The summary is based on all the lectures given. The summary contains many pictures to clarify the text. At the back of the summary are some examples of exam questions. By learning this summary, I obtained an 8.1 at the exam (note, the exam grade is still combined with a practical grade).
The co...
The visible light spectrum are wavelengths between 400 (blue) and 700 (red) nm
- But… color is a perception of the brain
- Short wavelength = higher energy and longer wavelength = lower energy
The eye
- Optic nerve → takes information from the retina to the brain
- With an otoscope you can look inside the eye
- Retina* → layer of tissue that converts light energy into
neural activity
- Focal point → the sharp point on the retina
- Fovea → located in a straight line behind the pupil
- Fovea is surrounded by the macula
- Located on the retina
- Optic disk → the area where the eye does not pick up light = blind spot
- The brain processes this blind spot using the sight of both eyes
- Lens → muscles can alter the shape of the lense so the
focal point is on the correct position
- If you look at far objects → the lens is more ‘flat’
- Far-sighted → focal point is behind the
retina when you look at close objects
- You need spectacles to already
bent the light a bit
- If you look at close object → the lens is more ‘round’
- Near-sighted → focal point is before the retina when you look
at objects far away
- You need a concave lens
- Inside the eyes there is fluid to keep the right pressure
→ when a lot of light enters the pupil/iris there is pupil constriction (light reflex)
- Pupil constriction → response that narrows the pupil when a lot of light enters the
retina
- This is consensual (happens in both eyes even if only one eye is shined on)
- Happens on level of the brain stem
→ each eye had its own visual field
- The nasal side is smaller than the temporal side
- Visual acuity → the ability of the eye to distinguish between two near point
(sharpness)
- Limited by the distance between photoreceptor cells (if light enters in between
the photoreceptor cells, the object cannot be seen)
*Retina
→ layer of tissue that converts light energy into neural activity
, - Consist of 5 cell types:
1. Photoreceptor cells** → respond to light and influence the membrane
potential of the bipolar cells connected to them
2. Bipolar cells → pass the light response to the ganglion cells
3. Ganglion cells → fires the action potential along the optic nerve in
response to light
4. Horizontal cells
5. Amacrine cells
**there are two types of photoreceptor cells
1. Cone photoreceptor cells → used under photopic conditions (daylight)
a. 3 different types: 1 for blue, 1 for green and 1 for red light
2. Rod photoreceptor cells → used under scotopic conditions (dark)
a. 1 type pigment (gray/no color)
b. Humans have 20x more rods than cones
c. Rods are 1000x more sensitive to light than cones
→ on the fovea, all cells above the photoreceptor cells ‘wave’ outside so the light can pass
the fovea directly without passing all the other layers
- On the fovea, there are more cones than rods
- Each photoreceptor cells is connected 1:1 to a ganglion cell (via a bipolar cell)
- At other position of the retina, more photoreceptors are connected to one
ganglion cell (so you can pick up more light)
What happens at the level of the photoreceptor cell?
- Under resting conditions:
- Neurons: -65 mV
- Rods: -30 mV
→ light (rod-to-cone) adaptation = when light hits:
- Takes 5 to 10 minutes and there is pupil constriction
- How does this work?
1. When light hits a G-protein coupled receptor (opsin) it changes the
conformational change of retinal → hyperpolarizing
a. Opsin = 7x transmembrane α-helix (one of them is retinal)
b. The conformation of retinal changes by light bleaching rhodopsin
(receptor protein)
2. Bleaching activates transducin (protein) and transducin activates the
phosphodiesterase enzyme: cGMP → GMP
a. cGMP normally keeps the Na+ channel open, but if cGMP decreases,
the Na+ channel will close
3. Hyperpolarizing reduces the flow of ions into the cell and the membrane
potential goes from -30 mV to -60 mV
→ after hyperpolarizing…
4. Ca2+ influx ends (because the Na+ channel is closed) and guanylyl cyclase is
not inhibited any more
a. Guanylyl cyclase converts GMP → cGMP and is inhibited by Ca 2+
, 5. This results in more cGMP production (guanylyl cyclase is not inhibited any
more)
6. The ion channels will open again due to high cAMP levels → you can see
contrast again
→ there is signal amplification
→ dark (cone-to-rod) adaptation = dark current
- Takes 20 to 25 minutes
- How does this work?
1. In dark conditions there is an
inflow of Na+ → depolarizing
a. Na+ is present in the
cell and cGMP keeps
the channel open
2. There is ‘unbleaching’ of
rhodopsin
3. Depolarizing releases
neurotransmitters (glutamate)
Human photopic and scotopic spectral sensitivity curves
- Spectral sensitivity curve → shows the relationship between wavelength and
brightness
- There are different curves for photopic (cone) and scotopic (rod) vision
- Blue and green are able to stimulate the retina in the dark (rods
optimum)
- Orange and yellow are able to stimulate the retina in the daylight
(cone optimum)
Young-Helmholtz trichromacy theory
→ the brain sees colors based on the comparison of three receptor types
- The three receptor types of cones (for blue, green and red)
Retinal processing and output
→ the most direct path for the information flow in the retina is from a photoreceptor cell to an
bipolar cell to the ganglion cell
- Light only on a specific region of the retina creates a strong signal → this is called the
receptive field
- Receptive field = center + surrounding
Retinal processing for bipolar cells → bipolar cells do not trigger action potentials, process
visual signals by integration of synapse and voltage-gated channels
- ON-bipolar cells → depolarized by spotlight in the center and hyperpolarized by
spotlight on the center and annulus (surrounding)
- Ganglion cells will pick up this signal
- OFF-bipolar cells → hyperpolarized by spotlight in the center and depolarized by
spotlight on the center and annulus
- Ganglion cells cannot pick up this signal
, Retinal output for ganglion cells → changes in ganglion cell membrane potential due to
center and surround responses
- ON-center ganglion cells → depolarized by spotlight in the center = active form
- When light hits the surrounding → ganglion activity is reduced
- OFF-center ganglion cells → hyperpolarized by spotlight in the center = inactive
form
- OFF-center ganglion cells become active when dark hits the center
- When dark hits the surrounding → ganglion activity is reduced
→ in both types (bipolar and ganglion cells): the response
is a balance between the activity of the center and
surrounding, this is called contrast amplification by lateral
inhibition
Ganglion cells
→ fires the action potential along the optic nerve in response to light and there are two types
1. Magnocellular ganglion cells (5%)
a. Have large receptive fields but are only shortly activated
b. Fast passage of action potentials to the optic nerve
2. Parvocellular ganglion cells (90%) → are connected to rods
a. Have small receptive fields but are long activated
b. Sensitive to different wavelengths of light
i. Color opponent cells* (nonM and nonP cells)
*Color opponent cells → certain wavelengths of light in the receptive field center will be
blocked by another wavelength in the receptive field surround
- Red and green → the center is activated by red, and green in the surround inhibits
the activation (R+G-)
- Blue and yellow → the center is activated by blue, and yellow in the surround inhibits
the activation (B+Y-)
→ so red and green and blue and yellow are the opposite colors of each other
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