CHAPTER 29 PSYCHOPHYSICAL METHODS

Psychophysical methods are the tools for measuring perception and performance. These tools are used to reveal basic perceptual processes, to assess observer performance, and to specify the required characteristics of a display. We are going to ignore this field's long and interesting history (Boring, 1942), and much theory as well (Gescheider, 1985; Macmillan and Creelman, 1991), and just present a user's guide. Use the supplied references for further reading. Consider the psychophysical evaluation of the suitability of a visual display for a particular purpose. A home television to be used for entertainment is most reasonably assessed in a "beauty contest" of subjective preference (Mertz, Fowler, and Christopher, 1950), whereas a medical imaging display must lead to accurate diagnoses (Swets and Pickett, 1982; Metz, 1986) and military aerial reconnaissance must lead to accurate vehicle identifications (Scott, 1968). In our experience, the first step toward defining a psychophysically answerable question is to formulate the problem as a task that the observer must perform. One can then" assess the contribution of various display parameters toward that performance. Where precise parametric assessment is desired it is often useful to substitute a simple laboratory task for the complex real-life activity, provided one can either demonstrate, or at least reasonably argue, that the laboratory results are predictive. Psychophysical measurement is usually understood to mean measurement of behavior to reveal internal processes. The experimenter is typically not interested in the behavior itself, such as pressing a button, which merely communicates a decision by the observer about the stimulus.* This chapter reviews the various decision tasks that may be used to measure perception and performance and evaluates their strengths and weaknesses. We