Retinal Processing and Output
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
ON
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