Visual system - 2 Flashcards
How can rods and cones signal difference intensities of light?
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).
What is dark current?
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.
Which second messenger is important in light transduction?
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.
What is the connectivity pattern in rods and cones to bipolar cells?
- Many rods are connected to one bipolar cell.
- One cone is connected to one bipolar stal.
Which cells in the retina are able to generate action potentials?
On- and off-center ganglion cells
Describe the pathway of the visual system (which way does visual information take to be processed).
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.
There are other pathways that process visual information. What other pathway is there?
The central visual pathway
What is the central visual pathway?
This pathway consists of multiple (parallel processing) pathways.
The central visual pathways also consist of a pathway that sends visual information to the hypothalamus. What is an important function of the hypothalamus?
Regulation of circadian rhythm
Why is there a connection between the processing of visual information and the regulation of the circadian rhythm by the hypothalamus?
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.
What happened when two men got stuck with their ship on the ice during polar night (70 days)?
They saw that crew members had “polar anemia”:
- Disturbances of sleep
- Fatigue
- Muslce atrophy
- Heart rhythm
- Cognitive symptoms: confusion, shortened attention
- Emotial symptoms: depression
How is the circadian rhythm regulated?
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.
What processes does the suprachiasmatic nucleus regulate?
- Sleep
- Physical activity
- Alertness
- Hormone levels
- Body temperature
- Immune function
- Digestive activity
What happens when there’s continuous exposure to light (nowadays very prevalent since we use smartphones all the time)?
- 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
The central visual pathways also consist of a pathway that sends visual information to the pretectum. What is the pretectum (function, location)?
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.
What happens to pupils upon light exposure and what happens when there’s “no” light?
- Pupils contract and become smaller during light exposure
- Pupils dilate and become bigger in the absence of light.
The central visual pathways also consist of a pathway that sends visual information to the superior colliculus. What is the superior colliculus (function, location)?
The superior colliculus is a structure lying on the roof of the midbrain. It regulates the movement of head and eyes.
To summarize, name the following functions of the four central visual pathways:
- Parallel processing in the hypothalamus
- Superior colliculus
- Pretectum
- Lateral geniculate nucleus (thalamus)
- 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
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.
Also for this picture, just study.
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?
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).
What is contralateral and ipisilateral?
- Contralateral → processing by the opposite hemisphere
- Ipsilateral → processing by the same hemisphere
Summary of what has been discussed so far.
Ok
What image is perceived when you cut the nerve at A?
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.
What image is perceived when you cut the nerve at B?
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.
What image is perceived when you cut the nerve at C?
Damage to the optic tract causes loss of vision of the left side.
What image is perceived when you cut the nerve at D?
Damage to the fiber tract from the lateral geniculate nucleus to the cortex causes partial loss of vision of the left side.
Is visual information from the left and right eye also kept separate in the lateral geniculate nucleus?
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.
What’s the only place where the information of the left and right eye is mixed?
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.
Describe characteristic of the lateral geniculate nucleus
- 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).
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.
- 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).
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?
Yes, see picture.
Describe characteristics of M- and P-ganglion cells in regard to their function and receptive fields.
- 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.
What is seen in this experiment?
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.
Summary of what has been discussed so far
Ok
How is the primary visual cortex organised?
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.
True or false:
There’s retinotopic mapping of the visual space in the visual cortex.
True
How did they map the retinotopic map?
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.
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.
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.
What is cortical magnification?
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.
Just two examples of cortical magnification.
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)?
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.
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)?
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).
There are many more cells in the visual cortex than in the lateral geniculate nucleus. Why is this?
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.
How are cells organized in visual cortex?
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.
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?
- 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.
How is color processed in the visual cortex?
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.
What happens when we see motion?
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.
What brain area is responsible for processing information for motion?
The middle temporal area, close to the visual cortex.
What brain area is responsible for perceiving color?
V4 of visual cortex
Describe the magnocellular pathway.
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).
Describe the parvocellular pathway
- 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).
What is the function of the ventral pathway?
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).
How do we see 3D images (depth)?
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).
Apart from monocular cues, there’s stereopsis and binocular disparity.
What is the meaning of stereopsis and binocular disparity?
- 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.
How are 3D movies filmed?
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.
Summary of what has been discussed.
Ok
