Basic Psychology: Psychophysics

Psychophysics also refers to a broad category of methods for studying a perceptual system. Threshold measurement, ideal observer analysis, and signal detection theory are all heavily used in modern applications.

Psychophysics has numerous and significant practical applications. For example, in the field of digital signal processing, psychophysics has aided in the development of lossy compression models and methods. These models explain why humans perceive very little loss of signal quality when lossy compression is used to format audio and video signals.

When psychological research in Nazi Germany came to a halt in the 1930s, both approaches were gradually replaced by the use of stimulus-response relationships as evidence for conscious or unconscious processing in the mind. [8] Charles S. Peirce studied and expanded on Fechner's work, which was aided by his student Joseph Jastrow, who went on to become a well-known experimental psychologist in his own right. Peirce and Jastrow mostly agreed with Fechner's empirical findings, but not entirely. A classic experiment by Peirce and Jastrow, in particular, rejected Fechner's estimation of a threshold of perception of weights as far too high. Peirce and Jastrow invented randomised experiments in their experiment: they randomly assigned volunteers to a blinded, repeated-measures design to assess their ability to discriminate weights. Peirce's experiment influenced other psychologists and educators, who established a research tradition of randomised experiments in laboratories and specialised textbooks in the 1900s. The Peirce–Jastrow experiments were part of Peirce's pragmaticism program's application to human perception; other studies looked at light perception, for example. "Mr. Peirce's logic courses gave me my first real experience of intellectual muscle," Jastrow wrote. Though I quickly joined Stanley Hall's laboratory of psychology, it was Peirce who gave me my first training in the handling of a psychological problem while also stimulating my self-esteem by entrusting me, then relatively innocent of any laboratory habits, with a real bit of research. He borrowed the apparatus for me, which I took to my room, installed at my window, and used to make the observations when the lighting conditions were favourable. The findings were published in the Proceedings of the National Academy of Sciences under our joint names. The demonstration that traces of sensory effect too slight to register in consciousness could nonetheless influence judgement may have been a persistent motive that induced me to write a book on The Subconscious years later." This work clearly distinguishes observable cognitive performance from conscious expression.

Modern approaches to sensory perception, such as research on vision, hearing, or touch, measure what the perceiver's judgement extracts from the stimulus, often ignoring the question of what sensations are being felt. One prominent method is based on signal detection theory and was developed for cases involving very weak stimuli. However, the subjectivist approach is still prevalent among those who follow in the footsteps of Stanley Smith Stevens (1906–1973). In contrast to Fechner's log-linear function (cf. Stevens' power law), Stevens revived the idea of a power law proposed by 19th century researchers. He also advocated for the assignment of numbers in proportion to the strength of stimuli, a technique known as magnitude estimation. Stevens also included techniques like magnitude production and cross-modality matching. He was against assigning stimulus strengths to points on a line labelled in order of strength. Nonetheless, that type of response is still common in applied psychophysics. Such multiple-category layouts are frequently referred to as Likert scaling, after the question items used by Likert to create multi-item psychometric scales, e.g., seven phrases ranging from "strongly agree" to "strongly disagree."

According to Omar Khaleefa , the mediaeval scientist Alhazen should be considered the founder of psychophysics. Although al-Haytham provided numerous subjective accounts of vision, there is no evidence that he used quantitative psychophysical techniques, and such claims have been refuted.

A threshold (or limen) is the point of intensity at which a participant can only detect the presence of a stimulus (absolute threshold) or the presence of a difference between two stimuli (differential threshold) (difference threshold ). Stimuli with intensities less than the threshold are deemed undetectable (hence: sub-liminal). Stimuli at values close to a threshold are frequently detectable some proportion of the time; thus, a threshold is defined as the point at which a stimulus, or change in a stimulus, is detected some proportion p of the time.

The ascending and descending methods are used alternately in experiments, and the thresholds are averaged. One disadvantage of these methods is that the subject may become accustomed to reporting that they perceive a stimulus and may continue to report in this manner even after the threshold has been reached (the error of habituation). In contrast, the subject may anticipate that the stimulus is about to become detectable or undetectable and make a hasty decision (the error of anticipation).

Georg von Békésy introduced the staircase procedure in his study of auditory perception in 1960 to avoid these potential pitfalls. In this method, the sound begins audible and gradually becomes quieter after each of the subject's responses, until the subject reports not hearing it. At that point, the sound is gradually increased in volume until the subject reports hearing it, at which point it is gradually reduced in volume. This allows the experimenter to "zero in" on the threshold.

The 1-up-N-down staircase is a popular staircase design (with fixed-step sizes). If the participant correctly responds N times in a row, the intensity of the stimulus is reduced by one step size. If the participant responds incorrectly, the stimulus intensity is increased by one size. The mean midpoint of all runs is used to calculate a threshold. Asymptotically, this estimate approaches the correct threshold.