Odd color sensitivity distribution phenomenon

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Dave Bell - 19 Jul 2007 04:09 GMT

I noticed something last week while on a couple of business flights, and
it has me curious.

We were taxiing through the airport just after sunset, and the ground
was backlighted by the sky light. Lining the runways were ground
markers, small glass pods about a foot off the ground, with an intense,
extremely deep blue/violet light from a very small source.

Looking at individuals markers from a hundred or several hundreds of
feet away, they were essentially point sources of nearly UV light. I
noticed at one point, that a light I was looking directly at disappeared
- went black. I looked more directly at an adjacent light, and the first
one came back on, in my peripheral vision. Sure enough, as I explored
the phenomenon, if I looked within about half a degree (diameter of the
full moon) of a light, it vanished from my central vision. But only the
deep blue lights did this; others, yellow or white, simply became
sharply defined, and didn't change color or intensity.

At first, I though it was an effect of fewer rods in the center of my
visual field, but realized I was seeing the lights in full color when I
saw them, so it had to be a cone issue. Why do I seem to have a
deficiency of receptors to that particular hue, dead center?

Dave

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Mike Ruskai - 19 Jul 2007 18:35 GMT

>I noticed something last week while on a couple of business flights, and
>it has me curious.
[quoted text clipped - 18 lines]
>saw them, so it had to be a cone issue. Why do I seem to have a
>deficiency of receptors to that particular hue, dead center?

You haven't mentioned if you were wearing glasses or not.

Seeing something with averted vision that's faint or even invisible
with direct vision is something quite familiar to amateur astronomers.

So is chromatic aberration, which every pair of glasses will give you
to one degree or another. My most recent pair has polycarb lenses,
which were the cheapest available (they're only a backup for my
contact lenses), and I've found them to be very highly dispersive.

Seeing blue/violet on the edge of a bright object viewed at an angle
is textbook chromatic aberration.

I'm not sure how that all fits in with your scenario, because it's
certainly true that there are vastly more rods than cones in your
non-direct visual field. But my guess is that there are enough to
give you a sense of color for the light which required more rods to
detect than your macula possesses.

Assuming you were wearing glasses, that's my theory - violet from
chromatic aberration, with a luminance too low to detect with cones
alone.

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- Mike

Ignore the Python in me to send e-mail.

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Dave Bell - 19 Jul 2007 21:24 GMT

> >I noticed something last week while on a couple of business flights, and
> >it has me curious.
[quoted text clipped - 20 lines]
>
> You haven't mentioned if you were wearing glasses or not.

Sorry! I forgot to mention anything about that, as I usually don't wear
glasses, and was not at the time.

> Seeing something with averted vision that's faint or even invisible with
> direct vision is something quite familiar to amateur astronomers.

"Seen with averted imagination"; yes, quite familiar with that.
This was somewhat different, in that it was only the *very center* of
vision that was insensitive to the blue light. Even a very small shift
off-center, and the light popped right back on.

> Seeing blue/violet on the edge of a bright object viewed at an angle is
> textbook chromatic aberration.

Of course, this blue/violet light was "real", not dispersion from white
light.

> I'm not sure how that all fits in with your scenario, because it's
> certainly true that there are vastly more rods than cones in your
> non-direct visual field. But my guess is that there are enough to give
> you a sense of color for the light which required more rods to detect
> than your macula possesses.

That sounds like it's on the right path. Perhaps I just don't realize how
small the macula is, in angular diameter...

Dave

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p.clarkii@gmail.com - 21 Jul 2007 05:28 GMT

> I noticed something last week while on a couple of business flights, and
> it has me curious.
[quoted text clipped - 20 lines]
>
> Dave

its not really so odd if you understand the histology/physiology of
the eye. as you know, the cone photoreceptors are predominant in the
macula region of the retina, but only about 2% of them are blue
cones. and the most important point is that the blue cones are LESS
concentrated at the fovea but seem to be more eccentrically located.
the red and green cones however are densest at the fovea. thus
perhaps the anatomical distribution of cone receptors within the human
eye is the explanation of the phenomenon you observed.

incidentally, the different distribution of the blue cone receptors
has caused some researchers to postulate that they have actually
evolved separately from the red/green cones-- perhaps from rods.

do more reading about it here, but you'll need to follow some links:
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html

I think this is actually some cool stuff, and it was the kind of thing
my PhD thesis was about.

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Dave Bell - 21 Jul 2007 07:23 GMT

>> I noticed something last week while on a couple of business flights, and
>> it has me curious.
[quoted text clipped - 39 lines]
> I think this is actually some cool stuff, and it was the kind of thing
> my PhD thesis was about.

Great answer! That sounds exactly like what would cause it...
As it happens, I was back to my retinologist for another follow-up check
today (the other eye, not my remaining good one), and posed the question
to him. It stumped him, for what it's worth.

Thanks!

Dave

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