Re: sclerotic rings

[rowe@lepomis.psych.upenn.edu (Mickey Rowe)]

Nicholas Roy Longrich <longrich@phoenix.Princeton.EDU> writes:

>       On the subject of eyes, I recall that some birds have a pair
> of focal points (fovea) in the eye, and so are able to set up a
> binocular system with each eye. 

I'm afraid that you're confused on a few issues.  First off, the
retina is on the back of a sphere, so all points on the retina are
"focal points" (i.e. the image is in focus at every point on the
retina).  Secondly, the bifoveate retinas of a lot of birds do *not*
image the same areas in space to different areas of the retina.  They
are two distinct areas of comparatively higher spatial acuity sampling
distinct areas of space.  The types of receptors and the processing
performed on the outputs from the receptors are different in the two
foveas, so the birds use each fovea for different tasks.  Many animals
(e.g. the cheetah) don't have a single foveal spot, they have a
"visual streak" -- a horizontal band of higher visual acuity.  Other
animals have no notable regions of retinal specialization at all
(i.e. no fovea and nothing of comparable significance; every piece of
the retina looks like every other).  The animals I've been studying
(green sunfish, _Lepomis cyanellus_; yes, that's how this computer was
named) have large scale variations across their retinas but no fovea.
They have much higher visual acuity for points directly in front of
their noses than for points off to the side or behind them, but the
variation occurs gradually across the retina.  When you hear things
like "two foveas", you have to put the statement into a bit of context
about a) what a fovea is, and b) what other variations exist on our
own particular (i.e. primate) theme.

> This would seem to be a convenient arrangement for the predatory
> dinosaurs, who didn't get around to binocular vision until pretty
> late in their history. More untestability.

I've read about a lot of weird eyes, and the only ones I can think of
that might come close to imaging the same part of a visual scene in
two different parts of the same eye are some surface feeding fish.  In
general, though, it seems that under normal circumstances their
specializations are adapted to do the opposite -- to map the image of
an object reflected from the water's lower surace onto the same
retinal position at which the direct rays are imaged.  I think a lot
of work still needs to be done on the optics of these eyes to
demonstrate that even this is the case, however.  

Getting back to dinosaurs, you should be careful about the statements
you make vis a vis binocularity.  The next time you go to an aquarium
or pet store with a lot of fish, look at the shapes of the pupils.
Many species of fish have tear-dropped shaped pupils with the point of
the tear facing their nose.  Although no one has done any hard core
analysis of the optics involved, it seems likely to me (and I'm not
alone in this -- I first heard the idea from Steve Easter) that this
specialization arose in order to increase the eye's aperture for light
coming from in front.  Sunfish eyes are pretty much on the sides of
their heads, but it's clear from watching them swim around that they
are most interested in what's directly in front of them (and as I
said, they have higher visual acuity for that region).  Furthermore,
the eyes of older sunfish tend to protrude from their orbits.  I
suspect that this also is an adaptation that allows them a better view
of the world in front of them.  We can't currently know if dinosaurs
had such specializations, but you also can't make facile
generalizations about binocular fields of view and how important it
was to the animal based on the bones alone.

-- 
Mickey Rowe     (rowe@lepomis.psych.upenn.edu)