Chapter 9 - Vision Flashcards
What is in the optical system of the eye?
Cornea and lens
What does the optical system do?
Projects physical image of the world onto the retina
What does the retina do with the optical image?
It converts it into a neural image
What constitutes the retina?
3 interconnected layers and 5 classes of neurons
What are the 3 stages of vertical information flow?
1) Transduction of the image by photoreceptors (physical energy (photons) to electrochemical energy)
2) photoreceptors synapse on bipolar cells
3) bipolar cells synapse on ganglion cells
What is the lateral information flow?
At each stage of the vertical transmission there are specialized laterally connecting neurons called horizontal cells and amacrine cells.
These cells modify forward transmission across the synaptic layers ( largely by inhibition)
When are rods and cones depolarized?
When they are in darkness
In the rod, what maintains sodium and potassium concentrations?
Na/K pump
What happens to channels in outer segment when in the light?
Cell hyperpolarizes
What is the role of the visual pigment molecules?
Absorb electromagnetic energy and divert the energy into a biological process
What could you compare the rhodopsin to?
Is like a G-protein receptor
What could you compare retinal?
To a ligand (it’s a chromophore)
What are the steps of phototransduction?
1) Light stimulation of rhodopsin leads to activation of a G-protein, transducin
2) Activated G-protein activates cGMP phosphodiesterase (PDE)
3) cGMP binds Na+ channels opening them
4) PDE hydrolyzes cGMP, reducing its concentration
5) this leads to closure of sodium channels
Explain dark reaction
1) cGMP is high in cytoplasm
2) inward current of Na+ through cGMP-Gated channels which depolarized cell. This is the dark current which is usually ~ -50pA (in current) keeping cell’s Vm at ~ -40mV
3) depolarization causes transmitter release (glutamate) at terminal region
Explain light reaction
1) Light activates rhodopsin
2) Transducin, the g-protein is active
3) phosphodiesterase (PDE), the effector enzyme, is activated
4) PDE activity reduces the cGMP level
5) cGMP-gated N’a+ channels close- no Na+ current (dark current)
6) Cell hyperpolarizes to ~ -70mV, transmitter release stops
1) OFF bipolars are … by …/… by light
2) ON bipolars are … by…/… by light
1) depolarized — glutamate—hyperpolarized
2) hyperpolarized—glutamate—-depolarized
What are the two ways of thinking about the receptive fields’ visual system?
1- the set of photoreceptors to which the cell is connected
2- the set of the visual field in which various visual stimuli can affect the discharge rate of the cell (slide 39)
What are the two pathways of the receptive field? What are the components of each?
A) vertical pathway: - photoreceptors
- bipolar cell
- ganglion
Forms the center of receptor field (blue)
B) horizontal pathway: - photoreceptors
- horizontal cells
- bipolar cells
- ganglion cell
Forms the inhibitory surround of receptive field
What are the different types of ganglion cells?
M: large receptive fields, not wavelength selective, respond well to large objects and movement, terminate in M (magnocellular) layers of LGN
P: small receptive fields, selective to particular wavelengths of light (color), concerned with analysis of fine detail and color, terminate in P (parvocellular) layers of LGN
What is scotopic vision?
Vision under dim-light (dark)
What is photopic vision?
Vision under well lit conditions
Name the parallel pathways
Magnocellular M(motion)
Parvocellular - P(shape, detail)
Koniocellular - non-M-non-P (color)
For every point of the visual field we have at least 10 ganglion cells. What are they?
- On-M
- Off-M
- On-P (2)
- Off-P (2)
- non-M-non-P (4)
What does LGN stand for?
Lateral geniculate nucleus
NonM-nonP ganglion cells project to which pet of the LGN
Koniocellular
What is disparity tuning?
Many binocular cells code for retinal disparity - differences in images from the left and right eye that the brain uses as a binocular cue (stereopsis) to determine depth or distance of an object.
What are the components of the horizontal organization of V1?
1) Occular dominance columns (cells in 4C are monocular)
2) orientation columns
3) cytochrome oxidase blobs
What are receptive field properties emerging in V1?
- Disparity tuning (for stereopsis, depth perception)
- orientation tuning (for form perception)
- direction selectivity (for lotion perception)
- wavelength (color) selectivity (from retinal P pathway)
- blobs - koniocellular pathway
Which pathways continue to be segregated in parallel pathways?
M and P pathways are processed in segregated in parallel pathways by the visuals system
What are the two streams of processing?
Dorsal and ventral
Magnocellular Pathway
M-type ganglion Magnocellular LGN (Layer 1,2) V1 Layer IV C alpha (monocular) V1 Layer IV B (binocular) dorsal pathway
Where is motion represented?
It is represented in middle temporal area MT
Are MT receptive fields larger than V1 r fields?
Yes, much larger
Which neurons respond to optic flow stimuli?
MST neurons represent the motion of the world as you move through the world
What is color constancy?
Our ability to determine the color of an object despite changing illumination
Where are the cells responsible for color constancy located?
In V4
Dorsal Pathway Explanation
“Where”
V1 V2 V3 MT - Motion MST - navigation other dorsal areas
Rods vs Cones
Rods: High sensitivity to light
One type
Active in low light (scotopic)
Concentrated in peripheral retina
Cones: Low sensitivity to light
Three types (red, green blue)
Active in bright light (photopic)
Concentrated in central retina (fovea)
P cell traits
95% of ganglion cells color sensitive smaller receptive fields slower adaptation detect shapes + details
M Cell Traits
~5% of ganglion cells color insensitive larger receptive fields rapid adaptation detect motion
Name 6 layers in LGN and what they do
1, 2 - Magnocellular
3, 4, 5, 6 - Parvocellular, some koniocellular
Ventral Pathway
"What" V1 V2 V3 V4 (color vision) Inferior Temporal (IT - face recognition)) other ventral areas
Topographic Mapping in Visual System - Name and explanation
Retinotopic: Retinal cells in are close together, and the same in each layer, at the LGN level and cortex level
Parvocellular Pathway
Perception of shape & detail
P-type ganglion LGN Layer 3-6 Parvocellular V1 Layer IV C Beta (monocular) V1 Layer II & III (binocular) Interblob
