Module 29 - Vision

Question 1

What is the sensory system hierarchy for the visual system?

Answer 1

Basic neurons in the eye and the retina such as the rods and cones → go through the optic nerve to a specialized area in thalamus called the lateral geniculate nucleus and ultimately lies in the primary visual cortex.

Question 2

What are the 3 main steps for the information to reach higher processing areas in the visual system?

Answer 2

• Eye and retina
• Primary visual cortex
• Visual association areas

Question 3

What is the Retina?

Answer 3

• Retina = receives light from the lens and converts it into neural signals that the brain can understand.
• It is a laminated neural component of the eye that contains photoreceptors = which are the specialized neurons that respond to light.
• It is the initial processing machinery for the visual pathway.

Question 4

What is the optic disc?

Answer 4

• This is the region where the axons leaving the retina to gather to form the optic nerve.
• There are no photoreceptors here → blind spot
• The blindspots are not superimposed, they are slightly different for each eye, therefore you have no functional deficits when you are using both eyes at once.

Question 5

What is the Fovea?

Answer 5

• The fovea is the central fixation point for each eye.
• Region of the retina with the highest visual acuity.

Question 6

What is the Macula?

Answer 6

• Oval region surrounding the fovea.
• It also has high visual acuity.
• Occupies the central 5 degrees of visual space.
• Detailed vision
• Small in size, but provides input to about half of the optic nerve fibers and cells in the visual cortex.
• It corresponds to the central 1-2 degrees of visual space

Question 7

What is the general pathway of the light to travel in the fovea and macula to be perceived as information?

Answer 7

• As you can see in the image the light is coming in a different direction than the information.
• Light will enter the area and hit the photoreceptors, and then the information will travel backward (back to where the light came from = as you can see in the image).

Question 8

What are photoreceptors?

Answer 8

• They are cells in the retina that respond to light
• They are specialized neurons.

Question 9

What are the two types of photoreceptors?

Answer 9

Rods and cones

Question 10

What are a few of the characteristics of RODS photoreceptors?

Answer 10

• Primarily found in the peripheral retina
• Achromatic → they do not see color
• High light sensitivity
• Nighttime vision, when light levels are low
• More rods than cones (20:1)!!!

Question 11

What are a few of the characteristics of CONES photoreceptors?

Answer 11

• High-acuity and colour vision during day-time when light levels are higher
• Primarily found in the central retina (fovea)
• Cones are less numerous than rods overall but there are more of them located in the fovea. This makes sense because the fovea has the highest level of acuity, therefore we need cells that are specialized for high acuity and color.

Question 12

Why do we have an acuity grading of the visual field?

Answer 12

• The two systems of photoreceptors both the Rods and the Cones allow the visual system to meet the conflicting demand for sensitivity and acuity.
• The way the photoreceptors are laid out allows for our visual field to meet the visual demands.
  
  • The optic disk = the blind spot = where the axons are coming out
  • Blurred peripheral vision = A high number of Rods for the peripheral vision and barely any for the fovea/central part
  • Central detailed vision = A high number of Cones for the fovea/central vision and barely any at the peripheries.

Question 13

What are the main players of the eyeball

Answer 13

Retina
Fovea
Optic disk
Photoreceptors
Rods/cones

Question 14

What are the main players for phototransduction?

Answer 14

Retina
Fovea
Optic disk
Photoreceptors

• Rods/cones

Question 15

How is light translated into a neural signal our brain can understand?

Answer 15

The photoreceptors will respond to light, and we know that photoreceptors provide information to ganglion cells.

Question 16

What is the receptive field for photoreceptors?

Answer 16

• All photoreceptors (cones and rods) CANNOT respond to everything at the same time.
• Otherwise, everything would look the exact same all the time → it is similar to touch, if you hit your pinky finger with a hammer your whole hand isn’t going to feel it.
  
  • But if you hit your finger in space where there isn’t a receptor field (which isn’t necessarily possible but for argument sake) you aren’t going to feel it.
    
    • For the eye or the visual system, it is the same but instead of this mechanical system or the hammer, you need to think of the visual system responding to light
• The receptive field of a neuron in the visual pathway is defined as the portion of the visual field where light causes excitation or inhibition of the cell.
• Each cone and rod has its own receptive field.

Question 17

What is a graded potential?

Answer 17

A graded potential is a change in membrane potential that varies in size. It is NOT an all or none phenomenon.

Question 18

Recap: If the information that is felt on the skin, is sufficient in magnitude an AP will happen and the information will travel up to the brain. This is how the sensory system, or in terms of touch works. NOW: There are a few differences when we talk about phototransduction. Talk about these differences…

Answer 18

• Photoreceptors (rods and cones) use graded potentials, not action potentials, to control the release of neurotransmitters onto post-synaptic neurons.
• In the eye, membrane potential changes are done in a graded response with varying degrees of light.
• Photoreceptors respond to light by hyperpolarizing. NOT depolarizing.
  
