Afferent Visual System

1. Afferent Visual System Dr. Daniel Fok PGY – 4 Adult Neurology

2. Outline • Anatomy of the afferent visual system • Eye • Optic nerve and tract • Cortex • Clinical History • Clinical Examination

3. Eye • To reach the retina, light must pass through the: • Ocular media • Tear film • Cornea • Anterior chamber • Lens • Posterior chamber vitreous humour • Ciliary muscle adjusts the shape of lens to focus light optimally from varying distances on retina (accommodation) • Visual image projected upside-down and backwards onto retina. Fig 1.1 The Eye. Prasad S, Galetta SL. Anatomy and physiology of the afferent visual system. Handbook of Clinical Neurology, Neuro-Ophthalmology Vol 102. Chapter 1. Elsevier BV 2011.

4. Vascular Supply of Eye • Majority of blood supply comes from the ophthalmic artery, the first branch of the internal carotid artery • Two groups of vessels: • Globe • Central retinal artery, muscular artery, anterior ciliary arteries, long and short posterior ciliary arteries • Other structures • Lacrimal artery, supraorbital artery, ethmoidal arteries, frontal artery, nasal artery.

5. Retina Fig 1.2 . Prasad S, Galetta SL. Anatomy and physiology of the afferent visual system. Handbook of Clinical Neurology, Neuro-Ophthalmology Vol 102. Chapter 1. Elsevier BV 2011.

6. Photoreceptors 4 types of photoreceptors in humans: 3 cones and the rods Rods are saturated at natural light intensities and is better for scotopic vision The Macula is located temporal to the optic nerve and is approx. 5.5mm in diameter Within the macula is the fovea which is 1.5mm in diameter and the foveola. Fovea has 200 000 cones/mm3 Fig 1.3 Prasad S, Galetta SL. Anatomy and physiology of the afferent visual system. Handbook of Clinical Neurology, Neuro-Ophthalmology Vol 102. Chapter 1. Elsevier BV 2011.

7. Ganglion Cells • Three main types of ganglion cells • 80% midget cells – P (parvocellular) pathway • 10% parasol cells – M (magnocellular) pathway • 10% other cells • Small bistratified ganglion cells K (koniocellular) pathway Fig 1.6 The Eye. Prasad S, Galetta SL. Anatomy and physiology of the afferent visual system. Handbook of Clinical Neurology, Neuro-Ophthalmology Vol 102. Chapter 1. Elsevier BV 2011.

8. Ganglion Cell Types • Cones = Midget cells = Parvocellular pathway • Rods = Parasol cells = Magnocellular pathway Table 1.1 Prasad S, Galetta SL. Anatomy and physiology of the afferent visual system. Handbook of Clinical Neurology, Neuro-Ophthalmology Vol 102. Chapter 1. Elsevier BV 2011.

9. Optic Nerve • 1.2 million retinal ganglion cell axons per optic nerve. • Optic nerve head = optic disc – 3-4 mm nasal to the fovea and is 1mm thick • Travels posteriorly through lamina cribrosa to exit back of globe. It then becomes myelinated and invested with meninges. • Exiting the orbit, it enters the optic canal within the lesser wing of the sphenoid bone for 6mm. Intracanalicular optic nerve rises at 45 degree angle and then exists the optic canal for 17 mm before reaching the chiasm Fig 1.7 Prasad S, Galetta SL. Anatomy and physiology of the afferent visual system. Handbook of Clinical Neurology, Neuro-Ophthalmology Vol 102. Chapter 1. Elsevier BV 2011.

10. Vascular Supply of Optic Nerve All of the blood supply is ultimately derived from the ophthalmic artery The anastomotic circle of Zinn-Haller provides circulation to the optic nerve head and is supplied by the posterior ciliary arteries, the pial arteriole plexus and the peripapillary choroid Prasad S, Galetta SL. Anatomy and physiology of the afferent visual system. Handbook of Clinical Neurology, Neuro-Ophthalmology Vol 102. Chapter 1. Elsevier BV 2011.

11. Optic Chiasm • Chiasmal decussation brings together information from the halves of each retina that view the same portion of the visual field. • Axons from nasal ganglion cells cross and join axons from temporal ganglion cells from the contralateral eye. 53% of fibers are crossed, 47% are uncrossed. Prasad S, Galetta SL. Anatomy and physiology of the afferent visual system. Handbook of Clinical Neurology, Neuro-Ophthalmology Vol 102. Chapter 1. Elsevier BV 2011.

