BHS 140.1 Vision Science I
Notetaker: Caitlyn McHugh
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Date:1/31/2012, 1st hour
Page1
Lecture 33
o Why and how does visual sensitivity increase in the dark?
 Role of the pupil – already discussed. Does contribute, but not to any significant degree.
 Photochemical explanation of dark adaptation
 Rods are a 106 times more sensitive than cones after 30 minutes in the dark
 Rods have greater gain than cones
 Rods display spatial summation
 Rods display temporal summation
o Spatial and temporal summation are processes that occur in the human
retina—related to both dark adapted and light adapted scenarios
 Worksheet #8: spatial summation
Dark Adaptation packet, p. 29
o If you expose the human retina to a powerful bleach, photopigment regenerates at the offset of the
bleach
o Summary of regeneration of visual pigment
 Stop the phototransduction cascade
 Phosphorylate Rhodopsin molecule in presence of rhodopsin kinase
 Allow the capping protein arrestin to fit on top of the phosphorylated rhodopsin
 Deactivated rhodopsin molecule chromophore must be regenerated
 Deactivated rhodopsin molecule wants to let go of chromophore in the all-trans
retinal form—needs to be released
 All-trans retinal migrates back to RPE to be regenerated into 11-cis retinal
 Need to have a source of 11-cis retinal—want to deliver 11-cis retinal back to rhodopsin
 Retinal being transported from RPE to outer segment of photoreceptors
 Similar in the cones
o Difference between rods and cones regenerative processes
 Different opsins between cones and rods
 Time—length of regeneration process
 Cones are faster
 Rods have a half-life of 5 min, cones have a half-life of 1.5 minutes
 Recovery of pigment explaining dark adaptation (DA) in more detail
o Expect sensitivity to recover as pigment regenerates, and expect
relationship between these two variables
o Hypothesis: systematic relationship between amount of pigment regenerated
and the threshold
 Aka the photochemical explanation for the recovery of sensitivity
with time in the dark
 Applies to both rods as well as cones
 As photopigment regenerates—thresholds get lower, sensitivity
should increase
 The relationship between amount of pigment that has regenerated
and the threshold at any point in the dark is not simple
o Figure 3-14 from Schwartz
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BHS 140.1 Vision Science I
Notetaker: Caitlyn McHugh
Date:1/31/2012, 1st hour
Page2
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Photochemical explanation for DA assumes that probability of
absorption of a photon of light will be greatest in the dark when all
pigment has been regenerated
 All pigment regenerated—probability of photon of absorption is
greatest
 In this particular example: what is the probability of photon
absorption? If 10000 photons are incident on photoreceptors, and
2000 photons are absorbed—probability of absorption is 20%
under fully dark adapted/fully regenerated conditions
 If bleach a certain fraction of pigment—ex. 50% of the pigment is
bleached. Example 2 on the figure
 Still have 10,000 photons incident upon photoreceptors
 Half of the visual pigment has been deactivated,
probability has to be less than 20%, in this case the
probability of absorption will be half
 The new probability of absorption is now 10%
 Here, 1000 quanta will be absorbed
 If human observer needed 2000 quanta to be absorbed in order to
reach a visual threshold, will the observer still see the test flash if
10,000 quanta is presented in the second case where ½ of the
pigment is bleached?
 No, would only get 1000 photons absorbed, would need
to increase the intensity of the incident light in order for
2000 photons to be absorbed
 If bleach half the pigment, it is expected that threshold will double
According to photochemical explanation, 50% bleach would result in only
half of the available quanta being absorbed--In order to bring observer to
threshold, need to double the intensity of incoming light
Plot time in the dark on X axis, log test flash threshold on the Y axis
 Results of 2 different subjects (p. 31)
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Visual normal is the solid line –rod-cone break is
prolonged because the data was obtained in very specific
conditions where a very great amount of pigment was
bleached—but is an otherwise normal rod-cone break
Both the adapting flash and test flash are white/broadband
6 log unit span for the visual normal
Dotted line is a rod monochromat
BHS 140.1 Vision Science I
Notetaker: Caitlyn McHugh
Date:1/31/2012, 1st hour
Page3
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New about this graph : ordinate on right hand side of the
graph—
o If bleached 100% of pigment, would have high
threshold
o If allowed all pigment to recover, threshold
would be very low
o If bleached 50% of pigment, would expect
threshold to double. If double the threshold, Log
(2) = 0.3—would expect threshold to go up by
0.3 units—however this doesn’t occur.
Threshold is increased by 1^10 units – very high!
 This just means that the relationship between amount of
pigment regenerated and TF threshold isn’t simple
 If bleach 30% of pigment, threshold won’t be raised by
30%, but by much more than this
 If exposed a rod monochromat to a light that bleaches 100% of
pigment—would be very painful! – threshold is elevated by 20 log
units
 If double amount of bleaching –get a huge increase in TF
threshold
 If you look at how much rhodopsin has been regenerated
by typical rod/cone break = 90% of photopigment has
been regenerated
 Rod cone break takes about 10 minutes, and it takes about
15-20 min more to achieve maximum sensitivity even
after 90% of pigment has been regenerated
Graph on. 32
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Proportion of pigment in bleached state
Have amount of threshold elevation over dark adapted threshold
If threshold measured at a particular bleach is the same as the
threshold in the dark adapted state, the numbers will be the same,
the ratio will equal 1 and the log will be zero
A value of 2 means that when 10% of pigment has been bleached,
threshold has been raised by 100 fold (inverse log of 2) with
respect to absolute dark adapted threshold
If bleach 40% of pigment, raise threshold 8 log units
Bleaching a little bit of pigment elevates threshold enormously
Does bleaching have a greater effect on rods than cones?
