The Visual System Flashcards
Discuss the major steps in phototransduction in the outer segments of rods
Phototransduction: the process by which light is converted to a change in membrane
potential by the photoreceptors.
Light absorption causes photoreceptors to hyperpolarize.
To catch light, membrane protein is packed at high density in surface membrane infoldings (cones) or intracellular membranous sacks (rods), photon has 50% chance of capture (absorbed by Vit A bound to protein).
Light stimulates rhodopsin, leads to G prot. transducin activation, which activates cGMP phosphodiesterase–>hydrolizes cGMP. Ultimately light reduces concentration of cGMP, leading to closure of Na channels (hyperpolarization)
Describe the receptive field properties of retinal ganglion cells.
Also give examples of the chain for OFF-center ganglion cell and ON-center ganglion cell
Photoreceptors talk to bipolar and horizontal cells.
Bipolar cells talk to ganglion cells. Only ganglion cells make APs, other cells talk by changes in membrane potential and altered NT release.
Retina wired to detect contrast (first level of edge/corner/shape detection).
Receptive field: the best stimulus to get sensory neur to change AP firing rate.
Ganglion cells: donut-shaped receptive fields.
On center ganglion cells: excited by light shining in their centers and inhibited by light in periphery.
Off center ganglion cells (opposite of on).
In fovea, receptive field as wide as single cone. Larger fields in periphery of retina.
Key determ in receptive field type of ganglion cells is the type of receptor on bipolar cells.
Remember: {photorecep are hyperpol by light (less NT), photorecep release glutamate; bipolar cells can be either excited (OFF center) or inhibited (ON center) by glutamate; Bipolar cells always make excitatory synapses on ganglion cells}
OFF center: if glutamate excites a bipolar cell, then shining light on a photoreceptor will lead to INHIBITION of the ganglion cell.
ON center: if glutamate inhibits a bipolar cell, light relieves inhibition, EXCITING the ganglion cells.
Describe color-opponent ganglion cells
In fovea, most bipolar cells are connected directly to one kind of cone in the field center (ie red) and indirectly via horizontal cells to cones with a different color preference (ie green) in the field surround–>creates RED ON center and GREEN OFF surround receptive field. (All combos of red-green on-off exist)
=Color opponent cells. (red green and blue yellow opponent cells exist (yellow by converging red and green cones)
Identify where color is processed in the cortex
Color info is separated out from spatial info in the retina, and is handled in central regions of hypercolumns called blobs.
Visual cortical area V4 is considered “color” area while V5 is considered motion area.
Discuss the receptive field characteristics of cortical simple and complex cells and describe how these receptive field properties are achieved by synaptic inputs from lower order cells
A simple cell might
have an ON area
that is a narrow line
at some preferred
orientation that is
flanked on each side
by OFF areas. Diffuse light is ineffective.
While some simple cells have ON-centers with OFF flanking lines, others are the
reverse.
Simple cells have receptive fields with antagonistic flanking regions; the
shape of the field is a straight line and the orientation of the line is crucial.
Hierarchical processing: Several cells with similar but spatially offset receptive fields (concentric cells) converge on a higher order cell to create an altogether new type of receptive field. (The cortical cell will then have a receptive field that is the sum of the LGN cells’ receptive fields)
Describe the meaning of a sensitive period in the development of the cortex and discuss the importance of this concept in diagnosing and treating abnormal development
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Indicate the conditions under which the effects of abnormal developmental experiences can be reversed.
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Intensity corresponds to___, wavelength corresponds to_______
intensity->brightness
wavelength->color
Focusing power of eye
Cornea 2/3
Lens 1/3 (under neural control, allows for focusing of nearby objects)
Lens suspended by zonule fibers attached to ciliary muscle
Lens gets fatter shape to focus on nearer objects.
What gives rise to the blind spot?
optic disc (where retinal ganglion cells group together in optic disc to form optic nerve)
5 types of neurons in retina
photoreceptors face the back of the eye (rods–color insensitive and work best in dim light, cones–color vision, work only in bright light, concentrated in fovea (not rods))
Bipolar cells
Horizontal cells
Ganglion cells (output cells of retina)
3 properties of light
reflection
absorption
refraction
What mediates the receptive field surround?
Horizontal cells
Behave as though they have excitatory receptors for glutamate released from photoreceptors and make inhibitory synapses on the photorecep in field center.
