Chapter 12 - Central Visual Pathways

Question 1

Ganglion cell axons exit the retina through ..

Answer 1

The optic disc.

Question 2

Ganglion cell axons exit the retina through the optic disc, where they bundle together to form..

Answer 2

the optic nerve.

Question 3

Axons in the optic nerve run a straight course to ..

Answer 3

the optic chiasm.

Question 4

In one sentence, what happens in the optic chiasm?

Answer 4

In humans, about 60% of the optic nerve fibers cross in the chiasm.

Question 5

Where is the optic chiasm?

Answer 5

At the base of the diencephalon.

Question 6

From the optic chiasm, where do they fibers go?

Answer 6

Toward the thalamus and midbrain.

Question 7

What is the name of the optic nerve fibers leaving the optic chiasm?

Answer 7

Optic tract.

Question 8

The ganglion cell axons in the optic tract reach a number of structures in the diencephalon and midbrain. The major target in the diencephalon is..

Answer 8

the dorsal lateral geniculate nucleus of the thalamus.

Question 9

From the lateral geniculate nucleus, visual information travels towards the cerebral cortex via the ..

Answer 9

internal capsule, or the optic radiation.

Question 10

The axons that travel along the internal capsule terminate in ..

Answer 10

The primary visual cortex (V1).

Question 11

The primary visual cortex is also called..

Answer 11

the striate cortex.

Question 12

The ganglion cell axons in the optic tract reach a number of structures in the diencephalon and midbrain. One target that is not the LGN is..

Answer 12

Pretectum: a collection of neurons that lies between the thalamus and the midbrain.

Question 13

The pretectum is mostly known for..

Answer 13

The pretectum, a collection of neurons that lies between the thalamus and the midbrain, is mostly known as the primary coordinating center for the pupillary light reflex.

Question 14

What is the pupillary light reflex?

Answer 14

The pretectum-coordinated reflex that is responsible for the reduction in the diameter of the pupil that occurs when sufficient light falls on the retina.

Question 15

The pretectum receives its innervation from the ganglion cell axons of the optic tract, and does, in turn, project to..

Answer 15

The Edinger-Westphal nucleus.

Question 16

What is the Edinger-Westphal nucleus?

Answer 16

A small group of nerve cells that lies close to the nucleus of the oculomotor nerve (cranial nerve III) in the midbrain. It receives its innervation from the pretectum.

Question 17

How is the Edinger-Westphal nucleus connected to the pupillary light reflex?

Answer 17

It contains the neurons that send their axons via the oculomotor nerve to terminate on neurons in the ciliary ganglion. The ciliary ganglion neurons innervate the constrictor muscle in the iris, which decreases the diameter of the pupil.

Question 18

Retinal ganglion cell axons travel to 4 targets you need to remember. Which and where?

Answer 18

1. Dorsal lateral geniculate nucleus of the thalamus
2. The pretectum between the thalamus and midbrain.
3. The suprachiasmatic nucleus of the hypothalamus.
4. The superior colliculus on the dorsal surface of the midbrain.

Question 19

Animals that have completely lost form vision still are able to maintain normal circadian rhythms. Why?

Answer 19

The light-information that is used for the maintenance of circadian rhythm by the hypothalamus stems from specialized photoreceptors that are different from the rods and cones used for vision.

Question 20

The informaton from the temporal retina runs ipsilaterally or contralaterally?

Answer 20

Ipsilaterally.

Question 21

The informaton from the nasal retina runs ipsilaterally or contralaterally?

Answer 21

Contralaterally.

Question 22

Why can we, by an analysis of visual field, with relative precision, find the site of neurological damage?

Answer 22

Because the spatial relationships in the retinas are maintained in central visual structures.

Question 23

Relatively large visual field deficits are called..

Answer 23

anopsias

Question 24

Relatively small visual field deficits are called…

Answer 24

scotomas

Question 25

Damage to the retina or to one of the optic nerves before it reaches the optic chiasm results in a loss of vision that is..

Answer 25

limited to the eye or origin.

Question 26

Damage in the region of the chiasm - or more centrally - results in specific types of deficits that involve the visual fields of which eye?

Answer 26

Both!

Question 27

What is a homonymous hemianopsia?

Answer 27

It is a damage to the visual system that presents itself as a complete loss of vision in the affected region of the binocular visual field. It is caused by lesions in the optic tract.

