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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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