It is said that in the country of the blind, the one-eyed man is king. Even though this is not the point of the idiom, it seems to imply that one-eyed men (or women for that matter) should be at a disadvantage in the country of those with two eyes. Although this is essentially true, most people have gravely mistaken ideas about one-eyed vision. In this article, I will try to explain how depth vision works, as well as explain why it is that one-eyed vision is not such a handicap that some people believe it to be. Since I am practically blind on one eye myself, there will be a section entirely dedicated to how I function because of this. If you are not interested in the technical stuff, you can jump down to this section straight away.
First and foremost, perception of depth comes from cues in the visual image on the retina. There are many cues and they can be sorted into two kinds: monocular and binocular. Monocular cues only require one eye to function properly, but can be used with two eyes as well. Binocular cues only work with two functioning eyes. Let me explain some cues for each kind of vision.
Linear perspective is a phenomenon known to most people. Parallel lines converging on a given point seem to disappear into the distance.A railroad track disappearing into the horizon ought to be the most obvious example of this.
Aerial perspective is haziness because of distance. Particles in the air make distant objects look more fuzzy, losing their sharpness. In experiments, it has been confirmed that two object of the same size, viewed at the same distance from the observer, but with different sharpness of the edges, will, if using monocular vision, appear to be at dissimilar distances.
The texture of known objects is important in defining distance. Simply by knowing what an object looks like, and seeing its surface texture, the resolution of the texture is a cue in itself. A higher resolution of course means that the object is closer to the observer.
Another cue is familiarity of size. Placing two playing cards at identical distances in front of the observer, one card will be perceived as being twice as far away if it is half the size of the other playing card. Another example is a football. Knowing how big a football is, the brain can assess its distance from the eyes simply by using the size of the representation of the football on the retina, and comparing it to the known size of the ball.
Motion parallax is what occurs when there is movement, either by the object or the observer. If two objects are placed at different distances from the observer, and the observer moves, the images of the two objects will move over the observer’s retina. Closer objects will move much more, which gives the brain information about their depth location.
Convergence of the eyes can be used to perceive depth. Focusing on very close objects, the line of sight from each eye converges on the object and this angle is used as a cue.
Stereopsis is the most important binocular cue and makes use of the fact that, since the eyes are placed slightly apart, the retinal images are slightly different. This means that the brain will be able to triangulate the distance to an object, i.e. compare the disparity between the two objects and calculate the distance.
In a laboratory, it is possible to eliminate most of these cues to test only one or two in order to see how the brain is able to perceive depth when lacking other cues. In reality, however, this is rarely the case. Instead, there is a plethora of different cues available (often all of those mentioned above, plus some more I have omitted). This means that the brain has to be able to integrate conflicting cues, because information from all of them will not match perfectly at all times.
Experiments have shown that the brain makes use of all available cues and add them together to give a perception of depth. This is sound from an evolutionary perspective, because relying on one of the cues alone could prove to be fatal. Relying on all of them simultaneously is much more secure and is bound to produce better depth perception.
However, it is noteworthy for this article that the binocular cues only are powerful at very close distances.This is obvious, because the farther away from the observer an object is located, the less the disparity of retinal images and the less the convergence of the eyes. Even for close distances, depth perception is mostly reliant on monocular cues. Having two eyes is important in some situation, but it is not essential. From an evolutionary perspective, having two eyes is also a case of redundancy and backup. Let me now turn to some personal examples.
No, having only one functioning eye is not a handicap. In fact, I do not think that is a disadvantage at all, save for some very specific situations. Some of my problems arise because my vision on my seeing eye is flawed as well, but that has got nothing to do with one-eyed vision or this article.
So, what can I do that some people think ought to be difficult? Most things. For instance, I played badminton for two years. To be certain, I was far from being the worst player. Sometimes I experience problems, but not enough to make it a serious disadvantage. I have little problem with other sports including balls, such as volleyball, football or anything like that. I can juggle as well, even if throwing balls over my left shoulder is difficult. Practicing martial arts is not a problem (I once heard someone say that it would be difficult to practice because a one-eyed person would be unable to hit someone, because he or she would lack depth perception; this is of course utter nonsense). If you want a rule of thumb, assume that one-eyed people can do everything two-eyed people can do. You will find exceptions, but much fewer than if your would inverse the rule.
So, what can I not do? I think size and familiarity is of paramount importance. In new situations with unfamiliar objects, it might take me a while to get used to them. I would probably make a crappy waiter, because judging relative distance between glasses and bottles when pouring is sometimes difficult (at least in unfamiliar situations, this is never a problem with known objects). Thread a needle might also cause problems, but this could be because of my bad vision in general and not because of my having only one relatively good eye.
Sometimes, people pose a question I find both flattering and frustrating: If you are as good at sports as you are now, how good would you be if you had perfect vision? This is flattering, because it means that people perceive me as being good in the first place, regardless of bad vision and only having one properly functioning eye. On the other hand, it is also frustrating, because it is theoretically impossible for me to have stereo vision (since I was blind at birth, my brain has not developed the necessary functions for my left eye). I will never find out what the answer to that question is, which makes me frustrated at times. However, I am perfectly satisfied with my situation most of the time. I have been endowed with other qualities and have no reason at all to complain of my lot. So being king in the country of the blind is not such a bad thing after all.
Eysenck, M.W., Keane, M.T. (2005). Cognitive Psychology: A Student’s Handbook. Hove: Psychology Press.