Visual system - 2

Question 1

How can rods and cones signal difference intensities of light?

Answer 1

Rods and cones hyperpolarize when they sense light. When the intensity of the light changes, this is also seen in a change in membrane potential (there’s a graded response of the membrane potential to a graded change in light).

Question 2

What is dark current?

Answer 2

Dark current is where the cGMP-gated sodium channels (cation channels) on rods and cones are open, since there is no light present. This results in the depolarization of rods and cones.

Question 3

Which second messenger is important in light transduction?

Answer 3

cGMP.

• When the concentration of cGMP increases due to the absence of light, cation channels are open. This causes depolarization of the rods and cones.
• When the concentration of cGMP decreases due to the presence of light, cation channels close. This causes hyperpolarization of the rods and cones.

Question 4

What is the connectivity pattern in rods and cones to bipolar cells?

Answer 4

• Many rods are connected to one bipolar cell.
• One cone is connected to one bipolar stal.

Question 5

Which cells in the retina are able to generate action potentials?

Answer 5

On- and off-center ganglion cells

Question 6

Describe the pathway of the visual system (which way does visual information take to be processed).

Answer 6

Neurons in the retina have axons that extend to the optic nerves. These neurons send their information to a specific nucleus of the thalamus (lateral geniculate nucleus). From the thalamus, information is send through optic radiations (axons of thalamus) to the visual cortex.

Question 7

There are other pathways that process visual information. What other pathway is there?

Answer 7

The central visual pathway

Question 8

What is the central visual pathway?

Answer 8

This pathway consists of multiple (parallel processing) pathways.

Question 9

The central visual pathways also consist of a pathway that sends visual information to the hypothalamus. What is an important function of the hypothalamus?

Answer 9

Regulation of circadian rhythm

Question 10

Why is there a connection between the processing of visual information and the regulation of the circadian rhythm by the hypothalamus?

Answer 10

Our body adapts to the day and to the night, therefore different functions are assigned to the body when it’s day and when it’s night.

Question 11

What happened when two men got stuck with their ship on the ice during polar night (70 days)?

Answer 11

They saw that crew members had “polar anemia”:

• Disturbances of sleep
• Fatigue
• Muslce atrophy
• Heart rhythm
• Cognitive symptoms: confusion, shortened attention
• Emotial symptoms: depression

Question 12

How is the circadian rhythm regulated?

Answer 12

In the early morning, you’re mostly exposed to blue-spectrum light. This light acts on specific ganglion cells in the retina, which are M-cells. M-cells contain melanopsins, which are light-sensitive photoreceptors. When these are activated, the information is directly sent to the suprachiasmatic nucleus of the hypothalamus.

Question 13

What processes does the suprachiasmatic nucleus regulate?

Answer 13

• Sleep
• Physical activity
• Alertness
• Hormone levels
• Body temperature
• Immune function
• Digestive activity

Question 14

What happens when there’s continuous exposure to light (nowadays very prevalent since we use smartphones all the time)?

Answer 14

• Decrease in melatonin
• Increase in blood sugar levels → prediabetic state
• Decrease of hormone level leptin, a hormone that leaves people feeling full after a meal

Question 15

The central visual pathways also consist of a pathway that sends visual information to the pretectum. What is the pretectum (function, location)?

Answer 15

The pretectum is a bilateral group of highly interconnected nuclei located near the junction of the midbrain and forebrain. It has an important function in the reflex control (contraction and dilation) of the muscles of the pupil and lens.

Question 16

What happens to pupils upon light exposure and what happens when there’s “no” light?

Answer 16

• Pupils contract and become smaller during light exposure
• Pupils dilate and become bigger in the absence of light.

Question 17

The central visual pathways also consist of a pathway that sends visual information to the superior colliculus. What is the superior colliculus (function, location)?

Answer 17

The superior colliculus is a structure lying on the roof of the midbrain. It regulates the movement of head and eyes.

