 | Stereoscopic perception is required by virtually all higher animals. For example, a pigeon flying over a garden may perceive a green rectangle. It will not have a WORD for this - such as the word "LAWN" - but it will have a "FEELING" or "CONCEPT" in its mind, by which it may recognise it again.
This is a pre-requisite for survival, such as to find bread-crumbs
on that lawn.
|
When seen from any other angle, the rectangle becomes a trapezium.
Two eyes produce two trapezia. Each is different. Stereopsis is
thus the creation of one "CONCEPT" from a pair of DISSIMILAR
figures.
These stereoscopic
pairs have a septum between them. You can view them "CROSS-EYED"
by squinting at a place halfway towards the septum. Do not view with your
head tilted. Try rocking your head slowly to obtain stereopsis. Move
further back to deepen the perspective.
On a misty day, it is still the same lawn - but the colour has changed.
To deal with all the variants, the brain produces "ACUTANCE" at the retina by allowing the first optic nerves that "fire" to take away much of the reagents, so that neighbouring nerves do not fire so well. This enhances the all-important edges of the image. Then the colour is removed.
The ability to "SQUINT" is controlled by the "LIMBIC SYSTEM" at the back of the brain, close to the monochrome vision centre. This area controls acquired reflexes of which the ability to see in depth is one, so we see depth by virtue of the "LUMINANCE", or "LUMA", of the two pictures rather than by their "CHROMA".
Amendment
The limbic system is shown in this diagram. It is illustrated in white. It can be seen that a "tree" of nerves is woven into the cerebellum. The posterior Limbic System tells you how close the object is, and gives you the urge to activate through that "tree" the acquired eyeball-moving reflexes (device drivers) of the cerebellum.
|
That luminance passes down through the layers of the "VISUAL CORTEX" , which trigger when detecting lines at different angles. In this way, the image is "DECONSTRUCTED" into lines, and the limbic reflexes make us squint until those lines overlap.
The following image has been constructed to have colours of equal "GREY-SCALE EQUIVALENCE", so that when the "CHROMA" are removed there is little for the brain to use for stereopsis. Unfortunately, the colour-balance of the human eye is constantly changing, from the 3,000 °K of tungsten light to the 11,000 °K at the top of a mountain. There is no way of guaranteeing a total absence of luminance. I cannot stop you seeing it in 3D if you try hard.
A better experiment might be to project such images to an audience whilst varying the red, green and blue brightness - asking each member of that audience to press a button when stereopsis is lost. This would allow the statistical study of grey-scale variation in humans.
 |
The next image has the primary colours in equal intensity. The eye sees green at about twice the brightness of red, which is about twice as bright as blue. Thus, the monochrome images in the monochrome centre of your brain will have plenty of contrast to use for stereopsis.
 |
So two monochrome images have become one monochrome cube, and the colours were "REINSERTED" into the description by your brain. However, WHICH colours were reinserted - the LEFT-EYE colours or the RIGHT-EYE colours? We have two colour specifications, but only one "CONCEPT" - the cube.
Consider a bunch of yellow roses:
It is well known that we can make yellow light by mixing red and green. What happens - as in some anaglyph systems - if we deliver the red by one eye, and the green by the other?
Do you see a red rose, a cabbage, a yellow rose or a rose that
FLICKERS
from one colour to another, depending on the focus of your mind?
So we PERCEIVE what is PROBABLY there. The yellow rose was not actually there in the two-tone stereogram. Yet what I see is a SILVERY yellow, after it has settled. Yellow mixed from ingredients via separate eyes is not the same yellow as that seen directly. My mind reserves judgement, and generates a separate CONCEPT for this yellow.
We can see that the mind, once it has generated a
"CONCEPT"
can use the
"CONCEPT"
of
"REPEATS"
to
"PERCEIVE"
a situation where there are many objects of a similar kind. Consider
a wall covered with a rose pattern, and having a light-switch:
We all have a
"BLIND-SPOT"
where the bundle of optic nerves leaves the retina - and those with
damaged eyes may have several. If the light-switch falls on the
blind spot, we will see only repeats.
