>From Dr. George Mastroianni, Research Psychologist, Natick RD&E Center:
As a target shooter with a little knowledge about vision, I would say that
while the practice described is sensible, the putative justification is
not. While it is the case that the response of photoreceptors habituates
with continuous exposure, our visual system has a built-in mechanism to
compensate for this effect: eye movements. Studies of stabilized retinal
images (created in a variety of clever ways) which eliminate the effects of
eye movements and cause the stimulus to stay in one place on the retina,
show that such images will indeed disappear under the right conditions.
Our visual system prevents this from happening in daily life by executing
frequent involuntary eye movements that serve to shift the retinal location
of an image, even during fixation.
Visual input during these movements is suppressed (a mechanism called
saccadic suppression) so that our view of the world is not jumpy and
discontinuous. This is why we are not aware of the eye movements.
Proper spatial "registration" of the image location is maintained because
the visual system seems to track and account for the eye movements.
In target shooting, there are a number of reasons to limit the amount of
time spent in trying to get a shot off - breath control, muscle fatique,
etc. Extended fixation without blinking could lead to corneal drying and
discomfort, possibly tearing to correct this, and conceivably a loss of
acuity as a result. These are good reasons to limit time trying to get a
shot off.
The idea that the brain can shift the perceived location of the image is
not correct - in any case, it would have no effect on the problem, which is
fatiguing of the photoreceptors. Only moving the image (by moving the
retina, hence, the eyeball) can relieve the effects on the cells. But when
the eye is moved, the brain "knows" it has been moved, and our view of the
world remains stable.
An interesting question: since both the sight and the target would be
affected by the "shift" in position suggested in the explanation offered in
the question, their relative position (the key to hitting the target) would
not change. Why, then, would we expect a diminution in accuracy?
Anyway, that's my off-the-cuff response. Good luck and good shooting.
------------------
-------------
Original Text
From: dsliney@aeha1.apgea.army.mil, on 5/8/97 9:45 AM:
From: "J.D. Moreland" <coa09@cc.keele.ac.uk>
Subject: Sighting Advice to Target Shooters
To: cvnet@skivs.ski.org
Date: Wed, 7 May 1997 08:38:10 +0100 (BST)
Cc: coa09@cc.keele.ac.uk (J.D. Moreland)
Dear Hoover,
Would you please post this message for help.
_____________________________________________________
Dear Colleagues,
I have had a letter from a Biology teacher in my daughter's
school asking for an opinion on a practical aspect of target
shooting. The letter (below) describes current practice and
its "theory". It queries the validity of that theory and asks
for ideas for an experimental approach that can be used to
test it.
If anyone can help with this, I'd be glad to hear from them.
Jack Moreland
"When target shooters are taking aim, they look with one eye,
the dominant one, through the rifle sight to check their aim.
They then adjust their body position to improve their aim.
This is repeated until they are satisfied with their position,
a process that may take anything from 10 seconds to several
minutes. They are taught, if coached properly, that they must
not stare down the sights for more than three or four seconds
at a time. They must look away for several seconds then sight
again in order to avoid fatiguing the retina. The reason given
is that prolonged fixed staring fatigues the retina and makes
the brain 'shift' the position so the image falls on a different
part of the retina. Effectively then, the shooter is aiming at a
perceptual mirage and the shot is inaccurate. I have been unable
to find any work published to substantiate this belief or any
method by which I could prove or disprove it or, better still,
quantify the effect."