12. Optic Chiasm • Among crossing (nasal) fibers, those originating in the macula lie in a superoposterior position within the chiasm. • Axons from the inferior nasal retina may bend slightly forward into the contralateral optic nerve, forming Wilbrand’s knee. The existence of Wilbrand’s knee is controversial.

13. Optic Tracts • Visual field and retina have inverse and reverse relationship (‘upside-down and backwards’) • Nasal fibers of the ipsilateral eye cross in the chiasm and join uncrossed temporal fibers of the contralateral eye and form the optic tract, which synapses in the lateral geniculate nucleus (LGN), form the optic radiations and terminate in the visual cortex (V1, Brodmann area 17) of the Occipital Lobe • Inferonasal retinal fibers decussate in the chiasm and travel anteriorly in the contralateral optic nerve before passing into the optic tract – forming Willbrand’s Knee Prasad S, Galetta SL. Anatomy and physiology of the afferent visual system. Handbook of Clinical Neurology, Neuro-Ophthalmology Vol 102. Chapter 1. Elsevier BV 2011.

14. Lateral Geniculate Nucleus Prasad S, Galetta SL. Anatomy and physiology of the afferent visual system. Handbook of Clinical Neurology, Neuro-Ophthalmology Vol 102. Chapter 1. Elsevier BV 2011.

15. Lateral Geniculate Nucleus • 5-10% of the synapses in the LGN are for retinal afferents. • LGN also receives extensive modulating connections from the thalamic reticular nucleus and layer 6 of the visual cortex • Pulvinar nucleus forms higher order relay receiving extensive descending cortical projections from both layers 5 and 6 of the visual cortex.

16. Superior Colliculi Deep layers received multimodal sensory inputs and help mediate saccadic eye movements through their efferent connections to the ocular motor system. Also receives reciprocal connections from cortical areas involved in saccade generation • Superior colliculi plays a role in generating orienting eye and head movements to sudden visual (and other sensory) stimuli. • Organized in superficial and deep layers • Superficial: solely process visual information with direct retinal inputs with a visuotopic map of the contralateral field • Efferent connections to thalamnic nuclei which are then relayed to cortical visual areas.

17. Pretectal Nuclei • A portion of the fibers in the optic tract subserve the pupillary light reflex and synapse at the pretectal nuclei in the midbrain through the brachium of the superior colliculi. • There is consensual innervation to both pretectai nuclei and each pretectal nucleus has dual connections to each Edinger-Wesetphal nucleus. • The Edinger-Westphal nuclei give rise to parasympathetic efferent fibers which travel with the oculomotor nerve and regulate pupillary size via pupillary constrictors.

18. Suprachiasmatic Nucleus • There is a retinal ganglion cell that contains photpigmentmelanopsin and demonstrates intrinsic responsiveness to light that is not mediated by rods or cones. These ganglio cells give rise to a separate unmyelinated pathway through the optic chiasm and tracts and transmits light information directly to the suprachiasmatic nucleus at the base of the anterior hypothalamus.

19. Optic Radiations • 2nd order neurons from the LGN to calcarine cortex. • Two major bundles: • Temporal radiations which take an anterior course through the temporal pole, termed Meyer’s Loop • Represent contralateral superior field • Parietal radiations represent the contralateral inferior field Prasad S, Galetta SL. Anatomy and Physiology of the Afferent Visual System. Handbook of Clinical Neurology, Vol I02. 2011 • Temporal radiation blood supply: anterior choroidal, proximal branches of MCA: lenticulostriate and inferior temporo-occipital arteries • Parietal radation blood supply: distal branches of MCA: angular artery, posterior temporal arteries. • Distal portions of optic radiations before entry into visual cortex supplied by superior tepmoro-occipital branch of the MCA and anterior temporal and calcarine branches from the PCA.

20. Calcarine Cortex (Primary Visual Cortex) Prasad S, Galetta SL. Anatomy and Physiology of the Afferent Visual System. Handbook of Clinical Neurology, Vol I02. 2011

21. Calcarine Cortex Prasad S, Galetta SL. Anatomy and Physiology of the Afferent Visual System. Handbook of Clinical Neurology, Vol I02. 2011

22. Occipital Lobe Vascular Supply Prasad S, Galetta SL. Anatomy and Physiology of the Afferent Visual System. Handbook of Clinical Neurology, Vol I02. 2011

23. Steroscopic Vision • Monocular inputs to the primary visual cortex are arranged in ocular dominance columns. The two eyes have a different view of visual space, resulting in a slight displacement of their respective retinal images. Prasad S, Galetta SL. Anatomy and Physiology of the Afferent Visual System. Handbook of Clinical Neurology, Vol I02. 2011

24. V1 Primary Visual Cortex • V1 neurons are selective for specific orientations of luminance contrast, forming the basis of image contour analysis. There is initial processing of colour composition, brightness and direction of motion as well.

