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

Psychophysics and Signal Detection

Now that we have reviewed the basic operation of the most commonly studied senses, we are in a better position to appreciate one of the oldest branches of psychology: psychophysics .

Classical Psychophysics

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

In the early 1800s, a group of scientists hoped to use scientific method to answer the question of how we perceive the world. They called their discipline psychophysics because it was aimed at understanding the interaction or interface between the physical world and the psychological world. Psychophysics was the first branch of science to use experimental methods to study mental responses.

Psychophysicists wanted to investigate the relationship between physical stimuli and internal experience. They began their research in the 1820s by measuring how strong a stimulus must 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.

How did researchers define the absolute threshold or limen?

Like a threshold underneath a door, a psychophysical threshold was supposed to be a boundary between outside and inside. However, researchers soon found out matters were not so simple. The threshold was not always at the same point. Sometimes people were more sensitive than at other times. So the researchers decided 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).

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

This definition of threshold varies from the popular conception of a threshold as an all-or-none point. If we speak of a sound so weak it is "below the threshold of hearing" most people would assume nobody can hear it. However, by the definition just outlined, people might often hear such a stimulus nearly half the time.

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."


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