Duplex retina Flashcards
Pupil size accounts for
1 log unit
Scotopic vision
Low levels of light, rod sensitive
Photopic vision
Bright levels of light, cone sensitive
Mesopic vision
Intermediate light levels, both cones and rods operate
Peak density of rods
150,000 at 20 degrees from fovea
Peak density of cones
115,000-225,000 at foveal center with the absence of S-cones
Peak density of s-cones
2,000 at 0.5 degrees from foveal center
How much rhodopsin in each eye
10^15
Retinal
Chromophore part of rhodopsin made from vitamin A
Opsin
Protein part of rhodopsin
Bleached state
When molecule of rhodopsin absorbs a photon of light, unable to absorb another photon
What is the half life of rhodopsin
5 minutes
Rhodopsin absorbs at
507 nm
Principle of univariance
Ability to match the response of a photoreceptor to a lower intensity light at a preferred wavelength to that of a higher intensity light at a less efficient wavelength
Detection threshold
The dimmest light stimulus of any given wavelength that a subject can detect
L-cone pigment and peak absorption
Erythrolabe
565 nm
Red
M-cone pigment and peak absorption
Chlorolabe
535 nm
Green
S-cone pigment and peak absorption
Cyanolabe
430 nm
Blue
Photopic peak absorption
555nm
S-cones make no contribution
Photochromatic interval
Difference in sensitivity between the scotopic and photopic systems for a given wavelength
Purkinje shift
Shift in relative brightness of suprathreshold stimuli as peak sensitivity moves from 555 to 507nm
Half-life of cones
1.5 minutes
Dark adaptation
Recovery of visual sensitivity over time following exposure to a bright light
When is the rod-cone break the steepest?
At wavelengths with large photochromatic intervals
Rod monochromacy
Condition characterized by a near absence of cones in the retina
Retinal densitometry
Measures the amount of bleached rhodopsin in percentages
Dowling-rushton equation
log(threshold at given time/threshold after complete da) = 20 for rods or 3 for cones * proportion of bleached photopigment
Light adaptation
Process of the visual system adjusting its performance to the ambient level of illumination when going from dark to light
Increment threshold procedure
Used to measure light adaptation, must detect a faint light stimulus against a uniform background as it grows progressively lighter
Increment
Difference between background luminance and stimulus luminance
Section one of light adaptation function
Dark light
Sensitivity is limited by neural nose inherent in the visual system
Theoretical
Neural noise
Visual light phenomena when in a completely dark room or with eyes closed
Section two of light adaptation function
De-Vries rose law
Sensitivity is limited by quantal fluctuations inherent in the background light source
De-Vries rose law
Increase in threshold is equal to the square root of background intensity
Section three of light adaptation function
Weber’s law
Relative sensitivity stays constant while absolute sensitivity decreases
Weber’s law
Increase in background intensity leads to a constant ratio of increment intensity to background
Section 4 of light adaptation function
Rod saturation
Rods unable to signal the presence of a stimulus even with infinite increase in brightness
At 10% bleached rhodopsin
Section 5 of light adaptation function
Photopic section
Follows weber’s law with an increased contrast sensitivity
Absolute sensitivity
How well a subject can go from seeing nothing to seeing something
Scotopic
Relative sensitivity
When a subject can detect a target against the background
Photopic
Static perimetry
Computes the increment thresholds at different locations of the visual field holding the background luminance constant
Good spatial summation
Scotopic
Poor spatial resolution
Scotopic
Good resolving capacity
Photopic
Poor spatial summation
Photopic
Poor sensitivity to target detection in dim light
Photopic
Larger receptive fields
Scotopic
Scotopic system critical period
100ms
Photopic system critical period
10ms
Ricco’s law
When the threshold number of quanta are within 10 minutes of arc, there is complete spatial summation and stimulus is detected
Stimulus intensity times area equals constant
Bloch’s law
Within the critical duration, the total number of quanta required to reach threshold remains constant for different flash durations
If flash duration exceeds critical, total quanta required increases
Stimulus intensity times duration equals constant
Relative luminous efficiency
The apparent brightness of the light to an individual
Stiles Crawford Effect 1
Directional sensitivity of cones
Light that enters straight through the center of the pupil is more effective in cone stimulation than if it enters peripherally
Intraocular scatter
Scattered light that strikes the cones at large oblique angles
Minimized due to SCE1
Duration of dark adaptation for experiment
At least 40 minutes
Where in visual field should test flash be presented?
20 degrees temporal to the fovea due to peak rod density
Size of the test flash?
10 arc minutes in diameter due to Ricco’s law
Duration of test flash?
1 millisecond due to bloch’s law
Stimulus wavelength for experiement?
510 nm due to scotopic sensitivity curve