Question 18

To summarize, name the following functions of the four central visual pathways:

• Parallel processing in the hypothalamus
• Superior colliculus
• Pretectum
• Lateral geniculate nucleus (thalamus)

Answer 18

• Parallel processing in the hypothalamus → circadian rhythm
• Superior colliculus → regulation of eye and head movement
• Pretectum → pupillary light reflex
• Lateral geniculate nucleus (thalamus) → visual processing

Question 19

Can’t think of a question for this slide of the powerpoint. Just study and know that images that we see are inverted and left-right reversed on the retina.

Answer 19

Also for this picture, just study.

Question 20

Besides the fact that images that we see are inverted and left-right reversed on the retina. What else happens to visual information that is processed in our brain?

Answer 20

There’s crossing over of fibers, visual information on the left side of the visual field is processed by the right optic tract (right hemisphere) and vice versa → visual information on the right side of the visual field is processed by the left optic tract (left hemisphere).

Question 21

What is contralateral and ipisilateral?

Answer 21

• Contralateral → processing by the opposite hemisphere
• Ipsilateral → processing by the same hemisphere

Question 22

Summary of what has been discussed so far.

Answer 22

Ok

Question 23

What image is perceived when you cut the nerve at A?

Answer 23

This would be like losing sight in the right eye. The entire right optic nerve would be cut and there would be a total loss of vision from the right eye.

Question 24

What image is perceived when you cut the nerve at B?

Answer 24

The optic chiasm would be damaged. In this case, the temporal (lateral) portions of the visual field would be lost. The crossing fibers are cut in this example.

Question 25

What image is perceived when you cut the nerve at C?

Answer 25

Damage to the optic tract causes loss of vision of the left side.

Question 26

What image is perceived when you cut the nerve at D?

Answer 26

Damage to the fiber tract from the lateral geniculate nucleus to the cortex causes partial loss of vision of the left side.

Question 27

Is visual information from the left and right eye also kept separate in the lateral geniculate nucleus?

Answer 27

Even though, the optic nerves of the left and right eye have crossing over, the information from both eyes is still kept separate. This can also be seen in this picture.

Question 28

What's the only place where the information of the left and right eye is mixed?

Answer 28

In the striate cortex (primary visual cortex). Note: the visual information that arrives in the striate cortex is still kept separate. Only, the processing of this information in the striate cortex will be mixed.

Question 29

Why is it important to keep information from the left and right eyes separate?

Answer 29

It's important for seeing depth. The visual field from one eye is somewhat different then the visual field of the other eye, so the brain processes and compares the two visual fields, which creates depth.

Question 30

What is the parvocellular pathway?

Answer 30

The parvocellular pathway is located in the lateral geniculate nucleus and carries information about small, slow, colorful things (high spatial frequency information; low temporal frequency information). It receives its information from cones (long- and medium-wavelength necessary for the perception of color and form (fine details).

Question 31

What is the magnocellular pathway?

Answer 31

The magnocellular pathway is located in the lateral geniculate nucleus and carries information about large, fast things (low spatial frequency, high temporal frequency) and is **colorblind**. It receives its information from rods (necessary for the perception of **movement**, depth and small differences in brightness)

Question 32

Describe characteristic of the lateral geniculate nucleus

Answer 32

* 6 layers * Cells have **monocular** input (from one eye) * Layers alternate inputs from each of two eyes * The top four layers are **parvocellular** layers (two times two layers from each eye → important for shape, size and details of object). * The bottom two are **magnocellular** layers, (two times one layer from each eye → important for information about movement).

Question 33

There are three different ganglion cells that project to the different layers of the lateral geniculate nucleus. Name these three types of ganglion cells, also describe what layers their axons project in the lateral geniculate nucleus and what the function is of these layers.

Answer 33

* P ganglion cells → project to **parvocellular** layers. These layers collect information from cones about color, shape and size of objects. * M ganglion cells → project to **magnocellular** layers. These layers collect information from rods about movement. * K ganglion cells → project to **koniocellular** layers (layers in between the other layers). They collect information from blue-sensitive cones (short-wavelength light).

Question 34

So if these three different types of ganglion cells have different axonal projections to the different lateral geniculate nucleus layers, do they also have different projections to the visual cortex?

Answer 34

Yes, see picture.

