Visual Perception Flashcards
Threshold
minimum amt of a variable that an individual can detect 50% of the time
1/2 way between random guessing & perfect.
Ex. smallest letter a pt can resolve at a distance, minimum level of light intensity an individual can detect
how does threshold relate to sensitivity
Inverse relationship
low threshold = high sensitivity
high threshold = low sensitivity
What are some methods that could determine an individual’s threshold?
Method of ascending limits
Method of descending limits
Method of constant stimuli
Stairstep method
Adjustment method
Forced choice method
Method of ascending limits
increasing brightness of lights incrementally from presentation to presentation
Ex. dark adaptation
Method of descending limits
decreases the brightness of lights incrementally from presentation to presentation
Ex. Snellen when check VA
Method of constant stimuli
varies the intensity of the light from presentation to presentation
What’s the greatest drawback of using the ascending and descending method?
anticipation
the method of constant stimuli fixes this issue but is not used clinically, stairstep method minimizes anticipation
stairstep method
gradually increase the light until pt detects light and gradually decreases the light until pt no longer sees the light
minimizes effects of anticipation
Ex. VF
Adjustment method
experimenter adjust light until it is percievable
ex. nagoanolmoscope
What’s a major drawback w/ minimal detection?
Strict and lax criteria
strict (false negative) = will not indicate they have light detection until they are absolutely sure they see the light (threshold is higher than expected)
lax (false positive) = will indicate they have detected a light as soon as they think they see a light (threshold is lower than expected)
because each individual uses different criteria for determining thresholds, the psychometric function may be skewed
Forced choice method minimizes this criterion
Forced choice method
Pt is shown 2 alternatives that are presentable at the same time
Ex. Teller acuity cards, Broken wheel
What’s the threshold for teller acuity cards?
75%
Which psychophysical method should be used for determining dark adaptation? What are the drawbacks? Is there a method that minimizes these drawbacks?
method of ascending limits
drawback- skewed by individuals’ threshold of criteria
Forced choice test minimizes these drawbacks
Signal detection theory
ability to detect signal vs. noise
Noise = random and Gaussian distributed, like a bell curve. Noise corrupts the signal making it difficult to detect the signal against the background
Signal = the visual system does not receive “pure” signal. instead, it receives a combo of signal and noise (S+N) which is also shaped a like a bell-shaped curve. The signal (S) must separate from S+N
great separation between N curve and the S+N curve means it will be easy for the visual system to separate the signal from the noise (greater detectability)
lax:
false positive
false negative
false positive
strict:
false positive
false negative
false negative
True positive
The disease is present and the test is positive
False positive
The disease is absent but the test is positive
True negative
The disease is absent and the test is negative
Radiometry
energy per time produced by a source of electromagnetic radiation
- Radiant power (W) (point source in all direction), energy per second produced by a light source
- Radiant intensity (Watts per solid angle). energy per second in a given direction (one direction)
- Radiance radiant intensity per projected area of light source, extended source that has an area (fluorescent tube of light)
- Irradiance: radiant power per unit area of a surface (light falling down on an object), deals with amt of light that reaches the target
Photometry
how our visual system responds to electromagnetic radiation , measure response to light NOT the light itself
- Luminous power (lumen) if light elicits a strong rxn by the visual system = higher value of lumenous power (the more lumens the brighter we percieve light)
- Luminous intensity = luminous power in a given direction (lumens per solid angle = candela)
- Luminance = luminous intensity per projected area of light source (candelas per square meter). It deals with spatially extended sources, can also be measured in foot lamberts
- Illuminance = luminous power per unit area of a surface ( lumens per square meter or lux), light falling on an object, foot-candles
one foot candle = 10.8 lux
Luminous Efficiency Function
V1 = brightest light, max value of V = 555nm (peak sensitivity of photopic luminous efficiency function occurs at this wavelength)
V0 = weakest light
For every watt of power of light source, our visual system responds with ___ .
