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STEREOSCOPIC VISION:
page 2

        Elementary Binocular Physiology

                                                       John Wattie

version 11/03/04      

 

 

     This section has been simplified and repetition is used to get the concepts across.

     PAGE 1

  1. Panoramic versus stereo vision
  2. Binocular vision
  3. Visual scanning.
  4. Eye convergence, focus and retinal differences
  5. Colour pseudo-stereo
  6. Monocular 3D
  7. Eye exercises

         PAGE 2 = THIS PAGE

  8. Brain wiring for 3D in animals and man.
  9. A stereoscopic pair of images

   Return to stereo contents page 

Animals with two eyes facing forwards see the world in 3 dimensions.

Animals with eyes facing sideways don't see good 3D, sometimes none, but they do see a great panorama

 

 

 

 

Complete crossover = panorama vision.

    Partial crossover =  binocular vision.

The retinas of the two eyes are formed in embryonic life as out-pouchings from the brain. The retinas of higher animals are actually part of the brain, rather than separate sense organs.

(The lens and cornea are modified skin).

Photoreceptor cells in the retina are linked to each other by nerve fibres in such a way  that analysis of the scenery is already underway in the retina. The impulses  sent to the rest of the brain, through the optic nerves, are already partly differentiated.

(Contrast enhancement of edges and straight lines, for example, starts in the retina and is responsible for the Mach effect).

Nerve impulses from the eyes eventually end up in the visual cortex. On the way they go through the optic chiasm where all or some of the nerve fibres cross to the other side, depending on the species of animal.

The nerve impulses reaching the brain cortex have already been analysed further, in the lateral geniculate ganglions. These are the oval things, with "dots" representing ganglion cells, shown on the diagram. Some optical reflexes occur at this level and in lower animals without a brain cortex all visual reactions are set up here.
 The wiring in the retinas and lateral geniculate ganglia is not as simple, as shown here. The nerve cells interconnect with each other, as well as sending impulses straight through to the visual cortex. These interconnections are further analysing the visual message from both retinas.

Visual information does not go only to the visual cortex. Six other brain regions, with different functions are, also linked to the eyes. A vital zone for 3D vision is the pretectal region, which controls convergence of the two eyes onto the plane of interest. In primates like us, a lot of eye control also comes from the visual cortex, as described in the panel to the right.

 A camera image is upside down and back to front. Light rays cross over in the lens, but the reversed picture is no problem because we just flip the photograph so it looks right. The "picture" on the retina is flipped by the eye's lens system just as it is in a camera, but our brain is not disturbed about that, it just thinks upside down.

Visual field split in half

Notice how our visual field is split vertically down the middle in each eye.  The left side of the world is seen by the right side of each eye and vice versa. The two half fields end up combined together on the opposite side of the brain. This crossed visual link, of nasal field from one eye with temporal field from the other, is why the left hand is controlled by the right side of the brain and vice versa. This keeps the nerve connections from visual centre to motor centre shorter and avoids overloading the corpus callosum, which joins the two sides of the brain together.

Primitive panorama vision

Simple animals with eyes on the sides of their heads also cross the information over to the opposite side of the brain. But all the information crosses over because they do not have our system for splitting each visual field down the middle. They cannot see in stereo because the visual field for each eye does not show the same thing. The two pictures from each eye are joined together in the brain to make one sweeping panorama. In some animals there is a gap in the middle of the panorama because neither eye has any part looking forward. A long nose does not help forward vision either. Sperm whales are a famous example and whalers take advantage of this by trying to sneak up on a whale from directly in front, or directly behind, where he is blind. Whales do not have the stenopoeic pupils of seals and do not see well out of the water.

Panorama plus binocular vision

Other animals, like horses, have eyes angling to the side but can also see forwards. However, they are said to be blind for things right in front of their noses, in the lower part of the visual field, which they overcome by tossing their heads around. They are also blind for things directly behind, which we do not find surprising, but their visual field does extend backwards almost to parallel with the sides of their heads. Riders are taught not to restrict head tossing, which is said to be vital for horse forward vision of the ground ahead.

The idea that horses have a ramp retina is no longer an accepted reason for head tossing. (The only animals with a ramp focus are old humans using graduated focus spectacles and they do indeed have to flip there heads up and down to see at different distances). 

Because they can see much further to the side  than we can, and even backwards, race horses have blinkers placed to restrict side and rear vision, so avoiding distraction from other horses' antics. Horses do have a poor version of binocular vision provided by only partial optic nerve crossover from the temporal visual fields common to both eyes. Humans have 50% information from each eye crossing over, but horses  have 85% (and dogs about 60%). The primitive condition is total cross-over, for pure panorama vision and no 3D.

