Visual System II Flashcards
What is the difference between on-center and off center retinal ganglion cells?
When light is put on the center of the retinal ganglion cell:
- On-center ganglion cell excites where the light hits. (respond to increase in luminence in the receptive field)
- Off-center ganglion cell excites where light didn’t hit when the light goes off. (respond to decrease in luminence in the receptive field)
How does vertical integration relate to on-centre and off-centre receptive fields and their activation?
Cone synapses onto two bipolar nerves, half are excitatory and other half are inhibitory.
- Cone releases neurotransmitter at rest, so at rest the off-center cells are depolarized and the on-center cells are hyperpolarized
- If light hits the center of the cell, the center cone cell is hyperpolarized, less transmitter comes across the synapses, so there is a reduced activity of the off-center bipolar cell. Less transmitter also means the on-center cell is less inhibited, and therefore more action
Vertical integration is critical for setting up properites for the _____ part of the receptive field, whereas the ______ part of the receptive field is done by horizontal integration.
- centre
- surround
Vertical integration is critical for setting up properites for the CENTER part of the receptive field, whereas the SURROUND part of the receptive field is done by horizontal integration.
What happens when theres a light spot in the surround cells?
- Less transmitter across surround sound cone cell and its synapse with the horizontal cell.
- This activates the horizontal cell.
- The horizontal cell inhibits the adjacent photoreceptor (the center part)
- on-center ganglion cell activity is high
Center part = _______ integration
Surround part and amacrine and horizontal cells = ______ integration
Center part = vertical integration
Surround part and amacrine and horizontal = horizontal integration
Do cones allow us to identify colour?
No, they DO NOT ALLOW US TO IDENTIFY COLOUR, they don’t have enough info.
One amount of light corresponds to multiple colours, so it can’t tell you, but rather can pinpoint WHERE something happened in terms of light.
What are the types of cones, and which ones are represented more abundantly?
Short (S) cones for blue light
Medium (M) cones for green light
Long (L) cones for red light
In abundancy: L > M > S
How can we indentify colour?
Individual photoreceptors are sensitive to wavelengths but cannot identify colour!
We need to combine photoreceptor signals to identify colour.
What is colour blindness, and what are the different kinds?
Colour blindness occurs when you are missing certain cones.
Anamalous trichromatism: missing all three cones (can’t see colour at all)
Dichromatism (something wrong with one or two cones):
- ‘protanopia’ = red
- ‘deuteranopia’ = green
- ‘Tritanopia’ = blue (which is very rare)
Deuteranamlaous most common
What is the condition called when you are missing the L cones. What can’t you see?
Protanopia (missing L cones, therefore can’t see reds)
What is the condition called when you are missing the M cones. What can’t you see?
Deuteranopia (missing M cones, therefore can’t see green)
What is the condition called when you are missing the S cones. What can’t you see?
Tritanopia (missing S cones, therefore can’t see blue)
What are the colour opponent theories?
Red (L) vs green (M)
and
Yellow (L+M) vs blue (S)
- the average of red and green (L + M)
Therefore:
- Red center, green surround
- Green center, red surround
- Yellow center, blue surround
- Blue center, yellow surround
Are center surround receptive fields achromatic?
Centre surround receptive fields can be achromatic or not (receptive to colour)
What are magnocellular ganglion neurons?
Large cell bodies support large axons with high myelination, fastest way for info to be transmitted throughout the brain
- Respond transiently to visual info (signals change. If there is no change (ex no movement) this pathway won’t be very active)
- Not high acuity, instead fast and looking for change
- Fovea is less interested/good at this
- Important for survival (can make quick decision if a tiger is attacking you for example)
What are parvocellular ganglion neurons?
- Smaller receptive fields, wavelength selective (important for colour vision)
- Less interested when something changes
- Fovea is good at this system/pathway
- Allows for high acuity (eg reading your notes)
What are the differences between magnocellular and parvocellular pathways of ganglion neurons?
Magnocellular pathways:
- large cell body
- larger receptive fields
- transient response to sustained illumination
- gross features of image and movement
- achromatic
Parvocellular pathway:
- more numerous
- smaller receptive fields
- wavelength selective (i.e., colour tuning)
- fine detail of image
T or F: The right and left signals from the eyes mix in the optic tract.
False: Right and left signals are in the optic tracts but they don’t mix there, kept segregated