680 lumens (unit of photometry)
** at 555nm there are 680 lumens/watt**
We use this to convert physical property to psychophysical property
Radiometry vs Photometry
Radiometry = Physical property
- Radiant power (W)
- Radiant intensity (W/solid angle)
- Radiance
- Irradiance
Photometry = perceptual property
- Luminous power (lumens)
- Luminous intensity (candela)
- Luminance (foot- lamberts)
- Illuminance (lux or foot-candles)
one foot candle = __ lux
10.8
Peak sensitivity of scotopic luminous efficiency? What does it correspond to?
507nm
507nm corresponds to 1700 lumens/watt
Abney’s law of additivity
if multiple wavelengths are present in a light source, you can add lumens from each wavelength to determine the total luminous power
Filters
absorb some of the light incident upon them
- this means that light leaving the filter has a different spectral composition (different wavelengths that make up the light)
Narrow band filters
only allow a smalll range of wavelenghts to pass through
specified by 2 quantities
1. location of peak: which wavelength range most easily passes through the filter
2. half-height bandwidth: indicates the selectivity of the filter. higher half-height bandwidth = less sensitive filter
Define the following filters:
Narrow band filter
Interference filter
Broadband filter
Long pass filter
Narrow band filter: allow only a small range of wavelengths to pass through
Interference filter: Allow only 1 wavelength to pass through
Broadband filter: Allow a large range of wavelengths to pass through
Long pass filter: allow only long wavelengths to pass through, blocking short wavelengths that could cause cataract formation
Which filter is used for sunglasses?
long pass filters
Sunglasses can act as a neutral density filter by equally reducing the transmission of all wavelengths
Neutral density filter
transmit all wavelengths equally, minimizing color distortion
this will decrease the amt of light transmitted from the eye to the visual cortex = slower neural response
Pulfrich phenomenon
Pt sees a pendulum swinging back and forth b/c each eye equally transmits visual signal to the visual cortex
When you place a ND over one eye the neural response of a visual cortex is SLOWED and the pendulum appears to move in an ellipse rather than back and forth plane
- place ND over right eye = counteRclockwise
- place ND over left eye
= cLockwise
Why would we use a ND filter in clinic?
measure density of APD
- place ND over unaffected eye until APD affected disappears
Lambert surface (cosine diffusers)
same luminance from every viewing angle (matte paper)
L = RE
L = luminance (foot-lamberts)
R = reflectance
E = illumination (foot-candles)
Specular surface
unequally scatter light in different directions (shiny piece of silver)
Retinal illumination
amt of light falling on the retina
T = LA
T = retina illumination (troland)
L = luminance of the viewed surface
A = Area of the pupil
General illumination
general illumination falling on a tilted surface is related to the intensity of light source by inverse square and cosine laws
E = Icos(angle)/d6^2
I = intensity of the point source
angle = angle of tilt
d = distance of point source to surface
A pt looks at a computer screen with a luminance of 50 nits. Her pupil size is 3mm. What is the amt of retinal illumination?
T = LA
A = pi(R)^2
A = pi(1.5mm)^2
T = (50)(7) = 350 trolands
scotopic vision
- Vision under dim lighting (primarily rods)
- lack of color discrimination
- good sensitivity
Photopic Vision
Vision under bright light conditions (primarily cones)
- excellent VA and color discrimination
- poor sensitivity
Principle of univariance
a photopigment responds to absorption of a photon (quantum of light) in a manner that is completely independent of the wavelength of a photon
- the wavelength of a photon only determines the probability of absorption of photon by the photopigment
- as a result, once photon is absorbed, all info about wavelength of the photon is lost
In simple terms, the principle of univariance means that the information about a color or brightness comes from the total amount of light hitting a receptor, regardless of what combination of colors or brightness levels produced it. So, a receptor (like a cell in the eye) can’t tell what combination of colors or brightness levels made the light, it just knows how much light there is.