Question 35

Describe characteristics of M- and P-ganglion cells in regard to their function and receptive fields.

Answer 35

* M-ganglion cells have a big receptive field and specialize in movement. * P-ganglion cells have a much smaller receptive field and specialize in perceiving color. Here, different P cells are sensitive to different colors, which makes it possible that they can perceive different colors.

Question 36

What is seen in this experiment?

Answer 36

Here, they use the red/green P-ganglion cells to measure their reaction to different colors. It's seen that when red is displayed as a color, the red/green P-ganglion cells respond by producing action potentials. This isn't seen when the color blue is displayed. And since white is a color similar to blue and red, the P-ganglion cell will be partially active.

Question 37

Summary of what has been discussed so far

Answer 37

Ok

Question 38

How is the primary visual cortex organised?

Answer 38

The primary visual cortex is termed as V1 (blue in picture), but there are also other visual areas (V2-V7) that are upstream of the primary visual cortex.

Question 39

True or false: There's retinotopic mapping of the visual space in the visual cortex.

Answer 39

True

Question 40

How did they map the retinotopic map?

Answer 40

They fed radioactive glucose to monkeys. Certain visual stimuli are given and when the associated brain areas are activated by this visual stimulus, they take up the radioactive glucose, which can be visualized. So with this, you can identify the brain areas that are activated by visual stimuli.

Question 41

Depicted in the picture are the visual field (what is seen by the eye) and how this is processed in the visual cortex. Describe how the visual field is translated into the visual cortex.

Answer 41

The red line on the right picture depicts the lower visual field, which is visualized in the upper calcarinus sulcus (red line in left picture). The small yellow dot on the visual field (right) is represented by a larger brain area in the visual cortex. This is because the fovea of the retina contains a lot of cones.

Question 42

What is cortical magnification?

Answer 42

It describes how many neurons in an area of the visual cortex are responsible for processing a stimulus of a given size, as a function of the visual field location. So the fovea, which covers 0,01% of the retina, contains many cones that are represented by a large part of the primary visual cortex.

Question 43

Just two examples of cortical magnification.

Answer 43

Question 44

How is the (striate) cortex organized (think of from what brain region visual information comes from and where the visual information in the cortex arrives)?

Answer 44

The cortex consists of columns and 6 layers, these 6 layers contain different cells. Here, input from the lateral geniculate nucleus projects to different layers of the striate cortex. The input mostly arrives in the _fourth_ layer (main input layer). In this fourth layer there are different cells that project to other layers of the cortex (**extrastriate cortical areas**). The deeper layers 5 and 6 are output layers for other (sub)cortical structures.

Question 45

What are the cells that process the information in the visual cortex (also think of how these cells are different to cells in the retina and lateral geniculate nucleus)?

Answer 45

**Simple cells**. Unlike cells in the retina and LGN, these cells are much more complex in there organisation (see picture). Here, the combination of different simple cells will result in a more complex _interpretation_ of visual information (rather than just interpreting the light like the cells in the retina do).

Question 46

There are many more cells in the visual cortex than in the lateral geniculate nucleus. Why is this?

Answer 46

That is because the information that is processed by the visual cortex becomes more complex. Here, each LGN neuron shares its information with many cortical simple cells.

Question 47

How are cells organized in visual cortex?

Answer 47

If you look at the cortex from above, you can see that each part of the cortex processes information from certain parts from the visual field. Therefore the visual cortex will contain different patches of cells that are sensitive to different orientations. So one cortical column will contain all the cells that process different orientations.

Question 48

So in humans (and monkeys) cells that respond to the same orientation are organized in patches. Does this also apply to the visual cortex of rats and cats?

Answer 48

* In rats cells that respond to different orientations are dispersed in the visual cortex (see picture). * In cats cells that respond to the same orientation are organized in patches, thus cats have the same orientation as humans and monkeys.

Question 49

How is color processed in the visual cortex?

Answer 49

There are separate cells in the visual cortex that process color, called blobs. These blobs are located in between the patches of simple cells that respond to the same orientation.

Question 50

Columns of neurons in the visual cortex are themselves collected into assemblies called _modules_. Describe what cells can be found in each module (these cells together are able to process the whole visual image).

