Retina Flashcards
Fundus
The back surface of the eye.
Optic disc
A white circle where all the veins, and arteries that feed the eye and where the axons of the ganglion cells leave the eye towards the brain.
Notice that this part of the eye has no photoreceptors and is therefore “blind”
Macula
The dark spot in the fungus, it is right behind the pupil at the centre of the eye.
It contains a high density of photoreceptors and is responsible for the central vision.
Fovea
The central part of the macula.
Almost no blood vessels and a high quantity of photoreceptors.
It is responsible for sharp central vision.
True or False
The optic nerve and the retina can only be imaged with intrusive methods.
False
Photoreceptors
Found at the back of the retina close to the pigment epithelium which gives the photoreceptors important nutrients.
Notice that the foremost layers of the retina are transparent and that the photoreceptors transduce light energy into neural energy.
Rods
- Photoreceptors specialized in night vision. They respond well to low luminance and do not process colour.
- Have rhodopsin
Cones
- Photoreceptors specialised in day-time vision. Respond well to high luminance and process colours.
- Cones have three different opsins which correspond to long, medium and short wavelengths.
True or False
Although the photoreceptors are behind the retina, the light manages to pass
True
True or False
You have much more cones in your retina than rodes
False
True or False
You have much more cones in your fovea
True
True or False
There are almost no cones outside your fovea
True
This means that peripheral vision has a poor definition of colours.
Visual Angle
Vision scientist measures the size of visual stimuli by measuring the size of an image that appears on the retina rather than the object itself.
In summary, the visual angle of an object is in function with the size of the object and the distance of the observer, and it corresponds to the size of the object on the retina.
True or False
The foveal area is more than 2 degree angle.
False
The foveal area is 1 degree angle.
By the rule of thumb, we can’t see much than 2 degrees angle
How do we “capture” photons?
When a photon hits a photoreceptor the process of photoactivation begins.
Photoreceptors
They are divided into 3 parts:
- Outer segment
- Inner segment
- Synaptic terminal
Visual pigment
Created in the inner segment of the photoreceptor and stored in the outer segment of the photoreceptor.
Pigments contain a retinal that allows them to “capture” the photos and a protein called opsin whose structure determines the wavelength of light to which the photoreceptor responds.
Melanopsin
Protein that some photoreceptors contain. This protein is responsible for monitoring ambient light levels and influences sleep/awake cycle
Photopic system
- Has 4 to 5 million cones
- Throughout the retina with a high concentration in the fovea.
- Hight acuity
- Low sensitivity
Scotopic system
- Has 90 million rods
- Outside the fovea
- Low acuity
- High sensitivity
Four mechanisms for dark adaptation
- Pupil changes its size. This happens fast but effects are limited
- Rods and cones with gradually become more sensitive to light.
- Duplex retina: rods will take over cones
- Neural circuits enhance contrast, making vision possible regardless of global luminance levels.
How will rods and cones become more sensitive to light?
- Cones will first gain a lot of sensitivity but will level off after 5 to 8 min. This is since cones don’t have a lot of storage so they will reach maximum sensitivity before rods.
- Rods will steadily increase their sensitivity, up to 25 min.
How many photons per second hit the eye in day-time?
How many at night?
1013 photons at day-light
107 photons at night
Cone-Rod break
When the number of photopigments in the cones and in the rods is the same.
Notice that at this point, the cones have already reached their max in photopigment storage, whereas the rods will continue increase the number.
Day light
All the photopigments get “bleached” in both cones and rods
Cinema room
At the beginning, all the photopigments are bleached if you are coming from bright light. In the cinema room, pigments start to regenerate.
For every 2 photopigments regenerated in the Rods, 5 photopigments regenerate for cones.
And this number increases as you lose a photopigment in each rod and cone.
How can we measure the sensitivity of rods and cones separately?
- Color
- Foveal or peripheral vision (eye trackers to see when they break fixation)
Transduction by photoreceptors
- Once photoactivation starts the photoreceptors become hyperpolarized. (Becomes more negative.)
- Changes in the photoreceptor are communicated to the bipolar cells in the form of graded potentials
- Bipolar cells synapse with retinal ganglion cells, which fire like regular cells (all or nothing fashion).
What does hyperpolarized mean?
This is the opposite of a neuron discharging. Normally is associated with the inhibition of neural activity.
Bipolar cell
Called like that since they seem to have one arm touching the photoreceptor and the other touching the ganglion cell.
Their main role is to transmit information from the photoreceptors to the ganglion cells.
They can do two things with the graded potential.
- Reverse the sign: ON-center bipolar cell
- Keep the sign: OFF-center bipolar cell
Graded potential
Temporary changes in the membrane voltage
Receptive Field
The region on the retina where stimuli influence a neuron’s firing rate.
They have a centre-surround organization.
This allows to increase contrast and it depends on horizontal cells.
Steps of Transduction/Transmission
1
Light hyperpolarizes the centre cone (negative)
Steps of Transduction/Transmission
2
On-center bipolar cell reverses the sign of the cone
Steps of Transduction/Transmission
3
Dark depolarises (positive) the surround cones
Steps of Transduction/Transmission
4
This activates horizontal cells which inturn inhibits all the cones.
Notice that the centre cone receives more negative.
