Throughout the evolutionary history of our eyes, nature kind-of screwed up at some point. Instead of having the optical nerves that lead to the brain processing centre come from behind the photoreceptor cells in the retina (like for instance in cephalopods), they start from the inside of the retina meaning that there are no photoreceptors at that point. This is the case for all vertebrates and creates a blind spot in our field of vision.

We can only detect our blind spot with our one eye closed. This is because, as seen in the picture, the blind spots fall in opposite angles in our field of vision, thus allowing each eye to cover up the blind spot of the other.

Even with our one eye closed though, our brains are good enough to “guess” what the information on that blind spot is based on the surroundings, and fill in the gaps pretty comfortably and with us never really noticing!

Here is a picture that will help you spot your blind spot (he he he). To do this, close your left eye, place your head about 25-35 cms away but in front of the big black circle on the right, and slowly start moving your right eye along the block of circles on the left side, focusing on them one at a time. At some point, you will notice that the big black circle has disappeared! If you keep moving your right eye further and further, it will come back on your vision field again.



There is also a good description and a number of other blind spot tests here and here.