What wavelength are the rods and cones EQUALLY sensitive?
650nm
Photochromatic interval
peak sensitivity for scoptic system
peak sensitivity for photopic system
equal sensitivity for photopic and scotopic system
difference between the sensitivities of the scotopic and photopic system
650 nm = rods and cones are equally sensitive
507nm = scoptic system
555nm = photopic system
Purkinje shift
- increase illumination = longer wavelengths appear brighter due to shift from scotopic system
- helps adapt to different lighting conditions (transition from scotopic to photopic conditions and vice versa)
(red and orange appear brighter in photopic conditions while red-green appears brighter in scotopic conditions)
Dark adaptation
- how threshold changes as we spend more time in the dark (regeneration of rods and cones)
- threshold and sensitivity has an inverse relationship
When is the rod-cone break absent?
650nm
Dark adaptation curve
determined by measuring the threshold at a certain wavelength
- transitioning from bright light to darkness
- rod-cone break = rods take over as primary source of seeing light
Dowling-Rushington Equation
- mathematically describes dark adaptation, it says that the photopigment regeneration is the sole cause of dark adaptation
Light adaptation
- how well an individual can pick out a spot of light from a background of slightly different intensity
- increase the intensity of light until just perceived light is detected against a background with slightly different intensity ( difference btw spot of light and background aka JND), the background intensity i sthen increased, and the pts threshold is determined again
When does rod PR saturation occur?
when 10% of rhodopsin molecule are bleached resulting in the closure of critical number of Na+ channels within the rod photoreceptor membranes
Fechner’s Log Law
assumes webers law holds true for stimuli intensities above the threshold
Stevens’ Power Law
Which system has greater sensitivity and poor resolution? what’s the ratio of rods and ganglion cells?
Scotopic system b/c it has larger pixels,
1:1 ratio
Which system has poor sensitivity and great resolution? What’s the ratio of cones to ganglion cell
photopic system
single or few cones with a single ganglion cell
Spatial summation
describes the relationship between the intensity and area of a stimulus that is below the threshold for spatial summation. It says that the total number of quanta (photons) for a stimulus is below the critical diameter constant
- critical diameter = size of the pixel in the visual system, scotopic system has a larger critical diameter. The size of the test light must be less than the critical diameter for spatial summation to occur
- if the critical diameter is greater than the size of the test light then you have to increase the intensity of the stimulus in order for the pt to detect test lights b/c photons from the stimulus are now falling on multiple ganglion cells
IA = C
I = stimulus
A = stimulus area
C = constant
Summary:
- stimuli less than the critical diameter can still be seen if the area is increased
- area of stimulus decr = increase intensity of light to see as long as total number of quanta (photon) is constant, and the area of the stimulus is below the critical diameter, the stimulus will be seen by the visual system due to spatial summation
**Easier to see a faint light if it has a larger area, or tiny speck of light with a brighter glow*
Ricco’s law
spatial summation
IA = C
Temporal summation
- when duration of flash exceeds 100ms, temporal summation of scotopic system no longer occurs = must increase intensity of light in order for the pt to see
- longer critical duration in scotopic system = greater sensitivity = poor resolution (multiple flashes of light are summed together within the time pixel
- shorter critical duration in photopic system = poor sensitivity (less time for ganglion to absorb light) = excellent resolution (light summed over multiple pixels
Summary
- stimuli less than the critical duration = decrease intensity of flash to be seen
- Decr duration of flash = incr intensity of stimulus to be seen
as long as the photons are constant and duration of flash is below critical duration, the flash of light will be perceived by the visual system due to temporal summation
adding up small bits of information to make a bigger picture
Bloch’s law
temporal summation (time)
describes the relationship between the intensity and duration of stimulus that is below the threshold for temporal summation. It states that the number of quanta (photons) for a stimulus is below critical duration is constant
It = C
Stiles-Crawford Effect
Cone photoreceptors are MOST sensitive to light that strike orthogonal to the surface (will look brighter compared to other angles)
Macular dz like ERM and ARMD may affect this
Trichromatic theory
There are 3 types of cones: L,M,S that absorb different wavelengths
- the absorption spectrum for each type of cone PR peaks at a different wavelength
S = cyanolobe = 426
M = chlorolabe = 530
L = erythrolabe = 557
Rods = rhodopsin = 507
Review: Principle of univariance
response of a cone PR to light is independent of the wavelength of light
- wavelength only determines the probability of absorption of light by the cone PR
- a single cone PR cannot discriminate between colors and intensity
- color discrimination is based on the relative response of all 3 cones to a light with a given wavelength which is based on their absorption spectra
WHich cone is the least robust against pathological damage?
