Flashcards in vision I-III Deck (62)
components of light
Wavelength: corresponds to color. Amplitude: corresponds to intensity or brightness
Components of eye that contribute to focusing/refractive power
cornea: 2/3. Lens: 1/3
size of pupil is controlled by
what is the blind spot
the optic disc contains no photoreceptors
Retinal neurons and flow of information
Photoreceptors (rods and cones) capture photons of light and convert (transduction) them to an electrical signal (change in membrane potential), which is passed synaptically to bipolar cells and horizontal cells, and then to the output cells of the retina, the ganglion cells.
Location of retinal neurons
From front (where light enters) to back: ganglion cells > bipolar and horizontal cells > cones and rods. This set up means that light must pass through all of the other cells to get to the photoreceptors, then the signal is passed "backwards" to the optic nerve. The other retinal cells are nearly transparent, making this possible
what is the fovea
region where acuity is greatest- cones are concentrated here and only work well in bright light
compare the anatomy of rods and cones
rods: Have intracellular membranous sacks/disks containing membrane proteins in the outer segment, and mitochondria/nucleus in inner segment, then a synaptic terminal. Longer than cones. Cones: Outer segment contains surface membrane infoldings, and the rest is the same as rods
What are the major steps in phototransduction
Photons are absorbed by rhodopsin which is membrane bound in disc > G protein, transducin, activation > cGMP phosphodiesterase activation > cGMP is degraded > nonselective cation channels in surface membrane close > cell is hyperpolarized
What is unique about ganglion cells
Only the ganglion cells make action potentials; all of the other retinal cells communicate by graded changes in membrane potential, which alter the rate of exocytosis of neurotransmitters in a graded fashion. Ganglion cells fire action potentials spontaneously in dark (cell is depolarized)
What is a receptive field
The best stimulus to get a sensory neuron to change action potential firing rate
What are the receptive field properties of retinal ganglion cells?
Donut shaped receptive fields. Two types: "On" center ganglion cells are excited by shining light in center (and inhibited by light in periphery). "Off" center ganglion cells are turned off by shining light in center (and turned on by light in periphery)
Compare receptive field in peripheral vs central vision
receptive field is 100 times larger in peripheral vision than in central vision
When is central vs peripheral vision used
Central vision (fovea, high conc of cones) is used in daylight wth high resolution. Peripheral vision (rods) is used in dark, with poor resolution.
compare numbers of rods and cones
100 million rods, 8 million cones
What neurotransmitter do photoreceptors release
What action does glutamate have on bipolar cells
Excitatory (Off center) or inhibitory (on center), due to different receptor types
will a bi polar cell with inhibitory glutamate receptors be inhibited or excited in the dark? In the light?
dark: tonically inhibited. Light: turned on
function of horizontal cells
Mediate receptive field: They behave as though they have excitatory receptors for glutamate from photoreceptors, and make inhibitory synapses on neighboring photoreceptors in receptive field.
center vs sourround in receptive field
the center is represented by rods which are connected to ganglion cells directly via bipolar cells The surround represents rods which are connected to bipolar cells, then horizontal cells, then the ganglion cell
In receptive field, which synapses are always excitatory
The surround photoreceptor to horizontal cell synapses, and bipolar cell to ganglion cell synapses
In receptive field, which synapses are always inhibitory
horizontal cell to photoreceptor synapses.
In receptive field, which synapses may be either excitatory or inhibitory
field center photoreceptor to bipolar cell
What is the rebound response in inhibitory area
When light is shone on an inhibitory area (center or surround), there will be a rebound response when the light is turned off. This is from the abrupt removal of inhibition
compare the responses in on-center vs off-center ganglion cells when light shines in the middle of receptive field, outside of receptive field only, small portion of outer receptive field, entire field
Middle only: on center fires Aps, off center has no Aps. Outside only: on center doesn’t fire AP's, off center fires Aps. Small portion of outer field: on center fires one AP, off center first few AP's. Entire field: on center fires a few AP's, off center fires a few AP's
Do ganlgion cells repond to absolute or relative light
Describe the decussation of optic nerve fibers
At chiasm, axons from nasal half of each eye crosses midline then continues as optic tract to lateral geniculate nucleus of thalamus.
