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order/pathway for vision
Light --> cornea --> pupil/iris --> aqueous humor (behind cornea but in front of lens) --> lens --> vitreous humor --> retina
duplex retina
use both rods AND cones
fovea
Cones no rods
Midget bipolar cell
One to one convergence
Small receptive field
High acuity
Low light sensitivity
prominent feature of fundus
Located near macula
Fovea is the approximate center of retina
Periphery
Rods
Diffuse bipolar cell
Many to one convergence
Large receptive field
Low acuity
High light sensitivity
**So we use rods to see when the light is low, and the cones take over when there is too much light for the rods to function well
light
- form of electromagnetic radiation
- Conceptualized as a wave OR a stream of photons
wavelength order
TV -> radio -> microwave -> heat -> Infared -> visible -> UV -> Xray -> Gamma ray
visible light is one part of EM spectrum
visible spectrum range
400-700 nm
1 nm
= 10-9 m
EM wavelength spectrum
logarithmic
speed of light
186,000 miles per second
Sky Color Explanation
color is because of scattering of sunlight by small particles
- Blue when the sun is high - short-wavelength (blue) light is scattered more strongly than other wavelengths
- Red at sunset, when the sun is near the horizon, because the sunlight must pass through more atmosphere near the Earth's surface, scattering more of the short-wavelength light allowing the longer-wavelength light to reach your eyes.
surface colors
light colored - reflected
dark colored - absorbed
if neither occurs it is TRANSMITTED through surface
cornea
- First tissue that light encounters
- Provides window to world
Transparent: light photons are transmitted through rather than reflected
- aqueous humos: a fluid derived from blood, filles space behind cornea to provide oxygen and blood
- Curved so it has a higher refractive index than air; most powerful refractive surface of eye
- Most powerful refractive surface in the eye is the cornea; contributes 2/3 refractive power
pupils
- To get to lens light must pass through pupil
- Pupil is simply hole in iris
- Low illumination: pupil large, depth of focus reduced, poor image quality
iris
- Muscular structure
- Gives eye color
- Pupillary light reflexes control size of pupil and amount of light reaching retina
- Light increases, iris expands
- Light decreases, iris contracts
lens
- Nonblood supply so it can be completely transparent
- Controlled by ciliary muscles
- accomodation occurs
Accommodation
- lens is only part of eye that can alter refractive power by changing its shape
- Change in focus
- Contraction of ciliary muscles
- lens attached to muscles through tiny fibers called Zonules of Zinn
- Ciliary muscles are relaxed, zonules are stretched, lens is flat
- To focus, ciliary muscles contract, reducing tension on zonules, enabling lens to bulge
- Fatter lens, more power it has, closer you can focus
power of lense
P=1/f where f is focal distance in meters
vitreous chamber
- Space between lens and retina
- Light goes from lens --> vitreous chamber --> retina
- Light refracted for the fourth and final time by vitreous humor
- Chamber composes 80% inner volume of eye
- Gel like and viscous and transparent
retina
- Only some light will reach retina: some lost in space or reflected (only ½ light from cornea reaches retina)
- surface on back of eye that contain actual photoreceptors that are sensory cells responsible for focusing light
- similar to cochlea in auditory system
- Neural structure of the eye where TRANSDUCTION takes place
- Layered sheet of clear neurons
Contains 100 million photoreceptors
rods
- 90 mil in each eye - more rods than cones
- Rods are absent from center of fovea
- Density increased peaks at about 20 degrees off fovea
- Drops with distance away from fovea
- Function well under DIM ILLUMINATION
- Rods all have same photopigment, can't sense color
cones
- 4/5 mil in each eye
- Become larger and sparser away from fovea
- require brighter illumination
- Has 3 photopigments; can sense color
four optical components of eye
cornea, aqueous humor, lens, vitreous humor
presbyopia
- Lens becomes harder and loses elasticity
- Opacities of lens are known at cataracts
