Visual pathways

Question 1

Overview of the visual pathway

Answer 1

• Begins with neural retina forming the optic nerve
• Partial decussation of fibers at optic chiasm leading to two optic tracts
• Optic tracts synapse onto the lateral gesticulate nucleus (LGN) in the thalamus
• Axons of the LGN form the optic radiations and terminate in the primary visual cortex (v1) of the occipital lobe

Question 2

3 functional layers of the retina

Answer 2

• Photoreceptors
• Bipolar cells and integrators (amacrine, horizontal)
• Retinal ganglion cells (form optic disc and axons make optic nerve)

Question 3

Retinal quadrants and visual fields (VFs)

Answer 3

• Two visual fields (one from R and L eye) and each VF is broken up into 2 halves: nasal (medial) and temporal (lateral)
• Each half is broken up into quadrants upper and lower
• Thus each VF has an upper nasal, an upper temporal, a lower nasal, and a lower temporal
• Two eyes means that the two VFs overlap to produce a single binocular field in the center and two peripheral rims of vision on both temporal extremes (of the VF)

Question 4

Inversion and reversal of the visual fields

Answer 4

• The right half of both visual fields is picked by the temporal side of the left eye and the nasal side of the right eye (left sides of both retinas)
• The left half of both VFs is picked up by the temporal side of the right eye and nasal side of the left eye (right sides of both retinas)
• This is reversal (things to the right are picked up by the left sides of each eye)
• The upper quadrants of each VF are picked up by the lower 1/2 of each retina
• The lower quadrants of each VF are picked up by the upper 1/2 of each retina
• This is inversion (things in the upper part of VF are picked up by the lower part of each retina)

Question 5

Optic nerves, chiasm, and tracts 1

Answer 5

• Each optic nerve carries both halves of the VF for that eye
• Therefore damage to an optic nerve results in loss of all vision in the ipsilateral eye (anopsia)
• Nasal fibers of each optic nerve (carrying info about the temporal 1/2 of each VF) cross at the optic chiasm (53%), temporal fibers remain uncrossed
• Therefore damage to the medial part of optic chiasm only affecting crossing fibers leads to loss of temporal half of both VFs (bitemporal hemianopsia)

Question 6

Optic nerves, chiasm, and tracts 2

Answer 6

• Each optic tract contains uncrossed fibers from the temporal half of the ipsilateral retinal (carrying the nasal half of the VF) and the crossed fibers from the nasal half of the contralateral retina (carrying the temporal half of the VF)
• At this point all info in one optic tract covers the same half of both visual fields (i.e. the right optic tract contains the L VFs for both the L and R eyes)
• Damage to an optic tract results in homonymous hemianopsia, where both eyes lose the L VF (if the R optic tract is damaged)

Question 7

Lateral genticulate nucleus

Answer 7

• LGN receives the axons from optic tracts, layers 2, 3, 5 (prime numbers) are the ones that receive info from ipsilateral eye (about contralateral VF)
• The axons that do not end on the LGN will mediate visual reflexes by synapsing on the brainstem
• In the LGN the fibers from the lower retinal quadrants (containing info about the upper quadrants of the VFs) are laterally to those form the upper retinal quadrants (which contain info about the lower quadrants of the VFs)
• Macular fibers are in the center of the peripheral fields
• Axons project to cortex as geniculocalcarine tract or optic radiations

Question 8

Optic radiations 1

Answer 8

• After leaving the LGN the optic radiations pass thru the posterior limb of the internal capsule for a short distance (posterior to the sensory and motor fibers)
• After the IC the fibers spread out in the white matter of the temporal and parietal lobes (breaks each VF half into quadrants- upper and lower)
• The fibers that run along the parietal lobe (superior fibers) project to v1 and are derived from the upper quadrant of the ipsilateral side of each retina (thus contain info about the contralateral lower quadrant of each VF)
• Therefore damage to the superior fibers result in loss of vision in the lower contralateral VF of each eye (contralateral inferior quadrantanopsia)

