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From Sensation to Perception

Roger Schank, an influential cognitive scientist, said simply, "We have in our minds a model of the world" (Schank, 1983, p.28). Perception is the act using the flow of information from the environment to guide the construction of a more or less accurate model of the world. 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)?

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

A possible answer to this question 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 light—easily observable in the animal's EEG (electroencephalograph). The powerful stimulus produced waves of activity throughout the nervous system. If the strobe light flashed 10 times per second, the neuroscientist could see 10-per-second rhythms throughout 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 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?

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 in the optic nerve, reflecting the physical stimulus, while the 12-per-second rhythm everywhere else in the brain, reflecting the cat's interpratation of the stimulus. The erroneous 12-per-second rhythm began as soon as the optic nerve reached a part of the brain called the lateral geniculate body, the first place in the central nervous system where visual information is processed.

What distinction between sensation and perception is suggested by John's experiment?

This suggests a clear distinction between sensation and perception. Sensation is the output of sensory systems. It is the coding of receptor activity by nerve impulses in the sensory nerves leading from the sense organs to the brain. Its origins are external to the brain. Perception, on the other hand, is centrally determined. It comes from activity of the brain that using information from the senses. Perception is a large-scale synthesis or act of construction. It can be influenced by ideas and memories as well as sensory inputs.

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