Early Visual Processing
Terms
undefined, object
copy deck
- What happens above the threshold?
- the retina doesn't care about the number of photons (brightness) but is more intereted in contrast between two objects
- What is positive contrast?
- light from the object is brighter than the surroundings
- What is negative contrast?
- light from the object is dimmer than its surroundings
- What do the optics of the eye exist for?
- to focus light on the retina
- T/F: The retina is a part of the CNS.
- true
- The retina is a ___ micron thick layer of nervous tissue with __ cell types and __ layers.
-
200 micron
5 types
10 layers - What is the retina?
- a sensing organ that captures photons of light
- __________ is important only at threshold.
- Brightness
- What are the 5 cell types in the retina?
-
1.rods and cones
2.horizontal cells
3.bipolar cells
4.amacrine cells
5.ganglion cells - What type of cells are in the outermost layer of the retina? What is this layer called?
-
photoreceptors (rods and cones);
nuclear layer - How many layers does light have to pass through to get to the photoreceptors?
- all of them (10)
- What do photoreceptor cells synapse on 1st? 2nd?
-
bipolar cells in the outer plexiform layer;
ganglion cells - What do horizontal cells connect?
- lateral connections between neighboring photreceptors
- What do amacrine cells do?
- make lateral connections between bipolar cell dendrites
- What kind of photoreceptor system does the retina have?
- dual (made of rods and cones)
- Do rods and cones do different things?
- no, they do the same thing under different conditions and they have different properties
- How much does the dual system expand visual range?
- 10 orders of magnitude
- What is the minimal pathway for a signal to leave the retina?
-
3 cells, 2 synapses;
photoreceptor --> bipolar cell --> ganglion cell - Where does a ganglion cell synapse?
- lateral geniculate nucleus
- What is the lateral circuit important for?
- sensing contrast, both color contrast and light/dark contrast
- What are the on/off pathways important for?
- determining light intensity increases and decreases
- How does the retina sense contrast (differences in edges)?
- through the on/off pathway
- T/F: There are two different pathways for determining light intensity increases and decreases.
- True
- How were rods and cones named?
- for their shape
- Rods contain a ____ concentration of what pigment?
- high; rhodopsin
- What is the pigment in cones? Do cones contain more or less pigment than rods?
-
cone opsin?;
about the same - What do the pigments in rods and cones do?
- capture photons and allow them to undergo a conformational change which causes a signal to be sent
- How do photoreceptors respond to light?
- by hyperpolarizing
- How do photoreceptors respond to dark?
- by depolarizing (get excited)
- Why are photoreceptors at the back of the retina?
- to be near the blood supply of the choroid (behind the retina pigmented epithelium) so they have easy access to the O2
- What is the optic nerve?
- a collection of all the ganglion axons leaving the eye to make a connection to the brain
- Are there photoreceptors on the optic nerve?
- no
- What does the fovea contain? What is it for?
-
a high density of cone receptors;
viewing of fine detail - Is damage in the fovea detrimental to vision?
- yes (because it only covers 1/2 micrometer squared)
- What percentage of photreceptors are rods?
- 95%
- T/F: Humans have a very tiny area of eye acuity.
- True
- How does the size of the cones change outside the fovea?
- they become larger
- How does the density of the rods change outside the fovea?
- the density peaks just outside the fovea and levels off further in the periphery of the retina
- Are rods sensitive? How many photons must fire for a rod to to generate a reliable electrical signal?
-
yes, highly;
one - How many rods synapse on a retinal bipolar cell?
- 1000s
- What is the pathway when a rod is stimulated by a single photon of light?
- light --> rod --> bipolar cell --> ganglion cell
- Rods are have (good/poor) spatial localization?
- poor
- Rods have (high/low) temporal resolution?
- low (slow)
- Cones have (high/low) temporal resolution?
- high (fast)
- A quickly flickering light in a dim room will appear as...
- a steady light source
- Cones have (high/low) sensitivity? Why?
- low (lower than rods); it takes more than one photon to generate a signal in cones
- What is responsible for color vision?
- cones
- Do cones adapt quickly or slowly to changes in intensity of light (i.e. coming out of a dark movie theater)?
- quickly (cones have a wide range of adaptation)
- Explain the rod signaling cascade?
-
the ligand (photon of light) activates 11-cis-retinal which undergoes a conformational change to create an active rhodopsin enzyme -->
rhodopsin is coupled to transducin (a G protein) -->
transducin is activated to activated a phospohdieserase which degrades cGMP -->
the concentration of cGMP modulates the membrane potential - What happens when phosphodiesterase cleaves cGMP?
- a membrane channel closes and prevents ions from entering and depolarizing the cell
- What does dark cause? light?
-
dark: depolarization
light: hyperpolarization - Describe the protein rhodopsin
- it is a 7 transmembrane domain G protein coupled receptor
- How many transducin does rhodopsin activate before inactivation?
- 500 (high sensitivity, slow recovery)
- How many transducin does opsin activate before inactivation?
- 50 (faster recovery but less sensitivity)
- Photoreceptors fire _______ and ______ responses not _______.
- graded; continuous; spikes
- What is the NT of photoreceptors?
