Visual Perception Flashcards
Person having low threshold …..
Has high sensitivity.
Threshold is inverse of sensitivity
Accurate way to determine threshold
Method of constant stimuli Stairstep method ( Humphery VF)
Example of force choice method
Teller acuity card
Sensitivity
Probability the test will accurately detect patients with disease
TP/TP+FN
Specifity
Probability the test will accurately detect patients without the disease
TN/TN+FP
Photometry
Property of light perception .
Luminous efficiency function defined as
V=1 brightest V=0 dim
Maximum value 555nm
Fo every watt of power of a light source, the visual system responds with 680 lumens
Scotopic lumens
507nm corresponds to 1700 lumens/watt
Abney’s law of additivity
Total number of lumens from multiple light source
Neutral density filter
Transmit all wavelength equally.
Sunglasses
Puff rich phenomenon
Pendulum appears to move in an ellipse rather than back and forth in a plan because of neutral density place on one eye .
Scotopic vs photopic
High sensitivity, low sensitivity .
Univariance
Photopigments respond to photon absorbed ( quantum) in a manner independent of the wavelength of the photon.
Photochromatic intervals
Difference between sensitivity of the Scotopic and photopic system. Zero at 650nm which both are sensitive to this wavelength .
Purkinje shift doesn’t exist in
Rods only visual system
Sole cause of dark adaptation
Photo pigment regeneration . Dowling-Rushton equation
JND
Difference between the intensity of the just perceivable spot of light compared to the intensity of the background .
Rocco’s Law
IA=C
If intensity of the stimulus increase, it can still be seen if the area is decreased as log as total number of quanta is constant
Bloch’s law
It=C
If duration of flash is increased , the stimulus can still be detected if the intensity of the stimulus is decreased.
Stiles-Crawford effect
Cone photoreceptors are most sensitive to rays of light that strike orthogonal to the surface.
Photoreceptors S M L rods
S-cyanolabe-440nm
M-chlorolabe-534nm
L-erythrolabe-564nm
Rods-rhodopsin -498nm
Protanopes
Missing L cones
Confuse red, orange, yellow and green. They see red very dim.
Inherited
Dueteranopes
Missing M cones.
Confuse red yellow orange and green
They do not have dim effect on red color
Inherited
Tritanopes
Missing S cones.
Blue-yellow defect
Usually acquired
Protanomalous trichromat
L con shifted toward short wavelength.
Red orange green all appear Green.
Red appear dimmer
Red weak
Deutranomalous trichromat
M cone shifted toward longer wavelength .
Red orange green all appear more Red.
Green weak
Copunctal point
All color confusion lines originate from one point.
Ishihara plates
Can only diagnose red-green defect.
Farnsworth D-15 test
Diagnose red-green and blue- yellow defects. Doesn’t distinguish between dichromats and anomalous trichromats.
Nagel Anomaloscope
Can distinguish between dichromats and anomalous trichromats.
Has test field (590nm, adjusted from 0-87) and mixture field (546nm and 670nm, adjusting from 0-73)
Normal color vision = mixture scale to 45 and test scale to 17
Protanopes result from Nagel test
Mixture of 73= will set the test field to very low settings.
Mixture of 0 = will set the test field to very high settings
Their mixture field fall on color confusion.
Deuteranopes Nagel test
Mixture field anywhere from 0-73= test field will set to 73.
They don’t have dimming of red.
Protanomalous trichromat
Will have to set mixture field higher than normal settings which is 45. They will set the test to 10 Because they r red weak.
Deuteranomalous trichromat
Will have to set mixture field lower than normal settings which is 45. They will set the test to 17 Because they r green weak.
Blue-yellow defect
Outer retinal disease
AMD
Glaucoma
Red-green defect
Inherited
Inner retinal disease
Optic neuritis, visual pathway disease.
Chromatopsia
Characterized by distortion of colors but not a deficiency
Visual pathway to detect motion
Primarily processed by the mangocellular pathway, V5 in middle temporal area.
Motion after effect
Perception of motion after moving stimuli is removed, in most cases is in opposite direction of the original motion stimuli. Common example is WaterFall illusion.
Example of low frequency cut off if temporal modulation transfer function
- purkinje tree
- Toxler phenomenon
Our visual system has poor sensitivity for very low frequency.
Crowding phenomenon is the result of
Simultaneous masking effect
Magnocellular pathway is responsible for
Saccadic suppression .
Damaged by glaucoma, which results patient with glaucoma may show decreased ability to process moving stimuli.
