vision

Question 1

what is exteroperceptive perception?

Answer 1

We dont have to be in immediate contact with the stimulus to process it.

Question 2

Why is the lateral geniculate nucleus (LGN) important for vision?

Answer 2

First synaptic relay in the primary visual pathway. Visual information that is processed by the photoreceptors is going to be relayed towards the primary visual cortex, but before will go in the thalamus LGN.

Question 3

**How are images formed in your retina?

Answer 3

There are three possible types of interaction between light rays and environment.

1) reflection: the light ray will bounce off some surface, example snow.
2) Absorption: there is a transfer of energy to the surface. Whatever that object happens to be, it is absorbing these light waves.
3) Refraction: this is the bending on the light rays. This happens when light rays travel from one transparent medium (air) to another water. This is how images are formed in your retina.

Question 4

What is special about the optic nerve?

Answer 4

The optic nerve is a bundle of axons that arises from the retina. The left optic nerve, from the left eyeball, contains information from both the left and right visual fields.

Question 5

Describe the structure of the eye (6)

Answer 5

Pupil: opening where light enter the eye, light enters and reaches the back of the eye (retina)
Sclera: white of the eye, it forms the tough wall of the eyeball.
Iris: gives color, also contains two muscles that change the size of the pupil opening. one muscle makes the pupil smaller, restricting the amount of light that comes in.
One makes the pupil larger, letting more light come in.
Cornea: Glassy transparent external surface of the eye. Doesnt contain blood vessles.
Extraocular muscles: move the eyeballs.

Question 6

What is the optic disk?

Answer 6

No light- no photoreceptors, it is the blind spot of the eye and has blood vessles that cast shadow on retina.

Question 7

Explain the fovea

Answer 7

The pit of the retina at the center of the macula (central vision). The fovea only contains cones. the cones are specialized for high acuity (detailed vision). The fovea is the center of the retina.

Question 8

Names 6 disorders caused by abnormalities of strucutre in the eyes.

Answer 8

1) Strabismus: imbalance of extracular muscle of the two eyes. Eyes point in opposite direction. Estropia: cross eyed. Exotropia: eyes that diverge, only one is lazy eye.
2) Cataracts: clouding of lens, light cant get through & reach the retina.
3) Glaucoma: evaluated intraocular pressure. progressice loss of vision, leading cause of blindness. Increase pressure in anterior chambre, aqueous humor which causes the eye to stress= damage the optic nerve.
4) Detached retina: retina pulls away from the underlying wall of the eye. After a punch. Vitreous fluid flow behind retina.
5) Retinitis pigmentosa: progressive degeneration of photoreceptors. Lose peripheral and lose rods. Total blindness
6) Macular degeneration: loss of central vision, no problem with peripheral vision. But there ability to recognize faces, read and watch tv are all affected.

Question 9

Explain what happens to the shape of eyes and image correction.

Answer 9

Perfect shaped eyeball: emmetropic: incoming light ray focus sharply on the back of the retina
Eyeball smaller than average: hyperopic: Focus on the far away objects but not near ones. Incoming light ray focus behind retina.
Bigger than average: Myoptic: Incoming light rays focus on the front of the retina.
Astigmatism: irregular shape of cornea, income light rays not refracted properly you end up with blurred vision.

Question 10

Visual field

Answer 10

The visual field is the amount of space viewed by the retina when the eye is fixated straight ahead.
The left visual field is imaged on the right side of the retina.
The right visual field is imaged on the left side of the retina.

Question 11

What is the only light sense of the cell in the retina?*

Answer 11

Photorecptors

Question 12

What is the only source of output from the retina?*

Answer 12

Ganglion cells

Question 13

Explain how the cells are organized by layers in the retina.

