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Summary: From Sensation to Perception

Receptors are the first link in the chain of information flowing into the sensory systems. They come in four varieties, sensitive to light, mechanical displacement, chemical shape, and magnetism. Magnetoreception is used by some animals during annual migrations.

The senses are optimized for each type of energy. They are about as sensitive as they can be without creating problems for the organism. Each sensory system responds most sensitively to a particular type of energy called the specific stimulus for that system.

Light is electromagnetic energy. Visible light is just a tiny portion of the overall electromagnetic spectrum that includes radio and television signals, microwave radiation, and X-rays.

Within the visible spectrum, light appears brighter or more intense when it contains more energy (represented on a graph as higher waves). Hue or color of pure wavelengths depends on frequency, from red (lowest) to violet (highest).

Sound consists of pressure waves moving through the air. Like light, it is represented graphically as a wave form. In the case of sound waves, the peak of the wave represents more pressure. The valley represents less pressure.

Higher frequencies of sound are perceived as having a higher pitch. A louder or more energetic sound is represented as a taller wave, in the graph of a sound wave.

Timbre is the word used to describe unique tonal quality of a sound. It corresponds to the distinctive mix of frequencies in a sound.

E. Roy John was curious about where sensation (the faithful encoding of external stimuli) ends and perception (the construction of a response in the brain) begins. He arranged a clever experiment in which cats had to decide whether a strobe was flashing at 10/sec or 12/sec frequencies.

John was interested in tracing the stimulus created by the strobe (a rhythmic wave) as it rippled through the perceptual system. He was particularly interested in what happened when the cats made a mistake, indicating they thought a 10/sec strobe was 12/sec, or vice versa.

John discovered as soon as the stimuli entered the brain the frequency of the wave seen in brain tissue reflected the cat's experience, reflected in its behavioral decision, not the actual frequency of the stimulus. Sensation occurred in the sense organs. Perception occurred in the brain.

Classical psychophysics was devoted to calculating the strength of a stimulus needed to make its way into conscious­ness (the absolute threshold) and also the amount of change that could be detected (the difference threshold).

Weber and Fechner (in the early to mid 1800s) and Stevens (in the mid-20th Century) provided three different approaches to graphing the relationship between external signal strength and human experience of signal strength.

As it turned out, psychophysical functions described how receptor cells and nearby neurons coded the intensity of external stimuli. The psychophysical functions showed the relationship between external signal strength and nerve impulses produced by sense organs.

After WW II, the Theory of Signal Detection was developed, to try to improve the reliability of information transmission in systems like telephones. It revealed that people could adjust their thresholds (their criteria for saying "Yes, I detect a stimulus") depending on whether it was important to avoid false positives or false negatives.

Next came a major insight. By putting people through a range of biasing conditions, asking them first to minimize false positives, then false negatives, scientists could filter out the effects of bias and produce a pure measure of observer sensitivity, called d-prime (d').

D-prime turned out to be incredibly useful. In any situation where people have to make a Yes/No judgment, such as recognition memory, the d' statistic can be used to describe the true sensitivity of the observer.

Research using d' showed that acupuncture did not alter sensitivity to pain; it changed people's bias so they reported less pain. Likewise, hypnosis did not improve memory sensitivity; rather, it changed people's bias toward accepting and reporting a memory.

Write to Dr. Dewey at

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