The Retina Flashcards
Where is the retina located?
Back of the eye
What colour cones correlate to which wavelengths?
Red/ yellow- long wavelength
Green, less to yellow- Medium wavelength
Blue- Short wavelength
What is the cause of Retinal colour blindness?
The absence of a particular cone type
Which part of the eye is the major component of the refractory materia?
Cornea- much more than the lens
Describe the function which the retina carries out?
The retina pre-processes the rod and cone signals via bipolar cells to ganglion cells
What functions do the ganglion cells then carry out
Pass the pre-processed signals to the brain
name each layer of the retina (8)
Pigment epithelium Photo-receptor outer segments Outer nuclear layer Outer plexiform layer Inner nuclear layer Inner plexiform layer Ganglion cell layer Nerve fiber layer
What two cells carry out lateral information flow and name their respective layers
Inner plexiform layer- Amacrine cell
Outer plexiform layer- Horizontal cell
How do the rods and cones transform the light into a neural signal?
Rhodopsin translates light into the closing of Na+ channels so that the membrane hyperpolarizes > neural signal that is sent to bipolar cells > ganglion cells
How are different versions of Rhodopsin (or photopsin) useful in different ways?
They are sensitive to different wavelengths
Why are dogs ‘colour blind’?
They only have 2 versions of rhodopsin (same as a mouse, goldfish and many birds have 5, chicken has 4)
How many versions do humans have?
4 (3 cones 1 rod)
Give four steps by which rhodopsin hyperpolarises the cell (diagram in copy)
- Light stimulation of rhodopsin leads to activation of G protein, transducin
- Activated G protein activates cGMP phosphodiesterase (PDE)
- PDE hydrolyzes cGMP, reducing its concentration
- This leads to the closure of Na+ channels
How does gender affect the probability of colour blindness
women are 10 times less likely to have retinal colour blindness (its not uncommon in males)
Name one of the best known tests to asses retinal colour blindness
Ishihara plates
Where are the majority of cones packed in the retina?
Fovea
What does this (cones in the fovea) indicate for our vision?
Colour vision seems to be present for the whole visual field, yet cones are almost exclusively confined to the central part of the visual field
Where are rods most dominant?
In the parafovea (beside the fovea) both reduce in the peripheal
Give two other reasons why the fovea allows sharper vision in the centre of your field of vision
cup shaped and highest density of photoreceptors
What does fundoscopy mean and what does it reveal?
Fundos is the back of your eye, fundoscopy is an image of the back of your eye.
Fundoscopy reveals that light has to pass a lot of obstacles to reach the photo-receptors: veins, vitreous body particles (‘bugs’)
How can you see the veins yourself?
Close eyes gently. Hold light source (phone) to the side of your eye / head so that its light is visible but not too strongly. Wiggle light up and down.
Name and describe the symptoms of a disease which affects the fovea
(Wet/ Dry) Macular degeneration; causes blank spots and distortions in the central vision, acuity loss.
What is the difference between wet and dry macular degeneration
In dry macular degeneration yellow deposits (drusen) accumulate in macula. In wet macular degeneration blood forms underneath the macula.
Name three potential causes of macular degeneration
Older age, smoking, diet, genetic
What do these toxic do which impedes vision?
Pigment epithelium (receptors) are lost due to accumulation of these toxic products
Light has to pass through the retinal network to reach the photo receptors, why this strange arangement?
The pigment epithelium absorbs the light rather than reflect it (letting the light scatter). This allows you to have very sharp vision
How is this different in certain animals? How is this different in regards to function (2)?
Cats have a reflective, rather than dark absorbant epithelium. This allows better low light vision ( because same ray of light hits more receptors) but an unsharper image
What causes the blind spot?
Retinal ganglion cell (RGC) fibers lying on top of it to pass through the optic disc and no receptors are present
How are the optic veins and arteries related to the blind spot and fovea in the eye?
They are more common over the blindspot than the fovea because you cant see through it anyway
What disease is related to the optic disk in the eye?
Glaucoma
Name two symptoms of glaucoma (1 happening inside eye, 1 experienced)
- Increased pressure inside of eye
- Damage of nerve fibers of the RGC’s: optic nerve
- Loss of peripheral vision first (but may vary) and progressively worse
What two types of glaucoma are there?
Narrow angle or open angle types (acute/ chronic)
What treatments are available for glaucoma? (2) What are their limitations?
Eyedrops, surgery (but lost RGCs are lost)
What is the cause of glaucoma
An increase of pressure of fluid inside the eye. Pressure is needed inside the eye for normal functioning but this pressure must not be too high. The increase in pressure is so bad that the optic nerves get compressed.
Give two reasons why it is difficult to catch glaucoma early
blind spots in visual field are often easy to miss and eye pressure is not something which is consciously felt
Why is there a need for data compression of visual information?
