Perception: The Visual System- Light, Optics, And The Eye; Retinal Processing And Early Vision

Front 1

Ether; which theories preceded dual-theory?

Back 1

a massless fabric if space through which light was thought to travel - this is what the particle theory was based on. It went particle theory then wave theory (diffraction) then dual-theory

Front 2

Dual- theory of Light

Back 2

the idea that the photons of light have wave-like motion when they travel. This provided an explanation for diffraction, and other phenomena such as electron emission when a light strikes and object

Front 3

electromagnetic radiation (3)

Back 3

-the energy radiated in the form of a wave that results form motton of a charged oscillating particle. it is found everywhere in nature.
- composed of 2 waveforms: electric and magnetic
- the waveforms are linked and perpendicular and vary over time

Front 4

Wavelength (4)

Back 4

- the electromagnetic waves are made by fluctuations in electric or magnetic fields as a function of space or distance
- one full cycle (separating two identical parts of the waveform) is known as a wavelength
- a variety of different units are used to describe wavelength such as meters for long ones and nanometers for small ones

Front 5

Electromagnetic spectrum

Back 5

-the spectrum of wavelengths of electromagnetic radiation
- shortest wavelength is gamma rays and longer ones like radio, tv, microwaves, with the longest being ELF waves

Front 6

visible light; Why can we only see this?

Back 6

400- 700 nanometers, in between ultraviolet and infrared rays; The emission spectrum of the sun is restricted to a narrow band spanning from 100- 4000 nm and the transmission spectrum of the atmosphere overlaps this, and the visible light band is where transmission efficiency is at its highest

Front 7

speed of light in a vacuum; light ray

Back 7

299,792 kilometers per second; the straight-line path of a travelling photon (a way of representing light propagation)

Front 8

optical infinity

Back 8

the distance from the point source, at which the diameter of the wave becomes so large that it essentially flattens out- about 6 meters

Front 9

inverse squares law

Back 9

relationship between light intensity and distance- intensity is inversely related to the square of the distance. For example at a distance of 3d, the intensity would diverge by 9 times. I= 1/dsquared

Front 10

What two things can happen when light strikes a surface?

Back 10

Absorption- when the objects atoms can convert the light energy int vibrational motion which is dissipated by as heat
reflection- the object doesn't turn it to vibrational energy but rather its atoms remit back the light energy

Front 11

How is colour related to absorption and reflection?

Back 11

if an object reflects long wave length light but absorbs the rest, the object will appear red. If it only reflects short wavelength light, it will appear blue... objects that reflect all visible light will be white and absorb all wavelengths will be black

Front 12

light scattering; 3 types of atmospheric scattering

Back 12

when light interacts with gaseous particles light is first absorbed and then re-emitted in a random direction;
1) Rayleigh scattering (particles of oxygen or nitrogen are small compared to wavelength of light, resulting in a greater amount of scattering of short-wavelength radiation vs long- the reason the sky looks blue and sunsets are deep red!! occurs above 4.5 km)
2) Mie Scattering (occurs with larger particles like dust, smoke,water, and is less wavelength specific. most of the scattering happeneds in direction of light propagation. Reason for the white glow around sun. under 4.5m)
3) non-selective (particles are much larger than wavelength of light. this is also wave-length unspecific and so all lengths are reflected same= white. reason clouds are white)

Front 13

Plane-polarized light; unpolarized light

Back 13

a type of light in which the electric field is restricted to only one plane of oscillation and so the magnetic field must be restricted to oscillate in the opposite perpendicular plane to it. so if electric is oscillating only in vertical plane, magnetic must oscillate horizontally;
most common form of light- where the electric field oscillates within all different planes

Front 14

What are 3 conditions that create polarized light?

