This section has been simplified and repetition is used to get the concepts across.
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.
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.
How the eye works: project for kids 10-14 years. (pdf file from New Zealand Optometrists).
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.
Escape from 3D: New Zealand Images