There are 130,000,000 photoreceptors but only 1,000,000 nerve fibres, if it was 1 for 1 it would be as thick as your whole brain
How is retinal information compressed
in regards to contrast between light and dark
How do photoreceptors respond to light and dark information differently?
Light > hyperpolarisation (closing of Na+ channels)
Dark > (opening of Na+ channels)
What kind of potential signal is this?
A graded potential signal
How is the photoreceptor signal converted at the bipolar cells? What is used in this conversion?
Converted to on and off signals using different glutamate receptor types at the synapse between receptor and bipolar
Describe how these on and off ganglion cells arise
The photoreceptors of the vertebrate retina all hyperpolarise to light (depolarise to dark), yield only graded potentials, and utilise the neurotransmitter glutamate.
The on and off systems originate at the level of the bipolar cells. The receptors make sign conserving synapses with the off bipolar cells and sign inverting synapses with ON bipolar cells (no action potential) that have a unique neurotransmitter receptor site (mGluR6)
What feature of our eyesight are horizontal cells important for?
Sensitivity to contrast
How do horizontal cells encode this sensitivity to contrast?
They receive feedback from a widespread region of receptors. They provide negative feedback from the receptors
What form of feedback do horizontal cells give?
Negative feedback on the widespread region of receptors
What would and would not cause bipolar cells to be activated when connected to a horizontal cell?
Uniform patch of dispersed light or diffuse light would not activated, light surrounded by a darker circle would activate them (contrast, see saved diagram)
What effect does this have on the receptive field of the bipolar cell?
Gives the centre surround (Mexican hat) profile of the bipolar (and subsequently ganglion cell) receptive field
distinguish between and on and off centre ganglion cell
On-centre is sensitive to light in the centre and dark in the surround while off centre ganglion cells are sensitive to dark in the middle and light in the surround
Therefore what is the prime functiona of the retinal network from photoreceptors to ganglion cells?
Luminance (identifying light/ dark) in the receptors to contrast in the bipolar and ganglion cells. From graded potentials (hyper/ depolarisation ) in cones, bipolars and horizontals to action potentials in ganglion cells because of long axons.
Retinal ganglion cells encode _______, ______ is discarded
Retinal ganglion cells encode contrast, luminance is discarded
How is this related to why we can perceive certain illusions? (2)
Because the retina encodes contrast, the perception of luminance is not veridical. Therefore in some illusions, colours in two areas can be perceived as different shades due to contrast when they are in fact the same shade. (Cornsweet Craik O’Brien illusion, Simultaneous Contrast illusions)
The Hermann Grid illusion (little dots appearing at crossroads between black squares) is a ‘side effect’ of the data compression by the retina (and higher visual areas), comparable to artefacts caused by JPEG compression. The ganglion cells have a lower response directly between the two squares than at the crossroads between them, leaving you to perceive them a bit darker.
What do the majority of rods connect to? What do these, then, connect to?
Rod bipolar cells which do not connect directly to RGC’s. They connect to cone driven bipolar cells via the Amacrine cells.
What implications do these connections have for the receptive fields?
Bipolars and RGC’s have overlapping receptive fields from cone and rod inputs
What input do horizontal cells have in this?
They give the rods a suppressive surround (mesican hat RF profile)
How do cones and rods differ in their sensitivity to photons of light?
A rod cell is sensitive enough to respond to a single photon of light and is about 100 times more sensitive to a single photon than cones.
What implications does this sensitivity for light have for rods and cones in regards to their activation and functioning?
Since rods require less light to function than cones, they are the primary source of visual information at night (scotopic vision). Cone cells require tens to hundreds of photons to become activated.
Explain with reference to the rods why vision is less clear in the dark despite the higher sensitivity to light (3)
They are sensitive to just a single wavelength, and hence are useless for colour vision. Rod bipolars receive input from multiple rods, hence have larger receptive fields. Therefore, vision in the dark is less sharp (also because rods sit mainly peripheral)
In the dark, the rods ‘take over’ vision. This is however only part of the process of Dark Adaptation. Name the four processes which take place when adapting to the dark
- Pupil dilation
- Cone > Rod transition
- ‘Bleaching’ (depletion) of pigment (opsin) in photoreceptors that becomes undone
- Less receptor signal > less negative feedback from horizontal cells
Name a disorder which affects the rods and cones in the eye
Retinitis Pigmentosa
What caused this disorder?
Genetic disorder (> 50 genes involved)
What symptoms are experienced in this disorder? (4 stages)
Night Blindness > loss of peripheral vision > tunnel vision > full blindness
What are the underlying factors behind the experienced symptoms of retinus pigmentosa? (2)
Progressive degeneration of receptors: rods first, followed by cones
Pigment deposits at affected parts of retina, depigmentation at vulnerable sites
What cure is available for this disorder?
