Lecture 6: Visual Pathways and Eye Movements

Question 1

What is a visual field vs. retinal field?

Answer 1

Visual field: area that a person is able to see when both eyes are fixed in one position

Retinal field: light passes from objects in the visual field, through the pupil to subtend an image upon the retina

Question 2

In the visual field, the object of attention is focused and centered at which location?

Answer 2

Fovea centralis and macula lutea

Question 3

What is found medial to the macula, and what leaves this region; what is absent from this region?

Answer 3

• Optic disc
• Region where retinal axons leave the eye as the optic nerve
• No photoreceptors here, which creates our blind spot

Question 4

Visual fields are subdivided into what 2 zones?

Answer 4

1) Binocular zone, broad central region seen by both eye

2) Monocular zone (R/L), seen only by the corresponding eye

Question 5

The location on the retina that an object in the visual field is the retinal field, how is each visual field divided within the retinal field?

Answer 5

• Each visual field is divided into retinal hemifields (nasal and temporal halves of retina)
• Each hemifield is divided into upper and lower, quadrants

Question 6

The image formed on the retina is inverted how?

Answer 6

In both the lateral and vertical dimensions

Question 7

Explain how the left and right half of the visual field forms an image on the temporal and nasal half of each retina.

Answer 7

• Left half forms image upon the nasal (right) half of left retina and the temporal (right) half of the right retina
• Right half forms image upon the nasal (left) half of right retina and the temporal (left) half of the left retina

Question 8

Where do the optic nerves partially decussate; which parts cross?

Answer 8

• Optic chiasm
• Nasal half of each retina —> contralateral optic tract
• Temporal half of each retina —> ipsilateral optic tract

Question 9

What is the optic tract composed of?

Answer 9

Fibers from temporal retina (ipsilateral) eye + fibers from nasal retina (contralateral eye)

*Only the nasal fibers decussate here

Question 10

What is the purpose of partial decussation of the optic nerve at the optic chiasm?

Answer 10

• Necessary to delivery information from contralateral visual field to each optic tract
• Brings together information from comparable areas of both retinas —-> DEPTH PERCEPTION

Question 11

Optic tract curves posteriorly around the cerebral peduncle and terminates where?

Answer 11

Lateral geniculate nucleus (LGN)

Question 12

How many layers make up the lateral geniculate nucleus and how do the fibers of the optic tract terminate here?

Answer 12

• 6 layers w/ myelinated fibers sandwiched between them
• Fibers terminate in a precise retinotopic pattern

Question 13

What forms the ventral base, and dorsal/lateral borders of the lateral geniculate nucleus (LGN)?

Answer 13

• Ventral base is formed by the incoming optic tract (retinogeniculate) fibers
• Dorsal and lateral borders formed by the outgoing optic radiations

Question 14

What is the Magnocellular (M) layers of the lateral geniculate nucleus (LGN); contains what size cells; receives what inputs?

Answer 14

• Layers 1 and 2 (ventral)
• Contains large cells

- Receive ganglion cell inputs relaying from rods —> larger receptive fields and thick, rapidly conducting axons, sensitive to moving stimuli

Question 15

The Magnocellular (M) layers of the lateral geniculate nucleus is sensitive to what stimuli?

Answer 15

Sensitive to moving stimuli

Question 16

What are the Parvocellular (P) layers of the lateral geniculate nucleus; what size cells; what inputs does it receive?

Answer 16

• Layers 3-6 (dorsal)
• Contains small cells
• Receive ganglion cell inputs relaying from cone —> small receptive fields, slower conducting axons, tonically responsive to stationary stimuli, high-acuity vision

Question 17

Ganglion cell axons that arise in the temporal retina remain uncrossed and terminate in which layers of the LGN on which side?

Answer 17

• Layers 2, 3, and 5
• Ipsilateral LGN

Question 18

Ganglion cell axons that arise in the nasal retina cross and terminate in which layers of the LGN on which side?

Answer 18

• Layers 1, 4, and 6
• Contralateral side

Question 19

Looking at the RIGHT visual field, the nasal retina of the right eye and temporal retina of the left eye terminate in which layers and which LGN?