25. Higher Order Visual Cortex Anatomy and Syndromes

26. Calcarine Cortex (Primary Visual Cortex) Walsh and Hoyt’s Clinical Neuro-Ophthalmology. 2005. Spencer S. Eccles Health Sciences Library.

27. Anton’s Syndrome • Anton-Babinski Syndrome, Cortical Blindness • Bilateral lesions in the Primary Visual Cortex • Complete visual loss on confrontation testing with anosognosia • Loss of blink to threat, loss of eye closure in response to bright lights, loss of OKN

28. Higher Order Visual Cortex • Neurons in higher visual areas have expanded receptive fields that span both hemifields. • Two dichotomous processing streams • Ventral “What” Pathway – Visual recognition of objects • Dorsal “Where” Pathway – Spatial relationships among objects. • Also involved with grasping or manipulating objects

29. Higher Order Visual Cortex • Lateral Occipital Area • Ventral Occipital temporal Cortex • Fusiform gyrus • Parahippocampal area • Connections also to: • Prefrontal Cortex • Amygdala

30. Introduction • Diverse types of visual functions are affected by extrastriate lesions, which are grouped into two broad families: • Ventral Occipitotemporal Cortex: “What” Ventral Stream Pathway • Intermediate level of this stream is involved in colour processing • Lesions here will give achromatopsia • Higher level recognition deficits can lead to: • General Visual Agnosia • Prosopagnosia • Pure alexia • Topographagnosia

31. Introduction • Dorsal Occipitoparietal Cortex “Where” Dorsal Stream Pathway • Intermediate level of this stream is involved with motion processing • Lesions here will give akinetopsia • High level deficits can lead to • Balint Syndrome: Simultagnosia, Optic Ataxia and Oculoar motor apraxia • Cortical Stereoblindness

32. Syndromes of the Ventral Stream Achromatopsia Apperceptive Visual Agnosia Associative Visual Agnosia

33. Achromatopsia • Acquired brain lesions can impair colour vision • Complete impairment is called achromatopsia • Patients see the world in grayscale • Partial defect is called dyschromatopsia • The world takes on a specific tint • Lesions in: • Occipitotemporal cortex • Bilateral lesions of the lingual gyri • Unilateral lesions may have a hemiachromatopsia • Often associated with superior field defects and higher order problems of recognition such as • Prospagnosia • Topographagnosia Barton JS. Higher Cortical Visual Deficits: Continuum Review Article. Continuum (MinneapMinn) 2014; 20(4):922 – 941.

34. Achromatopsia • Patients with achromatopsia may still be able to read Ishihara plates. • Best tests for achromatopsia involve the Farnsworth 100 Colour test or the D15, or the HRR Plates

35. General Visual Agnosia • Trouble recognizing objects. • If the object they cannot name is made to emit a sound or given to them to touch, they will recognize it, showing that the problem is limited to vision. • Apperceptive vs Associative

36. Apperceptive Visual Agnosia • Unable to form accurate visual representations of objects in their brain and without that information they cannot determine what the objects are. • Different forms: • Visual Form Agnosia – Able to perceive the shape of objects is impaired; may have difficulty identifying elementary geometric forms like triangles and squares. Can not copy a drawing of an object. • Seen with CO poisoning affecting occipital cortex, lateral occipital cortex. Also seen with posterior cortical atrophy and bilateral occipital ischemia

37. Apperceptive Visual Agnosia • Integrative agnosia – Perceive shape reasonable well; they can match similar shapes. Problem lies in linking or grouping the parts of an object to see the whole item, especially if the object is large and complex. Patients can get derailed by intersections of lines in drawings with overlapping figures, etc… • Seen with Posterior cortical atrophy or bilateral occipital infarctions • Transformation agnosia – failure to recognize objects that are shown from unusual viewpoints; have difficulty creating or using a mental representation of the three-dimensional shape of an object.