  • Light = hyperpolarized = low NT release onto post-synaptic neuron
  • Dark = relatively depolarized = high NT release onto post-synaptic neuron

Question 19

What are the two main differences between photoreceptors and other sensory afferents?

Answer 19

• Photoreceptors use GRADED POTENTIALS
• More light (more stimulus) causes HYPER-POLARIZATION (less NT release)

Question 20

Where do interneurons fit in the light pathway in the eyeball?

Answer 20

• Once photoreceptors (cones and rods) have responded to the light, the information will then travel through the retina through the bipolar cell layer and ganglion cell layer before it goes out the axon to the optic neuron. As they go through the bipolar and ganglion cell layer there are these interneurons called horizontal cells and amacrine cells
• The interneurons are quite similar to the stellate and basket cells in the cerebellum –> they have a lot of lateral inhibitory or excitatory connections with nearby bipolar and ganglion cells

Question 21

How are the interneurons in the eyeball similar to the stellate and basket cells in the cerebellum?

Answer 21

The interneurons are quite similar to the stellate and basket cells in the cerebellum –> they have a lot of lateral inhibitory or excitatory connections with nearby bipolar and ganglion cells

Question 22

Where do the horizontal and amacrine cells reside?

Answer 22

In the bipolar cell layer.

Question 23

What are the two types of cells within the ganglion cell layer?

Answer 23

Parasol (ganglion) cells
Midget (ganglion) cells

Question 24

Describe the parasol (ganglion) cells.

Answer 24

• Gross stimulus features and movement
• Large cell bodies
• Large receptive fields
• Large axons

Question 25

Describe the midget (ganglion) cells.

Answer 25

* Fine visual detail and color * Small cell bodies * Small receptive fields * Smaller axons * More NUMEROUS → compared to the parasol cells

Question 26

The photoreceptors will respond to light, and we know that photoreceptors provide information to ganglion cells. BUT how do these ganglion cells respond to light?

Answer 26

* Retinal ganglion cells have receptive fields (receptive field = area of the visual field that cell monitors/responds to) * There are on-center (off surround) and off-center receptive fields * On-centre (off surround): a light spot in the centre of receptive field excites * Off-center (on surround): a light spot in the center of the receptive field inhibits

Question 27

What happens to when light hits an on-center (off surround) ganglion cell?

Answer 27

* If we think of a hammer as the light (the stimulus) hitting an on-center cell, the light spot in the center of the receptive field will want to excite the cell and information is going to want to travel through the afferent and continue to the optic nerve to the brain.

Question 28

What happens to when light hits an off-center (on surround) ganglion cell?

Answer 28

* If we think of a hammer as the light (the stimulus) hitting an on-center cell, the light spot in the center of the receptive field will want to inhibit the cell and information is going to stop and will NOT travel through the afferent and continue to the optic nerve to the brain. * The cell is not receptive to information coming its way, it does not want to transmit any information any further if there is a light spot in there.

Question 29

What allows for these cellular characteristics of ganglion (on-center and off-center) cells?

Answer 29

Complex connections between photoreceptors, horizontal and amacrine cells, bipolar cells, and retinal ganglion cells allow for these cellular characteristics.

Question 30

RECAP! How does a cell respond to glutamate? Are there different kinds of glutamate or there are different kinds of receptors?

Answer 30

There are different kinds of receptors. The glutamate is the same but the cells will respond differently.

Question 31

How is glutamate involved in the on-center cell response?

Answer 31

* Light hyperpolarizes cone * Less glutamate is released * Glutamate is INHIBITORY to on-center bipolar cell * This results in more excitation of ganglion cell = more action potentials = less inhibition = more action potentials (double negative) * \*Removing its influence means bipolar cell now more excited (depolarized)

Question 32

How is glutamate involved in the off-center cell response?

Answer 32

* Light hyperpolarizes cone * Less glutamate is released * Glutamate is EXCITATORY to off-center bipolar cell * This results in less excitation of ganglion cell = fewer action potentials = less * \*Removing its influence means bipolar cell now less excited (hyperpolarized)

Question 33

The whole purpose of this configuration is that this center-surround arrangement of these ganglion cells receptive fields makes these neurons particularly sensitive to _______________ and relatively insensitive to the overall level of \_\_\_\_\_\_\_\_\_\_\_\_. This is how our retina adapts so it can respond effectively over an enormous range of different lighting,. This is how you can see in relative detail even when it starts to get a little dark outside.

