the anatomical blind spot

The area where the optic nerve connects to the retina in the back of each eye is known as the optic disk. There is a total absence of cones and rods in this area, and, consequently, each eye is completely blind in this spot. Under normal binocular vision conditions this is not a problem, because an object cannot be in the blind spot of both eyes at the same time. On the other hand, where the field of vision of one eye is obstructed by an object (windshield post), a visual target (another aircraft) could fall in the blind spot of the other eye and remain undetected.

In order to find the blind spot of the right eye, it is necessary to close the left eye, focus the right eye on a single point, and see if anything vanishes from vision some 20 degrees right of this point. The following diagram has a set of characters on the left hand side, and black circle on the right. Keeping your head motionless, with the right eye about 3 or 4 times as far from the page as the length of the red line, look at each character in turn, until the black circle vanishes.

With increasing age, the blind spot enlarges. You may find that the black circle disappears when several of the characters are looked at. The size and shape of the blind spot can be found if a large enough grid of characters is used.

The same test can be done for the left eye. Close the right eye, and look at each character until the black circle disappears.

Note that when the black circle vanishes, you see only a white background where the circle was. What happens if the background colour is different? Say, yellow.

The blind spot appears as yellow. This is interesting, because it means that, although my eye can't detect anything in the blind spot, something knows that it is surrounded by yellow, and has guessed that what is in the blind spot is probably yellow. Smart!

How smart? If a thick horizontal line is drawn through the blind spot, what happens then?

The answer, it seems, is that if the line passes right through the blind spot, whatever is making shrewd guesses about colours is also able to work out that a line going in one side and coming out the other probably continues through the middle. The black circle disappears, but the line remains.

So what happens when a pen or pencil is pushed into the blind spot? It seems that as the tip enters the blind spot, the pencil appears truncated, as if it were vanishing into something (which, after all, it is). But when the tip emerges at the other side, the visual processing system fills in the missing part between. The following animation mimics pushing a pencil into the blind spot.

The first conclusion drawn from this little experiment is that, although each eye has a blind spot, some sort of intelligence is used to give this area not only a likely colour, but also to fill in lines that pass through the blind spot - rather than just have a fuzzy grey area. The net result is that, with one eye closed, it isn't immediately obvious where the blind spot is, because it has been given a suitable colour, and even pattern, based on what is adjacent to it.

The second conclusion drawn is that what we see is not just what has appeared on the retina, but is an image that has been reprocessed, tidied up. And if the human visual cortex is able to tidy up the blind spot, then it may well be that the same is being done for the entire visual field - that what we get to 'see' is not what appears on the retina, like a photograph, but instead something which has a whole bunch of special effects added.

If so, then we can't trust our eyes. We're being given doctored information, massaged figures. The world that we see is not something out there, but a world that we invent. The world I see is my idea.

#### The Night Blind Spot

The "Night Blind Spot" appears under conditions of low ambient illumination due to the absence of rods in the fovea, and involves an area 5 to 10 degrees wide in the centre of the visual field. Therefore, if an object is viewed directly at night, it may go undetected or it may fade away after initial detection due to the night blind spot.

#### The Fovea

The fovea is the small depression located in the exact centre of the macula that contains a high concentration of cones but no rods, and this is where our vision is most sharp. While the normal field of vision for each eye is about 135 degrees vertically and about 160 degrees horizontally, only the fovea has the ability to perceive and send clear, sharply focused visual images to the brain. This foveal field of vision represents a small conical area of only about 1 degree. To fully appreciate how small a one-degree field is, and to demonstrate foveal field, take a quarter from your pocket and tape it to a flat piece of glass, such as a window. Now back off 4 1/2 feet from the mounted quarter and close one eye. The area of your field of view covered by the quarter is a one-degree field, similar to your foveal vision.

Now we know that you can see a lot more than just that one-degree cone. But, do you know how little detail you see outside of that foveal cone? For example, outside of a ten-degree cone, concentric to the foveal one-degree cone, you see only about one-tenth of what you can see within the foveal field. In terms of an oncoming aircraft, if you are capable of seeing an aircraft within your foveal field at 5,000 feet away, with peripheral vision you would detect it at 500 feet. Another example: using foveal vision we can clearly identify an aircraft flying at a distance of 7 miles; however, using peripheral vision (outside the foveal field) we would require a closer distance of .7 of a mile to recognize the same aircraft. That is why when you were learning to fly, your instructor always told you to "put your head on a swivel," to keep your eyes scanning the wide expanse of space in front of your aircraft.
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