The researchers found that about 90 photons had to enter the eye for a 60% success rate in responding. Since only about 10% of photons arriving at the eye actually reach the retina, this means that about 9 photons were actually required at the receptors. Since the photons would have been spread over about 350 rods, the experimenters were able to conclude statistically that the rods must be responding to single photons, even if the subjects were not able to see such photons when they arrived too infrequently.
Piecing these and your findings together, it hints to an interesting sub-question, what do we really mean when we ask the original question?
Can the human eye physically detect it? Seems like…yes?
Which suggests the subsequent physiological thresholds involved, various human signal processing chains etc. What a fascinating topic.
The choice of a 60% success rate is an interesting one, too.
If we could consciously see single photons, we would experience too much visual “noise” in very low light, and so this filter is a necessary adaptation, not a weakness.
From what I understand, the rhodopsin protein will sometimes become spontaneously activated without having to absorb light. Since a single activation is enough to trigger the next cell to fire, this makes the system so sensitive that it is noisy when it is dark. So it makes sense that rather than considering every triggering event as a positive signal, some later step in the signal processing chain would require a certain number photoreceptor cells to activate before it lets the signal pass through into our consciousness.
I have not looked further into the processing chain, so I don’t know at what level this threshold is applied. It could be that multiple neurons need to activate to provide enough of an impulse to activate the optic nerve - or it could be that the signal from a single event does propagate all the way to the visual cortex, and the visual cortex filters these low signals. I suspect that this is well known, but I shouldn’t allow myself to spend time on this today. I will try to find out when I do have time, as this is a very interesting topic!
What a fantastic read! Well done!
Did you see this in the OP article?
Piecing these and your findings together, it hints to an interesting sub-question, what do we really mean when we ask the original question?
Can the human eye physically detect it? Seems like…yes?
Which suggests the subsequent physiological thresholds involved, various human signal processing chains etc. What a fascinating topic.
The choice of a 60% success rate is an interesting one, too.
Thanks! I appreciate you reading it :)
The first paragraph points out the following:
From what I understand, the rhodopsin protein will sometimes become spontaneously activated without having to absorb light. Since a single activation is enough to trigger the next cell to fire, this makes the system so sensitive that it is noisy when it is dark. So it makes sense that rather than considering every triggering event as a positive signal, some later step in the signal processing chain would require a certain number photoreceptor cells to activate before it lets the signal pass through into our consciousness.
I have not looked further into the processing chain, so I don’t know at what level this threshold is applied. It could be that multiple neurons need to activate to provide enough of an impulse to activate the optic nerve - or it could be that the signal from a single event does propagate all the way to the visual cortex, and the visual cortex filters these low signals. I suspect that this is well known, but I shouldn’t allow myself to spend time on this today. I will try to find out when I do have time, as this is a very interesting topic!