Back 14

1) scattered light by a particle can be polarized depending on the direction of scattering.
2) through transmission through certain crystals that absorb more light in one plane than another like tourmaline (absorption)
3) under certain conditions, unpolarized light can become polarized after reflection, with the electric field orienting along the field causing the reflection ex polaroid sunglasses (reflection)

Front 15

refraction; refractive index

Back 15

bending of light as it moves from one medium to another; a way to indicate the speed of light in a particular medium- the greater the refractive index of a medium, the slower light travels in it and the greater the amount of refraction at its boundaries

Front 16

"normal"

Back 16

the imaginary line perpendicular to the boundary separating two mediums
I / - angle after entering medium
I /
I ------ - normal

Front 17

concave lens; convex lens; optic axis

Back 17

a diverging lens )( ; a converging lens () ; an imaginary line connecting the midpoints of the two refracting surfaces of a lens or other element ----)(-----optic axis

Front 18

How is Refractive power of a lens determined? What = greater RP? how can we change RP? How is it measured?

Back 18

- simply by how much refraction will occur at its two surfaces.
- a lens with a greater curve vs flatter will have greater refractive power
- to alter refractive power you could change the density of the material or you could alter the curvature of the two surfaces.
- measured in diopters= 1/focal length

Front 19

The greater the refractive power of a lens....

Back 19

the smaller the image distance. The smaller the object distance, the greater the image distance for any lens (therefore if you move closer to object, image is shifted behind retina)

Front 20

sclera; cornea; aqueous humour; vitreous humour; crystalline lens (lens); iris; pupil... so two refractive components of eye are...

Back 20

tough outter membrane - appears as white of eyeball; transparent part of eye (refractive);
liquid immediately behind cornea;
gelatinous liquid in front of retina;
separates the aqueous humour and vitreous humour (refractive);
opaque curtain-like structure (different colours);
hole in centre of the iris that lets light in;
lens and cornea (cornea provides 2/3 of the refractive power)

Front 21

Schematic Eye

Back 21

a theoretical model of understanding optical properties of eye : takes into account the optical properties of the cornea and lens to arrive details of image location and quality

Front 22

emmetropia

Back 22

a term often applied to an optically normal eye- in that incoming parallel light is brought to focus on the retina

Front 23

accommodation ; cillary muscle

Back 23

a change in the surface curvatures of the lens, allowing it to assume different refractive powers depending on object distance; controls lens movement by constricting to make the lens more round

Front 24

near point

Back 24

the greater the divergence of the incident rays from the source (due to the object being very close to eyes) the more the demand for the eye to focus= more contracting of cillary. the extent to which the lens will round is called the near point. for most people it is 20 cm. after this an object closer will seem blurred. In other words, it is the limit of maximum accommodation

Front 25

nodal point

Back 25

an entire cone of rays emitted from a point source can be collapsed into a single ray that passes through this nodal point. this confirms two things: retina image is inverted and horizontally flipped, but the retinal image is a faithful replica of the actual object

Front 26

Lens aberrations

Back 26

optical factors in lens refraction that produce imperfect imaging, especially in the peripheral. the farther away an image gets from the optic axis, the greater the degradation of the image

Front 27

retinal magnification; depth of field

Back 27

the size of the retinal image in comparison to the size of the object. as an object moves closer to eye, the retinal magnification increased rapidly; the range of object distances that produces a sharp image. the smaller the aperture (diameter of pupil) the greater the depth of field (the greater the range of distances that will be in focus)

Front 28

presbyopia; hyperopia; myopia how corrected?

Back 28

hardening of lens with age which reduces its accommodative ability, treated with a convex lens;
the length of eyeball is too short in comparison to its optical power - light focused behind retina (far sightedness, corrected with convex lens);
eyeball is too long given the refractive power of the eye- light focused in front go retina- nearsightedness, corrected with a concave lens

Front 29

far point for myopic people...

