Retinal Processing and Output

also: larger receptive fields, conduct action potentials more rapidly, and are more sensitive to low-contrast stimuli

How is light is passed beyond the eye to the rest of the brain?

receptive field (of a neuron)

the region within the visual field in which stimulation (light) can affect the neuron's response

Bipolar Cell Receptive Fields

two classes

OFF

based on the response to the glutamate released by photoreceptors

light (hyperpolarizat. of cone) effectively turns them off

hyperpolarize

“turned on” by light

depolarize

have ionotropic glutamate receptors

GPCR(metabotropic) receptors

Glu -> depolarization

Glu -> hyperpolarization

antagonistic center-surround receptive fields

response of a bipolar cell’s MP to light in the receptive field center is opposite to that of light in the surround

measured in millimeters across the retina or, more commonly, in degrees of visual angle

Ganglion Cell Receptive Fields

also ON and OFF

difference: fire APs ⚡

also: response to stimulation of the center is canceled by the response to stimulation of the surround

rather, mainly responsive to differences in contrast

can be further categorized

two major types

large M-type ganglion cells and smaller P-type ganglion cells

~ 90% of ganglion cells: P cells, ~5% M, 5% nonM–nonP (less well characterized)

visual response differs

M cells respond to stimulation of their receptive field centers with a transient burst of action potentials

P cells respond with a sustained discharge as long as the stimulus is on

also: larger receptive fields, conduct action potentials more rapidly, and are more sensitive to low-contrast stimuli

also: some (P and nonMnonP) are sensitive to differences in light wavelength

called color-opponent cells

response to one color in receptive field center is canceled by showing another color in receptive field surround

two types of opponency

red versus green and blue versus yellow

lack color opponency

responses to light are not color-specific

overall ganglion cell population sends information to the brain about three different spatial comparisons

light versus dark, red versus green, & blue versus yellow

one special type: ipRGCs

intrinsically photosensitive retinal ganglion cells

blind people unconsciously synchronize their behavior to daily changes in sunlight.

use melanopsin as a photopigment

unlike hyperpolarization to light in rods and cones, ipRGCs depolarize to light

very large dendritic fields (dendrites photosensitive): sum light input over much larger areas of the retina than rods and cones

not used in fine pattern vision, provide input to subcortical visual areas that synchronize circadian rhythms

Parallel Processing

different visual attributes are processed simultaneously using distinct pathways