Psychophysical Approach

Psychophysics is the study of sensory, perceptual, and cognitive systems, based on the evidence of human observers making judgments about what they see, hear, or feel. It is the oldest route to understanding the senses, with its roots in classical times and medieval Arab learning, flowering in the hands of 19th century geniuses such as Thomas Young, Hermann von Helmholtz, and Ernst Mach, and with increasing technical sophistication it is a key approach to understanding perception today. Psychophysical methods are also important in the accurate diagnosis of sensory and cerebral disorders and deficits (e.g., color deficiency and visual field loss) and in applications to human engineering problems. This entry draws its examples of the psychophysical approach from the study of visual perception. However, many advances in psychophysical methods, and some of the most sophisticated quantitative applications, have come from work on hearing, and the approaches described here have been effectively applied in the other senses and in studies of multisensory perception.

The term psychophysics was coined by Gustav Fechner in 1860 to denote the study of how subjective (“psycho-”) attributes of a stimulus relate to its physical properties. For Fechner and a number of his successors, the key question was the way that subjective magnitude of a stimulus (for example, a numerical assessment of a light's perceived brightness) related to its physical intensity (e.g., photometric luminance). However, this question has been a diminishing part of psychophysical research in recent decades.

Psychophysics in a much broader sense has been the use of subjective responses and judgments to make inferences about the structure and function of the visual system and other sensory systems. This approach, for the pioneers of the 19th century, provided their only tool for probing how sensory systems operate. More recently, electrophysiological and neuroimaging methods have allowed researchers to make physical measurements of the brain mechanisms of perception and cognition at work. Psychophysical methods have not been made obsolete by these advances in neu-roscience. They remain an approach of great power and precision (and relatively low cost) in analyzing how sensory systems work. More fundamentally, measures of neural activity alone can tell us little about the functional significance of that activity. In contrast, psychophysical judgments of distance, color, or motion must be tapping the systems that we use to gain information about these qualities of the world around us, and so have clear functional validity. Furthermore, the parallel evolution of psychophysical and neuroscience methods has opened the possibility of a new “psycho-physical” relation, directly testing the relationship between physical patterns of activity in the nervous system revealed by imaging and recording methods, and the associated patterns of subjective visual experience. Such linked studies offer a possible way to advance our understanding of the neural basis of consciousness from direct experimental evidence.