Back 29

is not optical infinity but rather located closer to the eye

Front 30

astigmatism

Back 30

one of the refractive components like the cornea maybe more curved in one direction that the other component= blurring of retinal image along the affected direction, can be corrected with a cylinder lens

Front 31

Choroid; retina; optic nerve; optic disk

Back 31

dark pigmented layer (blood supply); where photoreceptors are, transduction; carries neural signals to brain; hole in nasal retina for blood vessels- results in blind spot because there are no photoreceptors

Front 32

When looking at objects located at closer distances:

Back 32

we see increases accommodation, pupil constriction, and mergence (inward rotations of eyes)

Front 33

Albinism; cataracts

Back 33

absence of melanin pigment in choroid layer- photoreceptors exposed to extra light rays that aren't part of the primary image-forming rays;
Lens opacity which creates scattered light, first noted as a halo around bright lights

Front 34

where do photoreceptors lie?; fovea

Back 34

in front of the pigment epithelium. A certain amount of light bypasses the photoreceptors and is absorbed by this wall so that it doesn't interfere with vision; the retina can be divided in two halves- nasal (toward nose) and temporal (toward temple) the fovea is the midpoint dividing these halves

Front 35

Rods and Cones (what do they do, where are they located, how many, efficiency )

Back 35

-Rods are more numerous (120 million rods, 6 million cones) and mediate night vision. Cones mediate day vision
- density of cone photoreceptors is low in peripheral parts of nasal and temporal retina, but they are highly concentrated at the fovea
- Rods are more numerous in the periphery and there are almost no rods in the fovea
- while both are good at absorption, rods are more efficient at converting photons to neural signal and so they have lower thresholds

Front 36

describe phototransduction in the darkness, dark current (5)

Back 36

- in darkness, rhodopsin molecules are maintained in inactive form
- there is a steady flow of Na+ ions into the outer segment through c-GMP-gated sodium channels (there is lots of c-GMP in outer segment). The sodium channels are only open when there is enough c-GMP around
- the entrance of sodium in no light is called dark current
- as sodium comes in outer, k+ leaves inner segment
- the sodium- potassium pump kicks out na+ and brings in K+ to maintain -40 mv
- receptors release steady stream of glutamate which triggers bipolar cells connected to OFF neurons

Front 37

rhodopsin and the location of molecules that carry it (3)

Back 37

- photopigment which mediates transduction
- found in both rods and cones
- the receptors can be divided into two sections: the outer part and inner part. The outer is imbedded within fibrous matrix and contains a large concentration of rhodopsin molecules (disk region). Inner (folds region) does not

Front 38

Describe phototransduction in the light

Back 38

- first light photons captured by photoreceptors which pas through the inner section to the outer to generate an electric signal, mediated by rhodopsin
- rhodopsin is activated and interacts with a G-protein
- the G-protein activates a second messenger enzyme which turns cGMP into ordinary GMP, making in unable to bind with sodium channels
- this shuts sodium channels down (they need cGMP to stay open) so na+ doesn't enter, but K+ still leaves the inner segment. this leads to hyper polarization as low as -70mv in bright conditions!
- this causes a lesser glutamate release, which in turn actually depolarizes the bipolar cells connected to ON neurons

Front 39

How much light is absorbed by rhodopsin depends on what two factors?

Back 39

1) intensity of light- peaks at 550nm so is largely regulated by L and M cones
2) wavelength- maximum absorption around 500 nm

Front 40

absorption spectrum; spectral sensitivity function

Back 40

curve showing how much light is absorbed by rod photopigment across the entire visible spectrum, shown in relative absorption- peak by rods at 500nm; the relative ability of our visual system to function at different wavelengths across visible spectrum - peak at 500 nm for rods, 550 for cones

Front 41

microspectrophotometry - what did it confirm

Back 41

miniaturizing the spectrophotometry (changing wavelength to measure how much rhodopsin is absorbed) to apply it to individual cones. showed there are 3 types of cones: S-cone (short wavelength, peak 440), M-cone (medium wavelength, peak 530) and L- cone (long wavelength, peak 560)

Front 42

scotopic vision; photopic vision

Back 42

visual processes mediated by rods, highly sensitive, peaks at 500 nm; day vision mediated by cones which allows us to separate the different cone types to see colours, less sensitive, peaks at 550 nm
see page 303!!!

