Sensory Coding and Vision Flashcards
What does vision start with
Light
Light
Energy wave and particles (photons)
Light that reaches the retina passes through:
Retinal Ganglion Cells (RGCs)
Amacrine and Horizontal Cells
Bipolar Cells
Photoreceptors (Rods and Cones)
Retinal Ganglion Cells (RGCs)
Send information to the brain
Amacrine and Horizontal Cells
Lateral interactions within the retina
Bipolar Cells
Carry information from photoreceptors to retinal ganglion cells
Photoreceptors (rods and cones)
Transduce light signals
Visual Processing in the Retina
Signals from photoreceptors are transmitted through horizontal, bipolar, and amacrine cells to retinal ganglion cells
The RGCs send information down the visual pathway
Phototransduction
Light strikes a light-absorbing pigment molecule (rhodopsin) in disc of photoreceptor
Rhodopsin breaks into retinal and opsin
Opsin closes the Na+ gates, hyperpolarizing the photoreceptor
Stops glutamate release
-Glutamate inhibits some bipolar cells and excites other bipolar cells
-Depolarization/hyperpolarization sent onto ganglion cells
Rods
Highly sensitive to light- ideal for vision in dim environments
Respond similarly to different light wavelength
Can be viewed as sensitive Black and White sensors
Cones
Much less light sensitive, need more light to be activated
Come in 3 types, each sensitive to either red, blue, or green wavelengths
Color Blindness
A lack in one or more of the cone pigments causes color blindness
Vision in rats and raccoons (Nocturnal)
Very poor color vision- (few cones)
Can’t see red
Vision in dogs, cats, squirrels
Dichromatic (2 cone pigments)
Vision in primates
Mostly trichromatic
Fovea
Vision is the sharpest, corresponds to the center of the gaze
Non-photoreceptor cells pushed aside
Highest density of cones, few rods
Therefore, our focus of gaze is optimized for the day
At night, we are “blind” in the fovea. Look at stars slightly off
Receptive fields of RGCs
Light–> photoreceptor releases glutamate
- Excites some bipolar cells
- Inhibits other bipolar cells
- Excited BP cells excite some RGCs
- These RGCs are excited by light at the center- “on cells”
- Inhibited bipolar cells inhibit other RGCs
- These RGCs are inhibited by light at center- “off-cells”
Light hyperpolarized photoreceptor=less glutamate
Light on neighboring photoreceptor depolarizes photoreceptor=more glutamate
Due to lateral inhibition by horizontal and amacrine cells
If all light or all dark, RGCs are silent
Maximal response by RGC is if center opposite surround
1. “On-center off surround”
2. “Off-center on surround”
On-center Off-surround
Light in center excites
Light in surround inhibits
Off-center On-surround
Light in center inhibits
Light in surround excites
On-center Off-surround and Off-center On-surround receptive fields
Enhance sensitivity to edges
