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Sensation to Perception

Where do we draw the line between sensation and perception? Where does sensory information processing (the faithful coding of environmental features by sensory neurons) turn into perception (the construction of an interpreted model of the world)?

An answer came from ingenious research by E. Roy John in the 1960s and 1970s. John used a strobe light to produce photic driving: large waves of synchronized neural activity stimulated by a strobe light, easily visible on the EEG or "brain waves."

Powerful flashes of light produce waves of activity, starting in the eyes and going through the optic nerves into the brain, then throughout the brain. If a strobe light flashes 10 times per second, a neuroscientist can see 10-per-second rhythms all over the brain.

E. Roy John trained cats to jump into one side of a box when a strobe flashed 10 times per second, the other side when it flashed 12 times per second. He was most interested in what happened when the cats made a mistake.

What would happen if the cat thought it saw a 10-per-second flash (as indicated by the direction it jumped) but the physical stimulus was a 12-per-second flash? Would the brain reflect the actual stimulus in the environment or the cat's behavior?

Soon John had his answer. When the cat made an error, the actual rate of flashing showed up only before the signal reached the brain, in the optic nerve (the bundle of nerve axons leading from each eye to the brain).

If the strobe flashed 10 times per second but the cat behaved as though it saw a 12-per-second flash, then John saw 10 per second rhythms in the optic nerve, reflecting the physical stimulus. There were 12-per-second rhythms everywhere else in the cat's brain.

In essence, the 12-per-second rhythm was the cat's interpretation of the stimulus. Keep in mind the cat had probably seen both the 10- and 12-per-second strobes hundreds of times while being trained. The cat "knew" them both. Both were familiar.

The simplest interpretation is that, when the cat made an error, it was behaving according to what it thought it saw. If it thought it saw 12 flashes per second, it jumped into the corresponding side of the box, reflecting its training.

The first part of the cat's brain showing the erroneous 12-per-second rhythm was the lateral geniculate body. That is the first brain center where visual information is processed. Before that point, in the sensory nerves coming from the eyes, the rhythm reflected the actual physical stimulus (10 per second).

How did E. Roy John use photic driving, and what did he discover?

This suggests a clear distinction between sensation and perception. Sensation is the output of sensory systems. It is the coding of receptor activity into nerve impulses in the sensory nerves.

That process occurs without detailed guidance from the brain. It tends to be veridical (faithful to the outside stimulation).

Perception, on the other hand, is a brain event. It is a complex synthesis that the brain produces, guided by information from the senses.

To summarize, the sense organs are transducers, converting outside energy into a form that can be interpreted by neurons. Perception is the organized activity in the brain built up using that sensory information.

What clear distinction did John's research suggest between sensation and perception?

Many other findings are consistent with this simple insight. Sensory information is not piped directly into the brain. It is made available through the sense organs and sensory nerves.

Perception is a larger-scale synthesis that uses sensory information, among other sources of influence. As one would expect from such a conclusion, percept­ion can be influenced by expectations, memory, and knowledge.

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) means it is generally accurate in ways that matter to us. We can usually trust what our sensory and perceptual systems tell us.

However, perceptions are not always accurate. Hallucinations 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 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.

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.

What is a hallucination? Do normal people have hallucinations while awake?

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 may indicate a serious problem such as psychosis.

By contrast, illusions are persistent. They do not go away even if studied. Different people 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 determined by clever testing techniques.

twisted cord illusion
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). Your eyes tend to move inward or outward much as they would if following a spiral pattern. Even after you know it is not a spiral, the illusion persists.

Illusions and hallucinations both produce brain activity visible on brain scans. The activity is what one would predict from the person's experience, not necessarily corresponding to the outside world.

For example, schizophrenics who "hear voices" have activity in the language areas of the brain. These are the same areas used for normal voice perception.

A subjective triangle (one that 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 pattern of activation as a genuine spiral, on a brain scan.

Write to Dr. Dewey at

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