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