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Psychophysics

In the early 1800s, a group of scientists hoped to use the scientific method to answer the question of how we perceive the world. They called their discipline psychophysics because it was aimed at understanding the interface between the physical and psychological world.

Psychophysics was the first branch of science to use experimental methods to study mental responses. As such, it is an early ancestor of psychology, although it thrived decades before Wundt founded the first experimental psychology laboratory in 1879.

Psychophysicists wanted to investigate the relationship between physical stimuli and internal experience. They began their research in the 1820s by measuring how strong stimuli had to be, in order to be detected.

A stimulus too weak to perceive was called below threshold. A stimulus strong enough to detect was said to be above threshold. The threshold was conceived as the dividing line between events too weak to detect and events strong enough to detect.

Like a threshold underneath a door, a psychophysical threshold was supposed to be a boundary between outside and inside. However, researchers soon found that matters were not so simple.

What was the primary concern of the psychophysicists? How did they conceive of the threshold?

Sometimes people were more sensitive than at other times. Noise and other environmental conditions made a difference. Researchers had to use an approximation.

The threshold was declared to be the point at which a very weak stimulus could be detected 50% of the time. Technically this is called the absolute threshold or limen (pronounced LY-man).

How did researchers define the absolute threshold or limen?

The limen is the absolute threshold, so a subliminal (sub-LIM-in-al) stimulus–literally one that is below the limen–can be detected up to 49% of the time. Of course, this is not what people mean when they use the term subliminal in most cases. They mean "undetectable."

What is a literal definition of "subliminal"? What is the more common meaning?

 

The Difference Threshold (JND)

Another type of threshold is the difference threshold. The difference threshold is the smallest change in a stimulus which a person can detect 50% of the time.

The difference threshold is determined by changing the stimulus varying amounts. The subject is continually asked whether he or she can detect any difference in the stimulus. The smallest change a person can detect 50% of the time is the difference threshold or just-noticeable difference (JND).

What is the difference threshold? The JND?

In testing for just noticeable differences (JNDs) experimenters must use catch trials during which no change is made. Sometimes a subject will claim to perceive a change even when none occurs (during a catch trial).

People often perceive what they expect to perceive. If you tell a person to listen for a change in a sound, that person might claim to hear a change even if none occurred.

A student discovered this when his roommate bought a new stereo system:

After we studied sensitivity to changes in pitch and loudness of sound I executed an experiment on my unsuspecting roommate. He recently brought a very powerful and expensive amplifier from home to attach to my stereo receiver.

That night when he came in I told him it was all connected, and I turned it on. The lights all came on and the radio played.

About five seconds later he re­marked on how great it sounded and how much clearer it was. He was really excited.

What really happened was that I never connected the amp with the speakers. They were still connected to his old system. I just plugged the new system in.

All the lights and meters came on but the amp wasn't doing any work. My analysis is that he expected and wanted to hear a big difference, so he did. [Author's files]

Why are "catch trials" necessary? How is this illustrated by the student's essay?

By interspersing catch trials with genuine changes in a stimulus, an experimenter can determine the smallest amount of change that an individual can reliably distinguish from catch trials. The amount of change a person can detect half the time is defined as the difference thresh­old or Just Noticeable Difference (JND).

Psychophysical Functions

Early psychophysicists found a reliable relationship between magnitude of stimuli and size of JNDs. The larger the magnitude of the stimulus (for example, the brighter the light), the larger was the just-noticeable difference (JND).

If you compare a 25-watt incandescent bulb with a 50-watt bulb, they look very different. But if you compare a 175-watt bulb to a 200-watt bulb, they look almost the same.

In general, we are less sensitive to differences in signal strength as the intensity of a stimulus increases. This relationship was first summarized by Ernst Weber (VAY-ber) in 1834. His equation was called Weber's Law.

What was the general idea behind Weber's Law?

Weber's Law states that the JND increases in direct proportion to the intensity of a stimulus. But often this is not true when stimuli approach extreme values.

old picture of Fechner
Gustav Fechner

In 1860 Gustav Fech­ner proposed a new law to replace Weber's. Fechner's Law speci­fied that psychophys­ical func­tions would follow a logarithmic relationship.

A logarithmic relationship can produce a curve that drops as the magnitude of a stimulus approaches extreme values. (The curve in the following figure labeled "brightness" shows such a relationship.)

Fechner type curves
Psychophysical curves produced by different types of stimuli

Fechner's Law provides a better fit than Weber's Law for many types of stimuli. Fechner won respect in the scientific world for his careful research.

What problem in Weber's Law did Fechner's Law address?

Some historians of science say that Fechner, rather than Wundt, launched experimental psychology. Fechner published a book 10 years before Wundt started his laboratory.

While many sensory stimuli fit Fechner's law, some do not. Electric shock produces a curve that goes the wrong direction. The line labeled "electric shock" on the graph shows this.

People become more sensitive to electricity as the magnitude of a shock goes up. The same is true of judgments of heaviness.

What was the general idea of Stevens' Law?

The American psychologist S. S. Stevens proposed a formula that accounted for all these curves, even the curve for electric shock. He suggested that the strength of a sensation was related to the intensity of a physical stimulus raised to some power.

In other words, he proposed an exponential function. If the exponent were greater than one, the line would curve up, as with the lines labeled electric shock and heaviness.

If the exponent is one, the result is a straight line as described by Weber's Law. If the exponent is less than one, the result is a downward sloping curves like those expressed by Fechner's Law.

Stevens's work was done in the 1950s and 1960s. It represented an elegant solution to the problem of defining psychophysical relationships.

However, by the time Stevens's Power Law replaced Fechner's Law, the whole field of psychophysics was undergoing a dramatic change. It was replaced by a new approach called the Theory of Signal Detection.

In retrospect, the psychophysicists were studying the very beginnings of the process by which sensations are converted into perceptions. When a tiny probe is used to monitor activity of a receptor cell or an associated neuron, the number of nerve impulses it fires matches the classic psychophysical functions.

What were the early psychophysicists studying, as it turned out? Where does it take place?

This means the psychophysicists were studying something real and biological, but it was not perception. It was not taking place in the brain.

They were studying the relationship between external stimulus strength and the number of nerve impulses sent to the brain by sense organs. For that process of transduction, psychophysical functions work well. They accurately describe how inputs to sensory receptors relate to outputs sent to the brain.


Write to Dr. Dewey at psywww@gmail.com.


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