We are all somewhat blind
In all probability you have already heard about this famous phenomenon called the blind spot in our eyes. Whether you have or not, try this. It is a beautiful and simple demonstration of an amazing fact about your eyes. Given below is a picture.
Now close your left eye with one of your hands, and look at the red diamond on the left. Nothing strange so far. But now, start moving towards the screen, all along keeping your left eye closed and focusing on the red diamond. At some point when you move towards the screen, the blue circle on the right will disappear. The key is to keep your eye focused on the diamond. Did it disappear or not? If not, try again. You will get it the second or third time.
You can also make the diamond disappear, by closing your right eye and focusing your left eye on the blue circle. Now the diamond will disappear as you move closer to the screen.
This disappearance is a beauty. It is almost unbelievable. How many ever times I see it, I am not bored. You can also play around a bit, when you hit the blind spot, by shifting your focus alternately from one object to another. The blue dot is definitely there, but why is it hidden from us?
The Retina is that part of our eye which receives light from the outer world. This has the same function as that of a film in a film based camera. In a digital camera, this function is performed by what is called the CCD or CMOS which is, again, light sensitive, and converts the light received into electrons and charges. Based on the light that the retina receives, a series of chemical reactions happen which results in our brain perceiving the image. The paths through which these messages travel from the eyes to the brain are called the nerve fibres.
Now imagine how a digital camera would be designed.
The light rays(red lines) come and hit the “front” side of the CMOS which is light sensitive and the wires (pathways), represented by green lines, will be on the “back” side to carry the converted messages to the processing device and finally onto the memory card. This seems simple and it is natural to expect this in our eyes too.
But here is where evolution plays a joke on us. Our eyes have evolved in an unexpected way. The pathways that carry the messages from the light sensitive device (our retina) do not originate from the “back” side of the retina, but from the very “front” side where light rays hit initially. So at some point it has to go “back” through the retina to take the messages through the brain. If that is confusing, look at the picture below, which shows how a camera designed in this unexpected way would look. The light rays come and hit the “front” side. The pathways that carry the information come out from the same side, regroup, and then pass through the light blue circle along the vertical line so that it can go “behind” the device to transmit information to the memory card.
Yes, it is terribly messy. In our eyes, brain pathways are on the same side as the light sensitive side, and hence it has to go back through the retina, at some point. And at that point (analogous to the light blue circle in the pic above), light sensitive cells do not exist. Hence light that comes in that direction is lost. That is why the blue circle or the red diamond suddenly goes missing.
In the picture above from Wikipedia, a vertebrate eye, and an octopus’ eye is shown. The part numbered 4 in the left half of the picture shows the blind spot. That is the point where the nerves go back through the retina to go to the brain.
But look at the octopus’ eye. The light receptors are on the front, and the nerve fibres are at the back. This is how an eye should ideally be. Our eyes are imperfect. But we still manage. This is one of those quirks of evolution, like the laryngeal nerve, I mentioned sometime back. The blind spot does not cause us much harm, since we have two eyes, and I also remember it being mentioned that, our eyes wiggle a bit to compensate for this blind spot, though I am not sure where I read/heard it.
The blind spot itself is very interesting, but there is another curious thing about the little experiment we did. When the circle or the diamond disappears, you don’t see a hole in your vision, do you? What you see there is the green background. That is surprising. If no input goes to the eyes at the point where the blind spot exists, where did the green come in from? The answer is that the brain fills it up. It is now well known that what our eyes see is not always what our brain perceives. The brain does a lot of filling up and interpreting of what actually enters our eyes. If you dont trust me, look at these illusions on the website of the neuro-scientist V.S.Ramachandran, who has written a fabulous book, “Phantoms in the brain”, which talks about, among other things, phantom limbs, where people “feel” non-existent limbs. It is a must-read.
While on the topic of illusions, let me finish the post, with this incredible illusion that I was introduced to by the thalaivar, Richard Dawkins
Wasn’t that mind blowing?