- interferes with regularity of crystalline (class of proteins that make up lens and make lens transparent)
emmetropia
refractive power of four optical comp of eye is perfectly matched to eye
myopia
nearsidenes
eyeball too long
image of star focused in front of retina, star seen as blur instead of spot of light
Corrected with CONCAVE lense
hyperopia
farsightedness
eyeball too short
image focused behind retina, at young age can compensate and see clearly by accommodating
Corrected by conex lense
no trouble seeing far
ciliary muscles are ALWAYS doing work
find when you are young, however, prescription will just get worse and worse and worse
astigmatism
- if cornea is not spherical but shaped like a football, the result is astigmatism
- Vertical lines focused slightly in front of retina
- Horizontal lines focused slightly behind it
- Lenses with 2 focal points can correct
- Refractive surgery called Lasik
fundus
branching blood vessels called vascular tree; only place in body where one can see arteries and veins directly
- fundus of the eye is the back of the eye, opposite the lens, and is made up of the retina, optic disc, macula, fovea, choroid, & blood vessels
opic disk
Arteries and veins that feed retina, enter eye and where axons of ganglion cells leave eye via optic nerve
No photoreceptors
Blind spot
optical coherence tomography
imaging technique uses low-coherence light to capture high-resolution images from within light-scattering media
transduction
- Light energy is TRANSDUCED INTO NEURAL ENERGY once it hits retina and thus is interpreted by brain photoreceptors sense light
- stimulate neurons in the intermediate layers: including bipolar cells, horizontal cells, and amacrine cells
- neurons then connect with the frontmost layer of the retina, made up of ganglion cells
- axons of ganglion cells pass through the optic nerve to the brain
dark and light adaptation
Enter dark room from bright sunlight, number of photons of light entering eye reduced, trouble seeing, after 30 mins it returns to normal
four primary ways visual system adjusts to changes in illumination:
1) pupil size
2) photopigmentation regeneration
3) duplex retina: rods when light is low, cones when more light
4) neural circuitry: ganglion cell is most sensitive to differences in the intensity of the light in the center and in the surround of its receptive field
To sum up:
-First, we reduce the scale of the problem by regulating the amount of light entering the eyeball, by using different types of photoreceptors in different situations, and by effectively throwing away photons we don't need.
-Second, by responding to the contrast between adjacent retinal regions, the ganglion cells do their best to ignore whatever variation in overall light level is left over.
retina contains 5 major classes of neurons
photoreceptors, horizontal cells, bipolar cells, amacrine cells, and ganglion cells
photoreceptors
- capture light
- produce chemical change
- start a cascade of neural events ending in a visual sensation.
- send signals through synaptic terminals, specialized structures for contacting other retinal neurons
- terminals contain connections from the neurons that photoreceptors "talk to" the horizontal and bipolar cells
photoreceptors consist of:
- outer segment (which is adjacent to the pigment epithelium)
- an inner segment
- a synaptic terminal
visual pigment molecules
- made in the inner segment
- stored in the outer segment
-opsin: determines which wavelengths the pigment molecule absorbs
- Chromophore: captures light photons and determines color by absorbing specific wavelengths
- Opsin and chromophore are connected
4 types of visual pigment molecules: Rhodopsin, S cones, L cones, M cones
Rhodopsin
found in rods, concentrated mainly in the stack of discs in the outer segment
S cones
5/10% of total cone population - missing from fovea; blue
L cones
2x as many L cones as M cones; red
M cones
green
photoactivation
- photon from star makes its way to rod
- absorbed by rhodopsin
- transfers its energy to chromophore
- aka bleaching
hyperpolarization
- Photoactivation initiates a biochemical cascade of events
- closing cell membrane channels
- altering balance of current between inside/outside of rods outer segment
- making inside of cell negatively charged
