Visual Perception (Duplex Retina)
4 basic mechanisms for controlling the sensitivity of the visual system:
-duplicity theory (changing rods and cones) -pupil size -the amount of unbleached retinal photopigment -neural changes
Peak density of S cones is at _______degrees.
0.5 degrees
In the long wavelength region, the photochromatic interval is __________.
0; rods and cones are equally sensitive
Each molecule of rhodopsin is capable of absorbing ______ (quantum) photon of light sufficient to activate a rod.
1
10 activated rods are sufficient to activate ________________
1 ganglion cell
Pupillary changes accounts for only __________ of adaptation range
1 log unit
1 quantal absorption is sufficient to activate ____________
1 rod
Neural responsiveness accounts for only ___________ of adaptation range
1-2 log units
2 designs of retinal prosthesis
1. Epiretinal design - microelectrode array tacked on to NFL, stimulating ganglion cell axons 2. Subretinal design- microelectrode tacked on to depth of damaged photoreceptors stimulating outer aspects of ganglion and bipolar cells layer
2 sections in dark adaptation curve
1. Rapid reduction in threshold (increase in sensitivity) up to approx. 10 minutes, then plateaus (cone plateau) **Rod-cone break around 12 minutes** 2. Slow reduction in threshold out to 35 minutes, then plateaus (rod plateau)
In what 2 ways does spatial summation occur?
1. Ricco's Law 2. Piper's Law
Dark adaptation is abnormal in what cases?
1. Rod monochromat (no cones) 2. Congenital stationary night blindness (CSNB) 3. Oguchi's disease 4. Retinal diseases 5. Drugs 6. Alcohol intoxication
The visual system uses 4 mechanisms to adapt a wide range of light levels
1. Two different photoreceptors (rods and cones) 2. Pupil size change/adaptation in response to different luminance levels 3. Change in concentration of photopigments 4. Changes in neural (network) responsiveness
Being more light adapted causes:
1. a higher initial threshold 2. more prominent cone branch 3. rod-cone break 4. rod-cone break that occurs later 5. a longer time until complete rod dark adaptation occurs
Change in pupil size alters the retinal illuminance by ____________
1.2 log units
For cone pigments, every __________ minutes half the cones will still be bleached.
1.5 minutes
How long does it take for 50% of photopigments to recover?
1.5 minutes
50% bleaching of rhodopsin decreases quantal absorption by a factor of one-half (50%) which increases the threshold by __________________
10 log units
What is the adaptation range of the human visual system?
10 log units
A bright light bleaches 100% of your patients rhodopsin. What period of dark adaptation is required to ensure 75% of the rhodopsin has regenerated?
10 minutes
You have a light flash of 50 units on for 20msec. Assuming Bloch's law to be valid, if you now double the light flash luminance, what should the Time Duration be?
10 msec
Rod saturation occurs even when _____% of the rhodopsin is bleached
10%
Each disc contains how many molecules of rhodopsin?
10,000
The photopic system has a much shorter critical duration of ____________________
10-50msec
Each rod has how many discs?
1000
A fully dark-adapted patient can just detect a 30-millisecond flash of light containing 1000 quanta and presented 20 degrees superior to fixation. How many quanta would be required for detection if stimulus duration was increased to 60 milliseconds?
1000 quanta
The scotopic system has a critical duration of ______________
100msec
Scotopic vision occurs at luminance levels of __________
10^-2 to 10^-6 cd/ m^2
How many molecules of rhodopsin per eye?
10^15
Photopic vision occurs at luminance levels of ___________
10^2 to !0^6 cd/m^2
In the scotopic system, signals from _______ rods are summed by one ganglion cell (reaches threshold) to produce a signal that indicates the presence of a light.
15 rods
Most of visual system adaptation is largely due to the
2 independent systems made of 2 classes of photoreceptors (rods and cones)
Each decade ______________log unit higher threshold (lower sensitivity) of the rods and cones.
2-3 log units
Rods mostly packed ______________degrees from fovea
20 degrees
Less than ________% of the quanta incident on the retina are absorbed by rhodopsin.
20%
VA of ____________ is possible for rod monochromats under scotopic conditions.
20/200
A fully dark-adapted patient can just detect a flash of light containing 2000 quanta presented over retinal area of 4 minutes of arc (square stimulus). If the retinal area of the stimulus is increased to 8 square minutes of arc, then how many quanta would be required for detection of the light?