Question 28

Damage to middle portion of the optic chiasm results in what types of visual deficits?

Answer 28

The middle portion of the optic chiasm has the nerve fibers that cross over. The resulting loss of vision is confined to the temporal visual field of each eye, and is known as heteronomous hemianopsia.

Question 29

What is Meyer’s loop?

Answer 29

Meyer’s loop is a branch of the optic radiation (the pathway from the LGN to the striate cortex)

Question 30

Lesions in Meyer’s loop result in what type of visual deficiency?

Answer 30

A loss of a quadrant of the visual field.

Question 31

What is the unexplained phenomenon of macular sparing?

Answer 31

Injury to central visual structures can lead to this phenomenon: the loss of vision throughout wide areas of the visual field, with the exception of foveal vision.

Question 32

David Hubel and Torsten Wiesel showed in their pioneering studies that visual cortex neurons were not very stimulated by small spots of light/dark on the retina. They responded the most to..

Answer 32

light-dark bars or edges, and only if the bars were presented at a particular range of orientations within the cell’s receptive field.

Question 33

A neuron in the visual cortex can be described as having a preferred orientation. What is meant by this?

Answer 33

The responses of cortical neurons are tuned to the orientation of edges. The orientation to which a cell is most responsive is referred to as the neuron’s “preferred orientation”.

Question 34

Prefered or preferred?

Answer 34

Preferred!!

Question 35

By convention, neocortex is divided into how many cellular layers?

Answer 35

6!

Question 36

Where do the axons from the geniculate neurons terminate?

Answer 36

The axons of geniculate neurons terminate in alternating eye-specific ocular dominance columns.

Question 37

What is binocular disparity?

Answer 37

The disparity between the information about an object when it comes from both eyes. Since they look at the world from slightly different angles, the image that is the basis of the later processing is slightly off. This creates depth.

Question 38

Some neurons in the striate cortex respond strongly to stimuli that fall on non-corresponding parts of the retinas. Three classes of neurons are involved. Which?

Answer 38

Far cells, near cells and tuned zero.

Question 39

What are far cells?

Answer 39

Far cells are neurons in the striate cortex that respond maximally to the disparity caused by objects beyond the plane of fixation.

Question 40

What are near cells?

Answer 40

Near cells are neurons in the striate cortex that respond maximally to the disparity caused by objects in front of the plane of fixation.

Question 41

What are tuned zero?

Answer 41

Tuned zero is a class of neurons in the striate cortex that respond selectively to points that lie on the plane of fixation.

Question 42

The lateral geniculate nucleus has six layers. name them.

Answer 42

1-2 = Magnocellular layers.
3-6 = Parvocellular layers.
Between layers 1-6 lies the Koniocellular layers.

Question 43

Which layer is dorsal to the other - Parvocellular / Magnocellular.

Answer 43

The parvocellular layers are dorsal to the magnocellular layers.

Question 44

What is the main structural difference between the parvocellular and magnocellular layers?

Answer 44

The magnocellular layers are composed of large neurons, and the parvocellular layers are composed to small neurons.

Question 45

Which type of cell in the lateral geniculate responds to color differences?

Answer 45

The cells of the parvocellular layers.

Question 46

Damage to the magnocellular layer has what kinds of effects?

Answer 46

1. Little effect on visual acuity or color vision, but sharply reduces the ability to perceive rapidly changing stimuli.

Question 47

Damage to the parvocellular layeres has what kinds of effects?

Answer 47

No effect on motion perception, but severely impacts visual acuity and color perception.

Question 48

What is the function of the koniocellular layers?

Answer 48

Not clearly understood yet. Some aspects of color vision, especially information derived from short-wavelength-sensitive cones, may be transmitted via the K-cell rather than the P-cell pathway.

Question 49

Where is the ventral stream?

Answer 49

The ventral stream leads from the striate cortex into the inferior part of the temporal lobe.

Question 50

What is the dorsal stream?

Answer 50

The dorsal stream, which includes the middle temporal area, leads from striate cortex into the parietal lobe.

Question 51

What is the function of the dorsal stream?

Answer 51

This system is thought to be responsible for spatial aspects of vision, such as the analysis of motion and positional relationships between objects in the visual scene.

Question 52

What is the function of the ventral stream?

Answer 52

This system is thought to be responsible for high-resolution form vision and object recognition.