Answer 33

Answer: The whole purpose of this configuration is that this center-surround arrangement of these ganglion cells receptive fields, makes these neurons particularly sensitive to **luminance = light contrast** and relatively insensitive to the overall level of **illumination.** This is how our retina adapts so it can respond effectively over an enormous range of different lighting. This is how you can see in relative detail even when it starts to get a little dark outside.

Question 34

Ganglion cells are configured as either ON or OFF centers.

Answer 34

On-center: light = ????? Off-center: light = ????? ANSWER On-center: light = more action potentials Off center: light = less action potentials

Question 35

What are some (name 6) of the common visual disorders?

Answer 35

* Age related macular degenerations * Diabetic Retinopathy * Glaucoma * Cataracts * Homonymous field deficits (e.g. following stroke) * Agnosias (e.g. prosopagnosia)

Question 36

What are the 4 common visual disorders that are related to the eye and retina?

Answer 36

* Age related macular degenerations * Diabetic Retinopathy * Glaucoma * Cataracts

Question 37

What is age related macular degenerations (AMD)?

Answer 37

* Damage to macula. * Central vision affected * Objects are obstructed and unclear

Question 38

What is diabetic retinopathy?

Answer 38

* Excess blood sugar damages retinal blood vessels. * There is no specific to which blood vessel is damaged. * Patchy vision loss throughout the visual field. * There are areas where the rods and cones don’t receive blood supply and are not able to work * Vision is obstructed by macular edema.

Question 39

What is glaucoma?

Answer 39

Increased intraocular pressure = damage to optic nerve Often peripheral deficits

Question 40

What are cataracts?

Answer 40

Loss of lens transparency Objects are obstructed and unclear

Question 41

True or false: Progressively more complex response characteristics are seen as we move from the retina to higher visual processing centres.

Answer 41

TRUE

Question 42

How does information travel to the primary visual cortex? Describe the whole pathway.

Answer 42

1. As light enters the eyes, it forms an image on the retina, and then this image is inverted and reversed. 2. The light will pass through the rods and cones and afterwards pass into the opposite direction through the ganglion cells. 3. The information then comes out from the optic nerve. 4. The information is crossed at the optic chiasm. Only 60% of the fibers of each eye is crossed (for example, the optic tract on the left has 60% of fibers from the right eye and 40% of fibers from the left.) 5. As soon as the fibers pass through the optic chiasm, we now call this the optic tract. (the information that is contained in the tract is from BOTH eyes) 6. Then the information goes through the lateral geniculate nucleus, which is in the thalamus 7. Then it goes through the optic radiation which is within the internal capsule. 8. And then it ends up in the primary visual cortex, this is called V1 or brodmann's area 17

Question 43

Explain what does it mean that the image in the retina is inverted and reversed?

Answer 43

* Inverted and reverse, meaning that the image has been flipped sideways and then up/down. * Information from the upper visual space is projected on the lower retina * Information from the lower visual space is projected on the upper retina * Information has the same concept for left and right

Question 44

Where does the information from the optic nerve cross? Do all fibers cross over to the other side?

Answer 44

At the optic chiasm Only 60% of fibers cross to the other side

Question 45

True or false. Each optic tract is only made up of the fibers from the opposite eye.

Answer 45

FALSE = 60/40 %

Question 46

In which structure of the brain does the lateral geniculate nucleus reside in? And in which structure are optic radiations?

Answer 46

Thalamus = lateral geniculate nucleus Optic radiations = internal capsule.

Question 47

What are 2 synonyms for primary visual cortex?

Answer 47

V1 Brodmann's area 17

Question 48

Where is information from the left visual field processed?

Answer 48

Information from the left visual field is ultimately processed in the right hemisphere.

Question 49

Is information from the left visual field the same as saying the information from the left eye? Why or why not?

Answer 49

The left visual field --\> is not information necessarily from the left eye, because remember the left eye can see both sides of the world. If you cover up one eye, you can still see everything.

Question 50

Where is information from the superior visual field process?

Answer 50

Information from the superior (upper) visual field is processed in the inferior portion of the visual brain

Question 51

What are the four retinal projections that project to important subcortical regions?

Answer 51

Lateral geniculate nucleus = the primary one Pretectum Superior colliculus Hypothalamus

Question 52

What is the main function of the lateral geniculate nucleus?

Answer 52

It is a major subcortical center relaying visual information to the primary visual cortex Conscious awareness

Question 53

What is the main function (as a retinal projection) of the pretectum?

Answer 53

Controls pupillary light reflex This is generally consensual = it happens in both eyes

Question 54

What is the main function (as a retinal projection) of the superior colliculus?

Answer 54

Controls orienting eye movements

Question 55

What is the main function (as a retinal projection) of the hypothalamus

Answer 55

Regulates the circadian rhythms

Question 56

How is the LGN divided?

