Flashcards in Vision Deck (31)
Phototransduction is the
process by which light is converted to a change in membrane potential by photoreceptors.
photon is absorbed by
molecule retinal, a pigment in rhodopsin
absorption of photon results in
change in conformation of cis-retinal to trans-retinal
the conformational change of cis to trans rhodopsin results in
conformational change in rhodopsin to metarhodopsin
GTP binding protein called transducin
Before being inactivated by phosphorylation (after about 200 milliseconds), one transducin molecule can _____
activate about 500 other transducin molecules
Transducin is found
in high concentration in the disc membrane surrounding the rhodopsin molecules.
Each activated transducin molecule remains active for
tens of milliseconds before its bound GTP is converted back to GDP (transducin has intrinsic GTPase activity)
Transducin functions to
binds and activates a cGMP phosphodiesterase (PDE)
in large numbers attached to the cytoplasmic surfaces of disc membranes
cGMP PDE molecule is active:
long as the transducin bound to it remains active
(tens of milliseconds)
A decrease in the concentration of cGMP leads to a
closing of non-selective cation channels in the surface membrane
The final result of phototransduction is
the cell becomes hyperpolarized
Color opponent cells:
1. all combinations of red-green on-off opposing fields exist.
2. In addition to red-green opponents, there are blue-yellow opponent cells, thereby spanning the entire spectrum
(the yellow selectivity is created by converging both red and green cones).
Cones of different color preferences
converge in the retina to produce ganglion cells with receptive fields that are partial to particular colors.
where is color discrimination best
fovea, because all the photoreceptors are cones
how is the fovea's color discrimination best?
1. all the photoreceptors are cones.
2. most of the bipolar cells are connected:
a. directly to one kind of cone in the field center and
b. indirectly (via horizontal cells) to cones with a different color preference in the field surround.
3. This creates a RED ON-center and GREEN OFF-surround receptive field, which is passed along to the ganglion cells.
Color information is separated out from spatial information in the
The ____ is the primary visual pathway for the processing of form and color.
It extends from the retina through the lateral geniculate nucleus (LGN) of the thalamus to the primary visual cortex (V1)
geniculostriate: ganglion cells
carry information from the retina to the LGN are of two types, each named for the LGN layer to which they project.
Color processing is handled in
central regions of the hypercolumns called “blobs”
1. care ONLY about color
2. receive input from many color-opponent neurons
3. The positions of all these fields overlap entirely, so there is no spatial information in the color-only blob cell’s response.
4. DO NOT CARE ABOUT LIGHT
Color information from the blob cells travels
ventrally from V1 (primary visual cortex) to the temporal lobe,
coursing through regions of V2 and then onto area V4.
The stripe regions of V2 receive inputs from:
the color blobs in V1.
In area V4, anterior and inferior to the primary visual cortex (V1), cells have:
1. relatively large receptive fields in the central areas of the retina
2. respond only to fairly narrow bands of wavelengths over the visible spectrum, some as narrow as 10 mm.
Lesions in V4 can result in
impairments in color discrimination.
The parvocellular ganglion cells are ____
color-sensitive and project to the four parvocellular layers of the dLGN.
The slow-conducting parvocellular pathway mediates _______.
our perception of color and acuity