I hate to disagree with my friend and colleague, Laurence Harris, but I
would argue that what he rejected as a possibility is the indeed very
reason for "why you might not want to fixate a target for a prolonged
period of time before shooting at it". (I think my arguement comes close
to that of Patrick Cabe.)
He stated that "Now it might be argued that this conversion of position
from one reference frame (the retina) to another (e.g. the body or shooting
arm) is irrelevant since the system COULD solve the problem entirely
visually. The target and the sight (presumably with some kind of cross
hairs) are both in view and thus it could be argued that no other sense
than vision is required. You could just flail the gun around until the
cross-hairs and the target happened to line up and then pull the trigger.
It would not be necessary to know where the eye, head, body, target or gun
were at all!"
I would argue that the wording COULD should be changed to WOULD. and the
consequence of this solution is that the shooter SHOULD NOT be concerned
with the apparent location since he cannot correctly localize objects
located at different distances simultaneously (see references below). The
brain does indeed shift the visual direction of the target and the
cross-hairs, not because of the "fatigue", but in accordance with the
principle of visual direction when the non-viewing eye moves. (Again see
references below). For example, if the shooter changes the accommodation to
the target from the cross-hairs, the non-viewing eye moves
(accommodative-vergence) resulting in a change in the visual direction of
both the target and the cross-hairs. For a figure illustrating a stimulus
situation comparable to this situation, see Figure 4 in the ref. 3 below.
Perhaps, the suggestion to view the target and the cross-hairs for a brief
period is made to avoid the distraction of changes in visual direction that
accompany the change in accommodation. What is required for the shooter is
to have the target and the cross-hairs to appear colinear; where they
appear with respect to any other frames of reference is irrelevant for the
task. To constrast this view with that of Laurence, if we place a prism in
front of the viewing eye, accuracy should not change, as long as the
shooter is concern with the colinearity of the target and the cross-hairs.
This idea is testable, since it predicts that there would be an increase in
variable error with a long viewing time because of the "distraction".
If Laurence is arguing that the shooter would have a tendency to point the
rifle toward the apparent location of the target, then there should be an
increase in constant error with a long viewing time, whatever the reason
for mislocalization (his or mine). This idea is also testable but addresses
a different question.
1. Howard, I. P. & Rogers, B.J. (1995) Binocular vision and stereopsis.
New York: Oxford Press.
2. Ono, H. & Mapp, A. (1995). A restatement and modification of
Wells-Hering's laws of visual direction. Perception 24, 237-252.
3. Ono, H. & Barbeito, R. (1982). The cyclopean eye vs. the sighting
dominant eye as the center of visual direction. Perception and
Psychophysics, 32, 201-210.
H. Ono
Dept of Psychology
York University
4700 Keele St
North York, Ont. M3J 1P3
Tele:416-736-5118 ext. 66264
Fax: 416-736-5814
e-mail: hono@yorku.ca