2000 quanta (slide 115)
You have a light flash of 50 quanta presented over a retinal area of 6 arc minutes. Assuming Ricco's law to be valid, if you now double the light flash quanta, what should the Stimulus Area be?
3 minutes of arc
A bright light bleaches 100% of your patients photopigment. What period of dark adaptation is required to ensure 87.5% of the cone pigment has regenerated?
4.5 minutes
In the photopic system, signals from ____ cones are summed by one ganglion cell
5
Cones are present throughout the reinta, only ________% of the total cones are located in the fovea.
5%
S cones constitute what percentage of the cone population?
5-10%
For Rhodopsin, every __________ minutes half the rods will still be bleached.
5.2 min
Compared to the unbleached state (fully regenerated), when half of rhodopsin pigment is bleached, there is a _______chance that a quantum of light will be captured by rhodopsin.
50%
What is the half-life time of rhodopsin?
50% of rhodopsin will recover in 5 minutes from bleaching
What is peak absorption for rhodopsin?
507 nm (blue-green)
As lighting conditions change from scotopic to photopic, the wavelengths to which we are sensitive increases from ________________ to ________________.
507 to 555nm
Peak sensitivity of the rods is _______________
510 nm
Peak photopic spectral sensitivity is ________
555nm (yellow-green_
A fully dark-adapted patient can just detect a 20-millisecond flash of light containing 600 quanta and presented 20 degrees inferior to fixation. How many quanta would be required for detection if stimulus duration was decreased to 5 milliseconds?
600 quanta
What percentage of rhodopsin pigment has regenerated 10 minutes following a 100% bleaching?
75% regeneration
What percentage of cone photopigment has regenerated 3 minutes following a 100% bleaching?
75% regeneration (1.5 minutes --> 50% still bleached, 3 minutes --> 25% still bleached)
At rod-cone break, how much of rhodopsin has regenerated?
90%
Retinitis Pigmentosa (RP)
A progressive degeneration of the retina that affects night vision (first) and peripheral vision -no effective treatment rn -Vitamin A supplements slows progression -tissue transplant/gene therapy/retinal prostheses hold promise
Research suggests dark adaptation is abnormal in patients with _______________
ARMD
What factors affect dark adaptation?
Adapting light: wavelength, intensity, duration Test flash: wavelength, size and retinal location, task
Ricco's Law
Applies to stimulus up to 10 min of arc in diameter (critical diameter) IA=K (I= stimulus intensity, A= stimulus area, K=constant)
M-cones contain what photopigment?
Chlorolabe
What happens if the pupil is decentered? Do the cones point toward normal pupil or decentered pupil?
Cones are mobile and they orient themselves to light to maximize photon capture
What are the 3 cone photopigments?
Cyanolabe (426) Chlorlabe (530) Erythrolabe (557)
Post-receptor coding
Determines and replicates the encoded message produced by post-receptoral retinal processing and provides this input directly to ganglion cells -benefit: takes into account post-receptoral processing
L-cones contain what photopigment?
Erythrolabe
Which cones are most concentrated in the foveal center?
L and M cones
Photopic spectral sensitivity curve represents which 2 cones absorption?
M and L cones
Just because peak absorption of rhodopsin is 507 nm, does this mean quanta of other wavelengths cannot be absorbed by rhodopsin?
NO, other wavelengths are absorbed but with less probability
Principle of Univariance
Once a quantum of light (photon) is absorbed, all information regarding its wavelength is lost -So a photon of wavelength 680 or 507 nm once it bleaches rhodopsin, the effect is the same
What happens if the test stimulus size is very small and centrally fixated(0.5 degrees, instead of 10 degrees) in dark adaptation curve?
Only a cone function is obtained
Innermost aspects of rods and cones synapse in what layer?
Outer Plexiform Layer
Bloch's Law is the temporal equivalent of _________________________
Ricco's Law
The point during dark adaption at which rods become more sensitive than cones is ____________________
Rod-cone break
Which cone is not found in the central 0.3-0.4 degrees of the fovea?
S cones
Which cones make little or no contribution to spectral sensitiivity?
S cones
Why do military installations use red lights in situation rooms?