Steps of Transduction/Transmission
5
This amplifies the bipolar ON-cell activity and consequently the retinal ganglion cell activity.
What happens if you have light all over?
Then you get negative on the centre and surround cells. So the ganglion cell will not be as activated as having just light on the centre cell.
True or False
There are two types of bipolar cells but one type of ganglion cell.
False
There is two types of bipolar cells and two types of retinal ganglion cells.
There are the ON-center bipolar cells that reverse the sign of the photoreceptor and the ON-center retinal ganglion cells.
There are the OFF-center bipolar cells that do not reverse the sign or the photoreceptor and the OFF-center retinal ganglion cells.
Note: this allows for black on white to stand out and vice vers
True or False
The ON-center bipolar cells do not reverse the sign of the photoreceptor.
False
ON-center cels do reverse the sign but the OFF-center bipolar cells do not.
Kuffler
Mapped the mapped out the receptive fields of individual retinal ganglion cells of a cat.
True or False
“ON-center ganglion cells are excited when light falls in the centre of their receptive field and are inhibited when the light is in the surround.
For the OFF-centre ganglion cells is the opposite.”
True
Edge enhancement
The purpose of centre-surround organization is to help us detect edges
Midget bipolar cells
Receives information from ONE cone.
Diffuse bipolar cells
Receives information from MANY photoreceptors.
Parvocellular pathway
The parvocellular pathway is involved in fine acuity resolution, colour and shape processing.
It has poor temporal resolution but good spatial resolution.
Magnocellular pathway
The magnocellular pathway is involved in motion processing.
It has excellent temporal resolution but poor spatial resolution.
P ganglion cells
Connect the parvocellular pathway and receives the input from the midget bipolar cells.
M ganglion cells
Connect the magnocellular pathway and receives input from the diffuse bipolar cells.
Convergence
Diffuse bipolar cells have high convergence since they receive information from many photoreceptors.
On the other hand, midget bipolar cells have low convergence.
Diffuse: high convergence -> low acuity & high light sensitivity
Midget: low convergence -> high acuity & low light sensitivity
So the midget cells are in the FOVEA and the diffuse cells are in the PERIPHERY.
Visual Acuity
The smallest spatial detail that can be resolved.
For vision scientist:
The smallest visual angle of a cycle of grating (so the smallest visual angle at which you can identify a cycle grating, the better your vision).
20/20 vision
YOUR distance/normal
What does 20/15 mean?
It means that the smallest letter that the average can read at 15 feet you can read it at 20 feet which means that your sight is good!
What does 20/40 mean?
It means that the smallest letter that the average can read at 40 feet, you can read it at 20 feet. This means that your sight is not that good.
Spatial frequency
Cycles of a grating per unit of visual angle (in degrees)
Another way to think about it is the number of times that a pattern repeats itself per unit of area.
Gabor patches
Associate low, medium and high frequencies to the three images shown.
(a) low spatial frequency
(b) medium spatial frequency
(c) high spatial frequency
Why sine grating?
Because patterns with stripes and fuzzy boundaries are common: Books in shelves, trees in a forest, pencils in a cup
Any black and white picture can be described in terms of a weighted combination of different:
- Frequencies
- Contrast
- Phases
- Orientation
True or False
“Any black and white image can be described in terms of a weighted combination of different frequencies with different orientations.”
True
In an image, what do the low frequencies represent?
The general image
In an image, what do the high frequencies represent?
The details
Retinal ganglion cells (RGC) and stripes
Ganglion cells with centre surround respond preferentially to certain frequencies.
Notice that retinal ganglion cells are sensitive to :
- Frequencies
- Phases
- Contrast
but not orientation
Phase
The phase of a grating refers to its position within a receptive field.
Contrast
Intensity difference between the darkest and brightest portions of the patch.
Contrast sensitivity function
Visibility of a pattern in function with contrast and spatial frequency.
Contralateral representation of visual space
Information from the nasal part of the retina crosses to the other side of the brain.
Lateral Geniculate Nucleus (LGN)
way station
Types of layers in the LGN
Magnocellular (1,2)
Parvocellular (3 to 6)
Koniocellular
Koniocellular
Very small cells in between the magnocellular and parvocellular sections.
Not entirely sure about functionality.
Properties of the LGN
Controlateral representation: Left LGN receives information from the right visual field and vice versa.
Every LGN layer receives a signal from one eye only.
Within each layer of the LGN, the neurons are arranged into a retinotopic ma**p of the visual field. In other words, RGC with adjacent receptive fields connects to adjacent neurons in the LGN.
True or False
The LGN receives information from the retina, the thalamus, the brainstem, the cortex and itself.
True
Striate cortex
Part of the visual cortex that is responsible for processing visual information
Two important features of the striate cortex
- Retinotopic (topographic) mapping
- Cortical magnification:
- Dramatic scaling of information from different parts of the visual field
- Proportionally much more cortex to process the fovea than to process the periphery
Response properties of V1 neurons
By combining information from different ganglion cells it is possible to detect the orientation of lines.
Tilt after-effect
We have different populations of neurons that specialise in certain frequencies and orientations.
Adaptation
Looking at a certain pattern of tripes for a certain amount of time will “tire” the neurons and shift the balance in the opposite direction.