Question 9

Optic radiations 2

Answer 9

• The inferior fibers that run through the temporal lobe (around the inferior horn of the LV forming the loop of Meyer) project to v1 and are derived from the lower quadrant of the ipsilateral side of each retina (and thus contain info about the contralateral upper quadrant of each VF)
• Therefore damage to the inferior fibers result in loss of vision in the upper contralateral VF of each eye (contralateral superior quadrantanopsia)
• Lesions to both superior and inferior fibers (or v1) leads to contralateral hemianopsia, where the entire contralateral half of the VFs for each eye is lost (contralateral to side of lesion)

Question 10

Primary visual cortex (v1) 1

Answer 10

• Optic radiations from one side innervate the same side of v1 (do not decussate), meaning that each v1 receives info about the VFs of the contralateral side
• The cortex occupies the gyro within and adjacent to the calcarine sulcus on the medial surface of the occipital lobe
• Axons terminating in the upper calcarine sulcus are derived from the superior fibers from that side (i.e. have info about the lower contralateral quadrant of the VFs)

Question 11

Primary visual cortex (v1) 2

Answer 11

• Axons terminating in the lower calcarine are derived from the inferior fibers from that side (i.e. have info about the upper contralateral quadrant of the VFs)
• Macula is represented most posteriorly and the peripheral fields are more anterior (large cortical devotion to macula/fovea since it is the highest resolution)
• The cortex functions in visual perception and initial processing of visual info (sends out fibers to association cortices)

Question 12

Association cortices for vision

Answer 12

• Visual association cortex found in the medial and lateral surface of occipital lobe, receiving input from v1
• Functions of major attributes of visual stimuli (form, color, depth, location, motion
• Additional visual association areas receive input from visual association cortex, there are 2 additional visual association areas
• The first is the ventral pathway (what pathway- P cells) where the perception of shape and color are used for object ID
• The second is the dorsal pathway (where pathway- M cells) which analyzes motion and location of objects in the visual field
• P and M cells divide from each other at LGN

Question 13

Terms for visual defects

Answer 13

• Anopsia: loss of vision in entire field
• Hemi: loss of vision in 1/2 of a VF
• Bi: loss of vision in 2 parts of the VFs (usually bitemporal)
• Quadrant: loss of vision in 1/4 of VFs
• Homonymous: related to the same halves/quadrants of the two VFs (i.e. contralateral homonymous hemianopsia means both eyes lost info from 1/2 of their VF, with this loss being contralateral to the side of the lesion)

Question 14

General principles of lesions to eye pathways

Answer 14

• Lesions anterior to chiasm only affects corresponding eye
• Lesions posterior to chiasm affect both eyes, and of the homonymous side of the VFs
• Lesions at the chiasm affect both eyes but opposite sides of the VFs

Question 15

Damage to retina

Answer 15

• Focal damage results in scotomas (small blind spots) which may go unnoticed due to binocular compensation. Causes include degenerative diseases, infarcts, hemorrhage, diabetic neuropathy, infection
• Papilledema: edema of the optic disc due to increased intracranial pressure. Seen as blurring of the optic disc (mild) or obliteration of the vessels at the disc margin and hemorrhage (severe)
• Retinal detachment (often diabetic neuropathy) causes blindness

Question 16

Lesions of the optic nerve

Answer 16

• Anterior to the optic chiasm, damages will only affect the corresponding eye, but will result in complete blindness (loss of all VF) for that eye (anopsia)
• Can distinguish from glaucoma or other intraocular optic nerves b/c those will respect the horizontal meridian where as lesions to the optic nerve will destroy the entire VF
• There will be APD (afferent pupillary defect)
• Common cause: ischemic infarction of the optic nerve

Question 17

Lesions to the chiasm

Answer 17

• Will affect VFs of both eyes, but since it only damages the crossing fibers (those from the nasal region of the retina), only the temporal VFs will be lost from each eye
• This results in bitemporal hemianopsia
• Common causes: pituitary adenoma, meningioma

Question 18

Lesions to the optic tract

Answer 18

• Rare but will produce deficits in both eyes, of both L or R halves of each VF (depending on which side the lesion is- the half that is lost is always contralateral to the side of the lesion)
• Leads to contralateral homonymous hemianopsia
• Causes: tumor, infarct, demyelinating disease
• Usually APD as well