S cones, early stage glc pts show s cone VF loss on a b/y VF test even when complete VF loss is not readily apparent
Metamers
2 visual stimuli that are physically different but appear identical b/c they elicit the same response in all 3 cone PR
“colors made of different combinations of wavelengths but in our eyes it looks the same because our eyes cannot discriminate the different colors”
Grassman’s Law
provides general rules for metamers
- equal amounts of same wavelength of light are added to 2 metamers they will remain metamers
- if the intensity of 2 metamers is increased or decreased by the same amt, they will remain metamers (but will appear brighter or dimmer than before)
- stimuli A and B are metamers, stimuli B and C are metamers, then stimuli A and C are metamers
Color opponency theory
3 channels for color vision
- R/G
- B/Y
- Brightness (mediated by rods)
each channel treats 2 colors as opponents
- if R/G channels stimulated by equal amts of red and green = no color or net response is 0, same with b/y channel
perceived color for visual stimulus is dependent on what?
relative signals of each color channels
ex. short wavelengths will signal red and blue = purple
Sensitivity Effect of Opponency
normal color vision has 3 peaks 440nm, 520nm, 620nm
these peaks DO NOT correspond to S,M,L cones. This is a result of color opponent processing of M and L cone inputs which shifts location of peaks in sensitivity spectrum
What are the 3 characteristics of color?
Hue = related to wavelength
Saturation = desaturated color, colormetric purity
Brightness = related to V function
Colormetric purity
used to quantify saturation
high p value = highly saturated color
p = L (luminance of pure wavelength/ L(Luminance of pure wavelength)+L(luminance of white light)
p = L(l) / L(l)+L(w)
Munsell color appearance system
specifying color based on these 3 concepts:
- hue
- saturation
- brightness
Bezold-Brucke Phenomenon
our perception of hue is associated with a given wavelength of light changes as the intensity of the light increases
Ex. light wavelenghth of 540 appears green-yellowish, as the intensity of the light is increased it will look more yellow
Color constancy
colors appear the same under different lighting conditions
CIE color system
we can combine red (645), green (526), blue (444) light to create a stimulus that matches any other wavelength on the visible spectrum
allows us to quantify how much of each primary color is needed to create another color
Dichromat
missing one of the three cone photopigments
- Protanopes: missing erythrolabe (L cone), confused red orange green yellow, red colors appear dim (inherited)
- Deuteranopes: missing chlorolabe (M cones), confused red orange green yellow, no dimming of red color (inherited)
- Tritanopes: missing cyanolabe (S cones), confuse b/y (mostly acquired)
Anomalous Trichomat
have all 3 photopigments but absorption spectrum of one of the photopigments is displaced
Protanomalous trichromat: L cone (erythrolabe) is shifted towards the shorter wavelength, red orange green all appear green. REds appear dimmer (but not as strong as protanope), RED WEAK
Deuteranomalous trichromat: M cone (chlorolabe) is shifted towards a longer wavelength, red orange green all look more red, GREEN WEAK
X-chrom cls
red tinted lenses that act as long pass filters, resulting in shift of cone PR absorption towards longer wavelenght.
can be worn over one eye
Chromatic discrimination
how much we need to change a wavelength to perceive change of hue of the stimulus
Protanopes and deuteranopes have poor color discrimination at which wavelength?