Answer 13

Ganglion cell layers: inner most layer of the retina, cell body of the ganglion cells
Inner nuclear layer: cell bodies of bipolar cells, amacrine cells and horizontal cells.
Outer nuclear cells: cell bodies of photoreceptors, light sensative portion of photoreceptors of the retina.
Pigmented epithelium: outerouterouet most layer below the photoreceptor) plays important role in maintenance of photoreceptor and pigment.
Inner plexiform layer: bipolar and ganglion and amicrine cells
Outer plexiform layer: synape between photoreceptors and bipolar cells as well as horizontal cells.

Question 14

Compare Rods and Cones

Answer 14

Rods are very long & have a lot of disk, 1000x more sensitive to light, only one to contribute to vision at night, all same photopigment
Cones: Shorter outer segment & has few disk, most work in daytime, 3 types of cone all containing different type of photopigment, photopigment make it sensitive to different wavelengths of light.

Question 15

Why is the central part of your retina (fovea) way better for high resolution vision?

Answer 15

Only has cones

Very few photoreceptors feeds into each ganglion cell.

Question 16

Explain the synaptic transmission and the phototransduction for vision

Answer 16

Synaptic transmission; neurotransmitter is gonna bind to a G-protein coupled receptor, which will activate a g-protein. The g-protein is going to activate the membrane bound effector enzymes then the enzymes increase the intra-cellular level of second messenger. The second messenger will influence the conductance (open and close of ions).

Phototransduction: A light stimulates the photopigment, this will activate the g-protein and the g protein will affect the membrane effector enzyme which will change the level of intracellular second messenger to decrease the level. The sudden decrease of 2nd messenger will lead to closure of ion channel which will change the membrane potential.

Question 17

Bipolar cell receptive field

Answer 17

Off bipolar cells: glutamate released by photoreceptors - transmitter gated ion channels - Na influx - ESPS
Depolarizes the bipolar membrane potential in response to glutamate ( less negative value)
On bipolar cells: Glutamate released by photoreceptors - G protein coupled recptors- k efflux - IPSP
G protein will then open potassium channels on the membrane
K= leaves the cell causing the cell to hyperpolarize

OFF= depolarize and release (in response to the dark in the center of its receptive field)
ON= hyperpolarized less NT release

Question 18

Types of ganglion cells (appearance, connectivity and electrophysiological properties

Answer 18

M-type ganglion cells: magnocellular, much larger, make up 5% of the ganglion cells.
Will engage with anyon, responsible for processing location and night time vision
Large receptive type, conduct action potential much faster in the optic nerve
Much more sensitive to low contrast stimuli

P-type ganglion: parvocellular) they make up 95% of the ganglion cells.
One-on-one relationship, responsible for processing shape and color, sensitive to wavelengths of light and process color.
Smaller receptive field, conduct action potential slower in the optic nerve, less sensitive to low contrast stimuli, will continue to fire as long as there is a stimulus.

Question 19

explain the retinofugal projection

Answer 19

Neural pathway that leaves your eye
Optic Nerves: leave both of the eyes at the optic disk
which will form the
Optic Chiasm: base of the brain in front of the pituitary gland. Contains axons from the nasal portion of the retina, which crossover to the other side.
PARTIAL DECUSSATIONS
Following the partial decussation the axons will form the OPTIC TRACT

Question 20

Explain the left and right hemifields **

Answer 20

The full visual field is the entier region of space that can be seen with both eyes looking straight ahead.
Left optic tract contains info from right hemifield
Right optic tract contains info from left hemifield
Right visual field: Axons from left temporal hemiretina, right nasal hemiretina (left half of each eye) to the left optic tract

Left visual field: Axons from the right temporal hemiretina, left nasal hemiretina, right half of each eye, right optic tract.

Question 21

Show picture of new york What is damaged? Why?

Answer 21

Most of the axons make up the lGN of the thalamus. The neurons of the LGN are gonna send their axons to the primary visual cortex. The projection form the LGN to the visual cortex the optic radiation. Lesions anywhere in the retinal fugal projection (from the eye to the back of the brain) can cause blindness. The extent and where the blindness occurs is going to depend on location. We know that this is the pathway that mediates visual perception.

