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B&C from perception to consciousness deeltentamen 2
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Detailed summary of all lectures given on partial exams 2 of the course from perception to consciousness (Brain and cognition track)
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Perception to consciousnessLecture 1:
On the back of the head there is the visual cortex. First lecture will be on the eye.. the retina.
Retina looks a bit like a camera. Something is projected via the lens. Cornea has a much
stronger breaking index.
The image is samples through rods and cones.
The retina is a piece of brain that is sent forward and sitting outside of the brain. It is a piece
of neural tissue that is preprocessing neural codes.
- this network preprocesses the image and sents it via the gangliin cells via the optic
nerve to the brain.
- 3 cone types all sensitive to all different kinds of wavelengths.
o Short (blue) , medium (green) and long wave length (red) cones.
o Rods sensitive to the intermediate wave length. (greenish
part of the spectrum but not involved in seeing color).
- Rhodopsin translates light into the closing of channels so that the
membrane hyperpolarisez neural signal that is sent to bipolar ->
ganglion.
o Different versi9ons of rhodopsin are sensitive to different
wavelengths. Many varieties in animal kingdom.
▪ Humans have 4 types (3 cones, 1 for rods), goldfish
and many birds have 5, and dogs and mice have 3.
Ishihara plates:
Circles and numbers: retinal color blindness: absence of a particular cone
type.
- Color blindness among humans is not uncommon. In males 8 out
of 100 caucasians , 5 out of 100 asians.
- The probabibility is 10 times less in females because it is sex
linked.
The fovea is tightly packed with cones. Cup shaped, highest density
photoreceptors, mainly cones: sharpest vison, color vision.
Fundascopy reveals that light has to pass a lot of obstacles to reach the
photoreceptos: veins, vitreous body particles.
Funda = back of the eye.
Some diseases that particularly affect the fovea:
- Dry macular degeneration: yellow deposits in accumalte in
macula
- Wet macular degeneration: note blood underneath macula
Age related macular degeneration due to:
- Older age, smoking, diet, genetic
- Loss of central vision, acuity loss
, - Pigment epithelium (receptors) are lost due to accumulation of ..
In the back there is a layer called pigment epithelium (RPE). It is quite dark and full of
granuals with pigment and they absorb the light. If you would not have that the light would
scatter back to other photo receptor which results that the signal will activate
photoreceptors all over the place. THE RPE prevents the scatter to occur. So this way you can
have really really sharp vision.
- The disadvantage is that the light has to pass all those cells.
Cat pigment epithelium is light reflective instead of absorbent:
- Better low light vision (because ray of light hits more photoreceptors)
- But unsharper image (due to scatter).
RGC fibers lying on top causes the blind spot: the place were all retinal ganglion cells fibers
pass through the eye (optic disk) and no receptors are present.
Glaucoma:
- Increase of pressure inside the eye
- Narrow angle or open angle types/ acute, chroninc
- Damage of nerve fibers of the RGC’s: optic nerve
- Loss of peripheral vision first
- Treatment: eyedrops, surgery .
(the black dot is the blind spot).
Important to note: lost RGC are lost. Won’t be coming back.
Therefore over the age of 40 check your eye pressure!
Your eyes don’t feel more pressure so often when you find out it is too late.
The retina preprocesses the rod and cones signals via bipolar cells to the ganglion cells.
The need for data compression: each eye has 130.000.000 photoreceptors.
The optic nerve only contains 1.000.000 nerve fibers. Not all the signals can go to the optic
nerve. Therefore there has to be some kind of intelligent data compression.
Your eye mostly is interested in dark-light transitions. SO how does this work?
- The photoreceptor responds to light by hyperpolarization (closing of Na+ channels),
to dark by depolarization (opening of NA+ channels). : a graded potential signal
- When closing NA+ → hyperpolarization of cell potential (more negative).
- The photoreceptor signal is converted into ON and OFF singals at the bipolar cells.
Using different glutamate receptor types at the synapse
between receptor and bipolar.
A schematic of how ON and OFF ganglion cells arise
The photoreceptors of the vertribrate retina all hyperpolarize
to light , yield only graded potentials and utilize the
neurotransmitter glutamate.
The ON and OFF systems orginate at the level of bipolar cells.
The receptors make sign conserving synapses with the OFF
,bipolar cells and sing inverting synapses with ON bipolar cells that have unique
neurotransmitter receptor site (mGlueR6)
Vanaf slide 26:
Receptor field profiles on the bipolar receptive
field: center -surround (‘Mexican Hat’).
Ganglion cells only respond to contrast, not to
diffuse light.
Receptors to bipolar cells to ganglion cells.
Horizontal cells sum all activity and then give feedback
to the receptors.
So from luminance to contrasts.
From graded potentials to action potentials.
Cone = Color
So retinal ganglion cells encode contrast, luminance is
disregarded.
Luminance is not very useful you might say.
Because the retina encodes contrast, the perception of
luminance is not veridical.
- Contrast illusions.
- Herman Gridd illusion is a side effect of the data
compression buy the retina, comparable to artefacts
caused by JPEG compression.
Rods are primarily necessary for seeing in the dark. Rods also connect to bipolar cells.
Namely rod bipolar cells (RBC).
Difference between cone bipolar cell and RBC is that the RBC do not connect to the ganglion
cells directly. But uses the amacrine cell to connect to the ganglion cells.
So bipolars and RGC have overlapping receptive fields from cone and rod inputs.
Horizontal cells also give the rods a suppressive….
A rod cell is sensitive enough tio respond to a single photon of light and is about 100 times
more sensitive to a single photon than cones. Since rods require less light to functi0on than
cones, they are the primary source of visual information art night. (scotopic vision). Cone
cells tends to hundreds of photons to become activated:
- They are sensitive to a single wavelength, and hence are useless for color vision
- Rod bipolars receive input from multiple rods, hence have larger receptive fields
, - Therefore vision is the dark is less sharp.
In the dark, the rods take over the vision, This is however only part of the process of dark
adaptation:
- Pupil dilation
- Cone > rod transition
- Bleaching of pigment in photoreceptor that becomes undone
- Less receptor signal > less negative feedback from horizontal cells.
Retinitis pigmentosa:
- Genetic disorder (>50 genes involved)
- Progressive degeneration of receptors: rods first, followed by cones
- Pigment deposits at affected part of retina, depigmentation ar vulnerable sites
- Night blindness > loss of peripheral vision > tunnel vison > full blindness
- No cure (vitamin A? Stem cells?)
SO how the retina solved data compression problem?
1. Contrast coding
2. Rod ssignals pass through the same RGCs as cone cells.
Ganglion cells:
Two types cells: midget and parasol cells ( X and Y type cells).
- Midget: small receptive field, X type, single cone center and surround : color contrast
selective, contrast selective and slow sustained
responses.
o Input from 1 cone
o So midget cell can see color.
- Parasol: large receptive field, Y type, many cones
input to center and surround: not color selective, fast
transient response.
o Multiple cones.
So the yellow from the blue cell is actually green and red mix. You don’t have the opposite
because you cannot have two cells in the center.
So only:
- Red vs green
- Blue vs yellow.
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