Question 19

Lesions to optic radiations

Answer 19

• If the lesion is before the split of fibers (such as w/in the internal capsule), there will be the same effect as lesions to the optic tract
• Thus there will be contralateral homonymous hemianopsia
• This is usually associated w/ motor and sensory loss (3 H’s: hemiplegia, hemianesthesia, hemianopsia)

Question 20

Lesions to the superior or inferior fibers

Answer 20

• Temporal lobe pathology can lead to damage of the inferior fibers of the optic radiations
• Since the inferior fibers contain the info from the inferior ipsilateral quadrant of the retina, they thus contain info from superior contralateral quadrant of the VF
• Therefore damage to the inferior fibers leads to loss of vision in the superior contralateral VF, called contralateral superior quadranantopsia
• The superior fibers contain info from the superior ipsilateral quadrant of the retina, and thus contain info from the inferior contralateral quadrant of the VF
• Therefore damage to superior fibers results in loss of vision in the inferior contralateral VF, called contralateral inferior quadrantanopsia
• Due to tumors or infarcts of MCA

Question 21

Lesions to v1

Answer 21

• Lesions in v1 result in visual deficits that look almost identical to lesions in the optic tract (contralateral homonymous hemianopsia)
• The main difference is macular sparing due to anastomoses to the cortical regions for the macula from the MCA
• These result from lesions to the PCA

Question 22

Willinbrand’s knee 1

Answer 22

• The fibers from the inferior nasal area cross the chiasm and then run back anteriorly extending a little into the fibers of the contralateral optic nerve, then turn back and course into the optic tract
• A lesion of the optic nerve at the level of willinbrand’s knee produces a deficit in both eyes
• The optic nerve that is completely severed (ipsilateral to lesion) leads to complete anopsia of that eye

Question 23

Willinbrand’s knee 2

Answer 23

• The contralateral VF is mostly intact, except for these inferior nasal fibers that are damaged in the knee
• This damage leads to superior temporal field deficit in the contralateral eye’s VF
• This deficit is not the entire quadrant lost, but a portion of the superior temporal VF quadrant (due to damage of the inferior nasal retinal fibers)
• This is called a junctional scotoma

Question 24

Light reflex 1

Answer 24

• Shining a light onto one eye will constrict both pupils
• Light enters the optic nerves via both nasal and temporal retinal fibers
• The fibers split in the chiasm, so each tract has fibers carrying info about the light (the ipsilateral tract has temporal fibers and the contralateral tract has nasal fibers)
• These fibers do not synapse in the LGN, and instead they synapse in the pretectal nuclei at the level of the midbrain (above superior colliculi)

Question 25

Light reflex 2

Answer 25

- The axons of the pretectal nuclei project bilaterally (thru posterior commissure and periaqueductal gray) to the parasymp nuclei of III (Edinger-westphal) - This is the afferent limb of the light reflex - The efferent limb consists of parasympathetics from EW nuclei of III projecting to the ciliary ganglion, then to the sphincter muscles to constrict the pupils - Therefore, shining a light in one eye causes both pupils to constrict due to the bilateral innervation of EW by the pretectal nucleus

Question 26

Lesions affecting the light reflex

Answer 26

- Lesions to the right optic nerve leads to an absence of light reflex of both eyes when shining a light on the right eye - The light reflex will occur for both eyes when shining a light on the left eye - Lesions to the right CN III will lead to an absence of pupil constriction in the right eye, no matter where the light is being shined - Therefore the reflex is intact in the L eye when shining in the R eye - The reflex in the R eye is not intact when shining in the L eye (but the L eye reflex will be intact)

Question 27

Accommodation reflex 1

Answer 27

- Focusing on an object that is very close consists of 3 things: convergence of both eyes to midline (MR contracting), pupillary constriction (sphincter muscles contracting), and rounding of lens for focusing (contraction of ciliary body) - The afferent limb is CN II-> v1, and form v1 info goes to the integration center (accommodation center) - The accommodation center is not the pretectal nuclei, but also in the midbrain

Question 28

Accommodation reflex 2

Answer 28

- The efferent limbs consist of bilateral projections from the accommodation center to both EW nuclei and motor nuclei of III - The motor nuclei of III causes MR contraction and eye convergence - The EW projections of parasymp cause pupil constriction and contraction of ciliary muscles to relax and round the lens for focusing