Adelaide has been agog recently with reports of a miraculous event taking place in the small southern township of Yankalilla. The faithful have been witness to the manifestation of the Madonna and Child appearing on the rear wall of the small local Anglican Christ Church, an appearance which many claim is a great miracle.

Of course one must ask the obvious question – why should Mary, who is worshipped only by Catholics, appear in an Anglican Church? Did she lose her way on the astral plane?

Well, sorry to disappoint the believers, but the "apparition" at Yankalilla appears to be nothing more than a common type of visual illusion, what is called a simulacrum, an abstract pattern which produces the impression to people viewing it of some recognizable shape. These are quite common and can be found in or on both natural and artificial structures.

There are two types of simulacra. The first is the chiaroscura simulacrum, where abstract patterns of light and shadow combine to produce a recognizable figure or a face. The second type is where naturally occurring objects, such as rocks, have weathered and taken on a recognizable shape, such as the outline of an animal or a face.

The chiaroscura effect, by far the most common example of this phenomena, results from a complex mental process which enables us to "see" figures, or faces, in abstract or random patterns. As a result simulacra are quite common and can be "seen" in a diverse range of locations, such as patterned dies or wallpaper, in clouds, on hillsides, or in rock formations. Another very common location is amongst trees and rocks. Some examples of sylvan simulacra have been elfin faces, a sleeping puppy, and the Madonna. One can even see the simulacrum of a running man in the opening screen of Windows '95.

In 1988, in Melbourne, a simulacrum appeared on a wire fence beneath the south-eastern freeway, created by a random combination of flood debris. The fence was photographed by researchers to determine the height of water levels after a recent storm. The "face" was not noticed until the photo was turned on its side, and one of the researchers noticed what appeared to be a bearded face which looked like Jesus. There have been several examples where the chiaroscura effect has produced faces on snow covered hillsides, "faces" which were claimed to have been the "face" of Christ.

In the West such faces tend to be "identified" as Jesus, while other cultures identify them as figures from their own religious milieu. Thus, in a Taiwanese waterfall, where the combination of dark background rocks and cascading white water combine to produce what is apparently a human face, it is identified by the locals as the Buddhist goddess Kwan Yin.

There have been numerous reports of rocks whose rough surfaces have produced the appearance of human faces. Even one of the standing stones at Stonehenge has one surface which looks like a sunken "face", rather like a boxer with a broken nose.

Like the "image" at Yankalilla such abstract patterns are often found on the walls of buildings. In one example an image of a shaggy dog emerged from a newly plastered wall. The image bore a remarkable resemblance to the home owner's dog. As the plaster dried over several days the image gradually faded and disappeared.

The reason that such abstract patterns are perceived as "recognizable" images is closely related to the complex manner in which our brains process and interpret images of the external world, for, it is a fact that we do not "see" with our eyes, we "see" with our brain.

The eyes are complex receptors, which convert light from the outside world into at least four separate components, colour, depth, form and motion. These signals are conveyed to specific areas of the brain. Most travel to the primary visual cortex, but others are processed elsewhere in the brain. The decoded information is shunted between the various processing areas, combining all of the parts into a single image.

Just as a television picture is composed of thousands of "dots", so too, what we "see" is actually millions of separate pieces of information ingeniously blended together within the brain to create the impression of an integrated image.

In addition to the four "visual" elements of sight there are two other very important components of vision. The first is the memory component our ability to recall specific visual shapes and cues. Learned in early childhood, we refer to this knowledge throughout oar life.

Occasionally, however, we can encounter problems in the recognition of a shape or pattern, usually because there is insufficient detail to allow us to recognize the object. In such situations the brain compensates for the lack of detail by adding elements to supplement the missing detail until it finally produces, at least in the brain, a recognizable image.

Thus, this second component Einstellung or Mind-set is particularly important in the creation of illusions, for our visual experiences are greatly influenced by what we want to see! This process appears to be the one operating amongst those who have seen "miraculous" things at Yankalilla.

A famous example of this process involved the renowned American astronomer Percival Lowell. In 1877 the Italian astronomer Giovanni Schiaparelli observed marks on the surface of Mars, which he described in his reports as canali. This Italian word can mean either "channels" or "canals". Unfortunately, the word was translated into English as, "canals", a term which suggested artificial waterways. This error was enthusiastically embraced by Lowell who earnestly believed there was intelligent life on Mars.

Eager to provide additional evidence, Lowell commenced a vigil which was to occupy him for the rest of his life. He spent virtually every night peering through his telescope at the distant image of Mars, looking for these canals. Even though for most of the time the image of the planet was indistinct and blurred, he was convinced that he could see these canals. To him they appeared to wax and wane with the "Martian seasons". Because telescopic images were too poor to record on film, he carefully sketched these constantly changing "seasonal" patterns.

Although he produced hundreds of sketches of these "canals" other astronomers who looked at the Martian surface were unable to see these canals. The matter was finally resolved this century when the first interplanetary probes finally reached Mars. The pictures which they transmitted back to Earth showed no evidence of any Martian canals. They simply did not exist. They had existed only in Lowell's mind.

In similar fashion, during the Crusades, after Saladin recaptured Jerusalem, it was widely reported that many Christians witnessed that crucifixes and images of the saints shed tears of blood.

In more recent times during the infamous paranormal research programme, Project Alpha, subjects were asked to project thought-images onto the film in an 8mm movie camera. One subject produced quite spectacular results,

The subject of this particular experiment later admitted that he had produced this inexplicable "swirl" simply by spitting onto the lens of the camera, and allowing the camera to record the shapes made by the saliva as it dribbled down the lens. The wonderful things which the researcher "saw" were merely the products of his own subjective, and obviously extremely fertile, imagination.

Brierre de Boismont, in his book, Hallucinations: or, The Rational History of Apparitions, Visions, Dreams, Ecstasy, Magnetism, and Somnambulism mentioned an example of Mind-set. Walter Scott who had only shortly before received news that his friend Lord Byron had died, was in his library thinking about his friend, recalling past experiences with Byron, when,

Scott was especially struck by the precise accuracy with which his imagination had reproduced every detail and peculiarity of the clothing of the dead poet.

Illusions such as the one at Yankalilla are simply optical aberrations. The one at Christ Church is of extremely poor quality. Proper simulacra – even though they may take a little while to "see" them – usually produce a quality representation of the subject.

I have studied the newspaper photographs and television reports of this particular image and, despite the claims of the local minister that he can see the Madonna's eyes and other fine details, I am only able to see the most rudimentary outline.

I get a much better image when I turn the picture upside down then I can quite clearly see the Phantom's skull cave.

It seems that I am not the only one unable to see the miraculous figure. A number of people interviewed in the media admitted that they too were unable to see anything on the wall, or that all they can see is an indistinct outline which could be the Madonna and Child, or "anything else."

The evidence suggests quite strongly that the observers are unconsciously creating their own visual delusions; transforming what is simply a piece of rough plaster into something miraculous.