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
