Chapter 2-Light sensitivity Flashcards
what is the duplex retina
what must the 2 thigns operate
- one operates under dim conditions and is more sensitive to short wavelengths
- a second that operates under bright conditions and is more sensitive to long wavelenths
what is scotopic vision
rod mediated mech
operates under dim lighting conditions
doesnt perceive color
poor VA
what is photopic visoin
cone mediated mech
operates under bright lighting conditoins
perceives colors
best VA
what do rods contain in their outer segment?
how is that activated?
rhodopsin
it is very sensitve to light and it can be activated by only 1 photon
-can trigger rod to transmit signal to rod-bipolar cells
-DOES NOT mean we perceive this photon as light
what happens when rhodopsin is activated by a photon
becomes inactive and it is not sensitive to light anymore
BLEACHING is the inactive site
- cont. regenerates and reverts to the active state after some time
- after regnerated it is able to absorb another photon and etc.
how many rhodopsin molec does a single rod contain, what about the whole retina
how long does it take for all of these molec to become active (NOT BLEACHED)
10^7
10^15
40 min (in ocmplete darkness this is when we can measure scotopic threshold)
how can we obtain a similar function to scotopic spectral sensitivie for photopic conditions?
no dark adaptation, bright background and stimulus to stimulate cones
how long does it take for the regneration of the 3 cone opsins
5-10 min
what is the photochromatic interval
the difference btwn the scotopic and photopic spectral sensitivity function
in a radiometry, where will the output be
the radiant power (or radiant flux) of the source in Watt (W)
what does photometry deal w/
the effect a certain wavelength has on the visual system and how it compares w/ other wavelengths
or
how efficient a wavelength is at stimulating the visual system
photopic and scotopic luminosity functions
what is luminous power
the perceived power of light, not the physical power
what is abney’s law of additivity
add the total luminous power by adding all of the indiv wavelenghts
what is dark adaptation?
waht is light adaptation
dark: adaptation to dark conditions (or scotopic conditions(
light: adapatation to light conditions (or photopic)
what are the 2 def of dark adapattation
- how fast our visual system adapts to darkness after exposure to bright light
- how fast our sensitivity recovers after exposure to bright light
after bleaching w/ a very intense light the subject sets his/her threshold for up to _______ in complete darkness
40 min
what are the factors affecting dark adaptation curve
bleaching stimulus size stimulus location stimulus wavelength rhodopsin regneration
what is bleaching
the process when the photopigments become inactive and the regeneration process starts
what does the amt of bleached photopigment depend on
the intensity (I) and the duration (t) of the light used for bleaching
bleached photopigment = f(I,t)
what is the photochromatic interval
the difference in spectral sensitivity btwn photopic and scotopic system
what is the half life of rhodopsin regneration
cone?
Rods: T=5 min
in other words, in 5 min, 50% of the bleached rhodopsin is regnerated
Cones: 1.5 min
when causes dark adaptations to start sucking for pts?
- age-photoreceptor loss and thickening of bruch’s
- retinal/neural sources-RP, AMD, congenital stationary night blindness
- vascular changes
- choroideremia,
- gyrate atrophy - vit A deficiency
- chronic liver disease (cirrhosis)
- chronic GI tract dieases (malabsorption)
- alcoholism
what is the visual cycle also known as
retinoid cycle
when photopigments molec are active (ready to react to light) they are opsins bound with a …..
a photon is absorbed by that and it chnages its form to what?
chromophore called 11-cis-retinal (rhod etc)
a photon is absorbed by 11-cis-retinal which then changes its form to all trans retinal
what starts the phototransduction cascade
when rhodopsin is transformed to metarhodopsin
the photopigment is inactive until its all trans retinal is transformed back to
11 cis retinal
the all trans retinal undergoes a series of rxns to convert back to 11 cis retinal and combine w/ what again
opsins
known as the retinoid cycle (when all trans goes to a series of rxns)
what does RPE do for the visual cycle
- part of the bbb
- recycling of retinoid
- active transport of nutrients from choriocap to photorecep (vit A)
- phagocytosis of the aged OS tips
- contains melanin granules that absorb excess light and therefore reduce glare
what happens to the rod system as vit a decreases
what happens when it sig deceases
- rod system shows a slower rate of DA and photopigment regeneration
- abs threshold elevates (sensitity drops) and all rod function might be lost
what happens to the cone system as vit a decreases
what happens when it sig decreases
the cone system isnt too severely affected w/ normal DA threshold
-as it significantly decreases the cones degenerate
if vit a deficiecny is not too severe, how can it be reversed?
what does vit A restore w/in the RPE
effects can be reversed w/in days w/ oral vit a supplementation
-vit a restores 11 cis retinal w/in the RPE
what is weber’s law
the relationshiop btwn the initial intensity or quantity and the smallest detectable increment
-the increment threshold (just noticable difference) in a psychophyscial task will be a fixed fraction (weber fraction) of the background
our threshold for detecting an increment in the quantity or intensity of something changes depending on what
how much there is before we add the increment
weber’s law says that the just noticeable difference for a quantity is a …..
fixed percentage of this quantity
what is a visual field test
under what conditions
what is it useful for detecting/monitoring
measures the subject’s threshold on detecting a small light against a background
-done under photopic conditoins and proved to be useful for detecthing/monitor glaucoma