- Lowering calcium concentration
- reduces concentration of neurotransmitter (glutamate) molecules
- signals to bipolar cells that rod has captured photon
graded potentials
Photoreceptors pass info using GRADED POTENTIALS not all-or-none AP
photopic system
- 4/5 mil cones
- located everywhere
- high acuity
- low sensitivity
- fast response speed
- no saturation
- recovery in less than 5 mins
- fast light adaption
- trichromatic
scopotic system
- 90 mil rods
- outside fovea
- low acuity
- high sensitivity
- slow response speed
- saturate at twilight levels
- revovery in 40 mins
- slow light adapt
- no color
horizontal cells
- run perp to photoreceptors
- plays role in lateral inhibition
- enables signals that reach retinal ganglion cells to be based on differences in activation between nearby photoreceptors
amacrine cells
- perp to photoreceptors in inner layers of retina aka lateral inhibition
- Receive inputs from bipolar cells
- send signals to bipolar, amacrine, and retinal ganglion cells
- 40 types
lateral pathway
horizontal and amacrine
vertical pathway
photoreceptors, bipolar cells, and ganglion cells
peripheral vision
- bipolar cell receive input from up to 50 PR
- Pools this information
- Passes it on to a M Ganglion cell
- Convergence of information from many photoreceptors to a single bipolar cell is a characteristic of the rod pathway
- Most rods communicate with ganglion cells through diffuse bipolar cells
-largely accounts for the ability of the rod system to function well in dim lighting conditions
- High degree of neural convergence in peripheral vision causing important consequences for visual acuity
foveal vision
- Midget bipolar cells
- receive input from single cones
- pass this information on to single ganglion cells
- one-to-one pathways between cones and - - ganglion cells exist only in the fovea
- accounts for why images are seen most clearly in this part of retina
- each foveal cone contacts two bipolar cells (representing a divergence of information):
1) One depolarizes in response to an INCREASE in light captured by the cone and is called an ON bipolar cell
2) The other hyperpolarizes and is called an OFF bipolar cell.
ganglion cells
the final layer of retina
two types: P and M
the bistratified ganglion cells also known as koniocellular cells make up the final 20%
P ganglion cells
- midget bipolar cells send their signals to small ganglion cells (p cells)
- constitute 70% of the ganglion cells
- P ganglion cells have much smaller dendritic trees than do the M ganglion cells
- Smaller receptive fields than M cells
- finer resolution
- P cells provide information mainly about the contrast in the retinal image
M ganglion cells
- Diffuse bipolar cells project M ganglion cell
- dendrites spread out much more
- 8-10% of ganglion cells
- Listen to more photoreceptors; diffuse bipolar, horizontal, amacrine
- More sensitive
- Better able to detect visual stimuli under - -- low light conditions
- Signal information about how the image changes over time.
receptive fields
- ganglion cell has small window on world known as its receptive field
- region on the retina in which visual stimuli influence the neuron's firing rate
- excitatory, increasing the ganglion's firing rate
- inhibitory, decreasing the ganglion's firing rate
- Ganglion cells fire AP spontaneously even without stimulus
Kuffler experiment
- This area of the retina is called the "center" of the ganglion cell's receptive field
- The ganglion cell fires fastest when the size of the spot of light matches the size of the excitatory center
- Antagonistic interaction between center & surround is known as lateral inhibition and is mediated in part by horizontal cells.
- On center cells and Off center cells
- Retinal ganglion cells act as a filter by responding best to stimuli that are just the right size and less to stimuli that are larger or smaller.
- Second, ganglion cells are most sensitive to DIFFERENCES in the intensity of the light in the center and in the surround, and they are less affected by the average intensity of the light.
on center cells
- increases its firing rate when a light is turned on in the center of its receptive field
- decreases its firing rate when the light is turned on in the surround.