540nm b/c only M (protanopes) or L (dueteranopes) cones are responding to light
Tritanopes have poor color discrimination at
495nm b/c only the M cones are responding to light
Color confusion lines
color confusion lines on a CIE are lines on the x-y plane along which all colors are indistinguishable. This is because only one cone photoreceptor is responding to the wavelengths of light along the color confusion line, resulting in poor color discrimination
Alll color confusion lines originate from one point konwn as copunctal pt
Pseudoisochromatic plate test
must be conducted in daylight illumination or using macbeth illuminant C lamp in order for the results to be valid
Ishihara plates = dx r/g defects
HRR plates = dx b/y and r/g defects
Farnsworth D-15
- dichotomous test, consists of 15 colored chips
dx r/g and b/y - useful for plaquenil testing
Nagel Anomaloscope tests for what?
- dx r/g defects, CAN distinguish btw dichromats and anomalous trichromats
How does the nagel anomaloscope work?
There’s a test field (590nm) and mixture field (contains 670 and 546nm) that both can be adjusted
- pt is instructed to adjust the ratio in the mixture field and the radiance of the test field until both fields appear identical
- mixture field is adjusted from 0-73 (546-670nm)
- the test field can be adjusted from 0-87 (very dim to very bright yellowish light)
3 wavelengths used all fall on the color confusion lines for protanopes and deuteranopes
Normal color vision = mixture scale to 45 and test scale to 17 (will appear yellow)
Protanopes = mixture field set anywhere 0 - 73,
- mixture field set 73 (670), test field will be low setting
- mixture field set 0 (546), test field will be set high
(DIMMING OF RED)
Deuteranopes = mixture field set anywhere 0-73
- they will set test field to 17 regardless of the mixture field
(NO DIMMING OF RED)
Protanomalous trichromats = mixture field is set higher than normal b/c they are RED WEAK, they must add more 670nm light to the mixture field
Deuteranomalous trichromats = adjust mixture field lower than normal, b/c they are GREEN WEAK, they must add more 546nm light to the mixture field
Using a nagel anomaloscope
the mixture field is set to 0
and test field is 55
what’s the diagnosis?
mixture field is higher than normal (45)
protanomalous trichromat
using a nagel anomaloscope
the mixture field is set to 73
test field is set at a very low value
whats the dx?
protanope
using a nagel anomaloscope
the mixture field is set to 73
the test field is set to 17
what’s the dx?
deuteranope
using a nagel anomaloscope
mixture field is set to 73
and the test field is set to 5
what’s the diagnosis?
dueteranomalous trichromat
Inherited vs acquired color defects
Inherited (Genetic)
- most commonly r/g defects
- both eyes
- does not change drastically over time
- X-linked recessive
Acquired defects
- affect one eye more than the other
- may change over time
- b/y defect
- equal frequency M & F
Kollner’s rule
Outer retinal disease = b/y defects
(ex. macular dz)
inner retina, ON, or visual pathway dz = r/g defects
(ex. optic neuritis)
Most common achromatopsias?
rod monochromacy (only rods present)
- greatly reduced VA and color discrimination
Chromatopsias
characterized by distortions of color but still able to discriminate color
(ex. cataracts - acts as a yellow filter, after cataract sx objects appear more blue )
Contrast
C = Lmax - Lavg / Lavg
used to quantify our intuitive notion contrast
Lmax = max luminance (white part of sine wave grating)
Lavg = avg luminance of the grating
Contrast ranges from 0-100
Spatial contrast sensitivity function
type of spatial modulation transfer function (SMTF) - describes how well a particular system (a lens) transfers contrast from the input (an object) to the output (an image
- defocus lens = poor image contrast, particularly at higher spatial frequency more than moderate and low spatial frequency
Summary **
the spatial contrast sensitivity function (CSF) describes how well our eyes can see patterns of different sizes and contrasts. It shows that we’re best at seeing medium-sized patterns with moderate contrast. As patterns get either too small or too big, or if the contrast is too low, our ability to see them decreases. Understanding the CSF helps scientists and engineers design better images and displays that match how our eyes work.
converting snell from cpd
600/CPD = snellen denominator
How is CSF measured?
