Sightings: UFOs and Visual Perception

July 1998
by Mark B. Fineman, PhD

PENDELTON, Ore. June 25 [1947] (AP) – Nine bright saucer-like objects flying at “incredible speed” at 10,000 feet altitude were reported here today by Kenneth Arnold, Boise, Idaho, [a] pilot who said he could not hazard a guess as to what they were (cited by Peebles, 1994).

In December 1997 a crew chief aboard a military helicopter over Long Island Sound kept a sharp FAKE01lookout as the craft cruised along at 150 knots and an altitude of 3,000 feet. He spotted a shiny metallic object dead ahead and closing fast. Before he could warn the pilot of an impending collision with another aircraft, the metallic object darted by. It was a mylar party balloon (personal communication).

The New York Times (November 19, 1997) reported the results of the FBI investigation into the tragic loss of TWA Flight 800. Although 244 witnesses had reported seeing ascending lights immediately before the explosion, the investigators found no evidence of a missile strike or sabotage. On November 21, James Kallstrom, the head of the FBI probe, appeared on the Bob Grant WOR radio program. He was flooded with inquiries from irate callers who demanded to know how 244 eye witnesses could be wrong.

How could an experienced pilot so precisely describe the speed and altitude of “flying saucers?” How could a veteran crew chief mistake a mylar balloon for an airplane? How could 244 eye witnesses have been in error? Easily. For while human visual perception is remarkable in its breadth and precision, it is neither infallible nor unlimited. Of particular importance to these accounts is the role of depth perception.

Even skeptical examinations of UFO sightings can be superficial in their treatment of visual perception. Consider, for example, Klass’s (1981) comment that, “..the human perceptual system is such that it can play wild tricks on us under conditions of darkness, isolation, fatigue, or unusual stimulation.” Klass’s contention, like all of those that maintain a person’s eyes (or his brain, or his mind) can play tricks on him is superficial precisely because it portrays the person’s “perceptual system” ( or his eyes, brain, or mind) as somehow separate from the rest of his body. Thus, when visual perception doesn’t work quite right, it is explained away as a kind of disagreeable practical joke.

To be sure, we all understand, at least in a general way, the gist of the trickery thesis. We have all had moments when we have made erroneous observations. And yet, the usual reliability of our observations is made the more striking because of the infrequency of error.

Constancy and Illusion

Visual perception is noteworthy as a biological and behavioral process because of its truthfulness. In our everyday lives, the world around us appears stable, as indeed it is. This property of perception, called constancy, means that the way we judge an object is usually in agreement with a physical measurement of that object (see, for example, Matlin, 1988). Constancy is somewhat surprising since the representation of an object on the retina is usually in flux. For instance, as one moves around while viewing a fixed object, such as a table, the retinal image of that object may shrink or grow, its overall shape may change, as well as its location on the retina. Nevertheless, the table appears fixed in its location, its size remains constant, as does its shape, color, and so on.

Constancy is undoubtedly a good thing. How else could we navigate our world if it seemed to change capriciously? Because we are endowed with a stability of perception, we can make hypotheses and predictions about our surroundings and behave accordingly.

Students of perception have also been struck by exceptions to constancy, referred to as illusions. In this narrow sense of the word, an illusion is not a trick or an hallucination. It is simply a lawful exception to constancy, an instance when someone’s judgment of an object is not in agreement with a physical measurement of that object. It is a recognition that perception, strictly in its role as a measuring instrument, is sometimes in error. Hundreds of visual illusions have been identified, but one or two will suffice to make the point. Consider geometric illusions, which are drawings of lines and angles that are invariably misjudged by everyone. In the Poggendorff illusion, for example, the two collinear diagonal lines appear misaligned (they must actually be made physically misaligned in order to appear collinear)! In the corridor illusion, two objects contained within a drawn corridor appear to be unequal in size even when a ruler indicates otherwise. The general nature of perceptual constancy and illusion are well understood. These principles apply to the three examples given at the beginning of this article.