Example; Lesion in left optic, vision in left eye is lost but it doesn’t mean that we are completely blind in the visual field. (Partial blindness). Right eye can see portions of left visual field.
WHY?
The peripheral field is processed by the nasal hemiretina in the left eye, that is lost. The temporal hemiretina in the left eye is processing visual information from the right visual field. That is not completely lost because portion of the right eye can still process information.
If you damage pass the optic nerve and you cut either the tract of the radiation you loss complete blindness in the right visual hemifield. It affects the optic chiasm.

Question 22

What 3 organizational properties does LGN have?

Answer 22

The thalmus is made up of 6 distinct layers. The most ventral layer is 1 and the most dorsal is 6.
1. Everything is seperate: Each layer contains a representation of the contralateral visual field, but receives input from one eye only. The imput from both eyes are kept seperate.
Layers 2-3-5, are receiving imput from the ipsilateral eye. Temporal retina of the ipsilateral eye.
Layers 1-4-6: receive input from the contralateral eye.
2. Each layer of the LGN contains a topographic map of the retina.
3. The layers are not redundant, the cell types of each layer have clear distinction in terms of structure (axon diameter) and function ( visual infor that is processed).

Question 23

What is the main sort of imput from the LGN? **

Answer 23

Primary visual cortex (80% of excitatory imput comes from visual cortex) Corticofugal feedback pathway.

Question 24

Consequences of lesions in V1**

Answer 24

Hemianopia: complete lession of V1, complete damge to the left lGN, complete damage of the optic tract on left side

Quadrantanopia: loss of vision in a particular quadrant of the visual area, large lession in V1, Visual deficit in upper right quadrant

Scotoma: blind spot in visual field, usually unaware they have them

Question 25

Draw the magnocellular pathway, parvocellular pathway and blob pathway

Answer 25

Magnocellular pathway; m-type ganglion cells (retina)-- magnocellular LGN--- IVCD (V1) --- 
IVB (pyramid) 
1) binocular receptive fields
2) orientation selective
3) simple complex type
4) Direction
5) not wavelength sensitive 

Parvocellular pathway 
P-type ganglion cells (retina) --- parvocellular layer of LGN--- ivcb (v1) --- We find pyramidal neurons in II & III 
1) binocular repective fields
2) orientation selective
3) simple complex type
4) NOT DIRECTION sensitive
5) not wavelenght sensitive

Blob pathway Nonm/ nonP, ganglion cells in retina — pathway koniocellular layer LGN —-ii ( pervo) & iii (magno) blob V1— ii & iii: (pyramid)

1) monocelular receptive fields
2) center-surround + color opponent
3) not orientation pr drection sensitive
4) wavelenghts sensitive.

Question 26

What are the characterstics of what we see for Magnocellular, Parvocellular, and koniocellular.

Answer 26

Magnocellular; motion
Parvocellular: Shape
Koniocellular: Color

Question 27

Beyond the extrastriate**

Answer 27

Start at V1, we have two streams coming out of V1,
Dorsal stream: neurons are analyzing visual motion.
Ventral stream: involved in the perception of the visual world. Object recognition, shape & color. Each stream is dominated by a particular V1 pathway. Also receiving input to a lesser extent from other pathways, each stream receives input FROM EVERY PATHWAY

Question 28

What is Akinetopsia?

Answer 28

Can not process motion. Can discriminate color, objects and read/write. Bilateral injurty area MT & IP, Dorsal stream is affected.

Question 29

What is Achromatopia?

Answer 29

Damage cells in V4. Damage to ventral stream. Colorbind, difficulty seeing color. Cant remember the color of things as well. DAMGE IS BILATERAL.

Question 30

What is visual agnosia?

Answer 30

areas 20, 21, 38. disrupt the ability to identify complex visual stimuli. Not able to recognize objects visually but can recognize them through different sensory modalities.