QUESTION: Animal studies used for human applications...

Hey, all.

A couple of weeks back, I posted a question - it was really a question
for He Who Shall Not Be Named, and of course the question got buried in
another fracas of the type which seems to follow him like a cloud of
flies.

Neil Brooks did address my question, but unfortunately, the vocabulary
he used was considerably above my level. ;-)

My question is really fairly basic, and it concerns the applicability
of conducting visual studies on animals and extending the results of
those studies to humans.

Below is my initial query. The more I think about it, the more it bugs
me, and I'd really like to understand what the general concensus is.
And please - if anyone chooses to respond, please remember that I am a
lay person - so please use small words. )

Thanks!! (original post below)

BD

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In the absence of strong
supporting evidence, parallels between primates and humans are
EXTREMELY dangerous to draw in a medical context.

I have no CV in anything remotely medical, but
look at the behavior differences: I think it's fair to say that
primates have a lot more 'variation' in how they focus their eyes on
objects through the course of the day. Humans will spend hours and
hours looking at exactly the same thing (be it a book, a computer
screen, a TV)... Animals in the 'wild' are very likely NOT letting
their accomodation sit at a specific distance for hours on end. I
strongly suspect that animals who do not have persistent stimuli at a
static focal length will not maintain focus at any specific focal
length for much more than a few seconds.

We're all told that in our workplaces we should 'relax our eyes' once
and awhile, and take them off the screen for a few moments several
times a day. Do we need to ensure that the baboon down in the zoo takes

a break from studying his banana so that he doesn't get a headache?
Obviously not. Because he varies his focal length all the time -
looking from trees, to the bee buzzing around his lunch, to the bug he
just picked off of his wife's head, and at then to those dorky humans
who walk by staring at him all flippin' day.

Could this mean that the muscles around the primate lens cause the lens

to change shape more often than do humans? Sounds reasonable. Could
more frequent accomodation changes be a variable that will affect the
results of vision studies? I certainly expect so.

Regardless, before results of primate studies are deemed relevant, the
behavioural differences between the species (and there ARE differences)

should be quantified, measured, and eliminated from the list of
variables. For example - if we could teach monkeys to read, to watch TV

or enjoy movies, and thereby expose them to the same stimuli and
accomodation patterns as humans to, THEN we might be able to look at
primate studies and begin to draw analogies.

But given that we are dealing with different species, and that there
are easily quantifiable behavioral distinctions between these species,
I believe that the onus is on those who posit a relevance of the
information to establish that relevance before it is accepted.

If not, why can't we do studies on sloths, or penguins, or goats, and
extend the results to humans? Do not all mammals have similar
structures in their eyes? What about pigs? Pigs are often compared to
humans as being very 'biologically similar' - does this apply to eyes?
Can we start to run studies on pigs and infer relevance of the results
to humans?

 

Primates are frontal eyed foveate animals who do some near work with their
hands, and are thus very similar to human in certain regards. Visual
processing in some species of monkeys is very similar to human processes.
In fact, fairly recent work with functional imaging shows that many visual
brain areas in monkey identified through neurophysiological techniques have
an analogue in the human brain. There is no better general purpose animal
model of visual processing than certain nonhuman primate models.

This doesn't mean that every aspect of visual development and processing is
a perfect analogue for what goes on in human. For every model that someone
is attempting to show human relevance, at some point the relevance does
need to be confirmed. The assumption isn't automatically that an animal
model hold relevance to humans, especially when there is evidence to the
contrary.

 

There is no better general purpose animal model of visual processing than certain nonhuman primate models.

Thanks!

 

No problem. I should clarify a tad. By "visual processing" I'm
generally talking about downstream from the eyes (at least with respect
to the direction of information flow in the visual afferents)-- maybe
lateral geniculate nucleus and up into higher brain. That's not to say
that monkey models of eye, retina, and optic tract aren't good, but
different types of investigation merit different types of preps and
models.

For example, many biophysicists that work on retina really like the
turtle preparation for a variety of reasons. You can remove the eyes and
brain in a fashion that they maintain all of their normal physiological
connections, and keep the preparation alive with a minimum of fuss. Even
more importantly, that prep has many of the same architectural features
and local feedback networks that human retina has.

Then, there's the whole neuroethological approach, where you study fairly
strange behaviors in extremely specific preparations to try to learn very
basic stuff about nervous systems. For example, there are people
studying very low-level object recognition that use the jumping spider.
These spiders have shape-recognizers at a very low level (I'm not
recalling if its right at retina--or what passes for retina in a spider--
or slightly higher level. It can't be much higher level, because in
spider there isn't a much higher level) so they can identify mates before
they eat them. That's a pretty specific example, but another would be
mantle withdrawal in aplysia as a model of neural learning and
conditioned reflexes. Nobody would say that the aplysia mantle
withdrawal is a good model of anything a human would do, but the
conditioned reflex is a tool in our behavioral arsenal, so why not study
it in a very simple preparation where we have a shot at understanding it,
instead of in a very complicated high-level brain/spinal cord system.
That is, just like the guy looking under a streetlamp for something he
lost in a dark area a half block away, we study aplysia "because the
light is better" in that prep.

 

I'm generally talking about downstream from the eyes

And yes, on that point I would tend to agree, as I have no reason to
think that the neurological processes in the various species are all
that different - but I guess where my 'issue' is would be _at_ the eyes
- it seems to me that in order to use another species as a model for
our own (if that is indeed what's being attempted), we should likely
need to ensure that behaviors which can serve to modify or change the
eye need to be eliminated as variables. So if, as I say, the frequency
with which a primate changes its accomodation is so dissimilar from
humans as I expect it is, and if that can serve to have a long-term
effect on the lens and hence the vision of the subject... Oh well, I'm
just repeating myself now.

I guess that from a 'layman' perspective I can certainly see the
utility of modeling the visual processing systems of other animals for
use in understanding our own - but, again, from a strictly 'layman'
perspective I see some limitations when we specifically examine the
'physical' aspects of the system - such as the lens, the muscles of the
eye, etc. So if studies of the visual systems of other species examine
the effects of factors which could by their nature vary, dependent on
species-specific patterns of behavior... just seems to me that this is
a problem.

But, it may well be that most of the research done in this arena is, as
you say, upstream from the eye - in which case behavioral differences
are irrelevant.

Thanks again for indulging me!

BD

 

Again, no problem. Keep in mind that I'm no fan of basing myopia research
on a handful of decades-old monkey studies.

Another issue that really comes into play here is development. Different
systems of different species develop at different rates. I'm no expert
here, but it would seem that human babies are born with better developed
visual systems than many other species, some of which don't even open their
eyes until many days post partum. Some development is slow, other
development is fairly rapid. It would seem fairly difficult to "age-
match" development periods in different species, particularly if in one
species much of the development is occuring in utero, and in the other, out
in the cold, cold, world.

 

a handful of decades-old monkey studies.

I guess I'm more transparent than I thought. ;-)

It's true, my motivation for asking the question is not PURELY academic
in nature.