BHS 140.1 Vision Science I
Notetaker: Caitlyn McHugh
Date:1/31/2012, 1st hour
Page4
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Yes, much greater effect on rod threshold than on cone
threshold
 If bleach 50% of cone pigment, increase threshold less
than 100 x (lower than 2 log units on the Y axis)
 Whereas with rods, if bleach 50% of rhodopsin, threshold
increases about 10 log units
 Dowling-Rushton equation—explains Figure 9
o P =proportion of pigment in bleached state – if have 50% of pigment in
bleached state, can predict threshold
o Can determine the test flash threshold
o Don’t need to know how to use this formula for problems—just it’s name
o Relates amount of pigment in bleached state to test flash threshold
 FYI on p. 34 – won’t be tested on
 When you bleach pigment, there is a byproduct that is produced
 This byproduct hangs around, takes a long time to get rid of and keeps thresholds
elevated
Increase sensitivity
 Measured dark adaptation function – visual pigment has regenerated in cones to 100%
recovery, cone threshold is at its lowest
 Rod threshold is at its lowest after 30 minutes with 100% regenerated
 The fact that the pigment have regenerated doesn’t explain why rods are 1,000,000 times
more sensitive than cones even when both are fully dark adapted
 Rods are more sensitive than cones because of these 3 reasons:
 Gain
o Ex. From everyday life
 Stars are always emitting radiation/light
 The stars would be emitting the same amount of light during the
day and at night
 We see the stars at night, but not during the day
o Ex. From lab
 Had 9 targets going from 1 (brightest) to 9 (dimmest)
 Only people didn’t bleach well only saw up to 9
 Many got up to target 7
 When you walked into lab that day, all the LED’s would have been
on but no one would have seen #7
 After bleaching and dark adapting, we can see #7
 The intensity of #7 is fixed—nothing about it has changed
o Gain of the human photoreceptors is changing
o Gain: the response of the neuron to a fixed stimulus changes over time
 Can be an increase or a decrease in signal
o Gain should be flexible and able to be adjusted
o Want to boost/amplify a low signal and dampen or reduce a very intense
signal
o If the receptor output matches the input, then the gain is = 1
o If the receptor gives a response smaller than input signal, gain has been
reduced
o If receptor gives a response greater than input, there has been an increase in
gain
o If input to photoreceptor system is low/dim, want to amplify the signal
o Amplifying the signal is the same thing as increasing the gain
o Rods are going to be able to increase gain over time
 If they didn’t they would never been able to see target #7
 Over time rods get more sensitive/increase gain
o Gain control in rods
 Individual rods cannot adjust their gain
BHS 140.1 Vision Science I
Notetaker: Caitlyn McHugh
Date:1/31/2012, 1st hour
Page5
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Any amplification of a signal in rods doesn’t come from an
individual receptor
 Rods are physiologically wired to be an automatically high gain
system
 The actual gain of the receptor can’t be changed by itself
 When a photon of light is absorbed by a rod outer segment, the rod
will give an amplified response—the hyperpolarization response is
larger than what the signal will warrant
 Response of a rod to a few photons is much greater than the
hyperpolarization of a cone to the same number photons
 The hyperpolarization or dark current of the rod—the rods have a
response to photon absorption that is 20x greater than that of cones
 Gain of rods is much greater than that of cones
 This is an advantage—rods give amplification of incoming signal
when light is dim
 If amplify the incoming signal—won’t take much to get rod to
hyperpolarize to maximum extent
 Won’t take many photon absorption to get from -40 mV to -70 mV
and becomes saturated
 System will saturate quickly at low levels of illumination 
disadvantage of high gain system
 Don’t need rods to respond to moderate/bright light—we have
cones!
 Rod saturation at low levels of illumination is not a
problem unless one is a rod monochromat
 Takes 50-100 isomerations per rod to saturate an individual rod
o Gain control in cones
 Cones do not have as much gain as rods
 Don’t have to amplify dim signals
 Cones need to reduce their gain
 Cones have to respond to roughly a 1,000,000 fold change
in intensity from exceedingly bright lights to moderate
lights
 Want cone to reduce response to bright lights so that the
cone can signal a larger range
 Cones have the advantage of reducing gain within a single
receptor to regulate gain themselves—but need help from
other neurons
o P. 39—take home lesson
 There are 2 systems—a high gain, high sensitivity, low resolution
system (rods) and a low gain, low sensitivity, high resolution
system (cones)
 Advantage of rod having a high gain?—amplify dim signals, but
saturate to moderate levels of illumination
Spatial summation
Temporal summation