UV and visible difference and bird vision

[Andrea B.]

Subject: UV-absorbing, green-reflecting moss on tree trunk.

 

^{Before I respond, let me say how very much I enjoy discussing things like this.}

^{And kindly note that I can be persuaded and corrected when/if I have gotten something mixed-up or wrong.}

 

Jim: Surely the bird does have more information - precisely that there is no UV reflection.

 

If I look at at that green moss on the tree trunk, I have a physiological response only to the green wavelengths hitting my retinas. My eyes/brain have no physiological knowledge that the other primary visible wavelengths (red and blue) are being absorbed by that green moss. To repeat, there is no physiological detection of wavelengths which do not hit my eyes. The only reason I "know" about visible red & blue being absorbed is because I have studied the physics of light (photons, wavelengths, etc). The bird has an analogous physiological response to the UV-absorbing, green moss. The bird also physiologically detects green wavelengths but cannot detect absorbed UV wavelengths. It has no more physiological information than does the human when looking at green moss.

 

This remark does not address behavioural information. Who knows what the bird has behaviourally associated with reflected green or with green moss?

[Andrea B.]

I am also thinking that overall that UV sensitivity might only be a small part of the overall difference between bird vision and human vision - I need to think also of visual acuity, field of view and motion of the bird.

 

It's even more complex than that!

Do you have some reference papers on Avian vision. There is quite a lot available.

[Jim Lloyd]

Hi Andrea - I am glad you enjoy discussion, because I am often wrong, but I find getting to the right place is greatly helped by having freedom to say things ...

 

Anyway, I have to get back to writing my proposal soon - but I am still thinking that it is possible to have a sense of absence of something. We can be aware of silence, but only because we have hearing. We can be aware of darkness, but only because we have vision.

[Jim Lloyd]

Hi Andrea - I have had a look at a few papers, much need to do a lot more reading ...

[Andrea B.]

Jim: Imagine that I only have red and green receptors and my friend only has one receptor that spans the same red-green range.

 

^{I LOVE this thought experiment.}

 

Jim can physiologically detect two primary colours (+R-G and -R+G), call them Jim-red and Jim-green.

Jim can also detect "white" when both his R and G receptors are stimulated (+R+G). Call this Jim-white.

** Funny, isn't it, that in Jim's world our visual yellow is his white.

Jim can also detect, in relation to its surroundings, a black object. Black denoted -R-G. Call this Jim-black.

 

The friend who only has one receptor lives in a colourless black & white world because the friend's single receptor is either stimulated or it is not.Stimulation of the friend's single receptor produces Friend-white. Lack of stimulation of the friend's single receptor permits the Friend to detect the Friend-black of an object relative to its surroundings.

 

(We are disregarding intensity in all these discussions.)

 

Enter some objects of different colours. Let's label each one with Jim's bichromatic colours, the friend's monochromatic colours (B & W really) and the colour names from our larger trichromatic visual world. Assume for simplification that in our world all blue is absorbed (so I don't have to consider reflectivity cases outside Jim's world.)

 

1) +R-G = Jim-red = Friend-white = Our-red (+R-G-B ).

2) -R+G = Jim-green = Friend-white = Our-green (-R+G-B ).

3) +R+G = Jim-white = Friend-white = Our-yellow (+R+G-B ).

4) -R-G = Jim-black = Friend-black = Our-black (-R-G-B ).

 

 

 

Jim: If we lived in a yellow world (i.e. where all objects reflected red and green in the same ratio - and assuming it is illuminated by white light)....

 

In this world Jim would have a difficult time because everything would be +R+G which is Jim-white and not a kind of "yellow". There would not be any way to distinguish between objects in Jim's world. The entrance of the "red" ball would be a welcome relief -- but only to Jim. For Jim's long-suffering, monochromatic Friend in this +R+G world, everything is Friend-white including the introduced "red" ball which the Friend is going to have a darned hard time detecting.