perception exam 2 - textbook

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.