off center cells
- firing rates decrease when a light is turned on in a spot in the center of the receptive field
- increase when a light is turned on in a spot in the surrounding area
order cells
photoreceptor
horizontal
bipolar
amacrine
ganglion
pathways
rods -> diffuse bipolar -> M ganglion
cones -> midget bipolar -> P ganglion
contrast
the difference in illumination between the stripes and the background
visual angle
the angle that would be formed by lines going from top and bottom (or left and right, depending on the orientation of the stripes) of a cycle on the page, passing through the center of the lens, and ending on the retina
Bigger objects cast larger images on the retina than smaller objects
- larger the object is the larger its visual angle will be
Closer objects cast larger images on the retina than smaller objects
- closer the object is to the eye, the larger its visual angle w
cycle
one repetition of a black stripe and a white stripe
- if the receptors are spaced such that the whitest and blackest parts of the grating fall on separate cones, we should be able to make out the grating
- If the entire cycle falls on a single cone, we will see nothing but a gray field
- Shade found that wider stripes are NOT easier to distinguish the light stripes from dark stripes with decreased contrast
resolution acuity
represents one of the fundamental limits of spatial vision: it is the finest high-contrast detail that can be resolved
cones in fovea center seperation
0.5 minute of arc (0.008 degree)
vertical meridian asymmetry
Visual acuity falls off more rapidly along vertical midline of visual field than horizonal midline
visual cluster
Objects that can be easily identified in isolation seem indistinct and jumbled when surrounded by other objects
central vision slower than peripheral because
Foveal cones have longer axons than peripheral cones in order to allow dense packing in the central fovea, and the longer axons transmit slow signals better than fast ones
minimal visual acuity
is a limit in the ability to discern small changes in contrast
not used clinically
minimal recognizable acuity
refers to the angular size of the smallest feature that one can recognize or identify
the smallest letter that can be recognized
minimal resolvable acuity
refers to the smallest angular separation between neighboring objects that one can resolve
the finest stripes that can be resolved
minimal discriminable acuity
efers to the angular size of the smallest change in a feature (ex: change in size, position, or orientation) that one can discriminate
The smallest misalignment that we can discern is known as Vernier acuity
spatial frequency
number of times a pattern repeats in a given unit of space
number of cycles per degree of visual angle
Wider stripes = lower spatial frequency
contrast sensitivity function - CSF
- A function describing how the sensitivity to contrast (defined as the reciprocal of the contrast threshold) depends on the spatial frequency (size) of the stimulus.
- Shaped like upside down U
- Contrast threshold: The smallest amount of contrast required to detect a pattern.
- Contrast sensitivity just inverse of contrast %
Shade found that wider stripes are NOT easier to distinguish the light stripes from dark stripes with decreased contrast
on-off center retinal ganglion cells
responds to gratings of different spatial frequencies
- When the spatial frequency of the grating is too low, the ganglion cell responds weakly because part of the fat, bright bar of the grating lands in the inhibitory surround, damping the cell's response
- When the spatial frequency is too high (close together limited space between aka smaller stripes), the ganglion cell responds weakly because both dark and bright stripes fall within the receptive-field center, washing out the response
Optimal sized gratings BUT ganglion cell responds differently depending on phase
- tuned to spatial frequency; acts like a filter
magnocellular layers
Neurons in bottom 2 layers are physically larger than top
Receive input from M ganglion cells in the retina
Respond to large, fast-moving objects
parvocellular layers
Top four layers
Receive input from P ganglion cells
Processes details of stationary targets
inputs to eyes
1: Left LGN receives projections from left side of retina in BOTH eyes
2:
Layers 1, 4, 6 of right LGN receive input from left contralateral eye; going from bottom to top
Layers 2, 3, 5 receive input from right ipsilateral eye
names for V1
primary visual cortex, area 17, and striate cortex
LGN fibers
project mainly to layer 4C
Magnocellular axons coming into upper part of 4Ca
Parvocellular axons coming to lower part of 4CB
orientation tuning function of a cortical cell
- neuron fires vigorously when the line is oriented vertically; hardly at all when the line orientation is changed by 30 degrees
- this example is for a cell tuned to vertical lines, other cells are tuned to different orientation
- selective responsiveness orientation tuning
- Cortical cells also respond well to gratings
- Cortical cells more narrowly tuned than retinal ganglion cells
- Cortical cells respond well to moving lines
receptive fields of striate cortex neurons
- not circular, as they are in the retina and LGN
- they are elongated: as a result, they respond much more vigorously to bars, lines, edges, and gratings than to round spots of light.
