I would try to get more systematic about my posts from now on. For every two non-technical posts I would keep two technical posts.
This post would also be the first in a series of posts that in which I intend to write about some Visual Illusions only.
Before getting into subject of this post, it would be helpful to have a quick recap of the background.
The Blind Spot:
Consider a horizontal cross section of the human eye as shown below.
As seen in the above, the innermost membrane is the Retina, and it lines the walls of the posterior portion of the eye. When the eye is focused, light from the focused object is imaged onto the Retina. It thus acts as a screen. Pattern vision is caused by the distribution of discrete light receptors called rods and cones over the retinal surface.
Each eye has about 6-7 million cones, located primarily in the central portion of the Retina and they are highly sensitive to color. Humans can resolve fine details with cones as each cone is connected to its own nerve end. The vision due to cones is called Photopic or bright-light vision.
The number of rods is about 75-150 million andare distributed throughout the retina. The amount of details that can be resolved by rods is lesser as several of them are connected to the same nerve unlike in the cones. Vision due to rods is simply to give an overall picture of the field of view. Objects that seen in bright day light appear as color-less forms in moonlight as only the rods are stimulated. This type of vision is called Scotopic or dim-light vision.
As seen in the figure there is a portion on the retina which has no receptors (rods or cones), thus will not cause any sensation. This is called the blind spot.
Now because of the blind spot a certain field of vision is not perceived. We however do not notice it as the brain fills it with details from the surroundings or using information from the other eye.
The blind spots in both the eyes are arranged symmetrically so that the loss in field of vision in one eye will compensate for the other. This is shown by the figure below.
If the brain would not fill the lost field of vision with surrounding details and information from the other eye, then the blind spot would appear something like the black dot on the image below.
Now that means, if you close one eye then you can indeed detect the presence of the blind spot as the brain would not have sufficient information about the lost field of vision (though it would be good enough for us to not notice it normally). The presence of the blind spot can be demonstrated by the simple figure below.
Click on the above image to enlarge
Now enlarge the above image and close your right eye and focus your left eye on the X only. Don’t try to look at the O on the left. You’d just notice it at the periphery. The object of interest should only be X.
Now move towards the screen, at a certain point you will not see O in the periphery. If you go ahead of this point or behind it you’ll see O again, this specific point (a range actually) where you can not see O indicates the presence of the blind spot.
The Vanishing Head Illusion:
This leads to some interesting illusions, one of the most interesting being the so called vanishing head illusion.
As in the above figure. If the O is replaced by a head, the person would appear headless if the head falls on the blind spot.
Check the video below in full screen for best results.
View in Full Screen
We notice that Richard Wiseman on the left indeed appears headless and that field of view is filled up by the orange background when the blind spot falls. Then he does something even more interesting. He uses a black bar and moves it up and down in front of his face. Now instead of seeing the bar as discontinuous, the brain manages to show the bar as a continuous entity!