38. Associative Visual Agnosia • Form fairly accurate visual representations, but have difficulty matching this information to stored knowledge about what objects look like or their use. • Has trouble relating items with pictures, but can do so if told the names of objects instead. • Recognition of living organisms can be more affected than inanimate objects • Tested by Pyramid and Palm Trees Test: • Patients have to determine which two of three pictured items are related (ie. Hammer, screw driver, hockey stick). • Localization: Left or bilateral lesions of the parahippocampal, fusiform and lingual gyri

39. Associative Visual Agnosia • Prosopagnosia • Inability to recognize faces that were either previously known to the patient or recently learned. • Learn to recognize people by other cues (heavily on voices) • Examination: administer basic object recognition test that the patient does not have a general visual agnosia. Boston Naming Test is useful • Then confirm preserved knowledge about people. • Patients who cannot recognize faces or voices, and have reduced semantic knowledge about people have a multimodal person semantic deficit that is usually associated with anterior temporal lesions. • Test facial recognition with celebrities and anonymous faces and see what looks familiar. Prosopagnosia has reduced ability to make this distinction. Can also use the Warrington Recognition Memory Test • If the patient can state which faces are familiar but have trouble providing the names belonging to those faces, the diagnosis is prosopanomia, a much rarer condition.

40. Associative Visual Agnosia • Prosopagnosia • Lesions in the fusiform gyri or anterior temporal lobe; typically right or bilateral. • Benton Facial Recognition Test, Cambridge Face Perception Test. • Apperceptive variant – trouble perceiving the complex three-dimensional geometry of the face. • Associative/Amnestic Variant – have accurate perception and score well on the above tests. They have difficulty matching what they see and what they know.

41. Associative Visual Agnosia • Pure Alexia (Alexia without Agraphia) (Pure Word Blindness) • Reading network more heavily on the dominant hemisphere, and face network more heavily on the non-dominant hemisphere. • Lesions of the right fusiform gyrus cause prosopagnosia, lesions of the left fusiform gyrus cause alexia. • Alexia = loss of prior reading skill. • Can comprehend speech, converse with others normally, can write. • Some patients with alexia have a mild degree of anomia and a surface dysgraphia • Spelling mistakes with irregular works like yacht and colonel.

42. Associative Visual Agnosia • Pure Alexia • Always associated with left occipitotemporal lesions. • Most also have possible damage to the callosal fibers.

43. Associative Visual Agnosia • Topographagnosia • Get lost in familiar surroundings. • Landmark Agnosia has been associated with medial occipitotemporal lesions, particularly on the right (parahippocampal place area) • Difficulty recognizing landmarks • Some patients may recognize landmarks but have trouble encoding the spatial relationships between them in the cognitive map. Associated with involvement in the hippocampi and retrosplenial cortex.

44. Associative Visual Agnosia • Egocentric Disorientation: Loss of the ability to nagivate by using a sequence of verbal directions, requiring the sense of how one is oriented spatially relative to the environment.

45. Syndromes of the Dorsal Stream Akinetopsia Balint Syndrome Astereognosis

46. Akinetopsia • Lesion in the middle temporal area/visual cortical area 5 (V5). • Full blown akinetopsia appears with bilateral involvement of V5 in the lateral occipitotemporal cortex. • Unilateral lesions are usually asymptomatic, but with special computerized motion tests, they have impairments in the contralateral hemifield.

47. Balint Syndrome • Triad of: • Simultanagosnia • Optic Ataxia • Ocular Motor Apraxia • Localization: Bilateral Occipitoparietal lesions.

48. Simultanagnosia • Failure of the ability to pay attention to more than one object at a time. • If a display contains two items, the patient with simultanagnosia may be aware of only one. Even if both are located in the same place. • Examiner holds up a pen placed on top of a finger, the patient may be aware of the pen but not the finger. • Can test with a Starry Night Display Test, Cookie theft picture, and showing pictures of local and global capture.

49. Simultanagnosia • Linked to lesions of the medial occipitoparietal junction, cuneus, and intraparietal sulcus as well as a number of white matter tracts linking a visual attention network. Barton JS. Higher Cortical Visual Deficits: Continuum Review Article. Continuum (MinneapMinn) 2014; 20(4):922 – 941.

50. Optic Ataxia • Inaccurate reaching to targets under visual guidance. • Patients should be able to reach targets whose spatial coordinates are derived from other senses (ie. Parts of their own bodies) • Degraded analyses of position of objects in space • Lesions in the junction of Inferior Parietal Lobule and Superior Occipital Cortex. • Testing: accurate pointing to objects to the right or left of center with right and left hand (Deficit should be in the contralateral hemifield of lesion)