The eye: comparable to a camera
- Focus the sharpness of an image: done by the cornea and lens
- Optimal amount of light to reach the retina: done by the iris
* Parts of the eye:
- Cornea = transparent, slightly convex outer surface at the
centre of the eye. Breaks the light before the lens breaks the
light. Does not have any blood vessels, thus takes the nutrients from the
fluid behind it (aqueous humour) and fluid in front of it (tears). Fluid is
spread across the cornea when blinking the eyelid
- Pupil = opening that lets light enter the eye. Appears black, because there is
a layer of black pigmented cells in the back of the eye that absorb the light
- Iris = circular muscle whose pigmentation gives the eye its colour and
whose contraction lets the eye adapt continuously to changing light, thus
changing the diameter of the pupil
o Pupillary reflex = irises open wide when there is not much light, and
narrow when there is much light
- Lens = hold in place between the fluid in the in front (aqueous humour) and behind (vitreous
humour). Focuses the light rays onto the retina in the back of the eye
- Retina = converts the image formed by the light rays into nerve impulses
- Optic nerve = the axons of the retina’s ganglion cells that transmit the impulses from the eye
(i.d. the retina) to the brain (i.e. the V1).
Retina = a thin layer of nerve tissue
! The image focussed on the retain is inverted
! The retina is considered ‘a piece of brain’, because it is a neural tissue and part of the
processing of the image happens in the retina
1) Processing in the retina: there are three main layers separated by two
intermediate layers that make connections among the various neurons
2) Photoreceptors = deepest layer in the retina, consisting of rods and cones
o The only cells in the retina that can convert light into nerve impulses
3) Horizontal cells = receive information from photoreceptors and transmit it to a
number of surrounding bipolar cells
4) Bipolar neurons
5) Amacrine cells = receive input from bipolar cells and transmit it to ganglion
neuron: they activate the neurons that are in their surrounding
6) Ganglion neurons = only the axons of the ganglion
neurons exit the eye in the optic nerve to the brain
Rods and cones
* Are sensitive do different wavelengths:
- Rods: about medium wavelength
- Red/yellow cones: long wavelength
- Green(bit yellow) cones: medium wavelength
- Blue cones: short wavelength
,* Transform the light into a signal:
- Light activation leads to protein (rhodopsin/photopsin)
stimulation leading to cascade of events with eventually
closing of Na+ channels
- Closing of Na+ channels leads to membrane hyperpolarization
leading to a neural signal that is send from bipolar to ganglion
cells
- Different types of the protein leads to sensitivity to different
wavelengths/colours
o Other animals have different proteins
* Cones exists primarily in the fovea, but exists widely spaced in the periphery as well
* Rods are way more prevalent in the periphery
Disease: Retinal colour blindness = absence of a particular cone type
- Cannot see difference between certain colours (e.g. see a 3 instead of a 8)
- Not uncommon in humans, mostly in males (because is sex-linked genetic trait)
- Many tests to access colour blindness
The fovea (macular) = cup shaped, highest density photoreceptors (mainly cones), with sharpest
vision and colour vision
! Fundoscopy (look at the back of the eye) reveals that light passes a lot of
obstacles to reach the photoreceptors
- Veins/blood vessels
o The fovea has not much blood vessels, because there you
want sharp vision, so as least as possible obstacles
- Vitreous body particles (bugs) = proteins, blood links or other things
floating into the vitreous humour
- The retinal network itself
o Therefor in the back is retinal pigment epithelium (RPE), so the
light is not reflected and scattered to other photoreceptors
disrupting sharp vision
o It is necessary photoreceptors are embedded in RPE in humans
o Other animals (e.g. cats) have reflecting epithelium, so he same ray of light activates
several photoreceptors leading to more sight in dark, but less sharp vision
Disease: Dry macular degeneration / Wet macular
degeneration = proteins or other things are effecting your
fovea, pigment epithelium (receptors) are lost due to
accumulation of toxic products
- Loss of central vision, wholes in vision
- Can be caused by older age, smoking, diet, genetics
- There is some treatment with stem cells
, Optic nerve (optic disk)
* Blindspot = the place where all retinal ganglion cell fibres pass through the eye and no receptors
are present
* There is need for data compression: because there are 13.000.000 photoreceptors reduced to
1.000.000 nerve fibres passed to the brain, thus more sensitive to contrast
Disease: Glaucoma = increase of pressure inside the eye (kept by the aqueous humour
and vitreous humour), thus the optic nerve gets compressed and damaged nerve fibres
of the RGC
- Most often peripheral vision is first lost and last the centre of the visual field
o Thus most often not noticed in the begging (e.g. also don’t notice your
blind spot)
o RGC nerve fibres once they are lost cannot be repaired
- Can happen both acute or chronic/gradually
o Since gradually and not-noticed, best to check eye pressure regularly
(partly genetic)
- Treatment to reduce pressure: eyedrops, surgery
NEED FOR DATA SUPPRESSION:
1) Encode the contrast (with ON and OFF center surround receptive field profiles or retinal ganglion
cells)
* Photoreceptors: respond to light by hyperpolarization and to dark by depolarization, thus
difference is a graded potential signal (the more light, the more hyperpolarization and vice versa)
* Bipolar cells
- Signal for photoreceptor is converted into ON and OFF signals (thus the de- or
hyperpolarization itself, not the action potential) at the bipolar cells to the ganglion cells
- Two different possibilities for bipolar cells, using different neurotransmitter receptors
o Sign conserving synapses = hyperpolarization of photoreceptor is
hyperpolarization of bipolar cells (OFF-cells: depolarizes when light goes off)
o Sign inverting synapses = hyperpolarization of photoreceptors is depolarization of
bipolar cells (ON-cells: depolarizes when light goes on)
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