- pt is shown sine wave grating of known frequency and very low contrast below the pts threshold (will see a uniform luminance NOT white and black stripes)
- Pt increases the contrast grating until she can perceive sine wave grating (CSF is the reciprocal of threshold value)
- the procedure is repeated for sine-wave gratings of different frequencies in order to generate the entire CSF
What does a high-frequency cut-off indicate? Why does this happen?
there is a limit to our ability to resolve fine spatial details
- high frequency cut off represents the pts measured VA
- optical limitation (aberration still occurs with perfect focus)
- The density of photoreceptors is finite, thus our ability to resolve spatial detail is physically limited by the spacing of the PR
Is the frequency cut off higher in bright or dim illumination
Bright b/c cone PR are primarily responbile for high contrast VA
Most recognition acuity contains high or low contrast?
High
- even at high contrast, very small letters cannot be resolved by the visual system
Minimal angle of resolution (MAR)
represents the angle (measured from the eye) btw 2 bars that are just resolvable. MAR is related to the snellen fraction
SF (snellen fraction) = 1/MAR
60 arc minutes = 1 degree
A pt has a high-frequency cutoff of 15 CPD on the CSF. What is the pts snellen VA?
600/15 = 40
20/40
How would you get the snellen fraction in reduced form?
CPD/30 = snellen fraction reduced form
What are 2 reasons CSF shows a LOW frequency cut off?
- lateral inhibition in the retina
- ganglion cells in the retina shows a center-surround receptive field
- stimuli that falls in the center will excite the cell
- stimuli that falls outside the center will inhibit the cell
- a large stimulus falls on the peripheral part of the receptive field, resulting in inhibition of the cell, thus limiting our VA at low spatial frequencies
Fourier analysis and the visual system
simple idea of mathematics that says any fucntion can be broken down into a sum of sines and cosines
- visual system works similar to fourier analyzer
- visual system can break down almost any visual scene into its individual pixels and analyze each pixel separately and then resemble the pixels to create a complex visual scene
Summary **
Fourier analysis of the visual system is a way of breaking down images into simpler components called spatial frequencies. It’s like taking a complex picture and breaking it into basic patterns of different sizes. These patterns are akin to the building blocks of the image.
Mach bands
- strip of gradual change in luminance from one end to the other
- instead of perceiving a gradual luminance change, the pt perceives a light and dark band
- Mach bands occur b/c the gradual change in luminance along the strip is difficult to perceive due to its very low frequency
mach bands provide evidence of fourier analyses in the visual system
Summary **
Mach bands are a visual illusion that makes edges between light and dark areas appear even sharper than they really are. When you look at a transition between a light area and a dark area, such as where a shadow meets a lit surface, you might notice that the edge looks darker on the light side and lighter on the dark side. It’s like a little halo of brightness or darkness right at the edge.
This effect happens because of how our eyes and brains process contrast. When there’s a sudden change in brightness, our visual system amplifies the contrast to help us see the difference more clearly. Mach bands are an exaggerated version of this phenomenon. They occur because our eyes and brain enhance the contrast at the edge, making it appear even more distinct than it really is. This can help us perceive objects and edges more sharply, even in low-contrast situations.
