carolina, USA

<mgdavids@ncsu.edu>

Miller PE, Murphy CJ. Vision in Dogs. JAVMA. 1995;

207: 1623.

Miller PE, Murphy CJ. Equine Vision. In Equine

Ophthalmology ed. Gilger BC. 2nd ed. 2011: pp 398-

433.

Ofri R. Optics and Physiology of Vision. In Veterinary

Ophthalmology. ed. Gelatt KN 5th ed. 2013: 208-270,

Vision in Animals

Visual Pathways, Responses and

Reflexes: Relevant Structures

Optic n (CN II) – somatic afferent

Oculomotor n (CN III), Trochlear n (CN IV), Abducens n (CN VI) –

somatic efferent to extraocular muscles

Facial n (CN VII)– visceral efferent to eyelids

Rostral colliculi – brainstem center that mediates somatic reflexes in

response to visual stimuli

Cerebellum

Cerebro-cortex esp. occipital lobe

www.studyblue.com

Visual Pathway

Visual Cortex

Lateral Geniculate Body

Optic Radiation

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Visual Field

each cerebral hemisphere receives information

from contralateral visual field (“the area that can

be seen when the eye is directed forward”)

visual field

Visual Fiber (Retinotopic)

Segregation

nasal retinal fibers decussate at chiasm,

temporal retinal fibers remain ipsilateral

Nasal Retina

TemporalRetina

Fibers Remain Ipsilateral

Fibers Decussate

OD Visual FieldOS

nasal

hemifield

temporal

hemifield

Total visual field OD

nasal

fibers

temporal

fibers

Nasal Retina =Temporal hemifield

TemporalRetina =

Nasalhemifield

Fibers Decussate

Fibers Remain Ipsilateral

Visual Projection Pathway

When viewed with both eyes:

- target in right half of visual field (right

visual hemifield) right nasal

retina and left temporal retina left

optic tract left dLGN left

cerebral hemisphere

target in right half of visual field (right visual hemifield)

right nasal retina and left temporal retina left optic tract

left dLGN left cerebral hemisphere

www.dreamstime.com

target in left half of visual field (left visual hemifield) left

nasal retina and right temporal retina right optic tract

right dLGN right cerebral hemisphere

www.dreamstime.com

Visual Projection Pathway

Object in right visual

hemifield projects to:

- right nasal hemiretina

- left temporal

hemiretina

- left cerebrum

Object in left visual

hemifield projects to:

- left nasal hemiretina

- right temporal

hemiretina

- right cerebrum

Key Points from Previous Section

Fibers projecting from retina are organized in a

specific manner such that nasal fibers cross over

at chiasm, temporal fibers remain ipsilateral

This “retinotopic” organization continues to the

visual cortex, so each cerebral hemisphere

receives visual input from contralateral visual field

Assessing Visual Pathways

Brainstem reflexes:

- pupillary light reflex incl. chromatic

testing (past lecture)

- (optic) dazzle reflex (this lecture)

Optokinetic reflex (next lecture)

Electroretinogram

Visual-evoked response(Dr. Ron Ofri)

Cortical lesion = PLR/Dazzle normal; Vision Abnormal

Afferent Arm Lesion Before LGB = PLR/Dazzle and Vision Abnormal

Brainstem Reflexes and Visual Pathways

cortex

brainstem

(Optic) Dazzle Reflex Test

partial eyelid blink in response to bright light

- subcortical reflex

- eyelids may open then close

- contralateral closure < or sometimes absent

afferent = optic nerve (CN II) to rostral colliculi

(relay center for vision-associated somatic

reflexes)

efferent = facial n (CN VII)

present 1-2 days postnatally in dogs/cats

LGB

Rostral ColliculiPretectal Nuclei

CN VII

CN VII

nuclei

Obicularis oculi mm.

DAZZLE REFLEX

CN II

The Dazzle Reflex:

Ophthalmologists v. Neurologists

Ophthalmologists often use

to assess retinal function

with opaque ocular media

Neurologists often downplay

this reflex or use it only to

assess general level of

consciousness….not cited in

de Lahunta’s text

Progressive Retinal Atrophy/SARDS

Video courtesy Dr. Andras Komaromy

High intensity blue light elicits dazzle reflex = ipRGCs

Interpreting the Dazzle Reflex

In primates, dazzle reflex

not elicited with normal

flashlight (penlight), but

consistently present with

intense white light

Can intense white light

elicit a dazzle reflex

through ipRCG?

