Visual Systems 3: Central Pathways

Question 1

The central visual system extends from the ________ to the ___________.

Answer 1

• retina
• striate cortex

Question 2

Lateral Geniculate Nucleus:

Answer 2

• Layers are segregated according to
  component cell characteristics and
  input:
  
  • Magnocellular (ventral 2) layers:
    contain large cells that receive input from parasol ganglion cells (red)
  • Parvocellular (dorsal 4) layers: contain small cells and receive input from midget ganglion cells (blue)
• Functional divisions set up in the
  retina are retained

Question 3

What do the fibers of the opitc nerve do at the optic chiasm?

Answer 3

1. nasal fibers cross at the optic chiasm
2. temporal fibers do not cross at the optic chiasm

Question 4

Axons from the LGN innervate ____ in a ________ fashion

Answer 4

• V1
• retinotopic

Question 5

Meyer’s loops contains fibers from which part of the visual field?

Answer 5

• superior visual field

Question 6

What would happen if there was a lesion in Meyer’s loop?

Answer 6

• Superior visual field defects (Contralateral Upper Quadratonopia)

Question 7

• Visual field map is _______ on the cortex.
• Most V1 is buried in the __________.

Answer 7

• inverted
• calcarine sulcus

Question 8

The retina of the right eye has a retinotopic map of ___________________.

Answer 8

both the left and right visual fields

Question 9

Describe the visual fields and projections through the retina:

Answer 9

Consider the right retina:

• Temporal division:
  
  • receives information about the left visual field (solid line)
• Nasal division:
  
  • receives information about the right visual field (dotted line)
• Retinal ganglion cell axons of the right eye exit the retina through the optic disk to form the optic nerve

Question 10

The right optic nerve will contain information from _________________.

Answer 10

• one:
  
  • eye
• both:
  
  • left and right visual fields

Question 11

Describe the projections of the visual fields at the optic chiasm:

Answer 11

• Nasal retina: decussate
• Temporal retina: do not cross

Question 12

Fibers of the optic tract contain information of:

Answer 12

• one:
  
  • visual field (contralateral visual field)
• both:
  
  • left and the right eyes

Question 13

The right optic tract would contain information of the ____________ from the nasal division of the _______ and the temporal division of the __________.

Answer 13

• left visual field
• left retina
• right retina

Question 14

Damage to one of the optic nerves before it reaches the optic chiasm would result in:

Answer 14

• loss of vision in that eye of origin
• However, the patient would still be able to view the left and right visual fields using the undamaged eye

Question 15

Damage in the region of the optic chiasm would result in:

Answer 15

• loss of vision from the nasal retina of both eyes
• often patient complains of loss of peripheral vision (Bilateral hemianopia)

Question 16

Damage in the region of the optic tract would result in:

Answer 16

• loss of vision of the contralateral visual field represented in both eyes

Question 17

What does the light reflex do?

Answer 17

regulates pupillary size

Question 18

Direct pupillary light reflex:

Answer 18

The response in the stimulated eye

Question 19

Consensual pupillary light reflex:

Answer 19

The response in the unstimulated eye

Question 20

The fibers that control the light reflex DO NOT synapse in the _____.

Answer 20

LGN

Question 21

Central Pathway (light relfex):

Answer 21

• Afferent = CN II
• **Efferent **= CN III

Question 22

Ganglion cells of the retina:

Answer 22

project bilaterally to pretectal nuclei

Question 23

Pretectal nucleus of midbrain:

Answer 23

• projects crossed & uncrossed fibers ⇒ rostral Edinger‐ Westphal nucleus
  
  • in posterior commissure

Question 24

Edinger‐Westphal nucleus of midbrain:

Answer 24

• gives rise to preganglionic parasympathetic fibers
• which exit the midbrain with the oculomotor nerve
• synapse with postganglionic neurons at the ciliary ganglion

Question 25

Ciliary ganglion of orbit:

Answer 25

* gives rise to **postganglionic parasympathetic fibers** * which innervate the **sphincter pupillae muscle of the iris**

Question 26

What would happen if the **Edinger-Westphal nucleus were unilaterally lesioned**?

Answer 26

* There would be a **lack of pupillary response** on the **affected side**

Question 27

Lesion of the **pulvinar of the thalamus** can cause \_\_\_\_\_\_\_\_\_.