- glutamate
- When is glutamate released by photoreceptors?
- in the dark (light prevents release)
- What causes photoreceptors to partially depolarize in the dark?
- an influx of Na+ and Ca2+ ions
- What do you get when you apply a dim light to a rod? A light 2x as bright? will this continue?
-
a small, slow response;
2x the signal;
no because increasing the brightness beyond the threshold only prevents the channels from reopening for longer - Do rods or cones generate a faster response?
- cones (but more photons are required)
- These receptors allow for the response to be over quicker but also allow the response to be enhanced with repeated stimulus.
- cones
- Can both rods and cones be desensitized?
- yes
- Are cones more sensitive to flickering light when there is bright or dim background lighting?
- dim
- What is the Midget system?
- highest visual acuity and fidelity of signals carrying that message requires a private line
- What does convergent wiring do?
- increases sensitivity
- What does divergent wiring do?
- increases contrast
- What allows for the high fidelity and acuity of a visual signal?
- each photoreceptor has a separate neuronal pathway to the CNS
- A visual signal must transmit without ___________ in order to localize the origin of the signal.
- summation
- What is convergent wiring? spatial resolution? sensitivity?
-
when several photoreceptors send signals to one photoreceptor;
low;
sensitive - What is divergent wiring?
- when one photoreceptor signals to more than one bipolar cell
- What are the three types of signaling pathways in vison?
-
1.private line
2.divergence
3.convergence - What is the Push-Pull system? Does it generate a larger or smaller signal?
-
Divergent wiring: when the light is off the cell depolarizes and stimulates a ganglion to tell it the light is off, when the light is on the cell hyperpolarizes and stimulates a different ganglion to signal that the light is on;
larger - What are lateral pathways important for?
- determining the edge of an object
- What do feedback and feed forward synaptic interactions do?
- add flexibility and complexity
- What do spatial filters do?
- lateral inhibition
- What do temporal filters do?
- directional selectivity
- How does the lateral pathway allow for sharp edge detection?
- one neuron inhibits its neighbor via the lateral pathways
- What kind of wiring do On and Off pathways require?
- divergent
- What determines the response?
- receptor type (not the NT)
- In the On/Off system one of the bipolar cells has an _______ synapse. Which one?
-
inverting;
the ON cell - What does an inverted synapse mean?
- in the ON cell a secondary transmitter (cAMP) actually closes channels causing hyperpolarization
- Which bipolar cell in the ON/OFF system responds to the NT by opening channels? What does this lead to?
-
the OFF cell;
depolarization - What happens with light stimulation?
- the photoreceptor hyperpolarizes and stops releasing NT
- Why are they called midget bipolar cells?
- because of their small dendritic arbor
- What happens in the off pathway?
-
-light hyperpolarizes photoreceptor so less glutamate is released
-the off bipolar cell hyperpolarizes
-bipolar cell stops releasing NT so ganglion cell hyperpolarizes
-decrease in spike frequency - What happens in the On pathway?
- the ON cell has an inverting synapse
- Which system (on/off) has an inverting synapse?
- On pathway
- What kind of synapse does the amacrine cell have? What is it for?
-
inverted synapse;
to cancel out the stimulus from the bipolar cell - How do center and surround receptive fields work?
- they involve a midget ganglion cell that receives a signal from one central bipolar cell; bipolar cells around the central bipolar cell stimulate amacrine cells which feed onto the ganglion cell in an inverted fashion
- What happens if the bipolar cells around the central bipolar cell are also stimulated by light?
- the signal to the ganglion would decrease
- What does surround inhibition do to the spike frequency of the ganglion cell?
- decreases it
- How many different types of bipolar cells are there? How are they identified?
- 11; by morphology and if they respond to On or Off signals
- How many types of amacrine cells are there?
- more than 30
- What determines if a bipolar cell is an On or Off cell?
- where the bipolar cell dendrite resides in the sublamina
- Where do On responses reside?
- outermost ring of sublamina
- Where do Off responses reside?
- innermost ring of sublamina
- What identifies Midget ganglion cells?
- their small dendritic arbors
- The bistratified ganglion cell responds when ___ light goes on or ____ light goes off.
-
blue;
yellow - T/F: Rods have specific ganglion
- F, they have to piggyback their signals through the cone system using A II amacrine cells
- What is it called when rods and cones are both active?
- mesoptic lighting conditions
- T/F: All photoreceptors have their own ganglia.
- F, there is not enough space in the retina
- Rod system has a high degree of ___________ summation and uses the _____ ganglion cell to transmit signals to the brain.
-
spatial;
cone - The rod photoreceptor stimulates the ___________ which transfers the signal to the ___________ which stimulates the _______.
-
rod bipolar cell;
A2 Amacrine cell;
cone system - What do spikes encode?
- brightness of light by temporal frequency
- **T/F: Contrast is more important than uniform light.
- T
- **The retina has a _______ photoreceptor system.
- dual
- **What is the minimal pathway to the brain?
- photoreceptor --> bipolar cell --> ganglion cell
- **What two types of cells are important in lateral inhibition?
- horizontal and amacrine
- **What increases the differential between light increases and decreases?
- diverging pathways