A Constancy Primer: The Importance of Depth

The visual system, by which is meant the eyes and visual portions of the brain, take several variables into account in order to maintain the stability of the world around us. Among the constancies that are of particular relevance to the present discussion are those of size, shape, and velocity. Trying to untangle the puzzle of perceptual constancy has absorbed the efforts of researchers for more than a century. What follows is a brief introduction to some basic principles:

Euclid understood that by computing a small number of variables, such as the visual angle of an object, the diameter of the eyeball, and the distance to the object, the judged size of an object could be predicted. This geometrical relationship is called Euclid’s Law (Hochberg, 1971). Euclid’s theoretical formulation was given empirical support in laboratory studies, beginning with that of Holway and Boring (1941), in which the ability to judge object size was measured under various viewing conditions, including one in which distance information was effectively eliminated. With distance information present, subjects could accurately estimate the sizes of target disks. But when distance cues were eliminated, size constancy failed, and observers were reduced to judging size strictly on the basis of retinal image size. Therefore it is clear that for an observer deprived of distance information, a looming or receding object might well seem to grow or shrink. It is also entirely possible that a fixed object might appear to loom or recede from the same observer every time the observer himself approached or retreated from the object.

In summary, there is a convincing body of evidence that shows that the perceived size of an object is critically dependent upon the ability of an observer to judge its distance. Additional factors, like learning or past experience, may also influence size perception. It is also known that perceived size acts reciprocally to distance, i.e., the perceived size of an object is a source of information about its distance.

In normal circumstances the shape of an object is likewise reliably apprehended even though the shape of its retinal image may vary considerably from the actual shape of the object. Consider the elliptical retinal image of a circle viewed at a slant or the trapezoidal image shape of a square viewed at a slant. Here again, the distances to locations on the object are often taken into account, perhaps with learned properties of the object also contributing to shape constancy (Fineman, 1996). Notice how easy it is to misinterpret object shape in the absence of distance cues: Klass (1981) comments that luminous advertising signs carried aloft by airplanes or dirigibles are routinely reported to be cigar-shaped or otherwise elliptical. The relationship between depth and shape remains obscure, however. Even when depth information is knowable, judgments of object shape may persist in being pliable (Pizlo & Salach-Golyska, 1995).

Finally, there is constancy of velocity. As a moving object is viewed at greater and greater distances, its rate of retinal velocity decreases: The image takes longer and longer to traverse the retina. In fact, at very great distances the rate of retinal velocity is so diminished that a moving object may well appear stationary (celestial objects actually move but are perceived as motionless). At closer distances we usually perceive object velocity correctly even though retinal image velocity may fluctuate. Changes in retinal image velocity may also occur as the observer moves toward or away from the moving object.

Brown (1931) found that the constancy of velocity is attributable in large measure to a frame-of-reference concept. We tend to judge the velocity of an object in relationship to its surroundings. A car within a landscape, moving from one location to another at a fixed velocity takes a certain amount of time to make its passage, and that time to traverse the frame of surrounding objects does not change when viewed at different distances. But what would happen if distance information were drastically reduced or eliminated? Rock, Hill and Fineman (1967) found that in the absence of distance cues, velocity constancy vanishes and an observer is once again reduced to judging object velocity entirely on the basis of retinal image events, namely, retinal velocity. Indeed, to the observer deprived of distance cues a stimulus moving at a fixed rate of speed might seem to accelerate or slow down as the observer moved toward or away from the moving stimulus.

The Unreliable Expert Witness

What emerges from the substantial literature on constancy is the fact that when distance information is impoverished, the accuracy of perceptual judgment declines accordingly. This is true for all observers since the basic nature of perception is both lawful and universal. There is no a priori reason to believe that so-called expert or reliable witnesses are any better under these viewing conditions than anyone else. A policeman or airline pilot is as susceptible as you or I to the perceptual illusions that occur when depth perception fails. Hynek’s (1972) repeated emphasis on the reliability of UFO witnesses is therefore beside the point.