Stereo turn off by card tricks

Humans also have the panorama vision of less evolved animals but it is limited by both eyes pointing forwards so we cannot see so far to the sides and certainly not backwards. (The only "exception" is experienced school teachers working at the blackboard who can "sense" which students are playing up, but "eyes in the back of the head" is not a visual sense...) 

Both eyes have a wide-angle view, mainly limited by the nose low down in front, but differently limited for each eye.

You can easily prove we have also have the primitive split panorama vision possessed by animals with eyes on the side of the head.

Place a 55mm wide card in front of your face. Three fingers is about 55mm so you don't really need the card. 65mm is the distance between your eyes but if you use a 65mm card the trick only works when looking to the very far distance.

Now move the 55mm card forwards and back. Winking one eye and then the other shows the card is blocking a different part of each visual field. Soon you will find where the two blocked fields join, at around 15-30cm from your eyes. (This varies depending on the object you are viewing in your computer room and for a far distant object it never happens).

You can see the full visual field despite the card, which removes the temporal field of each retina but leaves the nasal fields intact. It seems you are looking through a transparent card because the temporal fields only see out of focus card while the nasal fields fill in the true scenery. (If you think this is back to front, think again - you have forgotten that light crosses over in the eyes).

Now look at nearby objects (which is why we are using a 55 mm card to allow your eyes to converge yet still see around the card). >Your stereo vision has gone! Things are still in 3D but only because of monocular clues. The world nearby is strangely flat. The distant world still looks OK partly because we do not have much stereo vision in the distance anyway. Binocular stereo is removed by the strip of card because the nasal visual fields are working alone and cannot be compared with the temporal field from the other eye.

Notice how the nasal field of each retina crosses over to the other side, so the human (and primate) nasal fields are the primitive panorama system used by animals with eyes on the sides of their heads.

By doing the inverse of a central 55 mm card, you can cut off the nasal fields. Cut out a 55mm window in the card (65mm if you are looking into the far distance). You see a poor panorama because it is a much reduced span, since the far lateral visual fields are lost behind the window frame formed by the card. >Once more, binocular vision is destroyed! Now you are seeing with the temporal fields individually and there is no nasal field from the other eye for the brain to compare. 

Again you don't need a card to do this. Just hold up both hands, palm to your face, and bring them in until the centre of your vision becomes monocular.

So 3D in primates depends on the nasal field from one eye being compared with the temporal field from the other. If passerine birds see in 3D, and it seems they probably can, it must be through a different wiring pattern; so that forward looking parts of the temporal fields can be compared with each other.

The bands in the occipital cortex visual "centre" are about 350 micrometer thick and are a magnified version of bands in the retina. (There is really no visual "centre" - many parts of the brain work together for vision)

  1. The cortical bands are revealed by injecting radioactive amino acid into one eye, from where the nerve fibres carry them to the brain, then auto-radiography shows  the bands in the cortex.

  2. Retinal bands are not visible. They are defined by the "wiring diagram" linking photoreceptors together. Retinal bands are mapped with a tiny point of light aimed at the retina and micro-electrodes record how the nerves respond. There is a sudden jump in nerve firing rates when the spot moves from one band to the next.

    Retinal bands require the eyes to be mirror-image versions of each other, so the nasal visual field lines up with the temporal field on the structurally equivalent halves of the retina. There is no problem with this - most animals are bilaterally symmetrical, or mirror image about the cranio-caudal plane. Note how the optic nerve lies on the nasal side of the macula in both eyes - they are indeed mirror images of each other.

(The cortical bands are further separated into  columns of nerve cells, about 30micrometer in diameter, which discriminate local 2D information.)

Cortical neurons which specialise in disparity between the two eyes have been identified with micro-electrodes. There are two sets:

  1. those which deal with big differences are in cortical layers II and III

  2. while the small differences are analysed in layers V and VI.

 Each of the two specialised layers has two sets of neurons:

  1. those which respond to disparity in front of the plane of fixation ("near neurons") and

  2. those behind the fixation plane ("far neurons")

This nerve cell separation of function reflects the two processes described above as necessary for 3D vision: 

  • Convergence to remove diplopia followed by 
  • Small Retinal Differences

 

The centre of consciousness.

Do not imagine there is a little chap sitting inside your brain seeing the world as interpreted by the eyes and brain. "Consciousness" is distributed all through the brain rather like a huge hologram. 

