How do photoreceptors distinguish between light intensities? - (Dec/17/2017 )

Hi,

I asked the same question here but I never got a response.

According to the Principle of Univariance, the input to a photoreceptor differs along two dimensions (wavelength and intensity) but the output can only vary along one dimension (the cell's graded potential). This means that there is inherent ambiguity in the output of the photoreceptor.

What I do not understand then is how it is possible for a rod (or cone) to distinguish between intensities (or wavelengths) when each photopigment can absorb a range of wavelengths at different probabilities.

For example, say a rod cell absorbs 50% of all photons at wavelength x and 25% at wavelength y. If 100 photons reach the rod cell at wavelength x we can roughly assume that 100*0.5=50 isomerizations will occur. Equally, if 200 photons reach the rod cell at wavelength y then we can roughly assume that 200*0.25=50 isomerizations will also occur. From the perspective of the cell's synaptic activity, the two responses will be the same despite the large difference in intensity.

Since there are an infinite number of permutations to this basic pattern it is unclear to me how the visual system is able to obtain any information about the intensity of light in rod cells. The same issue also appears present to me in cone cells (the usual explanation is that the relative amount of excitation/inhibition can be compared between cone cells, but this does not seem to solve my overall dilemma).

That is an interesting question.

I am definitely not an optical physiologist, but I will have a go at answering your question.

Basically as I see it you are actually asking two questions - how do we distinguish colours, and how do we determine intensity?

You are treating the eye as one object, but in reality the photo-reception happens in a huge number of individual cells in the retina -the rods and cones you have probably heard of.

For colours we determine these based on the combinations of receptor they trigger - we have three different colour receptors (cone types) that trigger based on a range of light wavelengths being absorbed (this is based on the chemical composition of the photo-absorbing chemicals in the cell). The proportion of each different cone type being triggered results in us perceiving (via a neural response) the different colours This is directly analogous to the RGB colour mixer where if you have a colour code of 0,0,255 (think of these as number of cells for the analogy) you would see blue. There is some overlap in the range of wavelengths absorbed and the strength of response to these wavelengths, which accounts for differences in colour perception between each eye on an individual and between people.

The triggering is an on/off response like you suggest and functions via a change in membrane potential. When not absorbing light of sufficient strength or of the right range of wavelengths of light the cell has a resting potential consisting of an electrical gradient. Upon triggering the cell depolarizes via a voltage gated ion channel and as a result sends glutamate (nerve signalling molecule) in a continuous stream until depolarization is complete (i.e. there is no electrical gradient to cause opening of the voltage gated ion channel). At this point the voltage gated ion channels close and the cell repolarizes. The rate of depolarization, the proportion of cells performing this step at any one time, and the rate or repeated depolarization account for the intensity signal.

And as I understand Bob1 talked about the behaviour and triggering of the photoreceptor cell signals themselves. But even at the retina, there are several layers containing interneurons that locally interpret many things we take as visual perception basics, but were found to be pretty difficult to achieve when we tried machines to do an image recognition. They are required to modulate areas of visual receptive fields that commonly do things such edge detection, (pre)perception of color, signal-to-noise filter and many others.

It is actually pretty similar to what happens when you want to detect changes in a fluorescent signal that is overlaping to different filters you have on the machine (this happens to almost all of them), you need to computationally substract the background, use some known values of overlaps. Retina neurons do the same but much more complex. The color vision for example is commonly simplyfied as RGB-like type, but it is a bit far from that (one reason is that absorption maximums of red and green cones is actually pretty close, after all they arose as mutant variation of one photoreceptor not so long ago) and also the distrubution in the retina of the three (or more specifically fovea) is not as uniform. The interneurons had to integrate all the raw data and sent to visual cortex an already pre-processed signals where the frequency is the intensity value (as usual with input nerve signal). Visual cortex then finalyze the image, for example with the color, the percieved color is imputed from the whole context of image and not from it's real "RGB" value, hence those popular "illusions" using same colored square on different backgrounds that are perceived as different colors.

For further understanding you IMO need to actually look at visual perception system as a whole first. There are some good starters on Wikipedia:

https://en.wikipedia.org/wiki/Receptive_field#Visual_system

https://en.wikipedia.org/wiki/Retinal_ganglion_cell

https://en.wikipedia.org/wiki/Photoreceptor_cell