An introduction to psychophysics

In the third of our ongoing series of guest features for students, Dr. Tom Stafford of the University of Sheffield introduces psychophysics.

How far away can you see a candle at night? Why can't you see it at the same distance during the day? How much do I have to turn up the volume before something seems twice as loud?

All these questions are about measuring the relation between physical qualities and the psychological impressions they cause. Psychophysics is the part of psychology which involves the systematic and precise investigation of these relationships.

Founded in the laboratory of German Gustav Fechner, psychophysics is one of the parents of modern experimental psychology. It demonstrated that mathematical analysis could be applied to subjective reports, and that principled relationships could be discovered between physical quantities and subjective impressions.

Let's take a close look at a famous example: Weber's Law, named after Ernst Weber, a colleague of Fechner's. This formula describes how changes in the subjective perception of stimulus intensity (e.g. how heavy a weight feels) are related to the actual change in stimulus magnitude (how much something actually weighs). You can look up the mathematics of this if you're interested, but a plain-language interpretation is that to increase the perceived intensity of a stimulus you need to increase its physical magnitude by a constant proportion, not a constant absolute amount.

Imagine: you can make an empty bag feel heavier by putting in a book, but a single book won't make a bag full of bricks feel heavier, even though in both cases you are adding the same amount of weight. Weber's Law gives you a mathematical way to calculate how much you would need to increase or decrease the physical weight to produce a subjective impression of a change in heaviness. It also allows you to compare sensitivity between the senses – showing, for example that we are more sensitive to brightness than loudness, because the proportional change needed to create a noticeable difference for lights is smaller than that needed for sounds.

As well as discovering many of the few laws that exist in psychology, psychophysics has generated methods and theories which are applied across all of experimental psychology, not just in the investigation of sensation and perception. In applying scientific measurement to subjective experience, the early psychophysicists were demonstrating a faith in empiricism, but they were also throwing themselves upon a dilemma - the attempt to relate the world of the measurable and objective to the subjective inner world of sensation. That dilemma is still just as relevant and profound today in all areas of psychology, and psychophysics is still vital as a toolkit for addressing it.

Read an excerpt of Fechner, G. (1860). Elements of psychophysics (HE Adler, Trans.). New York: Holt, Rinehart and Winston.

Link to the psychophysics introduction by Webvision.
Check out the other articles in this ongoing series, including "Why psychologists study twins", "A lyrical guide to using the web" and "Podcasts - a clickable list". Forthcoming in the series: "Systematic reviews" and "Virtual reality and online games" - stay tuned!
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