Front 43

Bleaching

Back 43

after a rhodopsin molecule absorbs light, it temporarily becomes unable to absorb more photons until the photopigment molecule is regenerated . As light increases, a greater proportion of rod rhodopsin becomes bleached until light can't be detects, which = the upper limit of scotopic vision, where rods are blind. this happens for cones too but only under crazy brightness like when we stare at the sun

Front 44

dark adaption- for rods and cones

Back 44

experiment in which detection threshold for a small light spot is measured at various times after bleaching to measure how fast we recover from bleaching. Cones adapt rather quickly- after 5 min the threshold is back to lowest limit, while rods take longer- up to 20 min after bleaching.

Front 45

retinal ganglion cells functions (3)

Back 45

- the only neurons in the retina whose axons leave the eyeball and carry signals to the brain
- first site in retina where action potentials are generated (activity in all other parts is just hyper or de polarization which doesn't quite produce action potential)
- emergence of parallel visual systems happens here

Front 46

3 layers of retinal neurone layout

Back 46

- photoreceptor layer (outermost region) inner nuclear layer (middle of retina), and ganglion cell layer (side, 1.25 million of them!) - so light goes through ganglion cells, and inner layer to reach photoreceptors;

Front 47

2 important retinal ganglion cells; spontaneous activity

Back 47

midget - small receptive field and compact dendritic field, represent 70% of total and are high concentration around fovea
Parasol- larger receptive field and dendritic field, 10% of total
spontaneous activity- ganglion cells fire in absence of light at a rate of 20-50 action potentials per second

Front 48

receptive field of retinal ganglion cells; ON/OFF vs OFF/ON cells

Back 48

- the area of retina that influences a neurone
- has two circular zones- an excitatory ON and inhibitory zone OFF
- if light falls in excitatory, firing will increase beyond spontaneous activity and inhibitory, firing rate goes below spontaneous
- centre of cell is excitatory and outer region inhibitory vs opposite

Front 49

centre-surround antagonism; result of this

Back 49

the two fields (ON and OFF) oppose each other in how they influence the rate of firing. if a light spot falls in centre of on/off ganglion cell, then = burst of action potentials. If we enlarge the light spot so it reached into Off zone, there will be a reduction in action potentials; result is that the retinal output sent to brain by ganglion is driven by contrast

Front 50

Bipolar cells

Back 50

situated in-between photoreceptors and ganglion cells and mediate the centre response signals

Front 51

horizontal cells, amacrine cells; lateral inhibition

Back 51

both are intermediate neurons between photoreceptors and ganglion cells which help to generate the surround response. Horizontal cells branch out laterally and make contract with adjacent photoreceptors, allowing them to gather signals for a large area of retina, and inhibit the photoreceptors linked to the centre response;
Amacrine cells also move laterally but at the bipolar and ganglion cell level
- both of these cells are responsible for lateral inhibition, where a neuron is inhibited by sensory input into surrounding areas through a network of lateral connections. This allows the ganglion cells to have centre-surround antagonism

Front 52

What are some reasons not all circuits are identical in the retina? (3)

Back 52

1) retina is uniformly thick except for the foveal pit at the centre is less thick
2) retinal wiring differs with eccentricity (deviation from centre)- the photoreceptors in centre of fovea connect to one bipolar which connects to one ganglion etc (spatial sampling). As you move away from the fovea, this becomes less true. as you get far away, one ganglion can receive input from a much larger number of photoreceptors. this also means that the ganglions at fovea have much smaller receptive field sizes than the others
3) Midget and parasol cells differences- midgets have smaller field sizes than parasol at ALL eccentrics from the retina and are higher in number

Front 53

Describe visual sensitivity (3)

