Limits of Vision: Absolute Luminance Detection Flashcards
Why is light considered a wave in some regards and a particle/photon in other regards?
It is dual natured.
It shows properties of a wave as light has the ability to defract.
It shows evidence of being a particle as a photoreceptor or sensor can only detect photons in discrete values i.e you can only absorb a whole number of protons- can’t absorb 0.5 of a proton.
What did Hecht, Shlaer, & Pirenne’s paper research? (1943)
How many photons does it take to trigger (‘hyperpolarise’) a rod photoreceptor.
Answer: It takes one photon to trigger a rod photoreceptor
How did Hecht, Schlaer, & Pirenne conduct their experiment?
First, the observer sat in the dark for 30 minutes (to dark adapt)
Then a light was presented 20 degrees to the left of the point of fixation
The light was:
Greenish (a wavelength of 510 nm ± 10 nm) .
Small (subtended an angle of 10 minutes of arc)
Brief (1 millisecond in duration)
The observer was asked to respond “yes” or “no” to say whether they saw the flash (2AFC)
The dimmest light detected 60% of the time was determined
What did Hecht, Schlaer, & Pirenne discover is the minimum number of detectable photons and from this how did they derive the number of photons needed to trigger one rod photoreceptor?
Observer “SS” (presumably author Simon Shlaer!) once managed to detect 54 photons with 60% reliability.
From this they discovered their range of detectable photons is 54 to 148.
They reasoned that not all of these photons will be absorbed by a rod. Specifically?
Some (~50%) will be reflected off the cornea (and other ocular media) and never reach the retina
Some will reach the rod but not be absorbed by rhodopsin
After much calculation, they conclude that only 10% of incoming photons are absorbed by rods
So we are actually able to detect 5 to 14 photons!
Given the stimulus size, those 5 to 14 photons are spread over ~500 rods
“if 7 quanta are absorbed by 500 rods, there is only a 4 percent probability that 2 quanta will be taken up by a single rod”
So a single rod is able to respond to (signal the presence of) a single photon of light
What did Baylor, Lau, and Yamb (1979) do to develop on Hecht,Schaler, and Pirenne?
An empirical follow up:
Attached an electrode to individual toad rod photoreceptors and…
Directly observed a response (change in membrane current) on around one third of trials
What did Koeing and Hofer (2011) do and what did they discover?
They did the same rod experiment and confirmed 50 +/- 16 photons could be detected at the cornea. (Confirming the work of Hect et al).
They also did the experiment with cones too though. Here they discovered cones able to detect 203±38 photons at the cornea. And after maths they worked out…
Cones require multiple photons to respond.
What did tinsley et al (2016) research?
Whether a person could respond to a single photon ( yes a rod can but can a person).
Over 2,420 trials, observers responded correctly 51.6% of the time
A (very) small but statistically significant(ish) difference (P = 0.055).
Thus technically yes
If somebody asked you what is the dimmest light a person can see what answers could you give?
A1. “A rod can detect a single photon” (Hecht et al, 1942)
A2. “A person can detect 50 photons at the cornea” (Hecht et al, 1942)
A3. “A person can (sometimes) detect a single photon” (Tinsley et al, 2016)
A4. “It depends”
Which 6 key factors affect our ability to detect luminance?
- State of the eye (i.e. whether it is dark adaptation)
- Stimulus location
- Stimulus wavelength
- Stimulus size (Ricco’s law)
- Stimulus duration (Bloch’s law)
- Experimenter criterion (target threshold)
What does 2AFC mean?
2 Alternate Forced Choice - it means you only had to options of answers to pick out of e.g. yes or no
What is retinal sensibility?
An outdated way of saying retinal sensitivity to sensory stimuli.
ie. retinal SENSITIVITY
Why did Hect et al choose those conditions (i.e a greenwavelength light tested at 20 degrees peripheral, etc)?
As those were the parameters which would yield maxiumum retianl sensibility based on evidence at the time.
(The only parameter we would slightly change is instead of the light being at 510nm we would now have slected it at 498 because that is the wavelength light is most sensitive to).
In Hect et al’s experiment why was the lightsource pointed at 20 degrees from fixation in order to test rod sensitivity?
In Hect et al ‘s experiment why did the observer sit in the dark for 30 mins before starting the experiment?
To dark adapt.
In Hect et al’s experiment why was the light source small?
Why was the light brief in Hecht et al’s experiment?
What is experimenter criterion?
It is essentially what the experimenter deems a significant enough result to either support or disprove their argument.
E.g. If I tell you I can shoot a basketball in a hoop yet could only do it 1 in 10000 times would you say I have the ability to shoot hoops. How about if i could do in 1 in 10 times?
There is no objectively “right” or “wrong” threshold level when measuring vision (re: lectures on psychophysics)
But it likely that an “ordinary” (unpractised) observer would require many hundreds or thousands of photons to perform at a “reasonable” level.