So: if a spot of light is shined on periphery of an ON-center ganglion recep field, photorecep in surround will hyperpolarize, reducing secretion of NT, reducing activation of excitatory recep on horizontal cells, which will hyperpolarize horizontal cells. This will DECREASE GABA secretion onto field center photoreceptors and will decrease inhibition of photoreceptors, causing them to release more NT. Since this is an ONcenter
cell, the receptors on the bipolar cells in the field center are inhibitory. So the inhibition of
the bipolar cells will increase when light shines on the periphery, which will reduce the bipolar
cell excitatory input to the ganglion cell, which will reduce the firing rate of the ganglion cell.
4 synapses of photorecep pathway. Which are excitatory and which are inhibitory?
2 are excitatory (NT depol cell): surround photoreceptor to horizontal cell synapses; and bipolar to ganglion cell synapses.
One is always inhibitory: horizontal cell to photoreceptor synapse
One is either excitatory (OFF center bipolar cells) or inhibitory (ON center bipolar cells) field center photoreceptor to bipolar cell.
What is the response to light in inhibitory area (center or surround) being turned off?
rebound response when light is turned off. (abrupt removal of inhibition)
What do ganglion cells care about?
CONTRAST
Pathway of optic nerve
Retinal ganglion cells form the optic nerve. Optic nerves cross at optic chiasm. Half of the axons from each eye cross to other side and continue in optic tract to lateral geniculate nucleus (LGN) of the thalamus.
At the chiasm, axons from the NASAL half of each retina cross over to the opposite side (decussate). So: right optic tract contains axons from RIGHT of each retina, which see the LEFT side of the world. The LGN represents the contralateral visual field.
After LGN, axons fan out in optic radiations to visual cortex in back of brain.
LGN layers
6 layers, no direct interaction between eyes at level of LGN (not binocular).
Layers 1, 4, 6 receive inputs from contralateral eye (decussates)
Layers 2, 3, 5 receive inputs from the ipsilateral
eye
Layers 1-2: inputs from magnocellular ganglion cells Magnocellular System Spatial Vision – Motion and depth 1. Low acuity (crude form) 2. Large receptive fields 3. Responsive to motion 4. No color vision (input from rods)
Layers 3-6: inputs from parvocellular ganglion cells Parvocellular System Object Vision – color, form, detail 1. High acuity (fine detail) 2. Small receptive fields 3. Not responsive to motion 4. Color vision (input from cones)
Explain the meaning of “ocular dominance” in cells of the visual cortex and indicate its physical basis.
LGN axons rad to V1 (primary visual cortex) in cortex (forms hypercolumns)
Layers 1 (top) thru 6.
LGN axons terminate in layer 4.
Columns divided in two parts, one half for each eye; these are called
ocular dominance columns.
About half of the cortical cells (near border b/t two eyes in hypercolumn) become BINOCULAR–receive inputs from both eyes.
All cells in a vertical column are sensitive to the same ORIENTATION of lines/light (different rays of pinwheels).
Color information is handled in central regions of hypercolumns called “BLOBS”.
Discuss the changes in ocular dominance caused by monocular deprivation, binocular deprivation, and alternating monocular deprivation.
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Binocular cells
Half of the cells in V1 receive input from LGN from both eyes (binocular). The receptive fields are virtually identical. Sensitive to and mediate depth perception
Complex cells
Have receptive fields like simple cells BUT they abstract for spatial POSITION. The line or edge can be anywhere within the RF.
Created by convergence of several simple cells whose positions are slightly offset. Any simple cell can then cause complex cell to fire.
Simple and complex cells are located together in the same hypercolumns.
(LGNs into layer 4 creating simple cells. Simple cells send axons up and down in same hypercolumn to higher and lower cortical layers, creating complex cells. Output of hypercolumn from 3-6 to higher order visual areas.
How many cone types do humans have?
3: blue (short), green (middle), red (long)
A single cone cannot encode color info (need relative activities of 3 types, and NS uses info to create cells in cortex to respond only to particular colors).
Parallel processing
Hierarchical processing
Parallel: Dissimilar dimensions (eg color and form) must be analyzed by separate but parallel neural systems.
Hierarchical: As we ascend the visual system, higher order cells survey lower order cells and abstract the collective properties. Eventually, cells that respond only to complete form (eg “face” cells)