The brain is the computer which enables an animal to interact with the world. "Consciousness" is a function of the whole brain. If bits of the brain drop out, you gradually get dumber and less able to function as an integrated animal (e.g. after a major stroke, or in patients with Alzheimer's disease.).

So where is the "soul"? Don't know - what is it anyway?

The thinking neocortex versus the reflex reptile brain.

There is a correlation between binocular vision and possessing a specialised brain cortex (neocortex). Primitive mammals like the rabbit have no binocular vision and do not have much of a cerebral cortex. Primates have major binocular vision and have a cortex which can analyse binocular information and combine it with 3D information from other senses, especially hearing and proprioception. This is used to direct our hands, which have a large motor area to control them in the brain cortex just in front of the visual cortex. The cortex is used for thinking and analysing, which we now know is a major part of interpreting the world in 3D. Dolphins have a bigger cortex than us, one of the major pieces of evidence they evolved on land, with binocular vision (like seals) but later returned to the sea. Foetal seals have eyes facing forwards, suitable for binocular vision, but they move further to the sides before birth. (Foetal changes often reflect evolution of an animal).

Failure to make use of 3D vision in art or photography is denying us a major brain evolution that separates us from lower animals. Lesser animals have visual reflexes based in the lateral geniculate bodies - the "reptile brain" of some authors. Humans are more than a sophisticated set of reptilian reflexes. 

We do not have the fast visual reflexes of a fly, because the neo-cortex is interposed in the visual circuit, thinking and analysing, which takes time but finally produces a much more detailed 3D image of the world than owned by a fly. Also as we have noted, fly eyes turn on and off faster than ours - they are operating a slow-motion camera with high frame rates.

The wiring for vision is exceedingly complex. Is detailed neural wiring built in to our genes? 

Only in part. The final detailed linking of retinal nerve fibres to the brain is directed by experience. It is one of the amazing things happening to infants as they develop. Lack of visual stimulus in infancy reduces the ability to see in stereo because the fine neural links for detecting retinal disparity have not been set up properly for that particular human.

Stereognosis is the ability to understand the 3 dimensional structure of objects by feeling them and playing with them in our hands. Infants train their stereoscopic vision by linking stereognosis to visual impressions of objects and toys. Interestingly, loss of stereognosis, called "astereognosis," can occur after strokes and is a symptom seen in Alzheimer's disease. ("Old-timers Disease").

Binocular vision in birds of prey

Owls have eyes pointing forwards, rather like we do, and have excellent binocular vision with neural crossover occurring behind the optic chiasm, in the thalamus. They do not have our well developed visual cortex but get by nicely with a Wulst, which does a similar job. Owl eyes do not move much in the orbits but they can turn their heads through almost 360 degrees. This keeps the stereophonic ears working in exact synchrony with the dark-adapted, stereoscopic eyes, so owls are deadly predators in the dark.

Raptors (falcons, eagles, hawks) have wonderful vision because their eyes are bigger than ours, with more tightly packed photoreceptors. The fovea is funnel shaped so even more photoreceptors are concentrated in the area of maximal focus. This makes their eyes work rather like a telephoto lens in the middle of the visual field but a wide angle lens further out. No wonder they can see small tasty animals while gliding far above the earth. Binocular vision is excellent, as they get close to the prey in a dive. Combine that with better colour differentiation than us (more hues, albeit over a smaller frequency range) and raptor vision is superb.

Vegetarian birds: victims of carnivorous animals

The temporal visual fields look at things in front of our noses. Birds with eyes angled to the sides of their heads often have a second fovea in the temporal field which provides forward and limited binocular vision. Sea birds often have a visual streak, which is a horizontal band of concentrated photoreceptors in each retina. This provides maximum resolution in a horizontal band across their world, which of course is the horizon. Less marked versions of the streak fovea are found in mammals. Including horses and dogs, where it improves panoramic vision along the ground level, which is where predators are likely to show up first.

So 3D in primates depends on the nasal field from one eye being compared with the temporal field of the other. If passerine birds see in 3D, and it seems they can, it must be through a different wiring pattern from us, so that forward looking parts of the temporal fields can be compared with each other.

How the eye works: project for kids 10-14 years. (pdf file from New Zealand Optometrists).

 

Go To   VENICE in 3D, but only if you have learned how to see X Stereo

The church above is in X stereo pairs. (cross-eye). When two X stereo images are joined, the centre pair of pictures are in U stereo (parallel eye). If you get two of these images fused then switch to an adjacent pair, you will have pseudo-stereo.

 

Return to binocular vision contents table

 3D contents page

Stereo Picture Gallery

Escape from 3D:  New Zealand Images