- this selective responsiveness orientation tuning: the cell is tuned to detect lines in a specific orientation
- circular receptive fields in the LGN transformed into the elongated receptive fields in striate cortex because (LGN) cells are lined up in a row, feeding into the elongated, linear arrangement of the striate cortex receptive fields.
ocular dominance
The property of the receptive fields of striate cortex neurons by which they demonstrate a preference, responding somewhat more rapidly when a stimulus is presented in one eye than when it is presented in the other.
simple cell
A cortical neuron whose receptive field has clearly defined excitatory and inhibitory regions
edge detector
most highly excited when there is light on one side of its receptive field and darkness on the other side
stripe detector
responds best to a line of light that has a particular width, surrounded on both sides by darkness
complex cells
A cortical neuron whose receptive field does not have clearly defined excitatory and inhibitory regions
each complex cell is tuned to a particular orientation and spatial frequency and shows an ocular preference
whereas a simple cell might respond only if a stripe is presented in the center of its receptive field, a complex cell will respond regardless of where the stripe is presented, so long as it is somewhere within the cell's receptive field
simple cells are "phase-sensitive" and complex cells are "phase-insensitive."
end stopping
The process by which a cell in the cortex increases its firing rate as the length of a bar increases until the bar fills up its receptive field, and then it decreases its firing rate as the bar is lengthened further.
columns
- a vertical arrangement of neurons
- Neurons within a single column tend to have similar receptive fields and similar orientation preferences.
- orientation is not the only property arranged in columns in the visual cortex
- Neurons that share the same eye preference (exhibiting ocular dominance) also have a columnar arrangement
hypercollumn
A 1-millimeter block of striate cortex containing two sets of columns, each covering every possible orientation (0-180 degrees), with one set preferring input from the left eye and one set preferring input from the right eye.
CO blobs
Regular arrays of "blobs" spaced about 0.5 millimeter apart in the striate cortex (V1), so named because their presence is visualized by staining with the enzyme cytochrome oxidase. They may function in color perception.
adaption
the diminished response that follows previous exposure to a stimulus
gives psychologists a noninvasive "electrode" they can use to probe the human brain.
Selective adaptation (i.e., adaptation to a limited set of stimuli) can provide insights into the properties of cortical neurons
Example: gratings oriented at 0 degrees (vertical) elicit the strongest response from the 0-degree selective cells
- Suppose we expose the visual system that contains these cells to a 20-degree grating for an extended period
- This adapting stimulus causes the 20-degree selective cells to be most active
- the difference between the lighter and darker bars corresponds to the degree of fatigue for each type of cell.
tilt aftereffect
The perceptual illusion of tilt, produced by adaptation to a pattern of a given orientation
spatial frequency channels
A pattern analyzer, implemented by an ensemble of cortical neurons, in which each set of neurons is tuned to a limited range of spatial frequencies.
preferential looking
one important method used by infant researchers
- an infant is shown two patches, one containing stripes and the other uniform gray, the infant will prefer to look at the stripes.
infant vision
- the rod system appears to be functional in early infancy
- It is now reasonably well established that by 2-3 months after birth, infants must have three functioning cone types
- Infants less than 1 month old fail to make chromatic discriminations; however, what remains unclear is whether these failures reflect immature cones or postreceptoral mechanisms
border ownership
distinguished by V2 cells; would differentiate betweenthe edge of a black square on a gray background and the edge of a gray square on a black background
where pathway
- heads up (or dorsally) into the parietal lobe
- visual areas in this pathway seem to be important for processing information relating to the LOCATION of objects in space and the ACTIONS required to interact with them (moving the hands, the eyes, and so on)
- an important role in the deployment of attention
what pathway
- heads down (ventrally) into the temporal lobe
- appears to be the locus for the explicit acts of object recognition
- As we move into the temporal lobe, receptive fields get much bigger and, as the pathway's name implies, WHAT the object is seems more important than WHERE it is
- contains IT or inferotemporal cortex
- what pathway moves through a succession of stages building a representation of your grandmother or your dinner or the Eiffel Tower out of the very specific, very localized spots, lines, and bars that interest cells
- seemingly simple acts of object identification require a lot from what pathway
IT cortex
- usual spots and lines didn't work well at all
- BUT silhouette of a monkey hand worked fantastically for some cells
hierarchical model of visual perception
- small networks of cells that might fire when you see your grandmother or some other highly specific object or type of object
- Grandmother cell - any cell that seems to be selectively responsive to one specific object
- IT cortex maintains close connections with parts of the brain involved in memory formation, notably the hippocampus.