No cure (Vitamin A? Stem cells?)
What two types of ganglion cells are there, describe differences in their structure and general ability
Midget cells (x type)- smaller, small receptive fields
Parasol cells (y type)- bigger, bigger receptive fields
What is the difference between midget and parasol cells in reference to cones?
Midget systems have a single cone centre and surround; colour contrast sensitive
Parasol systems have many cones input to center and surround; not colour selective
What is the difference between midget and parasol cells in terms of their response?
Midget: slow sustained responses
Parasol; Fast transient responses
Name different types of cones and their relevance to colour contrast sensitivity
L-cones are sensitive to red wavelengths
M-cones are sensitive to green wavelengths
S-cones are sensitive to blue wavelengths
How are these different types of cones presented in midget cell’s receptive fields?
one of these cones are in the centre, the surrounding can be made up of a few colours. The contrasts are either green surrounded (contrasted with) red, red surrounded by green or blue surrounded by yellow. Yellow cannot be in the centre because it is made up of green and red.
(different from painting because it is additive rather than mixing colours)
What illusions arise from this colour opponency?
colour opponent afterimages (4 dolphins)
Also our visual systems like these contrasts (complimentary colours in art)
Explain the concept of spatial frequency decomposition
Every image/ contour can be decomposed into the spatial frequencies it contains. Sharp edges (square waves) contain both low and high spatial frequencies, so do small spots of light (impulse function.)
Give two types of images which demonstrate the difference between frequency decomposition
Hybrid and mosaic images
How is spatial frequency relevant to contrast and brightness?
Sensitivity to spacial frequency depends on contrast and brightness; being able to perceive high spatial frequency depends on how distinct the contrast is and how bright it is
How does age affect spatial frequency?
An adult’s is better than a child but it also degrades over time (also differs between species- monkeys are closest to humans)
How is spatial frequency related to receptive field size?
Spatial frequency sensitivity depends on receptive field size. Center component of RGC’s sensitive from low to high SF’s, surround from low to intermediate SF’s. Combined, this gives the characteristic SF tuning of RGC’s
What spatial frequencies yield the best responses?
Medium frequency yields the strongest responses, low and high frequency yields the weaker responses
How are midget and parasol cells related to spatial frequencies?
Midget (X) cells: small receptive fields, tuned to high spatial frequencies Parasol (Y) cells: large receptive fields, tuned to low spatial frequencies
What part of the brain do the RGC’s mostly project to?
LGN
Which parts of the LGN do the individual ganglion cells project to?
Midget cells > Pavocellular layers of the LGN (small RF, high SF’s)
Parasol cells > Magnocelllar layers of the LGN (large RF’s, low SF’s)
M > P, P > M
Name some other characteristics of the parvocellular pathway (5)
- Has sustained response
- Sees colour
- Low contrast gain
- Higher spatial resolution
- Slower
Name some other characteristics of the Magnocellular pathway
Transient response Monochrome High contrast gain Lower spatial resolution Faster
How do these cells and layers compare in conduction speeds?
Y-type (parasol) RGC axons have faster conduction velocities than X-type (midget)
Magnocellular fibers of LGN faster than parvocellular fibers
What implications does this difference in conduction speed have for the image projected in our brain?
Two separate representations arrive at the visual cortex: M- vs P-type.
M-type (Y-RGC’s) arrives first
Describe the Navon task and what is demonstrates
The Navon task demonstrates global precedence (hierarchal letter stimuli). There is a letter or shape made up of smaller letter shapes or symbols. Subjects have to detect either the global target (large letter or shape) or the local target (small items)
Name two conclusions drawn from this Navon task
global targets are detected faster than local targets. Also congruent stimuli are faster than incongruent
Give a potential explanation for global precedence?
A potential explanation for Global Precedence is that the global information is primarily carried by the Y-type RGC’s(M-type LGNcells) whose signal arrives earlier in the brain
What limitations are there to this explanation?
note how the GP effect is a much larger RT difference than the difference in conduction speed (~20 vs ~2 ms)
Describe the hemispheric asymmetry in this experiment (2)
- Global information is faster in the right hemisphere (stimuli in left visual field)
- Local Information is faster in the left hemisphere (stimuli in right visual field)
(local- left (hemisphere))
What does findings do these differences in the hemispheric processing correspond to?
This corresponds to the finding that patients with right hemisphere damage have trouble copying the global shape, while patients with left hemisphere damage have trouble copying the local shapes
Therefore explain how your retina solves the data compression problem (4)
(130,000,000 photoreceptors in your retina and only 1,000,000 nerve fibres)
1) Through contrast coding (on and off center surround RG cells).
2) Rod signals pass through the same RGC’s as cone signals
3) Colours are coded as R/G G/R or B/Y contrasts
4) Parallel pathways (M/P>P/M) for colour/ bw, detail / global