We do not go about with holes in our vision. That which cannot be seen simply drops out of our perception.
Yet what has been seen becomes a collection of "CONCEPTS" ready to hang a name onto - as in Chomsky`s "UNIVERSAL GRAMMAR".
The pigeon will use the concept of a lawn, the concept of "MANY" or the concept of "FEW" to fly across suburbia, and like a homing-pigeon home in on the bread-crumbs many or few gardens away.
It will recognise those lawns because the chroma had been taken away, converted to the concept "GREEN", valid for all brightnesses and saturations, and then reinserted into the specification as a parameter of a "GENERIC" lawn.
The chroma centre of the brain is just above the limbic centre.
Thus, the "UNIVERSAL GRAMMAR" - even in animals - includes "NOUNS", like RECTANGLE and CUBE, "VERBS", like TO REPEAT and "ADJECTIVES", like MANY, FEW, GREEN and YELLOW.
"YELLOW", interestingly, is a "HARD-WIRED CONCEPT", in that there are yellow-sensitive cones in the retina of the human eye. Anybody born with a defect of these cones would experience the colour as "REDDISH GREEN", or "GREENISH RED", but through force of habit would describe it verbally as "YELLOW". It is the only physical secondary colour in the human eye.
The other "HARD-WIRED CONCEPTS" of colour in the human are "RED", "GREEN" and "BLUE" - the physical primaries - as well as "MONOCHROME", as seen by the rods of the retina in dim light. The last consists of concepts "BLACK" and "WHITE".
A concept may imply its inverse. Thus, if only the rods are firing we see "MONOCHROME", but if cones are involved we see "COLOUR". Mixtures, such as gray, are learned from experience of seeing.
If you look at your hand directly in front of you, your eyes will have same-sized images of that hand. If you look at your hand off-centre, the distance from one eye to the hand will differ from the distance from the other eye to that hand. The retinae will have images of different sizes. Note that you are free to swivel your eyes, because this does not change the viewing distance.
Within reasonable limits, your brain can accommodate different sizes
of images - because it is NOT OVERLAPPING THE TWO AREAS. The
image has ALREADY been deconstructed into
ANGLES
before it is presented to the limbic stereoptic reflex - so you only have
to match lines of any reasonable length at the correct angle within a
reasonable vertical range. If enough such visual clues are achieved,
your brain recognises stereopsis. Here the sizes are 9:10
But what about the stereopsis of GROSSLY DISSIMILAR images? In 1862, Joseph Towne wrote a paper where he had created apparatus to examine what happens when the mind seeks to make sense of a circle in one eye and a square in the other - each with its own colour.
He used pinhole oculars, to obviate any possibility of aberration due to optical geometry.
Without reading this work, I would probably not have had my mind drawn to the question of "CHROMA REINSERTION AFTER PERCEPTION".
What exactly happens? Does the colour of the circle fill the square?
Does the colour of the square fill the circle? Is the colour a
MIXTURE
or an
AVERAGE
of the colour of the square and circle? Or is the colour a
SMUDGE,
as in the
JPEG
system of image compression? Is this the psychological basis of JPEG?
To me, the result in inconclusive - I see a flickering REDDISH GREEN, not a YELLOW circle, even though I have yellow cones. Towne, however, gives his own views on the outcome.
Towne uses black and red, and describes seeing a "REDDISH BLACK" on page 86. One eye told his brain RED, the other told it BLACK, and he says so. However - unbeknown to Towne - black and red make DARK RED - a FREUDIAN SLIP.
There exist in nature a few objects whose appearance is critically dependent upon viewing angle. Steel, and diffraction gratings such as on a compact disc (CD) are examples. View a CD, and the outline will be steady, but the rainbow colours will be different for each eye. This leads to the "steel-like, glistening effect, with some tendency to alternate" in the middle of P. 97.