Does the LGN represent the ipsilateral or contralateral visual field
contralateral- Fibers which see the right field of view (but are located on the left side of each eye) synapse in the left lateral geniculate nucleus
List the pathway from the eyes to the brain
optic nerve > optic chiasm > optic tract > LGN > optic radiation > visual cortex
pattern of optic tract projections to cortex
retinotopic projection- lower half of each retina projects to lower half of each visual cortex, etc.
lateral geniculate nucleus layout
Layers 1,4,6 receive input from contralateral eye, while layers 2,3,5 receive input from ipsilateral eye. Additionally, layers 1 and 2 receive inputs from magnocellular ganglion cells and layers 3-6 receive input from parvocellular ganglion cells
What kinds of stimuli do parvocellular ganglion cells detect
Object visioin- color, form, and detail. High acuity, small receptive fields, not responsive to motion, color vision from cones
What kinds of stimuli do magnocellular ganglion cells detect
spatial vision- motion and depth. Low acuity, large receptive fields, responsive to motion and no color vision (input from rods)
Pathway of magnocellular and parvocellular systems
Segregated at LGN, travel in parallel but separate pathways through visual cortical areas
What are the receptive field characteristics of cortical simple and complex cells? How are these receptive field properties achieved by synaptic inputs from lower order cells?
Simple: stimulated by a narrow line of light/ on area that is flanked on each side by off areas. Diffuse light is entirely ineffective. The spatial position and orientation of the line is also crucial. Orientation columns, which are up and down like the hypercolumns, are organized as pinwheels horizontally, such that each orientation column has the same orientation, but adjacent orientation columns have a slightly different orientaion.
Draw the major features of a hypercolumn.
In primary visual cortex (V1), these are 1mm regions layered from layer 1 at surface of brain to layer 6 at border btw grey and white matter. Each column has two ocular dominance columns (one half for each eye), with about half of the cells being binocular and receiving inputs from both eyes (these are the cells at the interface btw left and right eye). Each column also has pinwheels radiating from central blobs which orient lines in the visual field. The blobs are central regions of the columns which handle color
Where do LGN axons terminate in the primary visual cortex
What are the receptive field characteristics of cortical simple cells
simple cells have straight line fields . Can have on centers and off flanking lines, or opposite. Diffuse light is entirely ineffective. The spatial position and orientation of the line is also crucial. Orientation columns, which are up and down like the hypercolumns, are organized as pinwheels horizontally, such that each orientation column has the same orientation, but adjacent orientation columns have a slightly different orientaion.
How are simple cell receptive fields achieved by synaptic inputs from lower order cells?
Hierarchical processing: Several cells with similar but
spatially offset receptive fields converge on a higher order cell to create an altogether new type of receptive field. Several overlapping ON-center LGN and ganglion cell receptive fields line up along a line, and many of these cells will converge on one cortical cell in V1. The cortical cell will have a receptive field that is the sum of the LGN cells receptive fields. Hierarchical processing: Several cells with similar but
spatially offset receptive fields converge on a higher order cell to create an altogether new type of receptive field. Several overlapping ON-center LGN and ganglion cell receptive fields line up along a line, and many of these cells will converge on one cortical cell in V1. The cortical cell will have a receptive field that is the sum of the LGN cells receptive fields.
function of binocular cells
receive inputs from LGN from both eyes. The receptive fields of the two eyes are identical (same orientation, same region of retina, same width of line, same on off organization). Mediate depth perception
What are the receptive field characteristics of cortical complex cells?
Complex cells have receptive fields like simple cells except they abstract for position, meaning position of the line is not as important
How are complex cell receptive fields achieved by synaptic inputs from lower order cells?
Convergence of several simple cells whose positions are slightly offset. The converging simple cells on the complex cell make excitatory synapses, and any single simple cell can cuase the complex cell to fire.
Where are simple and complex cells located
In the same hypercolumns- LGN axons terminate in layer 4, creating simple cells, then simple cells send axons up and down the same hypercolumn to higher and lower cortical layers, creating complex cells.
