Perception Flashcards
Threshold
Minimum stimulus that an individual can detect 50% of the time
Threshold is _____ of sensitivity
inverse
Method of ascending limits
Increase brightness until it is detected. how dark adaption occurs
Method of descending limits
Decrease brightness until cannot see any longer. Snellen VA.
Method of constant stimuli
The experimental varies the intensity randomly. Take a long time to complete
Stair step Method
Gradually increase light until deleted and then decrease until not detected. This is what is done in VF. Bracket testing
Adjust method
allows the patient to adjust the brightness.
Patients with strict criteria
will not indicate they see a light until absolute certain. Will have false high thresholds
Patients with lax criteria
will hit whenever they think they may have seen it. Will have lower than expected thresholds. False positives.
Threshold for forced choice
75%
Signal detection theory
How a patient picks signal from background noise. The more separated S is from S+N the easier to detect.
Sensitivity
TP/TP + FN (population with disease)
Specificity
TN/TN+FP (population with/o disease)
Positive Predictive Value
Probability patient has disease if test is possible. TP/TP+FP
Negative Predictive Value
Probability patient doesn’t have disease if test is negative. TN/TN+FN
Receiver Operating Characteristic Curve
How a patients criteria affects the detectability of the signal. Lax will be up on to right. Strict will be low on the left.
Radiometry
Energy per time produced by a source of electromagnetic radiation.
Radiant power
The energy per second produced by a light source
Radiant Intensity
Energy per second in a given direction
Radiance
Like a megaphone of projected light
Irradiance
Light on a surface
Photometry
How our visual system detects electromagnetic radiation.
Luminous power
Visual systems response to light. Higher lumen=higher response=higher perception of brightness.
Luminous efficiency function
V=1 is the brightest and V=0 is the least bright
When does V=1 with photopic system
555 nm with 680 lumens/watt.
When does V=1 for scotopic system
507 with 1700 lumens/watt
Abney’s law of additivity
Can ad the V’s together if more than one wavelength in a light
Narrow band filters
Only allow a small range of wavelengths to pass through. Location of peak specifies which can pass and half-height bandwidths-A higher number=less sensitive.
Half height bandwidth
Larger number=less sensitive
Interference filters
Highly sensitive (low half height bandwidth) that only transmit a single wavelength
Broad band filters
allow a wide range of filters
Long pass filters
Transmit only long wavelengths. Change color perceptions.
Neutral density filters
Transmit all wavelength equal so no change in color perception. Will decrease light transmitted to eye and therefore will have a slower neural response
Pulfrich Phenomenon
Pendulum moves in an circle instead of back and forth. Over left=clockwise. Over right=counterclockwise.
Lambert surface
Surface that shows the same luminance in every angle or scatters light equally in all directions. ie. matte paper.. L=RE. If Luminance is doublbled so will L
Specular Surface
Surface that unequally scatters light. i.e. shinny piece of silver.
Retinal Illumination
T=LA. L=luminance A=area of pupil.
Scotopic vision
Poor visual acuity, lack of color discrimination, good sensitivity but poor resolution.
Photopic vision
Good VA, good color, poor sensitivity but good resolution (think amount PR to ganglion)
Mesopic conditions
when both rods and cones are active
Principal of Univariance
Although wavelength of photon affects its absorption, once it is absorbed all facts about it are lost.
When is photo chromatic interval zero
650 nm. Both are equally sensitive.
Purkinje shift
As illumination increases longer wavelengths appear brighter due to shift from scotopic to photopic system.
Dark adaption curve
Expose to bright light and then track in dark. Cones first and then rods and then they take over
When would there be no rod/cone break
650 nm.
How would dark adaption curve shift for a stimulus of large wavelength?
It would take longer for rods to dominate as they are not sensitive at that wavelength.
Dowling Rushton equation
Looks at dark adaption and only take photopigment regeneration into account.
Light adaption
Light on a background of slightly different intensity. The intensity of the spot of light is adjusted until patient can just perceive the light against the background. The background intensity is then increased and patient’s threshold found again.
Laws in light adaption
De-Vires and then weber.
Fechner Log Law
Webers law holds true for stimuli above threshold.
Steven’s Power law
Perceived brightness scales with stimulus intensity rather than logarthimatically.