Answer 27

hemineglect

Question 28

Superior Colliculus:

Answer 28

* Mesencephalic laminated structure * Receives **input** primarily from the **parasol cells** * Produces **output to:** 1. **tectospinal tract:** controlling head and trunk movements 2. **brainstem nuclei: **controlling eye muscles

Question 29

What is the **orienting reflex** and where is it coordinated?

Answer 29

**_Superior Colliculus:_** * **Input:** * from **fast moving stimuli** in the **periphery** of your visual field * not high resolution area * To determine what it is: (e.g. **threat level**) 1. **Parasol retinal ganglion cells send a signal to the visual map** in the superior colliculus 2. Which then **sends outputs to move the head and the eyes** until the **high resolution area** of the retina is brought to **bear on the stimulus**

Question 30

Magnocellular cells (M cells):

Answer 30

* Large cell bodies * Large receptive fields * Responses are fast and transient * High sensitivity to contrast * Cannot transmit information about color * Transmit information with high temporal resolution

Question 31

Parvocellular cells (P cells):

Answer 31

* small cell bodies * small receptive fields * slower conduction velocity * lower sensitivity to changes * carry more information about details * can transmit information about color * provide information involving high spatial resolution

Question 32

What Brodmann's area corresponds with the **primary visual cortex (striate cortex)**?

Answer 32

Area 17

Question 33

Extrastriate Cortex (Visual Association Cortex): Charting

Answer 33

* BA 18 & 19 * V2, V3, V4 and V5 * now V6, V8, VP, and numerous subdivisions of above * **V5** is also referred to as area **Middle Temporal (MT)**

Question 34

Extrastriate Cortex (Visual Association Cortex): Receptive Fields

Answer 34

* **V1:** smallest * Generally get **larger the farther from V1** * In distal areas: can fill a whole quadrant of the visual field, or more * Large RFs in a small area of cortex means the **precision of the map is poor**, and**extrastriate cells are responsive over large areas**

Question 35

Function of V4:

Answer 35

color discrimination

Question 36

Function of V5/MT:

Answer 36

**Detection of motion:** * speed & direction

Question 37

**Cerebral Achromatopsia:**

Answer 37

* lack of color vision * **Bilateral damage to the human homologue of V4:** * produces **sudden, global impairment in color perception** * affecting the whole spectrum * **Note:** * different from color "blindness" * color “blindness” is caused by **hereditary defects in photopigment genes**

Question 38

**Cerebral Akinetopsia:**

Answer 38

* lack of motion vision * **Bilateral damage to the human homologue of MT:** * produces **sudden, global impairment in the ability to detect motion** * Patients see **changes in the position of items without seeing them move** to get to the new position. * Motion is detected as **“freeze frames”** in a film

Question 39

V1 projects heavily to \_\_.

Answer 39

V2

Question 40

What is the primary role of V2?

Answer 40

* **relay area** * although there is some processing

Question 41

Streams of Processing: **Ventral Pathway**

Answer 41

**V1 ⇒V2 ⇒V4** * **“What is it?”** * **Temporal lobe** * pathways for **recognition of objects** * e.g. shape, size, color, texture, etc. * **Lesions here impair recognition**

Question 42

Streams of Processing: **Dorsal Pathway**

Answer 42

V1 ⇒V2 ⇒MT * **“Where is it?”** * **Parietal lobe** * Pathways for **localization** * Especially involved in **directing visual attention to an object of interest** * e.g. 3‐D position, trajectory, orientation, etc. * **Lesions here impair the allocation of attention**, a deficit termed **“attentional neglect****”**

Question 43

Ventral Stream: Face Area in IT Cortex

Answer 43

* Further downstream from V4 * **Receptive fields are huge** * Cells have **complex visual stimulus requirements** for activation * sometimes including faces * **Damage produces complex deficits** * **not simple visual discrimination** problems * **Damage to a part of the fusiform gyrus:** * selective impairment in the **recognition of faces**

Question 44

**Visuospatial Neglect:**

Answer 44

* **Damage to parietal cortex**, especially in the right hemisphere, causes **“neglect” of the opposite half of visual space** * However, visual thresholds are **near normal when patient is forced to do tasks with only “neglected” hemifield**

Question 45

Fate of Dorsal and Ventral Streams:

Answer 45

* The **ventral stream** (which travels down the temporal lobe) **and the dorsal stream** (that heads to the parietal lobe) **convey high level signals** of objects and their location to **high level brain structures:** 1. hippocampus (memory), 2. prefrontal cortex (intention) 3. limbic system (emotion)