The long wavelength red light only stimulates cones, so when the red light goes out the person can immediately see in the dark because rods are dark adapted
Visual adaptation
The process whereby the visual system adjusts its operating level to the prevailing light level
Sensitivity is inverse of _______________
Threshold
Piper's Law
Used when stimulus diameter is greater than 10 arc minutes (10min of arc -24 degrees)
Stiles-Crawford effect 1
a property of the human eye that refers to the directional sensitivity of the cone photoreceptors
Duplicity theory
above a certain luminance level (0.03cd/m^2), the cone mechanism provides photopic vision. Below this level, rod mechanism provides scotopic vision. Range where two systems work together is mesopic range
Light quanta that are incident on the rhodopsin, but not transmitted have been ________________-
absorbed
What happens when there is an absence of phagocytosis of the photoreceptor discs?
accumulation of metabolic waste that damages rods and cones --> RP- retinitis pigmentosa
To generate the dark adaptation curve, the subject is exposed to bright adapting light(to bleach most of photopigment), then ________________________.
adapting light is turned off
rods can be dark adapted in relatively high-luminance conditions if the _______________________ primarily affects cones only.
adapting-light wavelength
At time zero in dark adaptation curve, both rods and cones______________
begin to dark adapt
Placement of subretinal implants subfoveally results in _______________visual outcomes than others.
better
A molecule of rhodopsin becomes ________________ when it absorbs light.
bleached
The sensitivity to light is related to the amount of ________________________
bleached photopigment
Under photopic lighting conditions: due to rod saturation, rod monochromats are essentially ___________________
blind
Under photopic conditions, light rays that strike cones perpendicular to their surface are perceived __________________ than those that do not strike perpendicular to the surface.
brighter
Once a molecule of rhodopsin is bleached- it will transmit a quantum of light incident on it, but it is not capable of _______________________
capturing another quantum
Inner segment in photoreceptors contain
cellular organelles (except nucleus)
Under normal conditions, cones point toward ___________ of the pupil.
center
Outer segment discs represent infoldings of the cellular membrane and are generated in the region of ____________________ and migrate outward.
ciliary connection
A more intense-adapting light makes the _________________ of the dark adaptation curve more prominent and prolongs dark adaptation.
cone branch
For the lowest two adapting calues, the rods were more sensitive than the cones as soon as the adapting light went off, so there was no _______________ to the curve
cone branch
If test flash contains only long wavelengths, the ______________ branch of the curve predominates.
cone branch
Which photopigments recover faster? (rods or cones)
cone photopigments
If the threshold task requires the subject to have good acuity, _____________ must be used.
cones
In which photoreceptor does the disk remain attached?
cones
For stimuli up to 10 min of arc in diameter (critical diameter), the total number of quanta necessary to detect is _____________
constant
Photopic light adaptation follow's Weber's law, this indicates that the photopic system is more sensitive to _________________ than the scotopic system, however its absolute sensitivity is less.
contrast
The scotopic system manifests total spatial summation for stimuli that fall within the _______________________.
critical diameter
The difference in spatial summation between the scotopic and photopic system manifests as a difference in the _______________________.
critical diameters
In order for two flashes of light to be temporally resolved as 2, they must be separated in time by at least the ___________________________
critical duration
S-cones contain what photopigment?
cyanolabe
Thresholds for photopic and scotopic spectral sensitivity are determined by having subjects ______________________
dark adapt (full regeneration of rhodopsin and cone pigments)
A larger (area) test flash generally yields a more prominent rod branch of the ___________________________.
dark adaptation curve
Rods discs are shed during the ____________ cones discs are shed during the _________.
day; night
During approximately the first 3 minutes after the adapting light goes off, the cone branch shows a rapid ____________ in threshold luminance.
decrease
Number of retinal cones remains stable as eye ages, but number of rods __________________
decreases
Light adaptaion
decreases the visual system's light sensitivity in response to an adapting light
Purkinje shift
difference in the peak sensitivity in systems; relative increase in brightness of longer wavelength stimuli as lighting changes from scotopic to photopic
In a completely dark adapted retina, the antagonistic receptive-field surround ____________________
disappears
Vitamin A deficiency increases thresholds in the rod portion of the curve but ________________ affect dark adaptation.
does not
Changes in the release of the neurotransmitter ________________ are involved in receptive-field changes of some ganglion cells.
dopamine
Why is the rod-cone break for the 465nm stimulus more prominent and earlier than for the 610nm stimulus?
due to the absorption properties (probability) of rhodopsin
equivalent background theory
during dark adaptation the threshold for detecting a spot is equivalent to the threshold for detecting the same spot against a background that bleaches the same fraction of rhodopsin
Even when the photoreceptors are not light adapted, neural adaptation has been found to occur in _______________________
ganglion cells
For Rods, the Testflash/stimulus appears ____________________ at threshold.
gray or colorless
The rod portion of the dark adaptation curve is more prominent if the test flash stimulates a retinal region with ________________________.
