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
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, 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
Optic Radiation
Lateral Geniculate Body www.studyblue.com 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 Temporal Retina Retina
Fibers Fibers Decussate Remain Ipsilateral OD Visual Field OS Total visual field OD temporal nasal hemifield hemifield
temporal nasal fibers fibers Nasal Temporal Retina = Retina = Temporal Nasal hemifield hemifield
Fibers Fibers Decussate 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
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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 Visual Projection Pathway
Object in right visual Object in left visual hemifield projects to: hemifield projects to: - right nasal hemiretina - left nasal hemiretina - left temporal - right temporal hemiretina hemiretina - left cerebrum - 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 (Dr. Ron Ofri) Visual-evoked response Brainstem Reflexes and Visual Pathways
Afferent Arm Lesion Before LGB = PLR/Dazzle and Vision Abnormal
brainstem
Cortical lesion = PLR/Dazzle normal; Vision cortex Abnormal (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 DAZZLE REFLEX
Obicularis oculi mm.
CN VII LGB Pretectal Nuclei Rostral Colliculi CN II CN VII nuclei 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 Visual Cortex Motor Cortex
Cerebellum LGB
CN VII Nuclei Visual input Obicularis oculi mm CN VII 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 E
This is a map of what can www.meadowsretina.com be seen (blue) and not seen (black) by patient, not a map of the affected retina Hemianopia (vision loss/deficit in ½ of visual field in one or both eyes) +/- E
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
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 Optic neuritis toxicosis www.ttuhcs.edu Optic Chiasmal Lesions
Left Right Left Right 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 Central Chiasmal Lesions E
Left Right 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 bitemporal hemianopia
Chiasmal lesion where crossing fibers affected
E www.ttuhcs.edu Pituitary Gland/Optic Chiasm
Humans = pituitary stalk directed Dogs = pituitary stalk directed rostroventrally, main mass of gland directly caudoventrally, main mass of gland is below chiasm (10mm), macroadenomas caudal (posterior) to chiasm, often involve central chiasm and are a macroadenomas or other tumors common cause of vision loss 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) Left Left
contralateral visual field affected: - contralateral 75% homonymous hemianopia fibers OS 25% - vision loss most obvious in fibers OD eye contralateral to lesion
visual field testing performed by directing menace response stimuli from lateral and medial to midline optic tract
E www.ttuhcs.edu 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
Left Left No effect on PLR
Contralateral homonymous hemianopia*
*As all unilateral, post chiasmal lesions….i.e. contralateral visual field lost Optic radiation
E www.ttuhcs.edu 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 Visual cortex
E www.ttuhcs.edu 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 E (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 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) Not E! 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 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 Menace pathway response with normal vision 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?