hmartin430 t1_j2vktwh wrote on January 4, 2023 at 6:14 AM

Reply to comment by Prestigious_Carpet29 in How close does one need to bring two coloured lights together to perceive a compound colour effect? by romxza

So I'm confused. If the fovea is responsible for highest resolution, and the fovea is packed full of cones which are color sensitive, how do we have more resolution with rods (black/white)?

My understanding was that rods are more sensitive to low light. That's why in the dark we have trouble discerning color. Also, our vision tends to be a bit less focused in the dark (more difficulty discerning details) because we're not getting much activation in the fovea, where our central vision is. In bright light, however, the cones will register the blue, green, red, far red wave lengths.

So we're sorta talking about two different "kinds" of light I guess?....white light/ambient light when it's daylight or you're in a lit room and then light in the form of the wave length being reflected off objects (like green curtains, say).

So wouldn't our color vision have higher resolution than our black/white vision, but that our color vision only works when the background intensity reaches a certain threshold. And our black/white vision has less resolution, but works in dimmer environments?

I'm no eye expert, but I spent a year working in a lab that was growing retinal organoids during a CIRM internship (super cool), it's been a few years so I could be remembering incorrectly, or perhaps understood incorrectly from the very beginning!

Prestigious_Carpet29 t1_j2w5jt0 wrote on January 4, 2023 at 10:36 AM

In response to hmartin430, my expertise is really in the use and application of CIE colour matching and display screen technologies, rather than the actual structure of the human eye.

My understanding too is that the rods are sensitive to low light (and saturate at higher light levels).

If we just consider the cones in the fovea, that comprises three types of cone: L,M,S (long, medium, short wavelength), which are very loosely red,green,blue. They are actually much broader bandwidth, with highly overlapping wavelength sensitivities than true RGB. The CIE colour matching functions (and resulting "chromaticity" coordinates) X,Y,Z are mathematically related to the L,M,S cone spectral sensitivities but are not quite the same thing (it's a long story...). The XYZ colour-matching functions are 'mathematically fudged' slightly such that the Y-coordinate represents luma (brightness) as well as (sort of) "green".

Grappling slightly for a consistent solution to all these things, I believe the answer is that in the fovea there is a highest density of M-cones, fewer L-cones, and fewer S-cones still. This means that our "luma" resolution is highest, red-green resolution is somewhat lower lower, and blue-yellow resolution the lowest. (In practice you need to match the luma (brightness) of the coloured test-stimuli to really demonstrate this effect, otherwise if "yellow" is much brighter than your "blue" it may be resolved in luma even if it isn't really resolved in chroma).

Again from a technological perspective, the Bayer colour filter array pattern used in the vast majority of electronic colour-camera sensors has twice as many green pixels as blue and red, which again maps to approaching human-eye properties to get the "best" visual image from finite technical resources.https://en.wikipedia.org/wiki/Bayer_filter

hmartin430 t1_j2x7fmz wrote on January 4, 2023 at 4:09 PM

Ah, so I think my issue might be that I have a lay person's understanding of brightness? I suppose I was thinking as brightness solely as the amplitude of waves. So like, low amplitude is only gonna excite the rods? High amplitude will allow the excitation of cones and at that point frequency will determine which cones are excited? It's been about 15 years since my last physics class, and it was a struggle lol. Definitely not my strength.

kilotesla t1_j2xlq9l wrote on January 4, 2023 at 5:39 PM

>low amplitude is only gonna excite the rods? High amplitude will allow the excitation of cones and at that point frequency will determine which cones are excited?

Yes, and yes, if that helps. Of course, the rods' response is not independent of frequency, but since there's only one type, you have no way to distinguish colors using them.