high rod density
Low threshold indicates _____________
high sensitivity
The rods are slower to dark adapt and have a threshold that is _________________ than the threshold of the cones during the initial period after light is turned off.
higher
Dark adaptation
increase in sensitivity (decrease in threshold) that occurs as a function of time in darkness -gradual improvement of vision after exposure to bright-adapting light
In Bloch's Law, beyond the critical duration, the light required for detection _____________________
increases
Threshold for flash of light is determined by:
increment (△I) presented on a background of a given Intensity (I_B)
Light adaptation can be studied by what technique?
increment threshold technique
Early in dark adaptation, the threshold for rods is __________________-
infinitely high
Magnitude of hyperpolarization depends on the ________________________________
intensity of the stimulus
A stimulus that emits only 10 quanta causes what?
is not detected; would be reflected or absorbed by tissues inner to the photoreceptors; not absorbed by rhodopsin
Increment threshold is also referred to as _________________________________
just notable difference
S cones are ___________ numerous than L and M cones.
less
Patients with rod monochromacy may benefit by wearing dark sunglasses during daylight conditions because the tinted lenses result in less _______________________________, thereby preventing rod saturation.
less bleaching of rhodopsin
Under scotopic conditions, the angle at which a light ray strikes a rod is much _________________ or relatively insignificant.
less critical
Changes in the concentration of photopigments
level of bleached photopigment has a big impact on visual adaptation
Photopic system shows _________________ summation over space and time.
limited
Rods are used for __________ luminance, and cones are used for ____________ luminance.
low (scotopic); high (photopic)
As the retinal illuminance of the adapting light was decreased, the threshold test flash luminance was __________________ immediately after the adapting light went off.
lower
The dark adaptation curve represents the lowest ________________at any given time.
lowest threshold (highest sensitivity)
Beyond 650 nm, the photopic system is marginally _______________ sensitive than the scotopic system.
more
Except in Long-wavelength (red) region of the spectrum, scotopic system is ___________ sensitive than photopic system.
more
A larger test flash generally yields a _______________ rod branch of the dark adaptation curve.
more prominent
A longer duration adapting light makes the cone branch _____________________ and prolongs dark adaptation.
more prominent
The rod portion of the dark adaptation curve is __________________ if the test flash stimulates a retinal region with high rod density.
more prominent
Early dark adaptation is a small, but very rapid decrease in threshold mediated by __________________ changes.
neural
The rise in threshold before the adapting light goes off is due to the duration of the ______________ response to the test flash.
neural
When test flash contains only wavelengths longer than 680nm, there is __________________-
no rod branch on curve
No matter how many flashes are presented within the critical duration, they will all be seen as _________________
one light
For 650nm dark adaptation curve, what changes?
only cone portion (rod aspect missing) -photochromatic interval=0
Stiles-Crawford function may be abnormal in retinal diseases that causes disruption of cone ___________________.
orientation
What are the 2 variants of erythrolabe?
peak absorptions at 552nm or 557 nm
cone synaptic ending
pedicle
Outer segment discs are continuously produced and shed and subsequently ______________________ by the RPE.
phagocytized
_____________________ of photoreceptor discs is essential for retinal health.
phagocytosis
The difference in sensitivity between scotopic and photopic systems for a given wavelength is referred to as the _______________________________
photochromatic interval
The human eye uses pure _____________vision in the range above 3.4 cd/m^2.
photopic
VA and Contrast sensitivity are superior in ____________________ conditions.
photopic
The scotopic system is better able to sum up information over time (temporal summation) than the ____________________.
photopic system
Which system is better able to distinguish 2 flashes of light separated by a brief interval in time? (superior temporal resolution)
photopic system
Disc structure in the outer segment of rods in cones contains __________________ that absorbs light quanta (photons).
photopigment
After approximately 30 minutes in the dark adaptation curve, the rod portion of the curve reaches a ___________________________ where there is little further decrease in threshold because rods are fully dark adapted.
plateau region
Retinal prosthesis
put a camera in glasses frame --> image transmitted to a micro-electrode array tacked onto the retinal NFL -these electrodes stimulate ganglion cells axons, mimicking signals produced when photoreceptors are activated
What other factors contribute to the dynamics of dark adaptation?
receptoral and postreceptoral
The absorbed curve is the _____________ of the transmission curve.
reciprocal
________________ may be most effective for rod monochromats to wear, due to the decreased effectiveness of long wavelength light at bleaching rhodopsin.
