Vision Flashcards
Describe the process of phototransduction in the outer segments of the rods
Photon absorbed by vitamin A on rhodopsin –> metarhodopsin –> activation of transducin (G protein) –> activation of cGMP PDE at cytoplasmic surface of disc membrane –> cGMP hydrolyzed to GMP –> closure of NSC channels –> hyperpolarization of rod membrane
What are the two types of ganglion cells in receptive fields?
ON-center ganglion cells and OFF-center ganglion cells
Describe ON-center ganglion cells
Excited by light shining in their centers, inhibited by light in their periphery
Describe OFF-center ganglion cells
Inhibited by light shining in their centers, excited by light in the periphery
Describe the sizes of the receptive fields in the fovea vs the periphery of the retina
Fovea = receptive field as wide as a single cone Periphery = large receptive field
What is the key determinant of receptive field type?
the type of glutamate receptor on bipolar cells. Excitatory –> OFF; Inhibitory –> ON
What type of synapses do bipolar cells make on ganglion cells?
Excitatory
what cells mediate the surround?
horizontal cells
What is the horizontal cell response to glutamate
inhibition of neighboring photoreceptors in the field center
What happens if light is shone on the PERIPHERY of an ON-center ganglion cell receptive field?
Hyperpolarized surround photoreceptors –> reduced glutamate secretion –> reduced activity of excitatory receptors on horizontals cells –> horizontal cell hyperpolarization –> decreased HC GABA secretion onto photoreceptors in field center –> decreased inhibition of photoreceptors –> increased glutamate release to bipolar cells –> inhibition (ON-center) of bipolar cells –> reduced excitatory input to ganglion cells –> reduced ganglion cell firing rate
Which synapses in the light-ganglion cell pathway are always excitatory?
surround photoreceptor + horizontal cell
Bipolar cell + ganglion cell
Which synapses in the light-ganglion cell pathway are always inhibitory?
horizontal cell + photoreceptor
Which synapse can be either excitatory or inhibitory?
Field center photoreceptor + bipolar cell
What effect does the light environment have on ganglion cells?
uniform illumination –> minimal firing
High contrast –> maximal firing
In which hypercolumn layer do LGN axons terminate?
layer 4
Describe ocular dominance columns
One for each eye. Ganglion cells at corresponding regions of retina from each eye –> LGN –> adjacent areas of cortex
What are binocular cells?
cortical cells that receive input from both eyes
If you view the hypercolumn from the cortical surface, what will you see?
Ocular dominance columns and binocular cells
What hypercolumn structures handle information about color?
blobs
Describe the visual field line orientations represented in hypercolumns
lie in different rays of pinwheels radiating from central blobs. Cells in vertical columns have the same line orientation sensitivity
Describe the receptive field of simple cells
ON area that is a narrow line at a preferred orientation, flanked by OFF areas
How are receptive fields of simple cells represented on V1 cortex?
Ganglion cells with OVERLAPPING ON-center receptive fields –> lined up –> converge on V1 cell (excitatory) –> V1 cell with receptive field equal to the sum of LGN receptive fields –> ON-center of a line with flanking OFF regions
What stimuli do complex cells prefer?
lines or edges moving across the visual field
How are complex cells created?
convergence of several simple cells with slightly offset positions
How are color-opponent ganglion cells made?
Cones of different color preferences converge in the retina –> bipolar cells –> ganglion cells with receptive fields that are partial to particular colors
Where in the eye are the highest concentration of cones
Fovea. All photoreceptors are cones
What is the role of V1 in color processing?
V1 contains blobs, which are excited/inhibited by color. Blobs = not highly tuned to specific colors; unresponsive to shapes and white light
What is the role of V4 in color processing?
color discrimination
How is color discrimination achieved?
Cones respond to narrow bands of wavelength, Lateral inhibition –> further narrowing of wavelength response –> color discrimination
What is the “sensitive period” in development of visual cortex?
Period of time when the connections to binocular cells in visual cortex can be altered by visual experience. Age 2-3
What is the effect of monocular deprivation during the sensitive period?
loss of all cortical connections to the deprived eye.
What is the effect of binocular deprivation during the sensitive period?
Normal V1 with disruption of higher-order visual cells. More nonresponsive cells, but most can be driven and are binocular
What is the prevailing theory regarding cortical connections during the sensitive period?
Due to competition between inputs from the two eyes.
One eye receives input –> that eye drives all cortical cells
Neither/both eyes receive input –> binocularly driven cells
What effect does strabismus have during the sensitive period?
One eye deviates –> two eyes looking at different parts of the visual world, receiving equal stimulation –> few binocular cells.
Nearly all cells driven by one eye or the other
How is ocular dominance represented in hypercolumns?
Banded appearance from Layer 4 LGN axon terminals. Activity-dependent
