T3D Re: Visual acuity = 1' of angle?

["Jim Crowell" <crowell@xxxxxxxxxxxxxxx>]

>
>
>>As the test pattern is a simple grid of black and white lines it seems to
>>me that the rods should be taking as much or more of a part in resolving
>>the image as the cones as they have evolved for black and white vision.
>
>But I think the "wiring" is different. If I recall correctly, typically
>a batch of rods are hooked up to each ("channel"?) that goes to the brain,
>while the cones in the fovea may be close to a 1:1 mapping

Thats' right, but the main difference is that cone system is specialized for
higher light levels (e.g. daylight) & the rod system for lower (twilight,
moonlight).  The pooling of rod signals you mention contributes to this, as
well as the fact that they seem to have a lower level of internal noise; if
_any_ rod that connects to a given retinal ganglion cell managazes to
capture a _single_ photon, there's a decent chance that that ganglion cell
will be activated.

At normal daytime light levels, the rods are all bleached out & can't
respond much to incoming photons, so rods don't contribute to daytime vision
at all.  The process of dark adaptation (by which you're gradually able to
seem more after you sit in darkness for a while) is a reflection of the
photopigment's being gradually transformed back into its responsive state.

It's true that rods can't signal color, because they only have a single kind
of photopigment (whereas each cone can have one of three different types). 
But the perception of black vs. white (luminance) in daylight is also a
function of the cones; your percept of brightness is a function of the
summed outputs of two of the cone types.

Oh, and it's also correct that there aren't any rods in the fovea (the
region of central vision), which is why astronomers learn not to look
directly at faint stars through the telescope but a little to the side
(where their rods can pick up the light that's too faint for the cones).

-Jim C.

-----------------------------------
Jim Crowell
Caltech Division of Biology
Mail Code 216-76
1200 E. California Blvd.
Pasadena, CA 91125
jim@xxxxxxxxxxxxxxx
http://vis.caltech.edu/~jim/Home.html


------------------------------