2 Dark Adaptation

What is the range that our visual system functions for retinal illuminance?

10 log units

What is the range that we can measure of our visual system for retinal illuminance?

6 log units

What are the 4 mechanisms that we use to adapt to light?

1. Duplex retina (two different photoreceptor system)
2. Change in pupil size
3. concentration and level of bleached photopigment
4. change in neural responsiveness

Which photoreceptor has high gain?

Rods

Which photoreceptor has low gain?

Cones

Which photoreceptor response to photons is amplified in dim signals?

Rods

Which photoreceptor responds to a wider range of lights?

Cones

Which photoreceptor has high sensitivity?

Rods

Which photoreceptor has low sensitivity?

Cones

Which photoreceptor has low resolution?

Rods

Which photoreceptor has high resolution?

Cones

T/F: the amount of bleached photopigment to bright light is a 1:1 ratio

False

What is retinal densitometry used for?

Measuring the proportion of pigment in bleached state by the log threshold number of quanta relative to dark-adapted threshold

Retinal densitometry includes the following EXCEPT:
a. the amount of photopigment that has been bleached
b. time course for dark adaptation and rhodopsin regeneration is the same
c. it explains the large increase in sensitivity with time
d. it explains th

c.
It does [NOT] explain the large increase in sensitivity with time

T/F: Bleaching of rhodopsin is a direct numerical change

F: its a log unit change

Which occurs quickly: light adaptation or dark adaptation

Light adaptation

Which increases in sensitivity: light adaptation or dark adaptation

Dark adaptation

What is the half life of cone photopigments?

1.5 minutes

During the dark adaptation experiment, why do we flash a bright light at the beginning?

To bleach all the rods/cones (stop function)

During the dark adaptation experiment, what is "time zero"?

The moment the bleaching light gets turned off and we begin to adapt

Why is the dark adaptation experiment considered a psychophysical measure?

We require the patient to push a button when they see the test flash - we don't know what they see, so they have to tell us

During the dark adaptation experiment, why do we only show the test flash for a short amount of time?

We don't want to rebleach them

During the dark adaptation experiment, which photoreceptor is the first to pick up the test flash?

Cones

At 3 minutes, what percentage of cones have recovered?

75%
(1.5 minutes = 50%, 3 minutes = 75%)

After 10 minutes, how much rhodopsin has regenerated?

75%

What is "rod-cone break"/Kohlrausch bend

The abrupt change in the slope of the dark adaptation curve where the cones cannot see the stimulus but the rods can

After the Kohlrausch bend, which of the following is false?
a. The light is dimmer
b. The light is colorless
c. The rods are more sensitive
d. The cones are regenerated
e. None of the above

e. They're all true

T/F: Rods threshold is higher than that of the cones in the first 10 minutes because they are slower to adapt

True

How do you read the photochromatic interval from the dark adaptation curve?

The difference between the cone plateau and the rod plateau

What does the cone plateau represent?

the minimum photopic threshold for a stimulus

What does the rod plateau represent?

The minimum scotopic threshold for the stimulus

T/F: the scotopic system is less sensitive than photopic system

F: photopic is less sensitive than scotopic

What happens if we reduce the brightness of the adapting/bleaching light?

The DA graph begins to look more like a rod-only response (no first plateau)

What happens if we reduce the time of the adapting/bleaching light?

The DA graph begins to look more like a rod-only response (no first plateau)

In the dark adaptation experiment, what happens if we use shorter wavelengths of adapting light (ex: red light)?

We will not be knocking out rhodopsin, so a rod-only response

In the dark adaptation experiment, what happens if we only show the flash stimulus at the foveal region?

Foveal = cones only, so we see cone DA curve

In the dark adaptation experiment, what happens if we flash a very small light?

Small light = foveal region only (cones)
we get a cone-like DA response

In the dark adaptation experiment, what happens if we use a shorter wavelength of flash stimulus (ex: red)?

We get a cone response

In the dark adaptation experiment, what happens if we flash letters instead of just a light?

Cone-function DA curve as we make the letters smaller

T/F: the dark adaptation test is done monocular only

F: can be either mono or binocular

In the Goldmann-Weekers dark adaptometer, how is the experiment carried out? [NOT ON EXAM]

At time zero, patietn looks at a dim red light near the center of the bowl. The tech gradually increases the intensity of the light until the patient first knocks on the table to indicate that they detect it. The tech will pull back on the control knob, p