Answer 19

• Nasal (R)–> Layers1, 4, 6ofleft LGN
• Temporal (L) –> Layers 2, 3, 5 of right LGN

Question 20

Looking at the LEFT visual field, the nasal retina of the left eye and temporal retina of the right eye terminate in which layers and which LGN?

Answer 20

• Nasal (L) —> Layers 1, 4, 6 of right LGN
• Temporal (R) –> Layers 2, 3, 5 of right LGN

Question 21

How is the same point in visual space able to be represented six times?

Answer 21

• Once in each layer of the LGN
• Optic tract axons branch in multiple layers even though they arise from same visual field

Question 22

What forms Optic Radiations; what else is this pathway referred to as?

Answer 22

- Secondary neurons from the LGN extend a large bundle of myelinated fibers

• Also referred to as the geniculostriate or geniculocalcarine pathway

Question 23

Where do the secondary neuron bundles (optic radiations) relay to, which is located where?

Answer 23

• Relay to the primay visual cortex (striate cortex)
• Located on the upper and lower banks of the calcarine sulcus

Question 24

Fibers from the lower quadrant of the contralateral hemifields originate from what part of LGN and target where?

Answer 24

• Originate from dorsomedial portion of LGN
• Arch caudally to pass through the retrolenticular limb of internal capsule
• Target superior bank of the calcarine sulcus, on the cuneus

Question 25

Fibers from the upper quadrant of the contralateral hemifields originate where in LGN and describe their route and target?

Answer 25

- Originate from ventrolateral portion of LGN - Arch rostrally, passing into white matter of temporal lobe to form a broad U-turn, the **Meyer loop** - Target **inferior bank** of the calcarine sulcus, on the **lingual gyrus**

Question 26

What is the Meyer loop and why is it clinically significant?

Answer 26

- Formed by fibers from the **upper quadrant of the contralateral hemifields** - These fibers arch into the white matter of **Temporal lobe** on way to **inferior bank** of calcarine sulcus, on lingual gyrus - Strokes or lesions to temporal lobe may interrupt this pathway and cause visual deficits

Question 27

Fibers conveying information from the macula and fovea originate from where in LGN and target what?

Answer 27

- Central regions of LGN - Pass to caudal portions of the visual cortex

Question 28

The macula is represented by disproportionally large volumes (relative to size) of the _____ and \_\_\_\_\_\_\_.

Answer 28

LGN and visual cortex

Question 29

The Macula is represented most \_\_\_\_\_\_\_\_\_, in the region of the _________ pole.

Answer 29

The Macula is represented most **posteriorly**, in the region of the **occipital pole**

Question 30

Visual pathways end retinotopically in the primary visual cortex/striate cortex above and below the calcarine sulcus, where do the inferior, superior, macular and peripheral fields project in this cortex?

Answer 30

- **Inferior** visual fields project to the cortex **above** the calcarine sulcus - **Superior** fields project to the cortex **below** the sulcus - **Macula** is represented more **posteriorly** and **peripheral fields** more **anteriorly**

Question 31

What Brodmann area is the primary visual cortex/striate cortex?

Answer 31

Brodmann's area **17**

Question 32

The striate cortex is surrounded by which Brodmann areas?

Answer 32

Areas 18 and 19 (comprise rest of occipital lobe)

Question 33

Brodmann's areas 18, 19, and related parts of the temporal and parietal lobes comprise what; what components of vision does this cortex help us with?

Answer 33

- Visual Association Cortex (**extrastriate cortex**) - Parieto-occipital-temporal area - Helps interpret: **location, motion, form, and color**

Question 34

Which visual cortical area is responsible for spatially directing head movements and visual reflexes; what inputs does it receive to help with this?

Answer 34

- Superior Colliculus - Retinal and Cortical input - Spinotectal (somatosensory) and auditory inputs \***Important in directing eye movements**

Question 35

How does the Superior Colliculus receive retinal input?

Answer 35

- Select fibers from each optic tract bypass the LGN - Pass over the medial geniculate nucleus in a bundle, the **brachium of the superior colliculus**

Question 36

The pretectal/pretectum area is a bilateral group of interconnected nuclei which respond to and mediate what; related to what reflex?