How does one hypercolumn relate to its neighbors
Because of the retinotopic projection, neighboring hypercolumns attend to neighboring retinal regions. The ocular dominance columns line up in stripes. The orientation column pinwheels spin out over the
cortical surface, interconnected with neighboring hypercolumns Because of the retinotopic projection, neighboring hypercolumns attend to neighboring retinal regions. The ocular dominance columns line up in stripes. The orientation column pinwheels spin out over the
cortical surface, interconnected with neighboring hypercolumns
For photoreceptors List: location, diffuse light, receptive field shape, orientation selective, binocular driven and position sensitive?
retina, diffuse light ok, tiny spot, not orientation selective, not binocularly drive, yes position sensitive
For ganglion cells List: location, diffuse light, receptive field shape, orientation selective, binocular driven and position sensitive?
retina, diffuse light so-so, donut, not orientation selective, not binocularly drive, yes position sensitive
For simple cells List: location, diffuse light, receptive field shape, orientation selective, binocular driven and position sensitive?
cortex, diffuse light no, bar, yes orientation selective, yes binocularly drive, yes position sensitive
For complex cells List: location, diffuse light, receptive field shape, orientation selective, binocular driven and position sensitive?
cortex, diffuse light no, edge, yes orientation selective, yes binocularly drive,no position sensitive
Types of cones
blue, green and red
What are color-opponent ganglion cells?
Ganglion cells with receptive fields that are partial to particular colors. In fovea, most biopolar cells are connected directly to one kind of cone (ie red) and indirectly, via horizontal cells, to cones with different color (ie green). This would create a red-ON center and Green-OFF sourround receptive field, for example, which is passed to the ganglion cells. All combos of red-green and blue-yellow on-off fields exist.
Where is color processed in the cortex?
ventral pathway - ending in temporal lobe. Blob cells play role
What is parallel processing
For different dimensions of the image (e.g. shape, color, motion, spatial information) we have
analogous systems that use hierarchical processing to construct higher levels of representation in
their dimensions. This means that dissimilar dimensions (color and form) are analyzed by separate, but parallal, neural systems
What is hierarchical processing
successive synaptic integration of highly specific synaptic inputs to construct higher and higher levels of representation of the retinal image
dorsal vs ventral parallel vision pathways and functions
The Dorsal Pathway travels from V1 dorsally through thick stripe of V2, then onto V5 (MT, middle temporal) and finally to the parietal lobe and is responsible for spatial vision, including motion and depth perception. The Ventral Pathway travels ventrally from V1 to stripe and interstripe of V2, then to V4 and finally to the temporal lobe and is responsible for object vision, including color, form, and pattern vision
Stripe regions of V2 receive input from where?
blobs in V1- only care abour color. They do not have center-surround anatomy but rather a uniform area within which light of one color excites cell and light of another color inhibits cell
Categories of ocular dominance
Category 1 cells: monocular, driven only by the eye contralateral to cortical cell. Category 4: binocular, driven equally by both eyes. Category 7: monocular, driven only by eye ipsilateral to cortical cell. All other categories contain some degree of binocular cells
How is cortical wiring of binocularly driven cells determined
its genetic- when cell receives inputs from both eyes, the receptive field positions and orientations in the two eyes are identical from birth
what happens with monocular deprivatioin
if one eye of a kittne is closed for even a few days, the cortical cells lose all connections to the deprived eye andalmost all cells become monocular. If one eye of a cat is closed for a few days, there is no change. This suggested disuse atrophy as an explanation for visual cortex wiring
Critical/sensitive period in cortex development
Period of time when connections can be altered by visual experience. Lasts about 3 months in kittens, with maximal sensitivity at 4-6 weeks. In humans, it is about 2-3 years Once the connections are lost, they can not be recovered.
What happens with binocular deprivation
When both eyes are closed in a kitten for a short amount of time, the primary visual cortex is mostly normal, about 50% binocular cells, but the cats were behaviorally blind (higher order visual cells were completely disrupted). Showed competition btw converging synaptic inputs from two eyes is mechanism of wiring in visual cortex
What happens with alternating monocular deprivation
The first eye that is deprived will lose cortical connection. When the first eye is then kept open and the second eye is then deprived, the first eye recovers and the newly deprived eye loses its connections with the cortex. This suggests that there is active suppression by the active eye.