TESTS OF THE CONVENTIONAL WISDOM in VISION
PROCESSES IN BIOLOGICAL VISION
Date: 02 JULY 2009
[1.3.1] Some Simple Reality Checks
Before going too far, exploring some simple arm chair experiments in human vision
is useful. This will better enable the reader to evaluate some statements of
folklore, found in both the popular and scientific literature, that have risen
to the level of an axiom.
[184.108.40.206] Adaptation--There is frequent reference to the synchronous adaptation
of both eyes in response to a change in light level to either eye. While in a
dimly lit room, less than one-half moon outside, close one eye and turn a light
on for a few seconds to a minute. Turn off the light. Now observe how much
detail you can see with the open eye. Close the first eye and open the other eye.
Observe how much detail is visible. Alternate eyes a few times. The level of
adaptation is clearly quite different in the two eyes; although looking in a
mirror will confirm that the irises of both eyes have contracted to the level
established by the eye open to the higher light level. Conclusion, the irises
of both eyes normally operate in synchronism, however, the major adaptation
process in vision operates individually in each eye and is independent of the iris.
[220.127.116.11] Peripheral Color Vision--Look straight ahead while bringing a reasonably
large (1/2 inch square) object of red or blue from behind you into your peripheral
field of view. Note the angle when you first see the color of the object. This
experiment should clearly show you have color vision at angles greater than 60
degrees from the optical axis, the area limited to "rod" vision in both the
scientific and popular literature. The status of "rods" will be developed later.
[18.104.22.168] Foveal vs peripheral night vision--On a clear night, take your star map
and go outside to look at the stars. It is best to be thoroughly dark adapted
(at least 20 minutes to be fair). However, the point can be made with less dark
adaptation. Our object is to see how much more sensitive our peripheral vision
is to our foveal vision. The most sensitive part of our field of view is about
five degrees temporally (toward our ears in the horizontal plane) from our point
of fixation. This is the location of the optical axis of the eye. The
photoreceptors located there are about 50% larger in diameter than in the fovea.
Our object is to see what is the dimmest known star we can see using our most
sensitive area and using our fovea. Don't use a red star in this initial
experiment. Most people will find the difference in stellar magnitude between
these two stars is less than one. One stellar magnitude is a factor of 2.5:1.
This is a difficult experiment because of the small difference in sensitivity
actually involved. Try using the stars of Ursa Major (the big dipper). Can
you see all four stars forming the cup of the dipper with both your foveal and
your most sensitive vision.
Clearly, our night foveal vision is not limited by "cones" in the fovea, with
a sensitivity 1000 times less than "rods," as frequently stated in the literature.
It is limited by the smaller cross-sectional area of the photoreceptors in the
fovea and the poorer performance of the elliptical lenses of the eye five
degrees from the optical axis when the iris is fully opened (related to the
Stiles-Crawford Effect). The first cause introduces a factor of about 2:1
and the second cause accounts for a factor of less than 1.5:1.
[22.214.171.124] Night Color Vision--Step outside on any clear night and look up at the
stars. Do you see any colored planets or stars? Mars, "the red planet", should
be easy to see somewhere along the ecliptic if it is in the sky. There are
many colored stars, and many of these have names given to them in ancient times.
According to the Field Book of the Skies, "There is a wide variety of tints
easily seen with the unaided eye." These usually range from green to yellow
to red. Can you recognize the color of a star while looking directly at it?
Do you have color vision under low light conditions in your fovea?
[126.96.36.199] Eye as a Camera--While looking straight ahead and without moving your
head, note closely the field of view you perceive. Without moving your head or
blinking, move your line of sight 5-10 degrees to either side and back. Try up
and down by 5-10 degrees. Did the overall scene that you perceive move? Would
you expect this result if you took a picture with a camera and then took a second
picture with the camera turned 5-10 degrees? The process of detecting and
perceiving information by the eye will be addressed later.
[188.8.131.52] Eye as a Change Detector--During your next eye examination, while
sitting at the Visual Field Analyzer --a device that presents a uniform white
field to the eye-- repeat a simple experiment fully explored by Yarbus in the
1950's. Before the technician inserts a stimulus into this uniform white field,
note that in the absence of any blinking or head movement, you begin to observe
a darkening of your field of view after 1-3 seconds. If carefully done, you
will become completely blind during this experiment. This is not the normal
result obtained with a camera. It is the normal response expected of a sensor
that is a "change detector."
[184.108.40.206] Bright Adaptation as opposed to Dark Adaptation--After
attending an afternoon movie and becoming fully dark adapted, walk quickly out
of the lobby of a small theater (or out the emergency exit) into the bright
sunlight. Neglecting the pain for the moment, how long does it take before you
can see effectively? It probably takes a few seconds, the period required for
"Bright Adaptation." Both "Bright" and "Dark" adaptation need to be considered,
explained and quantified.
[220.127.116.11] Noise Limited performance-- [[or photon noise limited performance ]]
These simple experiments will be addressed later to help the reader put into
perspective and better understand some of the results presented.
The numbers in [brackets] refer to the paragraph numbers in the main text.