Depth perception
- Monocular
- Binocular
Monocular depth cues
Pictorial depth cues (STAL)
- angles
- lighting
- texture
- shadows
Motion parallax = relative mvmt btw objects provides clues about positional relationship btw the objects
Binocular depth cues
comparison of location of the image formed on each retina
Uncrossed retinal disparity
- image falls on nasal side of each fovea = objects perceived to be farther away
Crossed retinal disparity
- image falls on temporal side of the retina = objects perceived to be closer
Panum’s fusional area and diplopia
INSIDE panums fusional area = stereopsis
OUTSIDE panums fusional area = diplopia
sequential flashing lights
object appear to be moving (not true motion)
(ex. Christmas tree lights)
Beta mvmt = if T (time interval) is chosen properly, spot of light will appear to be moving
No motion - if T is very small or very large
Phi mvmt = T is slightly too large or too small, the pt will perceive partial motion
Magnocellular pathway
- which area of the visual cortex is primarily responsible?
motion
V5/MT primarily responsible for motion detection
- cells in V5 respond to global stimuli (ex. random dot kinematograms).
- electrically stimulating this area will alter the perception of motion (in monkeys)
Random dot kinematograms
stimulus that contains randomly moving dots
- we can use this to measure percept of motion by change direction and mvmt of dots or by measuring how far the dots must move in order for a pt to perceive motion (minimum/max displacement thresholds)
Motion after effects (MAE)
- percept of motion lingers after the stimulus of motion is removed, the perceived motion moves in the OPPOSITE direction (ex. waterfall illusion)
Direction sensitive motion detector cells
- located in V5
- some are excited w/ upward mvmt and some downard mvmt
- stimulus appears stationary when opponent detector cells respond equally
- When a pt views a waterfall - downward motion detector cells are excited, after prolonged exposure, the downward motion detector cells adapt and become less sensitive
- when a pt views a stationary object, the waterfall will appear to move upwards b.c the upward motion detector cells are responding at a higher rate
Above what speed our VA drops b/c our pursuit eye mvmts are not capable of tracking object that are moving?
60-80 degrees/sec
Temporal perception
similar to spatial perception but now dealing w/ changes in visual scenes over time (change in luminance over time)
- often measured with a flickering stimulus (similar to sine wave grating) - change in luminance in space
Temporal modulation transfer function (TMTF)
similar to SMTF from spatial vision
- based on percentage modulation for a flickering stimulus with a given frequency
- plot sensitivity at different temporal frequencies (plot looks similar to CSF, with a high and low frequency cut off)
Percentage modulation
form of “temporal” contrast
Pm = 100 x A/Lavg
A = amplitude of stimulus
Lavg = time-averaged luminance
Pm = percentage modulation
Low-frequency cutoff TMTF
- our vision cannot detect very slow temporal change, we cannot watch the grass grow b/c the change happens on a very slow time scale
(ex. we cant see grass grow)
Ex. Purkinje Tree, Troxler phenomenon
Purnkinje tree
shine a light on our closed eyelid, we’ll be able to make out the structure of the retinal vessels b/c the light results in higher frequency changes in the images on our retina
Troxler phenomenon
our visual system has poor sensitivity for very low-frequency stimuli b/c of lateral inhibition in the retina, thus if we continuously stare at an object it will eventually fade away.
to combat this our eyes continuously move during fixation, this results in temporal changes in retinal illumination that exceed threshold so the object will remain visible
High-frequency limit TMTF
- our visual system has a high frequency cut off - frequency that can be resolved at max modulation
- because neurons take a finite, nonzero time to respond to stimuli and therefore time “pixels” are not infinitesimally small
smallest details that our eyes can distinguish
What condition can cause profound loss of moderate and high temporal frequency?
Glaucoma
Critical flicker fusion frequency
Above CFF = perceive constant light
Below CFF = perceive flickering light
Consider a stimulus that flickers at a frequency 1.5 greater than the pts CFF. At what frequency will the pt perceive the stimulus flickering?
