Ch. 3: Duplex Retina Flashcards
what is the light adaptation range of the human eye?
10 log units
although pupil size constitutes for 1 log unit of light adaptation range, what constitutes the rest?
rods and cones
what characterizes scotopic vision? and what photoreceptor mediates it?
sensitivity to very dim lights, poor visual acuity (20/200) and absence of color discrimination. Rods
what are the characteristics of photopic vision? and what photoreceptor mediates it?
occurs under bright conditions, excellent acuity (20/20) and ability to color discriminate. Cones
what type of vision occurs when both rods and cones are being used?
mesopic vision
what type of ending does a rod contain?
spherule
what type of ending does a cone contain?
pedicle
what photoreceptor contain free-floating discs as they breakaway and migrate outward?
rods
how many rhodopsin molecules does each human eye contain (on average)?
10^15 molecules per eye
after being bleached, how long would it take 50% of the rhodopsin in the eye to revert to an unbleached state?
5 minutes
at what wavelength is rhodopsin most probable to absorb a quanta?
507 nm
what is threshold for a stimulus?
the minimum amount of energy required for detection of a stimulus
does low threshold indicate high sensitivity?
yes
what percentage of quanta from any given stimulus is actually absorbed by rhodopsin?
less than 20%
what are the 3 fundamental photopigments in cones?
cyanolabe, chlorolabe and erythrolabe
what type of cone is cyanolabe?
S-cone (426nm)
what type of cone is chlorolabe?
M-cone (530nm)
what type of cone is erythrolabe?
L-cone (557nm)
after being bleached how long does it take 50% of cones to revert to unbleached state?
1.5 minutes
at what point does the photopic spectral sensitivity curve peak?
555 nm
is it true that S-cones make little, if any, contribution to spectral sensitivity?
yes
what is the photochromatic interval?
difference in sensitivity between scotopic and photopic systems
where are rods most densely packed and what is the approximate density?
20 degrees from the fovea. peak density of 150,000 rods/mm^2
is it true that number of cones stays stable as we age, but the number of rods decrease with age?
true
what percentage of cones are located in the fovea?
5% (of the total number of cones in the retina)
do the ratio of L-cones to M-cones remain 1:1 for all individuals?
no they can range from 1:1 to 16:1
in dark adaptation it takes approximately 35 minutes for the eye to become adapted to a dark environment, how rapid is the initial reduction in which the the curve plateau’s at the cone plateau? (420 nm curve)
10 mins
how long does it take to reach the rod-cone break? (420 nm curve)
12 minutes
what occurs during the rod-cone break?
the point at which rods become more sensitive than cones (prior to this point cones detect the stimulus and after the rods detect stimulus)
does cone plateau represent the minimum scotopic threshold?
no. it represent the minimum photopic threshold (rods represent scotopic)
early in dark adaptation is rhodopsin more likely to absorb a quantum of 465 nm or 610nm?
465 nm because sensitivity of scotopic system surpasses that of the photopic system (it would take longer to absorb 610 nm due to the regeneration properties of rhodopsin.
T/F? During dark adaptation, the regeneration of photopigment increases the probability of quantal absorptions, thereby increasing sensitivity.
true
what happens to quantal absorption and threshold when 50% of rhodopsin is bleached?
quantal absorption decreases by one-half
absorption increases by factor of 10^10
how is increment threshold plotted?
performed for background light levels ranging from darkness to brightness, resulting in a light adaptation curve that shows increment threshold as a function of background adapting intensity.
in a graph depicting the log of increment threshold, what does Weber’s Law explain?
as the background brightness is increased, the increment intensity must be increased such that the ratio of the increment intensity to the background intensity remain constant
what percentage of rhodopsin is bleached in rod saturation?
only about 10%
is the Weber fraction under scotopic conditions 0.015?
no it is 0.14.
0.015 represents photopic conditions
under what conditions is contrast sensitivity higher?
under photopic conditions (0.015)
is it true that humans “detect” stimulus better under scotopic conditions?
yes
why do rods produce great sensitivity but poor resolution?
because they are connected in such a manner to sum up information over space
T/F? Many cones (more than rods) report to one ganglion cell?
False. Many rods report to one ganglion cell
what is the difference between sensitivity and resolution?
sensitivity: a stimulus is seen
resolution: only one stimulus is seen
is it true that the scotopic system has excellent sensitivity yet poor spatial resolution?
yes
what is the equation for Ricco’s Law?
(stimulus intensity)*(stimulus area)= constant
which system (photopic or scotopic) has a smaller critical diameter?
photopic system has a smaller critical diameter (reflects the reduced spatial summation capability of the photopic system)
which system has superior temporal (time-related) resolution?
photopic system (able to distinguish 2 flashes of light separated by a brief interval in time)
what is the temporal summation period of the scotopic system?
100 ms
what is significant about Bloch’s Law?
multiple flashes presented within this critical duration are not resolved, and only one flash is seen.
what is the critical duration of the photopic system?
10-50 ms
at what angle must a light ray strike a cone to be maximally effective?
perpendicular to its surface
what is the Stiles-Crawford Effect of the First Kind?
light rays that strike cones perpendicular to their surface (pinhole centered) are perceived as brighter than those that do not strike perpendicular to the surface
