On November 7th about a dozen United Airlines employees, including a few pilots, claim they saw an unusual disc-shaped object in the sky over O’hare International Airport, Chicago. The witnesses insist that what they saw was not a terrestrial aircraft of any kind, that it was anywhere from 6-24 feet in diameter, and that after hovering for a few minutes it vanished at impossible speed through the clouds.
Should we find it compelling that all of the witnesses insist that what they saw was real and that it was clearly visible? Should we lend credibility to the fact that some of the witnesses were airline pilots? How about the fact that the witnesses all seem sincere and were emotionally affected by the experience?

No. Because despite all this the witnesses suffer from a critical flaw that makes them unreliable as eye witnesses. They are human.

The human visual system (although by far not the best in the animal kingdom) is a remarkable instrument indeed. But it is far from perfect. It is very much unlike the passive process by which film creates an image when exposed to light. It is something more like a digital camera, where the light strikes a surface that creates an electrical signal that is then processed into an image. But the visual processing of the human brain is far more complex.

Moreover, the purpose of vision is not just to create a static image of the world, but a dynamic image with processing that allows us to estimate distance, relative size, and movement. Further processing adds the perception of color, three-dimensionality, and enhances contrast. Finally, higher order visual processing gives meaning to the images that we see, fitting them to known patterns and assigning them conceptual and emotional content.

Our brains use visual cues to make reasonable judgments about what we are seeing, and then fixes those judgments in place, assigning context and meaning to the images accordingly. So when the visual cues suggest something is small and far away, it really seems far away to us. Optical illusions exploit one or more of the details of visual processing to trick the brain into seeing something it isn’t.

Once the brain makes its processing choices (right or wrong), the resulting visual experience seems as real to us as any other experience. The brain doesn’t tell us what choices it made, or how confident it is in those choices. It simply makes them, and presents the result to our awareness with utter confidence. It even fudges the details, (making them up if necessary) cheating in effect to enhance the match between the raw visual data and what it thinks it’s seeing.

A critical visual cue to estimating size, distance, and velocity is how one object appears relative to others, or a fixed background. This creates a special difficulty in judging objects against the sky – without any visual reference our brains simply do not have enough information to make good judgments. It will do the best it can, but it will likely make mistakes, perhaps profound mistakes. Small near objects may appear to be large and distant, and therefore their movement might seem to be impossibly fast, for example.

Our brains are illusion generating machines. But they can also be rational machines, in that they are designed to test what we see against our working model of reality (a process that is literally called reality-testing). So when faced with illusions – things that fail our reality testing – we can often realize that what we are seeing is a “trick of the light” or somehow not real. But when people believe that curious but strangely shy aliens are buzzing around the earth in quaintly designed flying saucers, that is the “reality” that their brains assign to optical illusions in the sky.