Eyewitness accounts of UFOs are predominately of the type Hynek (1977) called “nocturnal lights,” followed by the less frequent “daylight disks.” Since these are observations in the real world, it might reasonably be asked how reliable distance perception is under the circumstances. Bear in mind that whether seen in daylight or darkness, these are reports of amorphous objects in the sky. And that open sky offers precious little information about distance.

Briefly, human beings and other species depend upon a number of sources of information, widely referred to as depth or distance cues, to judge the distances to objects. Some of these are physiological depth cues, like the convergence of the eyes as objects approach the observer, changes in the shape of the lens for objects at different distances (called accommodation) and stereoscopic depth perception. The range of distances for which accommodation and convergence are effective is quite limited, only a few meters (Hochberg, 1971). Stereopsis may function to a distance of 1,100 meters or so (Gogel & Mertz, 1989).

Pictorial depth cues are more contextual. They are called pictorial because they were first described by renaissance artists as a means to depict depth in paintings, but the same sources of information are also present in the retinal images. Included here are cues familiar to almost everyone: linear perspective, changes in texture, relative size, the partial occlusion of one object by another (interposition), and so on. Organisms also infer distance from the motions of objects and their color and location in the visual field. For a fuller discussion of depth cues see Gregory (1972), Kaufman (1974), or Fineman (1996).

Sedgwick’s (1986) splendid analysis of space perception examined the variability of distance perception. He observed that, “The variability of observers’ estimates of distance under reduced conditions is very high… It seems likely that as efforts to eliminate information for distance are made more and more thorough, observers become more and more responsive to subtle influences on their assumptions and criteria that may not be under the deliberate control of the experimenter.”

It must also be pointed out that in the absence of depth information, objects tend to be seen as equidistant (Gogel and Mertz, 1989). Two ambiguously distanced objects, regardless of their objective distances from an observer, will be seen as equally distant from the observer. A light seen near the horizon, for example, tends to be reported at the same distance as the terrain along the horizon. A bright planet, luminous space debris, or weather balloon will be seen as equidistant to terrestrial features, like mountains, buildings, or the horizon. The distance error could easily warp judgments of size, shape and velocity.

Now re-examine the three examples given at the beginning of the article. Could an experienced pilot have mistaken bright lights juxtaposed against a mountainous terrain as a formation of fantastically speeding objects at an altitude of 10,000 feet? Could a veteran crew chief have mistaken a mylar balloon for an impending collision with another aircraft? Could 244 sincere eye witnesses have all seen evidence of a missile strike that never happened? Absolutely.


1) Brown, J. F. (1931). The visual perception of velocity. Psychologische Forschung, 14, 199-232.
2) Fineman, M. (1996). The nature of visual illusion. Mineola, NY: Dover.
3) Gogel, W. C., & Mertz, D. L. (1989). The contribution of heuristic processes to the moon illusion. In M. Hershenson (Ed.), The moon illusion. Hillsdale, NJ: Lawrence Erlbaum.
4) Gregory, R. L. (1972). Eye and brain (3rd Ed.). New York: McGraw-Hill.
5) Hochberg, J. (1971. Perception II. Space and movement. In J. W. Kling & L. A. Riggs (Ed.). Woodworth & Schlosberg’s experimental psychology (3rd Ed.). New York: Holt, Rinehart, & Winston.
6) Holway, A. H., & Boring, E. G. (1941). Determinants of apparent size with distance variant. American Journal of Psychology, 54, 21-37.
7) Hynek, J. A. (1972). The UFO experience. Chicago: Henry Regnery.
8) Hynek, J. A. (1977). The Hynek UFO report. New York: Barnes and Noble.
9) Kaufman, L. (1974). Sight and mind: An introduction to visual perception. New York: Oxford.
10) Klass, P. (1981). UFOs. In G. O. Abel & B. Singer (Ed.), Science and the paranormal (pp. 310 – 328). New York: Scribner’s.
11) Matlin, M. W. (1988). Sensation and perception (2nd Ed.). Boston: Allyn and Bacon.
12) Peebles, C. (1994). Watch the skies. Washington, DC: Smithsonian
13) Pizlo, Z., & Salach-Golyska, M. (1995). 3-D shape perception. Perception & Psychophysics, 1995, 57(5), 692-714.
14) Rock, I., Hill A. L., & Fineman, M. (1967). Speed constancy as a function of size constancy. Perception and Psychophysics, 4(1), 37 – 40.
15) Sedgwick, H. A. (1986). Space Perception. In K. R. Boff, L. Kaufman, & J. P. Thomas (Ed.) Handbook of perception and human performance, Vol. I: Sensory processes and perception. New York: Wiley.