Answer 56

The LGN is divided into 6 layers that are distinguished on the basis of cell size. \*You can think of these layers a little bit like the rex lamina within the dorsal horn of the spinal cord. Axons of the retinal ganglion cells form synapses on neurons within the LGN and which then project to VI.

Question 57

What are the 6 layers of the LGN?

Answer 57

* Magnocellular layers = ventral 2 layers * Information received from Parasol Cells * Motion and spatial analysis * Parvocellular layers = 4 layers * Information received from midget cells * Detailed form and colour

Question 58

What happens when parts of the visual system are damaged?

Answer 58

Damage within the primary visual pathway, means field deficits

Question 59

What are homonymous field deficits? What is a possible cause? What are some examples?

Answer 59

* Homonymous field deficits means deficits within the same visual field. Cause = following a stroke

Question 60

What are the response characteristics of cells in the primary visual cortex?

Answer 60

* Rather than responding only to light, we see that the cells within the primary visual cortex respond to different characteristics within a higher level characteristic. * For example, * Preferred orientation = these cells all respond to hard edges. Within all of these cells that respond to lines, some cells respond preferentially to a preferred orientation. * Therefore if you take the graph on the right hand side and look at the neuron that is in the middle, you can see that where it responds the most, its spike rate is when the stimulus is at its preferred orientation. * All edge orientations are roughly equally represented in the visual cortex, all these lines are assigned a cell, and have a given orientation. * Different cells have different characteristics which are encoded into their activity. There are also neurons that act in a similar fashion for direction of motion, right versus left and others.

Question 61

Where does the information get processed after the primary visual cortex?

Answer 61

* Once information has been processed along those cells that have different characteristics, the information will then go to secondary visual areas, or these visual association cortices. * There is 1 in the parietal lobe and another in the temporal lobe * Parietal/Dorsal stream * Temporal/Ventral stream

Question 62

What kind of information is processed through the parietal/Dorsal stream?

Answer 62

* It is processing WHERE? * The analysis of motion and spatial relations * \*Stroke = will typically happen in the parietal lobe because this is where things like motion and spatial relations are analysed

Question 63

What kind of information is processed through the Temporal/Ventral stream?

Answer 63

It is processing the WHAT? Analysis of form and color

Question 64

Recall what are the divisions of labour in the primary visual pathway? (i.e. the 6 layers of the LGN which are distinguished on the basis of cell size) Why is this important?

Answer 64

* Magnocellular layers = ventral 2 layers * Information received from Parasol Cells * Motion and spatial analysis * Parvocellular layers = 4 layers * Information received from midget cells * Detailed form and colour * This is important because we are seeing a continuation, it continues to be the case that different cells and different processing streams emphasize different aspects of visual information .

Question 65

Explain the visual pathway of colour.

Answer 65

If information was colour it would be processed in the midget cells --\> they would go to the parvocellular layers of the thalamus which then goes to V1 and V2 in the what pathway = the inferior occipital-temporal cortex.

Question 66

Explain the visual pathway of form.

Answer 66

If information was form it would be processed in the midget cells → they would go to the parvocellular layers of the thalamus which then goes to V1 and V2 in the what pathway = the inferior occipital-temporal cortex.

Question 67

Explain the visual pathway of motion/spatial analysis.

Answer 67

If information is motion/spatial analysis it was processed by the parasol cells, it would go into the magnocellular layers and then to V1 and V2 and then the where pathway = dorsolateral parieto-occipital cortex.

Question 68

Give an example of the WHAT pathway to explain how it works

Answer 68

* We have talked about that within the primary visual cortex, we have some cells responding to different orientations of a line. * We also have recognition neurons within the temporal lobe, and these recognition neurons preferentially respond to different visual stimuli. So for example, FACES * Study on monkeys: * There are recognition neurons that preferentially respond to different visual stimuli (faces) * Response is maximal when preferred stimuli (faces) are present * A population of cells with similar response characteristics allows us to perceive complex stimuli * If you look at the images from 1-5… we see that these recognition cells fired when a picture of a monkey face was shown 1 and 4… There was a little firing when a human was shown since we do look like monkeys. BUT there was no firing of these neurons for a hand. * As an example, a lesion in the right inferior temporal lobe produces prosopagnosia which is an inability to recognize faces

Question 69

What is prosopagnosia?

Answer 69

Prosopagnosia is the inability to recognize faces.

Question 70

What happens when you suffer from a lesion in the right inferior temporal lobe?

Answer 70

Prosopagnosia which is an inability to recognize faces.

Question 71

What are the general cells and corresponding structures of how the visual information is progressively more complex when moving from the retina to higher visual processing centres?

Answer 71

Answer = image

Question 72

Explain how the sensory system for touch and the sensory system for vision works the same way.

Answer 72

Answer = image