red tints
Bleaching
reduction in sensitivity (closure of sodium channels) of photoreceptors in response to light stimulus -photopigment color becomes almost transparent
Stiles-Crawford Effect 2
refers to changes in Hue and Saturation, of monochromatic light as the point of entry into the pupil is changed
With increase in age, the time of rod-cone break __________________
remains constant
Human scotopic spectral sensitivity is determined by the absorption characteristics of ___________________
rhodopsin
What is the rod pigment contained within the discs of the rod outer segment?
rhodopsin
Scotopic sensitivity curve is the same as ____________________
rhodopsin absorption spectrum
In which photorecptor does the disk break away and become free-floating as they migrate?
rods
After approx. 12 minutes in the dark adaptation curve, what happens?
rods become more sensitive than cones
The human eye uses pure ________________ vision in the range below 0.034 cd/m^2.
scotopic
Visual sensitivity (and absolute sensitivity) is superior in ___________________ conditions.
scotopic
The large number of rhodopsin molecules provides tremendous ability to capture light and contributes to ____________________
scotopic vision
Which visual system has an absence of color discrimination?
scotopic vision
Critical duration is longer under _______________ conditions
scotopic; which means that the scotopic system is designed to collect photons over time
Detecting the test flash at the same time the adapting light is turned off is a ___________________ task
signal detection
Photopigment-related changes during dark adaptation tend to be relatively _________________, but changes in sensitivity related to neural adaptation tend to be relatively ________________.
slow; fast
Rods begin to dark adapt at the same time as cones, but they are initially less sensitive and they are _________________ to dark adapt.
slower
Critical diameter of the photopic system is _________________ than that of scotopic system, reflecting the reduced spatial summation of the photopic system.
smaller
In order to detect a stimulus with 10 quantal absorptions, they must occur within certain ________________ and ________________ constraints.
space (spatial summation) and time (temporal summation)
Rods are connected in such a manner as to sump up information over _________
space; produces greater sensitivity, but poor resolution
Scotopic system loses info in spatial summation. Excellent sensitivity, but poor ______________________.
spatial resolution
Scotopic system to a greater extent (than the photopic system) sums up information over space leading to greater ______________________________
spatial summation
rod synaptic ending
spherule
Bloch's Law
states that, within the critical duration (of the photopic/scotopic systems), the total number of quanta needed to reach threshold remains the same for different flash durations and cannot be resolved (seen as a single stimulus) It=K (I= stimulus intensity, t= stimulus duration, K=constant)
The Weber's fraction is affected by ________________________________
stimulus size, duration, wavelength, and retinal location
1 activated ganglion cell causes
stimulus to be detected
Summation over time is referred to as _______.
temporal summation
Scotopic spectral sensitivity
the ability to detect stimuli (light) under scotopic conditions -determined by the rhodopsin absorption curve
Weber's Law
the change in the increment threshold (△I) to the background intensity is constant (I_B)
If the total quanta exceeds the detection threshold -->
the light will be seen
Spectral sensitivity
the relative efficiency of detection of light as a function of the wavelength of the signal
Critical Diameter
the retinal area over which light is summed
If one. measures a dark adaptation curve of a rod monochromat (a person who lacks cones), the dark adaptation curve does not have a break in it and shows only_____________________________
the rod branch of the curve
Critical duration (period)
the time period over which light is summed
For the first 10 minutes of dark adaptation curve what happens?
there is an upper cone branch during which the test flash is detected by cones
Limitations of retinal prosthesis
these devices are designed to stimulate ganglion cells with a signal that roughly replicated photoreceptors -Ignores the important processing that occurs within the retina
The outer and inner segments of photoreceptors are connected by ______.
thin ciliary processes
Detection threshold is determined as a function of ____________ for a stimulus flashed against a totally dark background.
time
In order for the quanta from two flashes to sum, they must be presented in _____________succession.
very rapid
Cones manifests connections that maximize______________________
visual resolution
Both of the Stiles-Crawford effects of the first and second kind are highly ___________________ dependent.
wavelength
The critical duration is not affected by ____________________
wavelength
At cone threshold, the test flash appears either ____________________ depending on its wavelength composition.
white or colored
Increasing the diameter of the stimulus beyond 24 degrees _____________improve detection.
will not
The ability to recover visual sensitivity from bright lights changes how after age 40?
worsens after 40
Over a period of 35 minutes in the dark adaptation curve, threshold reduces (improves) by ________________
~5 log units
DeVries- Rose Law
△I= (I_B)^1/2
