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Visual Pathways 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 <[email protected]> Vision in Animals 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 www.dreamstime.com 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
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