Above CFF = the light will appear constant
Laws that describe the properties of CFF
Ferry-Porter Law
Granit-Harper Law
Broca-Sulzer Effect
Brucke-Bartley Effect
Talbot-Plateau Law
Ferry Porter Law
High frequency CFF scales linearly with log of retinal illumination this is likely b/c the response of the retina speeds up following increased light adaptation
Granit-Harper Law
the high-frequency CFF increases as the stimulus area is increased; this is because the peripheral retina (larger time pixels) is better at detecting flicker than the central retina
Broca-Sulzer Effect
Light flashes above the threshold appear brightest when they last for 50-100msec. Longer or shorter flashes of light with the same intensity appear dimmer
Brucke-Bartley effect
flickering light appears brighter than a steady light of the same avg luminance (Lavg)
Talbot-Plateau Law
Stimulus that is flickering at a frequency greater than the CFF (appears constant) appears equally bright as a non-flickering stimulus with a luminance equal to the time-avg luminance of the flickering stimulus
Masking (define the following)
Forward masking
Backward masking
Paracontrast
Metacontrast
Simultaneous masking
Forward masking: mask is presented before the target
Backward masking: mask is presented after the target
Paracontrast: maks first and target 2nd, with both being close to each other in space
Metacontrast: target first, mask second, with both being close to each other in space
Simultaneous masking: mask and target appear at the same time
Which mask effect is responsible for the crowding phenomenon that occurs in pts with amblyopia
simultaneous masking - explains why pts often have a better VA when they are shown single letters instead of lines of letters
Entopic Phenomena
visual phenomena that originate from the eye itself
Purkinje image
Light reflects off the front and back surface of the lens and cornea, forming unintended “ghost image”
I = anterior cornea, virtual/upright/small, very bright (reflection)
II = posterior cornea, virtual/upright/very small, bright (reflection and refraction)
III = Anterior lens, virtual/upright/large, dim
IV = posterior lens, real/inverted/smallest, very dim
I-III = created from convex surface
IV = concave surface (that’s why its unique)
Purnkinje image largest to smallest following accommodation
I > II > III > IV
Image III moves forward, image IV moves backward (toward the retina)
What does Moore’s lightening streak indicate?
Retinal break or detachment, vitreous syneresis and traction
- vertical streaks or flashes of light in the peripheral VF
Purnkinje tree
pt ability to perceive retinal blood vessels if we shine a moving light on the retina
Haidinger’s Brush
due to birefringence of the radial Henle’s fiber layer in the macula
- used to diagnose eccentric fixation
- pt stares at a blue object through a rotating polaroid, she will perceive a rotating yellow brush centered around point of fixation
- if pt uses eccentric fixation, he will see a haidingers brush centered on a point other than the fixation target
Maxwell’s spot
- based on fact that blue wavelengths of light are primarily absorbed by pigment within the macula rather than the cone photoreceptors
- If pt fixates a purple light (composed of blue and red wavelengths), he will perceive a red spot located at the point of fixation b/c all of the blue light has been absorbed by the macular pigment
- maxwell’s spot can also be used to dx eccentric fixation
- if pt has eccentric fixation, red spot will be located away from the point of fixation
What are 2 test used to dx eccentric fixation?
- Maxwell’s spot
- Haidinger’s brush
Phosphenes
flash of light due to mechanical or electrical stimulation of the retina, or 2’ to neural noise
ex. eye rubbing
Blue arcs of the retina
- most commonly seen when a pts views a dim light in a dark room
- blue arcs will originate form the source of light and extend towards the physiological blind spot
Magno vs Parvo pathways
Parvocellular pathway: detection of detail and color, associated with object size/shape/details. includes neurons in central retina that are sensitive to high spatial frequencies (CONES)
Magnocellular pathway: detection of motion, includes peripheral retinal neurons that are sensitive to low spatial frequencies (RODS)
Which pathway is affected first w/ glaucoma and saccadic suppression?
Magnocelluar pathway
Parvocellular pathway
Magnocellular
vision is suppressed just before, during, and after a saccadic eye mvmt to prevent blurring of an image
