Re: presbyopia re: Evolution question - the eye.
- From: "Suzanne" <shiloh7@xxxxxxxxx>
- Date: Sun, 31 May 2009 01:26:44 -0500
"John S. Wilkins" <john@xxxxxxxxxxxxx> wrote in message
news:1j00ntj.1llqx2j171qsa0N%john@xxxxxxxxxxxxxxxx
William Morse <wdNOSPAmorse@xxxxxxxxxxxxxxxx> wrote:I was one that needed the multi-focal lens glasses
On Mon, 18 May 2009 20:19:35 -0700, rnorman@xxxxxxxxx wrote:
On May 18, 10:37 pm, William Morse
<wdNOSPAmo...@xxxxxxxxxxxxxxxx>
wrote:
On Mon, 18 May 2009 16:13:06 +0000, Paul J Gans wrote:
In talk.origins Kent Paul Dolan <xanth...@xxxxxxxx> wrote:
William Morse wrote:
So it remains an open question as to why older humans
should lose
their ability to focus on nearby objects. One would think
that
they would be less involved in activities such as hunting
game,
and gathering food far from the village where spotting say
a berry
patch from distance would be valuable. They would seem to
be more
involved in caring for children and teaching crafts,
activities
where one would expect them to lose their ability to focus
on far
away objects.
Perhaps the problem here is a misunderstanding of what
presbyopia is.
It is not a loss of ability to refocus the eye away from
infinity,
but to refocus the eye from the distance at which it is
_customarily_
focused. More or less, the lens "freezes in place". As a
computer
user since 1961, my now 20 year existing presbyopia means that
my
fixed distance of presbyopic focus is the usual distance of a
monitor
from my face. I can focus neither farther nor nearer without
prosthetic help.
[I'm getting a new prosthetic tomorrow morning for another
vision
problem, my second plastic lens insert by surgery to replace
natural
eye lenses gone blurry from cataracts.]
As a result, the lifelong flint knapper would most likely
still be
able to do that job, but would lose the ability to forecast
weather
from the clouds or spot game at a distance, or to do close
beadwork
sewing.
There is also the fact that not everything that the body does,
especially as it deteriorates, is due to evolution. Some of
it is a
spandrel effect.
Much of what happens to us in our older years has only a minor
effect
on our reproductive success.
This was also pointed out by Richard Norman, who also added a
nice
reference for which I thank him. And I am aware that there need
not be
a Just So story explaining every detail of evolution as an
adaptation
in response to natural selection, that much of it has to do with
what
selection actually sees, and what limits there are on what
selection
can in fact select for. But I remain a staunch adaptationist,
and look
first for selective explanations, which is why I like the
explanation
offered by Kent Paul Dolan. Age-related changes reduce our
ability to
change focus, but the focus that becomes fixed is the customary
one. I
would like to see if there are references that support that.
As I already said, the actual mechanism of accomodation is
somewhat
complex and controversial. See, for example, Wikipedia
"accomodation"
or "presbyopia". However you should know that the "relaxed" eye
focuses
at a distance. In order to change the focus to near vision, the
ciliary
muscle must be contracted. The eye drops used in an
optometrist/opthalmologist's office are used not only to dilate
the
pupil but to eliminate accomodation so that the eye becomes fixed
for
distance vision.
In that sense, distance vision is the "customary" situation.
According to the environmental theory of near-sightedness,
factors
change the growth of the eyeball so that the relaxed "distance"
vision
really focuses on nearby objects. The accomodation process can
then
allow you to focus even closer but there is no way you cause the
eye to
focus on something farther away than what the relaxed eye does.
Thanks. As usual, you are a wealth of useful information. If I
didn't
know better, I would conclude that you made a living teaching this
stuff :-) Based on your discussion, it makes sense that the general
aging
trend is towards farsightedness. On a side note I, along with my
brother, have one eye which is quite near-sighted and one eye which
is
somewhat near-sighted but probably well within the "normal" range
of
vision. (In this respect I note that "eagle eyed" seems to be a
common
term, which indicates that a range of visual acuity, or at least of
normal focal distance, was common in pre-corrective lens
societies.) I
had always thought this strange until I found that this strategy is
one
used for prescribing contact lenses for elderly patients in lieu of
bifocal or multifocal glasses.
I used to be able to read 6pt type at 6". I now have trouble with
14pt
type at arm's length. I wouldn't mind, but I also can't see well at
a
distance either. Basically I am perfectly adapted to deal with large
objects between 18" and 6'. So long as nothing else in the world
interacts with me, my fitness is very high.
because I was always doing things where I needed
a variety of middle ground focals. For example,
I played handbells, and the music was farther away
from my eyes than if you were reading a book. One
solution is tri-focals but then you have to change
back and forth all the time between glasses. So
it depends upon what your particular needs are. I
never dreamed I'd get cataracts, but I did. I now have
a multi-focal lens implant which sees near, far, and
everything in-between as well, called the RestorLens.
In a few years they may even have better solutions.
I'm very happy with the ones that I have.
Suzanne
.
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