Glossary of SFS - Early Visual Processing

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Type of contast in which the light from the object is brighter than the surroundings.
Type of contast in which the light from the object is dimmer than the surroundings.
Above threshold, the retina is less concerned with the number of photons and more interested in what?
The contrast between two objects
Is the inner or outer layer:
1. Closest to the vitreous humor?
2. Clostest to sclera; photoreceptors are here
1. inner
2. outer
Photoreceptors synapse on what type of cells and where?
bipolar cells in the outer plexiform layer
Bipolar cells synpase on what?
Ganglion cells
Photoreceptors in retina.
Rods and cones. Allows for expansion of the visual range of the perception of light.
Ganglion cells typically synapse where?
Lateral geniculate nucleus
Circuit involving horizontal and amacrine cells allowing lateral inhibition and center-surround organization.
Lateral circuit; important for sensing contrast (color and light/dark)
Important in determining light intensity increases and decreases.
On/Off pathways
Visual pigment in rods
Visual pigment in cones
Photoreceptors ___________ in the light and ____________ in the dark.
hyperpolarize; depolarize
Area of retina with a high density of cones and allows viewing of fine detail. Damage here can be detrimental to vision
95% of photoreceptors in humans.
Photoreceptors requiring only one photon to fire. High degree of spatial summation. Very sensitive with poor spatial localization.
What kind of photoreceptors have slow temporal resolution?
Photoreceptors with high temporal resolution but less sensitivity. Wide range of adaptation due to circuit and internal photoreceptor adaptation. Color vision.
Difference in rod and cone function is due to what?
Differences in the signaling cascades.
Rod signaling cascade pathway.
11-cis-retinal (light>conf. change) -> activates rhodopsin (GPCR) -> activates g-protein transducin -> activates phosphodiesterase which degrades cGMP.
What modulates the membrane potential?
Concentration of cGMP. Cleavage of cGMP causes membrane channel to close preventing ions from entering and depolarizing.
1. Activates transducin 500 fold. High sensitivity, slow recovery.
2. Activates transducin 50 fold. Faster recovery. Less sensitivity.
1. Rhodopsin
2. Opsin
Increasing the brightness beyond the threshold does what to channels?
Prevents the channels from reopening for longer.
Photoreceptors fire what type of responses?
Graded and continuous, not spikes
NT of photoreceptors released in the dark.
Photoreceptors that generate a faster response, but require more photons.
What location in the eye does each photoreceptor have its own bipolar cell and ganglion cell?
1. Wiring pattern increasing SENSITIVITY.
2. Wiring pattern increasing CONTRAST.
1. Convergent (several photorecptors to one photoreceptor). Low spatial res, but sensitive.
2. divergent. One photoreceptor to more than one bipolar cell. On/off pathways.
Push-Pull system
Depolarization will stimulate one ganglion cell, while hyperpolarization will stimulate another. Generates larger signal differential to maximize contrast.
________ pathways formed by horizontal and amacrine cells. ______ inhibition important in determining edges of an object.
Lateral; lateral. Active photoreceptor inhibits its neighbors for sharp border between activation/inactivation.
On/off pathways require ______ wiring. Response is determined by ________.
Divergent; receptor type (not the NT)
Has an inverting synapse. A secondary transmitter actually closes channels (hyperpolarization). Bipolar cell in other pathway has receptor that responds by opening channels.
ON cell.
The only photorecptor in the fovea that the bipolar cell receives input from.
Midget bipolar cells (small dendritic arbor)
Off pathway.
Light hyperpolarizes > less glutamate released > off bipolar cell hyperpol. since no longer stimulated by NT > ganglion cell hyperpolarizes decreasing spike frequency
On pathway.
Remember inverting synapse. Dark; photoreceptor depol., so releases glutamine causing hyperpol in bipolar cell > ganglion cell hyperpolarized - spike freq. decreases. When light hyperpolarizes photoreceptor, ceases to release NT > channels in bipolar cell open, depolarizes > stimulates ganglion cell increasing spike frequency.
Requires amacrine and horizontal cells to mediate lateral inhibition.
Center and surround organization.
Response from light stimulation.
Has an inverted synapse and feeds back to ganglion cell to cancel out stimulus from bipolar cell.
Amacrine cell (bipolar cell stimulates ganglion cell and amacrine cell)
Fields that involve a midget ganglion cell receiving a signal from one central bipolar cell.
Center and surround receptive fields
Bipolar cells around the central bipolar cell in 3D space stimulate ________ cells feeding onto the ganglion cell in a(n) ________ fashion.
amacrine; inverted
If bipolar cells around central bipolar cell are also stimulated by light, the signal to the ganglion would actually _________.
In the fovea, ganglion cells are highly __________.
dispersed (from fovea photoreceptors)
Determines if cell is an On or Off bipolar cell.
Where the bipolar cell dendrite resides in the sublamina
On responses are in the __________ ring.
Off are in the __________ ring.
outermost; innermost
Identified by their small dendritic arbors.
Midget ganglion cells
Responds when blue light goes on or yellow light goes off.
Bistratified ganglion cell.
Do not have specific ganglions, but piggyback signals through the cone system using AII amacrine cells.
Lighting conditions when rods and cones are both active
Pathway of rod stimulation.
Rod photoreceptor stimulates rod bipolar cell which transfers signal to A2 amacrine cell, stimulating the cone system.
Rod system has a high degree of spatial summation.
Encode brightness of light by temporal frequency.
Leads to centersurround receptive fields by lateral inhibition.

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