NC State Dazzleometer

Rostral Colliculi - Mediated Reflexes

Optic dazzle reflex

Coordination of eye movements in response

to visual stimulus (rostral colliculi to CN III,

IV, VI)

Turning of head and neck in response to

visual stimulation (motor fibers in spinotectal

tract)

Reticular activation system (activates cortex)

Assessing Vision and Visual Pathways

menace response

obstacle or maze course- A novel method for objective vision testing in canine models of

inherited retinal disease Invest Ophthalmol Vis Science 2008; 49:

3568-3576.

visual placing reaction

- hold dog in air, advance to table edge...both forelegs will

extend

visual cliff

- clear plexiglas extended over edge of table…visual animal

stops at table edge

- large animals = present birth

- dogs/cats = 4 weeks

patching or occluding for unilateral vision loss

Menace Response

Cortically mediated eyelid closure +/- head

withdrawal and globe retraction

- complex “response” not reflex

Pathway involves all 5 divisions of the brain:

- Retina optic nerve/tract/radiation visual cortex

intercerebral cortices pathways motor cortex

descending pathways to brainstem CN VII

Undefined connection with cerebellum:

- may result from pathway passing through cerebellum OR

from loss of cerebellar facilitation/modulation of motor

cortex

LGB

Visual CortexMotor Cortex

CN VII

Cerebellum

CN VII Nuclei

Visual inputObicularis

oculi mm

Menace Response - Limitations

False positive response – wind current

Uncooperative patient, cats

Closest equivalent in humans is “hand

motion”…visual acuity is 20/20,000 on

Snellen Eye Chart

Key Points from Previous Sections

Visual pathway lesions in the brainstem generally affect vision and

PLR/dazzle whereas cortical lesion affects vision but not

PLR/dazzle

The optic dazzle reflex is mediated by the rostral colliculi in the

mid-brainstem; may be elicited through ipRGCs

The rostral colliculi also mediates reflexive changes in conjugate

gaze and reflexive turning of head in response to visual stimuli

The menace response pathway involves all 5 major divisions of

the brain, but is a poor indicator of visual acuity

Automated Visual Field Testing

www.meadowsretina.comThis is a map of what can

be seen (blue) and not

seen (black) by patient, not

a map of the affected retina

E

Hemianopia (vision loss/deficit in ½ of

visual field in one or both eyes)

homonymous hemianopia:

- loss of one hemivisual field (e.g. loss of left visual field from loss of nasal retinal fibers in left eye and temporal fibers in right eye)

- *any unilateral lesion caudal to chiasm

www.ttuhcs.edu

+/- E

Characteristics of Visual Pathway and

Menace Pathway* Lesions

Retina

Pre-chiasmal optic nerve

Optic chiasm

Optic tract

Optic radiation

Occipital (visual) cortex

Parietal and frontal (motor) lobe*

Cerebellum*

Unilateral

Prechiasmal

Lesion

effects on PLR

ipsilateral visual deficits (menace response)

ipsilateral optic dazzle deficits

Enrofloxacin

toxicosisOptic neuritis

www.ttuhcs.edu

Optic Chiasmal Lesions

total lesions cause bilateral

PLR and visual deficits

PLR deficits may be

recognized before visual

deficits

proximity to

hypothalamus/pituitary

gland = abnormalities in

behavior, appetite,

temperature regulation,

endocrine function and

visceral motor activities

Left Right Left Right

Central Chiasmal Lesions

common in humans

with pituitary

macroadenomas

may cause

heteronymous

hemianopia e.g. only

crossing fibers

affected:

- bitemporal hemianopia

…crossing fibers from

nasal hemiretina

involved, loss of both

temporal visual fields

Left Right

E

www.ttuhcs.edu

bitemporal

hemianopia

Chiasmal lesion where

crossing fibers affected

E

Pituitary Gland/Optic Chiasm

Humans = pituitary stalk directed

rostroventrally, main mass of gland directly

below chiasm (10mm), macroadenomas

often involve central chiasm and are a

common cause of vision loss

Dogs = pituitary stalk directed

caudoventrally, main mass of gland is

caudal (posterior) to chiasm,

macroadenomas or other tumors

uncommon cause of vision loss

Misrouting of Visual

Pathways in Siamese Cats

Siamese cats and White Tigers (partial albinism)

