Visual Psychophysics

by

Joshua A. Solomon

• LAST REVIEWED: 29 October 2013
• LAST MODIFIED: 29 October 2013
• DOI: 10.1093/obo/9780199828340-0128

Introduction

Paradigms for attacking the problem of how vision works continue to develop and mutate, and the boundary between them can be indistinct. Nonetheless, this article represents an attempt to delineate one such paradigm: visual psychophysics. Unlike, for example, anatomical paradigms, the psychophysical paradigm requires a complete organism. Most of the studies discussed below involve human beings, but psychophysics can be used to study the vision of other organisms too. At minimum, the organism must be told or taught how to respond to some sort of stimulus. (Studies of reflexes such as the knee-jerk response are not psychophysical.) It is the decision processes preceding those responses that are of primary interest to a psychophysicist. The word “psychophysics” comes from Fechner’s Elemente der Psychophysik (originally published in 1860, translated into English as Elements of Psychophysics, by Helmut E. Adler in 1966), in which the paradigm was initially codified. All sensory processes are amenable to psychophysical examination. Consequently, several of the key advances in the both the theory and the practice of psychophysics were made by scientists trying to understand something other than vision. Much of that work will be ignored here. What this article does contain, after brief discussion of theory and practice, is a catalogue of psychophysical investigation, loosely organized according to stimulus properties (“features”) that are unquestionably encoded at a very low level in the hierarchy of visual processing: luminance, spectral content, contrast, and spatio-temporal position. Discussion will focus on how visual stimuli differing in these feature contents are detected, discriminated, and appear to human observers. Separate sections describe the criteria necessary for a stimulus property to be considered a visual feature and the effects of spatial and temporal context on detection and appearance. Some related areas of investigation and their relationship to visual psychophysics are described at the end.

Theory

The impact of Fechner’s empirical attitude toward mental events cannot be overstated (see Fechner 1966). Prior to Elemente, psychological science was based either on physiology or philosophy. Although Fechner explicitly refused credit for inventing any psychophysical methods, he did classify each one as either a method of just-noticeable difference, a method of average error, or a method of right and wrong cases. The first two of these methods survive today as the method of limits and the method of adjustment. However, more recent assays of the field invariably suggest new taxonomies, encompassing new methodologies and better distinguishing between certain flavors of the classic ones. The most important types of psychophysical measurement not considered by Fechner are response times (reviewed by Luce 1986), confidence ratings (analyzed by Galvin, et al. 2003), and magnitude estimations (popularized by Stevens 1961; criticized by Laming 1997). However, advances in the field of statistics have proven to be even more valuable for contemporary psychophysicists. This is because organisms do not always respond in the same way to the same stimuli. Thurstone 1994 was among the first researchers to characterize the stochastic nature of psychophysical responses, but the mathematical tools for summarizing this variability are still being developed today. Two particularly noteworthy summaries of advances in this regard are Green and Swets 1966 and Macmillan and Creelman 2005.

• Fechner, Gustav T. 1966. Elements of psychophysics. Vol. 1. Translated by Helmut E. Adler. New York: Holt, Rinehart & Winston.

  English translation of Elemente der Psychophysik, first published in 1860. According to Fechner, the whole of Elemente “evolved on the basis of and in connection with” (p. xxx) an idealistic interpretation of immortality. Fechner may not have had an undying soul, but his influence refuses to disappear. His greatest contribution to psychology was the notion that mental events can be measured in terms of the stimuli that elicit them.

• Galvin, Susan J., John V. Podd, Vit Drga, and John Whitmore. 2003. Type 2 tasks in the theory of signal detectability: Discrimination between correct and incorrect decisions. Psychonomic Bulletin and Review 10.4: 843–876.

  DOI: 10.3758/BF03196546

  Distinguishes between confidence in stimulus characteristics and confidence in response accuracy, and thoroughly models both from the perspective of Signal-Detection Theory.

• Green, David M., and John A. Swets. 1966. Signal detection theory and psychophysics. New York, NY: Wiley.

  With very few and explicit assumptions regarding the sources of response variability, the authors develop a mathematical framework for characterizing sensory computations with what was then revolutionary precision. It has become the cornerstone of contemporary psychophysical analyses.

• Laming, Donald. 1997. The measurement of sensation. New York: Oxford Univ. Press.

  DOI: 10.1093/acprof:oso/9780198523420.001.0001

  Trenchant critique of magnitude estimation and related techniques. Also contains refinements of and amendments to the author’s idiosyncratic theories of visual discrimination from previous publications. The empirical support is selective but persuasive.

• Luce, R. Duncan. 1986. Response times: Their role in inferring elementary mental organization. Oxford: Oxford Univ. Press.

  A compendium of analytical tools, tricks for getting sensible data, and caveats about over-interpretation.

• Macmillan, Neil A., and C. Douglas Creelman. 2005. Detection theory: A user’s guide. 2d ed. Mahwah, NJ: Lawrence Erlbaum.

  Textbook and methodological handbook for all types of psychophysical measurement.

• Stevens, Stanley S. 1961. To honor Fechner and repeal his law. Science 133:80–88.

  DOI: 10.1126/science.133.3446.80

  Whereas Fechner was interested in sensory intensity, what he measured was the ability to discriminate between similar sensations. Stevens introduced magnitude estimation as a way to actually measure apparent intensities. This paper may be his most accessible. It contains a succinct compendium of magnitude estimation’s “applications and validations.”

• Thurstone, Louis L. 1994. A law of comparative judgment. Psychological Review 101:266–270.

  DOI: 10.1037/0033-295X.101.2.266

  First published in 1927. The five “cases” introduced in this short essay are collections of increasingly restrictive assumptions about the sources of response variability. Read this to understand why it makes sense to call a 21st-century psychophysical model “Thurstone Case V.”