Answer 36

- Respond to **varying intensities** of **illuminance** - Mediate **non-conscious** behavioral responses to acute changes in light - Important in the **pupillary light reflex**, illumination of 1 eye results in bilateral pupillary constriction

Question 37

Voluntary eye movements are controlled by which frontal eye field and where is it located?

Answer 37

- Frontal eye fields (**area 8**) - Posterior portion of the middle frontal gyrus

Question 38

Which fibers from the frontal eye field will descend to the superior colliculus and control the lower motor neurons for voluntary eye movements; when testing these movements what state does the patient need to be in?

Answer 38

- Corticotectal fibers - Control LMN's of CN III, IV, and VI - Patient **must** be **concious** (**aroused, alert and attentive**)

Question 39

Which eye fields control Nonvolitional movements and where do the fibers project from; what type of eye movements fit this category?

Answer 39

- **Occipital eye fields** in the visual association cortex proejct **corticotectal fibers** to the superior colliculus - Controls **unconcious (nonvolitional)** eye movements, may be **associated** w/ following a target

Question 40

What is Hemianopia?

Answer 40

Blindness (-anopia or -anopsia) in **one-half** of the visual field

Question 41

What is Quadrantanopia?

Answer 41

Blindness of a quadrant of the visual field

Question 42

What is Homonymous vs Heteronymous visual fields?

Answer 42

**Homonymous:** conditions in visual field losses are **similar** in both eyes **Heteronymous:** conditions in which the two eyes have **non-overlapping** field losses

Question 43

How does damage anterior to chiasm, at the chiasm, and behind the chiasm affect visual field?

Answer 43

- **Anterior** to chiasm affects **only** the **ipsilateral eye** - **At** the chiasm causes **heteronymous** deficits - **Behind** the chiasm causes **homonymous** deficits

Question 44

A lesion where would cause this deficit; what is the name of the deficit?

Answer 44

Lesion of right optic nerve = **Monocular blindness**

Question 45

A lesion where would cause this deficit; what is the name of this deficit?

Answer 45

Lesion of optic chiasm = **non-homonymous bitemporal hemianopia**

Question 46

A lesion where would cause this deficit; what is the name of this deficit?

Answer 46

- Lesion of right optic tract or right Geniculocalcarine tract = **contralateral homonymous hemianopia** - Indicated by #3 and #6 on the figure

Question 47

A lesion where would cause this deficit; what is the name of this deficit?

Answer 47

- Lesion temporal lobe (Meyer's loop) = **Superior left homonymous quadrantanopia** **-** Indicated by #4 on figure

Question 48

A lesion where would cause this deficit; what is the name of this deficit?

Answer 48

- Parietal lobe lesion = **Inferior left homonymous quadrantanopia** - Indicated by #5 on figure

Question 49

A lesion where would cause this deficit; what is the name of this deficit?

Answer 49

- Lesion of both banks of calcarine fissure on the right = **Contralateral (left) homonymous hemianopia (with macular sparing)** - Indicated by #9 on the figure - Damage to **one occipital lobe** or **occlusion of PCA**

Question 50

A lesion where would cause this deficit; what is the name of this deficit?

Answer 50

Lesion of Inferior bank of calcarine fissure = **Superior left homonymous quadrantanopia (w/ macular sparing)**

Question 51

Which artery is associated with Associative Visual Agnosia, what is affected, and what is the presentation of this disorder?

Answer 51

- Occlusion of **posterior cerebral artery** leads to secondary infarction of the left occipital love and posterior corpus callosum - **Disconnects** the **language** area from the **visual association** cortex - Patient cannot name or describe an object in the visual field, **BUT** he/she can recognize and demonstrate its use - Patient may also be alexic (unable to read) and writing ability may be affected (agraphia)

Question 52

A lesion to the left temporal lobe affecting Meyer's loop would cause what type of visual deficit?

Answer 52

Superior right homonymous quadrantanopia

Question 53

What is Associative visual agnosia and infarction of which artery to which artery causes this?

Answer 53

- Infarction of **left occipital lobe** secondary to occlusion of **PCA** - May disconnect the language area from the visual association cortex - Pt cannot name or describe an object in visual field, he can recognize and demonstate its use