Answer 50

Each module contains: * One complete set of orientation columns * One set of ocular dominance columns (right and left eye) * Several blobs

Question 51

So to summarize: how can we describe the organization of the cortical column?

Answer 51

* Visual cortex is organized in cortical columns containing 6 layers * Information arrives in layer 4 * Each point in visual space is mapped to a certain column. Each column contains a set of cortical neurons specialized for processing different attributes of the visual stimulus.

Question 52

What happens when we see motion?

Answer 52

We do not perceive a moving object as a moving object, but we follow the moving object with our eyes. So while the object is moving, you perceive different visual fields. Thus the image remains stationary on the retina but we perceive movement because our eyes move and follow the object.

Question 53

What brain area is responsible for processing information for motion?

Answer 53

The middle temporal area, close to the visual cortex.

Question 54

What brain area is responsible for perceiving color?

Answer 54

V4 of visual cortex

Question 55

Describe the magnocellular pathway.

Answer 55

This pathway is responsible for processing and perceiving **motion**. * The perception of motions begins in the retina, here M-cells are activated and give rise to the magnocellular pathway in the lateral geniculate nucleus. * After this, information is sent to the visual cortex (V4). * From visual cortex it goes to middle temporal area (MT) and other brain regions. * Finally the information joins in the _dorsal parietal pathway_ for **spatial** information ("where" pathway).

Question 56

Describe the parvocellular pathway

Answer 56

* The perception of objects (**shape and color** and with this also **facial recognition)** begins in the retina. Here P-cells are activated that give rise to the parvocellular pathway in lateral geniculate nucleus. * After this, information is send to the striate cortex (V4). * Finally this pathway joins ventral (temporal pathway) for object recognition (“what” pathway).

Question 57

What is the function of the ventral pathway?

Answer 57

The ventral pathway goes to the temporal lobe, which contains **concept neurons**. Here, one specific concept neuron only recognizes one specific concept (e.g. the face or (sound of the) name of Jennifer Aniston).

Question 58

What are monocular cues?

Answer 58

Monocular cues are all the ways that a single eye helps you see and process what you're looking at.

Question 59

How do we see 3D images (depth)?

Answer 59

There are different types of monocular cues that help in perceiving depth. Examples are: * Familiar size * Occlusion (the nearer objects are in front). * Linear perspective (converging lines) * Size perspective (the smaller the object, the more distant it is) * Shades and illumination (closer objects are brighter). * Motion (objects that move towards you, move quicker).

Question 60

Apart from monocular cues, there's stereopsis and binocular disparity. What is the meaning of stereopsis and binocular disparity?

Answer 60

* **Stereopsis** (→ depth perception) is the visual ability to perceive the world in three dimensions (3D). * **Binocular disparity** means that the image is perceived differently by the right eye compared to the left eye. The combination of the two visual fields, creates depth.

Question 61

!!!!!!!!! How is depth processed in the brain? So for depth perception we need cells that detect and calculate differences between images. These cells are located in V1 and called far and near cells. These cells calculate the difference between the image on the retina. Object in the middle of both eyes (P) has the same distance to both eyes, so the difference is 0. Here zero disparity cells are sensitive to the absence of difference and are activated. But if we take object (Q) there's difference between images perceived on both eyes. Here binocular disparity cells are activated due to the difference in image

Answer 61

!!!!!!! For depth perception we need cells that are able to detect and calculte the differences between the two (different) visual fields perceived by the left and right eye. These cells are located in V1 and are called **far**- and **near cells**. * Far cells respond to disparities in planes further away from the plane of fixation. * Near cells respond to disparities in planes closer than the plane of fixation.

Question 62

How are 3D movies filmed?

Answer 62

3D movies are filmed by a special camera with two lenses (similar to the left and right eye), here the image is projected as polarized light (blue and green). When you watch a 3D movie with polarized glasses, one glass only passes one of the lights, while the other glass only passes the other light. So in this way, how the movie is filmed is also the way you see it when you have polarized glasses on.

Question 63

Summary of what has been discussed.

Answer 63

Ok