Optic Chiasm: X Marks the Spot:
• Emerging from the retina of your eye, the axons (colored red in the left image) come together at the blind spot, where about 1 million of them exit the eye to form the optic nerve. The blind spot has no visual cells, but you don’t notice it because when both eyes are open, they compensate for each other. To “see” your blind spot, follow the instructions on this link : http://goo.gl/HKB2s
• At the bottom of the brain, the optic nerves cross over in the chiasma (from the Greek χιάζω ‘to mark with an X’, after the Greek letter ‘Χ’, chi). Because we have binocular vision, signals from part of one eye cross over to the opposite side of the brain while the rest is deciphered by the same side. This means that each half of the brain receives visual signals from both eyes. The chiasma of the mouse brain (right image) shows nearly complete crossing over because rodents have poor binocular vision.
Last week, we were learning the 12 cranial nerves. When I mentioned the optic chiasma, my students didn’t understand what that meant; this is a beautiful photo showing the X. Hopefully, it will be more clear to my group when they dissect the sheep brain.
do u know Rajini u look like sunitha wiliams
Patrick Armstrong My favorite argument against intelligent design/creationism, that – if God is so smart, why make first one eye with the nerve and capillary connections run behind the sensors, and then another with the connections run overtop the sensors? That’s really, really dumb design, not intelligent design! 🙂
Gert Sønderby in my view it is much more likely that we do not properly understand the merits of the evolved (vertebrate) eye design. If it really is a seriously dumb design then one has to ask, why so much of organic design is so beautiful – way beyond anything we come up with.
Let me take another example. Why do cats’ eyes have slit shaped pupils, but most vertebrates have round ones? I could make a spurious argument that the cat eye is bad design compared to the round pupil of most vertebrates. Round pupils have all kinds of better optical properties! In the cat-human eye comparison one can find a convincing advantage for the slit pupil. Isn’t it more likely that our engineering knowledge is not yet up to explaining the advantages of the arrangement in vertebrate eyes?
“The blind spot has no visual cells, but you don’t notice it because when both eyes are open, they compensate for each other.” So why don’t I notice my blind spot when I cover one eye? Rajini Rao, it’s so easy to slip into thinking that an explanation is correct when the explanation gives the right answer.
James Crook *BZZZT* Wrong. It’s a dumb, but functioning design, and evolution never throws anything out that works. As a result, squids see better than mammals, but mammals see just fine. (Incidentally, birds have better eyesight than mammals, as a rule.)
As for cat eyes (and horse eyes, and the many other species whose pupils are not circular), they’re adapted to a purpose. Cats have high visual acuity in a horizontal zone directly ahead, and a high adaptability to light levels. Horses have a similar high visual acuity, but in in a plane around them. Primates have circular forward-oriented vision areas.
Each fits the needs of the animal – cats are nocturnal and crepuscular hunters, horses are plains herbivores whose main defense is early warning and escape, and primates are fructivores and omnivores with need for good close-range visual acuity. Evolution equipped each of them with the eye best suited, given what was there to work with. But still, all of them have a network of fine capillaries drawn over their cones and rods. A network that, in a sensible design, would be behind them.
And you don’t notice your blind spot under most circumstances because your brain compensates. But draw a dot on paper, cover one eye, and watch it disappear when held at the right angle, and the blind spot can’t see it, and the brain can’t compensate.
James Crook , when you cover one eye, you will only notice your blind spot when the object is small enough and at the right distance to be focused exactly on the blind spot. That’s why the demos of blind spots have you focus on one point with one eye, while you move your face towards the screen (or page). At some precise point (usually about a foot away), a second object that was visible in your peripheral vision will disappear. Check out this link for more information: http://serendip.brynmawr.edu/bb/blindspot1.html
Feel free to explain it better, if you are able!
Since cat eyes were brought up, I found this fascinating snippet on Wiki: “In Siamese cats with certain genotypes of the albino gene, this wiring is disrupted, with less of the nerve-crossing than is normal, as a number of scholars have reported. To compensate for lack of crossing in their brains, they cross their eyes (strabismus). This is also seen in albino tigers, as Guillery & Kaas report.”
I’ll have to look up the connection between albinism and strabismus, since it is not at all obvious (linked genes?).
Rajini Rao we’re on the same page about the blind spot. I use my eyes alternately and don’t have binocular vision. And I don’t ‘see’ the blind spot except under the special conditions. Certainly not apparent like a stuck pixel on a screen would be. The page you refer to talks about the brain ‘making up’ the missing part of the image.
James Crook , it must be difficult to have impaired stereoscopic vision. Is it due to amblyopia or something else? My eyes use monovision..which sounds worse than it is.. simply that one eye is corrected for distance seeing and the other for reading. It works really well for me, but I’ve heard other people who struggle with getting the images to make sense.
My eyes won’t cooperate either, presumably due to significant hyperopia and a bit of strabismus (both of which are correctable). Though the only real effect for me seems to be slightly impaired distance estimation (particularly in the 1-10m range), and inability to see false 3D, as in movies and images. Otherwise, my brain seems to compensate, somehow.
I use my left eye less than my right, and that eye is less good. I think it’s ‘voluntary alternating esotropia’ – I can choose which eye to use. My optician once asked me to come as a diagnostic challenge for a student he was teaching. Like Gert I can’t see the 3D in false 3D or dot stereograms. I’m OK at judging distances when things are stationary. Not good at judging exactly where a moving ball is.
In “Fixing My Gaze” http://www.amazon.com/Fixing-My-Gaze-Scientists-Dimensions/dp/0465020739 a neuroscientist describes her experience of gaining / retraining 3D vision in her late 40’s. It sounds like a lot more time and work than I would be prepared to put into it, but intriguing that it could be possible at all.