This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

Hallucinations, Illusions, and Perceptions

Most people never think about the fact that our perceptual processes are synthetic, assembled by the brain. We only reflect on this when we consider dreams, illusions, and hallucinations. Then the constructed aspect of perception become obvious. The fact that we can treat normal waking perception as veridical (faithful to the outside world) is a tribute to its usual accuracy. Generally speaking, we can trust what our perceptual systems tell us.

What is a hallucination? Can ordinary, sane people have hallucinations while awake?

Hallucinations in general show the creative nature of perceptual events. A hallucination is a perceptual event that is accepted as real although it does not correspond to reality. By this definition, a dream is a hallucination, because usually a dreamer believes in it while asleep. Hallucinations also occur during waking states, even in normal people. You might think you hear someone calling your name, when nobody called. That is a simple auditory hallucination. Or you might glimpse a pair of pants left on a chair, briefly mistaking it for a person sitting on the chair. That is a visual hallucination. People often "see something out of the corner of the eye" only to turn, look more closely, and realize it is something entirely different.

Hallucinations tend to be one-of-a-kind, individual, private events. It is hard to reproduce them or do research on them. Highly hypnotizable subjects can be induced to hallucinate vividly as if dreaming, but that is rare even among those who can be hypnotized. Long-lasting or persistent hallucinations during wakefulness are very abnormal and typically indicate a serious brain disorder such as schizophrenia.

Why is it easier to do research on illusions than hallucinations?

Different from a hallucination is an illusion. Illusions are persistent errors produced by an environmental stimulus. You can stare at an illusion and it does not go away. Different people can see them while awake and alert. Cultural background and worldview do not affect the majority of visual illusions. Even non-human animals see illusions much the way humans do, as nearly as this can be determined by clever testing conditions.

Fraser's "twisted cord" illusion

The illustration above shows Fraser's "twisted cord" illusion from 1908. The figure looks like a spiral, but actually it consists of concentric circles. If you trace around one of the circles, you can prove to yourself it is not actually a spiral. What makes it look like a spiral? Each circle consists of repeated segments that draw your eye inward (clockwise) or outward (counterclockwise) so your eyes tend to jump from ring to ring much as they would if following a spiral pattern. However, even after you know it is not a spiral, the illusion persists.

Illusions and hallucinations produce brain activity visible on brain scans. The activity is what one would predict from the person's perception and experience, not from what is actually present in the outside world. For example, hallucinated voices are accompanied by activity in the speech and auditory areas of the brain. An illusary triangle (one which appears to be a triangle although no contours are present) produces the same brain activity as a genuine triangle with contours. One would expect Fraser's illusion to produce the same brain activity as perception of a genuine spiral rather than concentric rings.

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Copyright © 2007-2011 Russ Dewey