An anaglyph RED-GREEN movie made from a black-and white original will be black-and-white. Made from a colour original, however, it will have red lips for one eye and black for the other. As the eyes hunt for stereopsis, the glistening alternation happens. However, on a static image the lips are clearly REDDISH BLACK.
Towne was an artist - and a great one. However, he was not a scientist and did not have modern optical or neurological knowledge. That blue and yellow are discordant is obvious if one realises that the brain is being told by one eye that the red, yellow and green cones -BUT NOT THE BLUE - are firing, whilst the other eye says ONLY THE BLUE.
As an artist, he will have thought in terms of mixing PAINT. Yellow and blue - "ARTIST`S PRIMARIES" - make GREEN. However, yellow and blue light - "PHYSICAL COMPLEMENTS" make WHITE.
Today we know about colour temperature, and the colour balance of the eyes. They are balanced INDEPENDENTLY. Towne is spot-on when he says that the experiment of covering each eye with a different colour filter will not work (p. 93).
Similarly, if squares and circles - CONCEPTS - are all separate, the brain lists them separately. However, if a concept NOSE is within concept FACE, they will be catalogued together by the brain. The eyes wander over the CONCEPT, or OBJECT until they find a void. If they must leap that void, what is seen is a separate concept - with a fresh catalogue of properties.
The "SENSORIUM is now known to be the monochrome centre, the colour centre and the limbic system at the top of the cerebellum (at the back, behind the cerebrum).
Towne comes close to discounting stereopsis as a phenomenon. The eyes have four muscles, and when they are pulled together the lens becomes flat - focusing far. If the muscles are imbalanced, the eye shifts its gaze. Thus stereopsis and focus are related. Towne uses a biconvex lens (p. 95), and FEELS that the image has depth, even when using only one eye. This is due to focus shift and distortion. It is not 3D.
A mistake that he failed to notice, therefore, is that the "ADJUSTIVE STEREOSCOPE" is essentailly no more than a free-viewing aid. The FOCUS will be at 16 inches whilst if the centres of the pictures are just over 2 inches apart, STEREOPSIS will be at infinity.
When you are talking to somebody near an obstruction, such that only one of your eyes can see one of his ears, you normally do not notice. The face is three-dimensional, and the 3D impression SPREADS. That which cannot be seen (the right eye view of the ear) simply slips away from your awareness.
Partial obstruction of vision is an everyday event, so nature HAS
to work this way - to gloss-over the temporary shortage of visual data.
Towne criticises Sir Charles Wheatstone on page 94 for saying that the stereo effect is most dramatic at about seven inches. He says how SELDOM we examine objects from seven inches. Yet we do this EVERY TIME WE EAT. Stereopsis is for short-range co-ordination. Distant views have little stereo depth - so the lanscape painter need have no fear of obsolescence!
What caused the period after the first lecture by WHEATSTONE to BRIDGE a span of 24 years? Towne was clearly thinking and experimenting. These thoughts are certainly no hasty conjectures!
Towne - as a fine sculptor himself - liked to examine the geometrical
properties of stereograms of sculptures. So what would be more appropriate
than his OWN sculpture of Addison?
I first came across the work of Joseph Towne when I was looking for images of Thomas Addison for a website on Addison`s disease. I found the remarkably lifelike marble bust. This I photographed and processed by computer to produce the only modern colour photograph of Thomas Addison - at http://wehner.org/addison/images/himself.jpg. Use BACK to return.
I then sought the report by Dr. Samuel Wilks on the first 25 cases to be authenticated in Britain. This was in the 1862 edition of Guy`s Hospital Reports.
To my surprise, it also contained the article by Towne on visual
perception. My reaction was amazement - he`s one of us - a
STEREOSCOPIST! My admiration was complete.
(C) 2002 Charles Douglas
Wehner.
Use freely but do not plagiarise.