Back 53

- humans can detect as little as 10 photons
- cones are more sensitive to long wavelengths, and the only way rods will dominate once 650 nm is in Nyctalopia (night blindness)
-covergence effects sensitivity- the ganglion in the centre split up inputs- say split 15 between 3 ganglion- whereas in the periphery 15 inputs may fall upon one ganglion. This makes the periphery more sensitive to light, but have poorer spatial sensitivity/ resolution

Front 54

photochromatic interval (2)

Back 54

- the difference vertically between the scotopic and photopic curves which indicated the different between just seeing a light (scotopic) and being able to tell its colour (photopic )
- the difference is greatest at short wavelengths and decreases at longer ones

Front 55

Purkinje shift, why does it occur

Back 55

t-ransition in visual sensitivity
- green colours will actually seem brighter under scotopic vision and yellow/red will seem dimmer. There is a shift 50 nm when you go from photopic to scotopic vision and so lower wavelengths end up in the high sensitivity area for scotopic
- occurs because the scotopic system is more sensitive to shorter than longer wavelengths

Front 56

Riccos law; Blochs law

Back 56

the product of light intensity and stimulation area (total amount of light) must reach a critical level for detection to occur. The greater the area, the threshold for intensity is lowered, but only to a point; the product of light intensity and light duration must reach a critical value to produce a detect threshold. As stimulus duration decreases, a larger intensity is needed for detection

Front 57

increment threshold for contrast, differences between rods and cones

Back 57

the minimum contrast needed for a light spot to be detected against background f particular intensity. At low background intensity, rods mediate and as background intensity increases cones activate. for rod-mediated vision, k=.14 so a 14% increase in intensity is needed to detect light over its background, whereas in photopic conditions, k=.2 so only a 2% increase is needed

Front 58

Acuity (resolution) (best, two-point spread function, snellen , gratings, contrast sensitivity function )

Back 58

- best resolution is of course in the rod-free fovea and worsens at peripherals
- 2-point spread is a measure of acuity, how far apart two light spots must be to be distinguished. For optimal resolution between two points, there must be an unstimualted receptor in between each stimulated one (this is how retina works in life)
- snellen chart is the eye chart. a person with a minimum angle of resolution MAR at 1minute arc will have 20/20 vision
- Sinusoidal grating patterns are used also to determine resolution (more reliable than snellen) and the spatial frequencies of the bars are plotted against sensitivity (the reciprocal of the threshold) in a contrast sensitivity function (CSF). The lower the contrast threshold one has the higher the sensitivity (S= 1/threshold)

Front 59

Nyuist Limit

Back 59

-the highest spatial frequency that can be sampled.
- it is determined by the spacing of the photoreceptors and exists because there is a limit to how small the receptive field size in the fovea (and everywhere else ) will be.
- At the fovea it is 60 cycles/degree

Front 60

3 factors that affect human CSF

Back 60

light level- going form photopic to scotopic results in lower contrast sensitivity and thus shifts CSF to lower frequencies
age- as age increases, the high frequency cut-off is affected but not lower end, meaning only our ability to process image detail declines with age
disease-myopia lowers cut-off, cataracts, glaucoma etc

Front 61

Two phenomenon that can be explained by centre-surround mechanism: lightness contrast, why does it happen; light consistency, why does it happen

Back 61

the lightness of an object can appear different intron of different backgrounds. because ganglion cells output is a summated response from centre and surround continents of its receptive field;
the relative consistency or similarity in the lightness perception of objects despite large changes in environmental illumination. this happens because both the intensity of stimulation through receptive field centre and the response curve detected to the background intensity change equally as ambient illumination changes

Front 62

Mach Band illusion (2)

Back 62

-explained by centre-surround.
-our tendency to see light at dark bands as boundaries where the intensity changes abruptly
- shows that under certain circumstances, perceptual impressions do not match reality

Front 63

Hermann grids

Back 63

- thing with dots in corner of the squares
- i dunno...

Front 64

what is the consequence of eccentricity?

Back 64

objects like words on a page that are farther from the fixation point must be larger to be recognized

Front 65

rod monochromacy

Back 65

condition where vision is provided by the rods only without cone contribution