The following are Letters To The Editor concerning this article


As anecdotal evidence seems so emotionally compelling, may I contribute an anecdote to underscore some points made by Mark B. Fineman in his excellent “Sightings: UFO’s and Visual Perception?” There is no UFO in my story–or not exactly.

During high school I lived in Times Beach, Missouri (yes, that Times Beach) and the adjacent town of Eureka. Off rural Highway 109 was a lonely place my friends called “Zombie Road.”

Really named Lawler-Ford, it was a narrow lane winding through miles of woods, once a route for trucks hauling stone from a remote quarry. Even by day it seemed enveloped in a dreamlike silence and half-light. You could never see past the trees and brush at the next curve. If you met another car, one of you would have to back up to one of the few wide places, or to the beginning of the road, in order to pass.

We agreed that the spookiest thing about Zombie Road was that it never looked the same shape or seemed the same length twice, even on the return trip from the turnaround at the chained entrance to the stone company’s property. At times we had the claustrophobic feeling that it would never end and that we would drive on forever into deeper darkness and silence.

We especially enjoyed scaring ourselves by driving it at night and repeating the usual urban-legend horror stories about teenagers on isolated roads. Of course the villain of the tales was always “the Zombie” who waylaid young lovers on Zombie Road.

One night we parked at the turnaround and ventured from the car to explore a “haunted shack” supposedly nearby. We were picking our way cautiously through starlit weeds when a light appeared about twenty feet before us. It seemed as bright as a single headlamp, but advanced at chest-height through high weeds. My first thought was that we had been caught trespassing and would be ordered to leave. But no warning came. In fact, the silence was unnerving. The light glided toward us through the weedtops as smoothly and as soundlessly as if it were on an invisible track. Most of us stepped back, staring.

But two of our party had whispered, “Let’s get out of here!” and were already stumbling back toward the car. Their oddly efficient panic rather forced us to accompany them, and soon we were running from the slow, silent light, which reached the road behind us just as we slammed our doors and drove away.

The return trip on Zombie Road was short that night, though we had a sober moment passing an apparently unoccupied old car that was pulled off the road. (It had not been there before.)

Later we learned (what most people reading this will have guessed) that the entire adventure had been staged. The two people who fled in panic had organized the dramatic scene. The surprise was that, in their eyes, it had utterly failed. They had planned terrors for us. Two sturdy young men had parked up the road (it was their car we saw) and sneaked round through the woods to surprise us. They had costumed themselves as the Zombie and his henchman, both brandishing weapons. They had growled and grunted and crashed as noisily as possible while pursuing us at top speed through the dry brush. To make sure they were seen, one of them had waved a flashlight.

Yet we had registered none of it–not the violent speed, not the noise, not the weapons, not the scary disguises. In fact, we had all been impressed by the unnatural silence of the free-floating light. Though we witnesses (even the conspirators) all agreed about what we had seen and heard, and even though we considered ourselves to have been in a state of heightened alertness at the time (uninfluenced, I should add, by alcohol or other drugs), we had in effect been unable to hear or see, let alone judge speed, scale or distance in any meaningful way.

It is not from disrespect that I doubt other people’s accounts of anomalous experiences. It is in part because I have learned firsthand how unreliable my own perceptions can be.

Gary Brockman
Madison, CT