Albino animals

Chediak-Higashi syndrome

some temporal retinal fibers cross over causing visual ambiguity

binasal heteronymous hemianopia (deficit in both temporal hemiretinas)

convergent strabismus (esotropia) and nystagmus may be attempts to compensate for visual ambiguity

Unilateral Optic Tract Lesions

effects on PLR (more dilated pupil contralateral to lesion)

contralateral visual field affected:

- contralateral homonymous hemianopia

- vision loss most obvious in eye contralateral to lesion

visual field testing performed by directing menace response stimuli from lateral and medial to midline

Left Left

75%

fibers OS

25%

fibers OD

www.ttuhcs.eduE

optic tract

Lesions in Optic Tract

proximity to internal capsule (ascending and descending fibers to and from cortex)

concurrent contralateral postural reaction deficits with normal gait (proprioceptive pathways)

Bilateral Retina, Optic Chiasm

Optic Nerve or Tract Lesions bilateral mydriasis,

PLR deficits, visual

deficits

commensurate

with lesion

lesions in both

retinas more

common than (>)

chiasm > both

optic nerves >

caudal

commissure >

optic tracts

Lesions of Optic Radiation

No effect on PLR

Contralateral

homonymous

hemianopia*

*As all unilateral, post

chiasmal lesions….i.e.

contralateral visual

field lost

Left Left

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Optic radiation

E

Lesions of Optic Radiation

proximity to caudal

limb of internal

capsule (ascending

and descending fibers

from cortex):

- complete contralateral

homonymous

hemianopia

- contralateral

hemiplegia and

hemianesthesia

Unilateral Visual Cortex Lesions

no effects on PLR

complete,

contralateral

homonymous

hemianopia

*as with all unilateral

postchiasmal lesions

www.ttuhcs.eduE

Visual cortex

Visual Cortex Projections

To opposite visual

cortex via corpus

callosum

To motor cortex of both

cerebral hemispheres

To cerebellum

To rostral midbrain

- rostral colliculi and to

CN III, IV, VI

Visual Cortex Connections

(de Lahunta)

Visual cortex and rostral midbrain (rostral colliculi, CN III, IV,

VI nuclei, gaze centers) have extensive interaction in

mediating visually guided behavior:

- Total bilateral rostral colliculectomy caused inattention to all visual stimuli and

loss of visual placing and menace

- Mesencephalic tegmentum (CN III, IV, VI, and gaze centers) lesions cause

loss of visual perception of movement and spatial orientation

- Unilateral lesion of the tegmentum cause contralateral deficit and postural

dystonia (severe torsion of head) from vision loss

- Bilateral removal of visual cortex, cats can still detect objects in lateral

visual field via direct rostral collicular projections from eye

E

Bilateral Visual Cortex Lesions

complications of anesthesia

(hypoxia)

- implicated with use of mouth

gags in cats (restriction of blood

flow to cerebrum)

(Vet Rec 2012;193;p367-373)

metabolic storage diseases in dogs

and cats

polioencephalomalacia (thiamine

deficiency)

lead intoxication in ruminants

cranial trauma (ischemia, humans)

Hubel and Wiesel

Neurophysiologists who studied structure and

function of visual cortex

1981 Nobel Prize in Physiology or Medicine

Their work is considered one of the greatest

contributions to neurobiology of the 20th century

https://www.youtube.com/watch?v=IOHayh06LJ4

Not E!

Parietal and Frontal Lobe Lesions

menace deficit in contralateral visual field

no loss of vision

Menace pathway

Cerebellar Lesions

Contralateral or

bilateral mydriasis

with normal PLR

Ipsilateral or bilateral

absence of menace

response with normal

vision

Menace pathway

Key Points from Previous Section

Any unilateral lesion caudal to the chiasm will produce a homonymous

hemianopia

Lesions in the optic tract and optic radiation are generally associated

with other neurologic deficits (proprioceptive and/or gait)

Siamese cats have a hemianopia caused by some temporal retinal

fibers crossing over at the chiasm, resulting in visual ambiguity, medial

strabismus and nystagmus

Use of mouth gags in cats may cause visual cortex hypoxia

Parietal lobe, frontal lobe, and cerebellar lesions may cause menace

response deficits without loss of vision

Hubel and Wiesel rock!

QUESTIONS?