9 - Visual System 3: Central Pathways

Question 1

What are the components of the visual system?

Answer 1

Image falls on the retina and is encoded in axons which track back and cross chiasm.

Half of the axons cross at the chiasm and and half don’t.

Fiber tract goes back to the lateral geniculate nucleus (LGN). Then axons flow out and take two paths, the superior or inferior optic radiation, to the striate (primary visual cortex).

Question 2

What structure do signals from the eye go to after the optic chiasm?

Answer 2

The lateral geniculate nucleus (LGN).

Question 3

What is the path of the inferior optic radiation? Why is this clinically relevant?

Answer 3

It loops way out and around the lateral ventricle and if within the temporal lobe. (Called Meyes loop)

Temporal lobe seizure treatment may be to cut some tracts of the temporal lobe.

Question 4

When images fall on the retina, everything is ______ and _______.

Answer 4

Backwards and upside down.

Question 5

The nasal retina takes information from the _____ part of the visual field while the temporal retina gets information from the ____ part of the visual field. Where do these paths go?

Answer 5

Nasal retina from the lateral visual field, temporal retina from the medial side of the visual field.

Nasal retina crosses at the chiasm, while temporal retina stays ipsilateral and does NOT cross.

Question 6

What is the region of highest visual acuity? Why is this?

Answer 6

The macula, because it’s going to a large region of the cortex.

This means that a lot of the processor is dedicated to what’s going on in the macula.

Question 7

Where do the superior and inferior optic radiations go to?

Answer 7

Superior: to superior aspect of calcarine sulcus.

Inferior: to inferior aspect of calcarine sulcus.

This means that the visual field map is inverted on the cortex. And that most of V1 is burried in the calcarine sulcus.

Question 8

Axons from the LGN innervate ____ in a retinotopic fashion?

Answer 8

V1

Question 9

What results from a unilateral optic nerve lesion?

Answer 9

Blindness in the affected eye only.

Question 10

What results from a lesion of the optic chiasm?

Answer 10

Bitemporal hemianopia because it interrupts the fibers from the nasal portions of the retina (only those that cross) which represents temporal visual fields.

It’s like you have blinders on because you don’t have peripheral vision.

Question 11

What results from a unilateral optic tract lesion?

Answer 11

Homonymous hemianopia because it interrupts fibers from the temporal portions of the retina on the ipsilateral side and the nasal portions on the opposite side. Thus the right side of both visual fields are affected.

Question 12

What results from a unilateral lesion of the optic radiation in the anterior temporal lobe (meyer’s loop)?

Answer 12

Contralateral upper quadrantanopia because affected fibers wind around the inferior horn of the lateral ventricle in the temporal lobe and are separated from fibers that come from the lower half of the visual field.

Loss is contralateral to the lesion.

Question 13

What results from a unilateral lesion in the medial part of the optic radiation in the parietal lobe?

Answer 13

Contralateral lower quadrantanopia because the affected fibers cause superior to those for the upper quadrant via Meyer’s loop.

Question 14

What results from an occipital lobe lesion? What is spared? What commonly causes this?

Answer 14

Homonymous hemianopia

Because the optic radiation fans out widely before entering the visual cortex, lesions of the occipital lobe have been described to spare foveal vision.

Most commonly caused by intracerebral hemorrhage.

Question 15

What results from a lesion of cortical areas of the occipital pole that represent the macula?

Answer 15

Homonyomus hemianopia central scotoma.

This only knocks out the macula, very rare.

Question 16

What is the superior colliculus?

Answer 16

Responsible for an orienting reflex that brings you input from fast moving stimuli in the periphery in your visual field.

This is why you can see a car coming at you when you’re crossing the street.

Question 17

The division between neurons that carry motion related signals and those that carry color and form content is maintained from retina through what?

Answer 17

The lateral geniculate nucleus and striate cortex.

These are superimposed on a retinotopic representation and are mapped.

Question 18

What are brodmann’s areas 17 and 4?

Answer 18

17: Primary visual cortex
4: Primary motor cortex

Question 19

What are brodmann’s areas 18 and 19?

Answer 19

Technically considered extra-striate cortical areas.

They contain anywhere from 9-20 discrete cortical visual areas.

Question 20

How is the striate cortex and extrastriate cortex mapped?

Answer 20

Striate cortex is V1

Subsequent regions are called V2, V3, V4, V5 and so on.

V5 also called middle temporal (MT).

Question 21

What is associated with the best visual signal?

Answer 21

When axons from the retina project on large areas of volume in the cortex.

Question 22

What is the function of the extrastriate cortical region V4?

Answer 22

Involved in color discrimination tasks.

Loss of V4 results in loss of color vision.

Question 23

What is the function of the extrastriate region V5/MT?

Answer 23

Damage to this region has no effect on the ability to discriminate visual stimuli based on motion but MT/V5 damage IS associated with impairment in the detection of motion.

Impacts how we perceive motion. Lesions here result in the inability to sense motion and these pts see life in freeze frames.

Question 24

What is cerebral Achromatopsia?

Answer 24

A meaning no, chrom meaning color, opsia meaning sight. Damage to V4.

Bilateral damage to the human homolog of V4 produces sudden, global impairment in color perception, affecting the whole spectrum.

Distinct from colorblindess that’s caused by hereditary defects in photopigment genes.

Question 25

What is cerebral akinetopsia?

Answer 25

A meaning no, kine meaning movement, and opsia meaning sight. Damage to MT/V5.

Bilateral damage to the human homologue of MT produces sudden, global impairment in the ability to detect motion.

Patients see changes in position without seeing them move to get to the new positions. Motion detected as “Freeze frames”

Question 26

In V1, we see an anatomical segregation based on _____, ____, and ______. If lesions of the extrastriate areas suggest that the function is segregateed, what does that mean?

Answer 26

Color, form, and motion.

This means that outputs from V1 are also segregated.

Question 27

Where does V1 project heavily to?

Answer 27

V2.

Question 28

What is the function of V2 of the extrastriate cortex?

Answer 28

While some processing occurs through V2, it’s largely considered a relay area.

Question 29

What is the function of the parietal lobe in processing visual information? What results from lesions here?

Answer 29

Especially involved in directing visual attention to an object of interest.

Lesions here impair the allocation of attention, a term called “attentional neglect”

Question 30

What is the function of the temporal lobe in processing visual information? What do lesions here result in?

Answer 30

Contains the pathways for recognition of obejcts (shape, color, size, texture)

Lesions here impair recognition.

Question 31

What is the function of the inferotemporal cortex (IT)? Where is it located? What does damage to this region result in?

Answer 31

Further downstream from V4 and helps you identify faces and form and give you names to things.

Very large receptive field.

Damage may result in an inability to name things or a hard time interpreting what things are.

Question 32

What is the function of the dorsal and ventral streams? What are the hippocampus, prefrontal cortex, and limbic system involved in?

Answer 32

The ventral stream travels from the temporal lobe and the dorsal stream that heads to the parietal lobe.

These convey high level signals of objects and their location to high level brain structures and help us make executive decisions.

Hippocampus: memory
Prefrontal cortex: intention
Limbic system: emotion