Spatial summation of photopic and scotopic
Scotopic has great spatial summation due to large pixels but poor resolution. Photopic has low spatial summation but great resolution.
Critical diameter
The number of photons necessary to detect a given stimulus is the same for all stimulus sizes less than the critical diameter. Equal to the size of the pixel.
Riccos Law
describes that the number of photons given a stimulus is constant when below critical diameter
If the diameter is decreased the Intensity must be____
increased
Critical Duration
The number of photons necessary to detect a given flash of light is the same for all duration of flashes that are less than the critical duration.
Critical duration of scotopic vs. photopic
Critical duration is longer for scotopic system compared to photopic due to larger pixel in scotopic
Bloch Law
Similar to Ricos law but for temporal summation. It=C1
Stile-Crawford Effect
The angle at which light strikes a photoreceptor affects the perception of light.
Which ray angle are cones most sensitive to?
Orthogonal to the surface.
Trichromatic Theory
There are 3 different types of cone photoreceptors.
S-cone
cyanolabe. 440 nm
M-cone
Chlorolabe. 534
L-cone
Erthyrolab. 564
Rhodopism
498 nm
Metamer
Two visual stimuli that are physically different but appear identical because they elicit the same response in all 3 cone photoreceptors.
Color Opponency Theory
There are three channels for color vision. Red Green, blue yellow, brightness. Each channel treats two colors as opponents.
R/G channel
Signal red for long and short wavelengths and green for medium
B/Y channel
Will signal yellow for medium and long wavelengths and blue for short.
Where do color opponent channels begin
At the ganglion cells.
Saturation
How vivid is the color
Chroma
Saturation
Value
Brightnss
Bezold-Brucke Phenomenon
The perception of hue associated with a given wavelength changes as the intensity of light is increased.
Color constancy
Colors will appear the same even in different lightening conditions. Due to processing to maintain this.
CIE Color system
You can combine red, green, and blue to create a stimulus that matches any other wavelength. 645, 536, 444.
Red wavelength
645
Green wavelength
526
Blue wavelength
444 nm
Protonopes
Missing the erytholabe pigment (564 nm). R-G color blindness. Red hues are very dim.
Duternopes
Missing the chloro pigment (534 nm). R-G color blindness with no dimming.
Tritanopes
Missing the cyan pigment (555 nm). B-Y color blindness.
Protonomalous trichromat
The erthylabe pigment is shifted lower (like wearing a green filter). Red weak.
Deuteranomalous trichromat
The cholorlabe pigment is shifter higher (like wearing a green filter) Green weak.
X-chrom CL
Red tinted lenses that allow long pass filters. Shift the cone photoreceptor absorption spectra towards longer wavelengths. Can wear over one eye to increase color discrimination.
Chromatic Discrimination
How much we need to change the wavelength of light in order to perceive a change in the hue of the stimulus.
Color confusion lines
Only one photoreceptor is responding to the wavelengths on this line.
Ichihara plates
No triatonpes and no nopes vs. anamolous
HHR
No nope vs. anomalous but yes tritanopes.
Kollner’s Rule
Outer retinal disease typically cause B/Y defects and inner retinal disease, ON, or visual pathway cause red-green defects.
Chromatopias
Distortion of color but still have an ability to see colors
A blurred lens will result in _____ image contrast, particularly at ______ spatial frequencies
Higher
High spatial frequency cutoff
40-60 CPD
Low spatial frequency cuttoff
4 CPD
Snellen denominator to CPD
600/CPD=snellen denominator
Why is there a high frequency cutoff?
Optical limitations and finit density of photoreceptors
Why is there a low frequency cutoff?
Lateral inhibition of the retina
March bands
If you gradually change luminance from end to the other you will actually just see march bands.
Monocular Depth Cues
Pictorial depth cues, motion parallax
Binocular Depth Cues
Uncrossed regional disparity (falling on nasal. Further away), crossed retinal disparity (falling on temporal. near)
Motion detection
MT and V5
Troxler phenomenon
Poor sensitivity for very low frequency stimuli and images will fade if we stare at it.
Critical Flicker Fusion Frequency
If below=flicker. If above=constant
Posterior lens perking image
Only one that is real, inverted, smallest. Formed by a concave.
Maxwell’s spot
Blue wavelengths absorbed by the macula. Patient fixates a purple light he will perceive a red spot located at point of fixation because all blue light absorbed by macula
