Psychophysics and Analyses of Signal Detection

Psychophysics is the branch of psychology concerned with the relation between the physical properties of events and the sensations they give rise to. The subject has two major fields of inquiry: global psychophysics, which studies readily dis­ criminable stimuli that differ by large amounts, and local psychophysics, which studies the fine differences between barely discriminable stimuli (Luce & Krum­ hansl, 1988). Global psychophysics is concerned with psychological scaling, whereas local psychophysics is concerned with detection and discrimination. Be­ cause this chapter is concerned with signal detection theory, it outlines some of the basic methods oflocal psychophysics and will exclude from consideration the methods of global psychophysics. It will discuss how to study and measure the limits of human sensory systems. The relation between methods for investigating human operant behavior and methods for measuring the limits of the human senses may not seem obvious. In fact, however, modern psychophysical procedures allow an objective analysis of human sensory systems; those methods do not invite a consideration of the pri­ vate accompaniments of detection or discrimination. In this analysis, an experi­ menter presents one or more events and records whether the observer, after ap­ propriate instruction, responds differentially to those events. In studying, for example, whether a person discriminates between two aurally presented sinu­ soidal waveforms that differ in frequency, the psychophysicist does not need to ask whether the observer perceived different pitches associated with each wave­ form. The pitch of a sinusoid is a private experience that may or may not accom­ pany the presentation of the waveform. The question of interest to the psy­ chophysicist is the discrimination of waveforms by an observer: The experimenter the difference, and the psychophysicist wishes to find out, through proper procedures, whether the observer knows the difference too. Like much research