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Santa Cruz "Mystery Spot" Explained

By Patricia McBroom, Public Affairs
posted September 9, 1998

Magnetic anomalies emanating from the Bermuda Triangle. Anti-gravitational forces caused by UFOs. An "ineffable, natural phenomenon that cannot be described or explained."

Supernatural gabble gushes from the house of illusions in Santa Cruz known as the "Mystery Spot," one of more than a dozen places in the nation where stunning visual illusions compel people to reach for metaphysical explanations to describe their experiences.

At Santa Cruz's "Mystery Spot," balls roll uphill, chairs sit on walls and people lean over so far they can't see their shoes, yet they don't fall down.

Nineteenth-century psychologists had theories to explain illusions like this, but the explanations left considerable room for mystery.

Now, Berkeley psychologists have generated a new theory based on experimental data that goes much further in explaining all the effects of the phenomena known collectively as "the mystery spot."

Central to their thesis is a new emphasis on the human need to establish horizontal and vertical orientations and the extent to which people take their cues from the immediate context if they can't see the earth's horizon.

"All the visual illusions in the Mystery House derive from the fact that the house is tilted," said William Prinzmetal, adjunct associate professor of psychology. He conducted the studies with colleague Arthur Shimamura, also a psychology professor.

"You know the house is tilted, but you don't know how much. Everything is tilted. You can't look outside and get a horizon, so you think that what you see is right. It's very compelling," said Prinzmetal, an expert on perception who has been to the Mystery Spot a dozen times. Although he has studied these illusions, he said his visual perceptions still are distorted when he goes into the house, which is tilted at a 20-degree angle from the ground.

It doesn't take a scientist to know that cockeyed rooms affect perception. If floors are slanted, for instance, people will hang pictures on a slant.

But what has not been known before is that when the perceiver's body also is tilted, the distorting impact on vision is greatly magnified -- up to two or three times the effect of slanting the visual field alone.

"In the tilted condition, you are much more affected by the immediate visual context," said Prinzmetal, who has tested dozens of subjects in a laboratory chair tilted at a 30-degree angle. In that position, he tests their ability to line up vertical dots in a slanted matrix in a darkened room where they have no clue to the true horizon. With their bodies tilted, he said, people's perceptual distortion more than doubles, compared to when they see the same matrix from a level chair.

"We are such visual animals," said Prinzmetal. "The mechanism in us that's responsible for determining the horizontal and vertical is mostly affected by what we see. If the context is screwy, that will throw off what we see as vertical and horizontal."

He said that other cues to people's horizontal orientation, such as the vestibular system in the inner ear and bodily sensations of gravity, appear to become less functional in the tilted condition, leaving visual context as the dominant cue.

Prinzmetal contends that understanding the principles of the Mystery Spot is critical for understanding other visual illusions that have remained unexplained for more than a century.

These illusions can make lines appear longer or shorter than they are, or straight lines appear curved and curved lines appear straight, among other distortions of reality.

Many of these visual illusions are also increased by sitting in the tilted chair, said Prinzmetal.

One critical application of the new research is to improve the flying of airplanes.

All cockpits carry an "artificial horizon," essentially a leveler, that pilots use when the real horizon is not visible. It is the only clue to the horizon when the cockpit is tilted -- as the chair was tilted in the laboratory. Pilots are trained to ignore the visual context of the cockpit and fasten their eyes on the leveler.

Unfortunately, they don't always follow that rule, said Prinzmetal, adding that planes have crashed because the pilot thought he was flying level when actually he was at an angle.

That is said to have happened in one story circulating through government aeronautic circles. According to the tale, the last words heard on the tape retrieved from a crashed airliner was the navigator shouting, "Look at your artificial horizon!" and the pilot responding, "I can't. It's broken!"

Prinzmetal said that scientists at the National Aeronautics and Space Administration, with new appreciation for the strength of these visual illusions, now are working to make the displays for artificial horizons much more obvious.

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