# Psychophysics Psychophysics is the scientific study of the relationship between physical stimuli and the sensations/perceptions they produce—bridging the objective world of physics and the subjective world of psychology. Founded by [[Gustav Fechner]] in 1860 with *Elemente der Psychophysik*, it was one of the first truly quantitative approaches to psychology. The core question: how does a physical change (in light intensity, sound frequency, weight) translate into a psychological change (in brightness, pitch, heaviness)? Weber's Law and Fechner's Law describe these relationships mathematically. Psychophysics introduced rigorous experimental methods still used today: thresholds (minimum detectable stimulus), just noticeable differences (JNDs), and methods of limits, adjustment, and constant stimuli. [[Signal Detection Theory]] later refined these approaches by separating sensitivity from response bias. The field revealed that perception isn't a simple mirror of reality—sensation grows logarithmically (Fechner) or as a power function (Stevens) with stimulus intensity. Modern psychophysics extends to multisensory integration, neural correlates of perception, and applications in display technology, audiology, and user experience research. ## The Psychophysical Relationship ``` ┌─────────────────────────────────────────────────────────────┐ │ PSYCHOPHYSICS: STIMULUS TO SENSATION │ ├─────────────────────────────────────────────────────────────┤ │ │ │ PHYSICAL WORLD PSYCHOLOGICAL WORLD │ │ ┌─────────────────┐ ┌─────────────────┐ │ │ │ │ │ │ │ │ │ Light (lumens) │───────▶│ Brightness │ │ │ │ Sound (dB) │───────▶│ Loudness │ │ │ │ Weight (grams) │───────▶│ Heaviness │ │ │ │ Pressure │───────▶│ Touch │ │ │ │ │ │ │ │ │ └─────────────────┘ └─────────────────┘ │ │ │ │ KEY QUESTION: What is the mathematical function? │ │ │ │ WEBER'S LAW: ΔI/I = k (constant ratio) │ │ FECHNER'S LAW: S = k·log(I) (logarithmic) │ │ STEVENS' LAW: S = k·I^n (power function) │ │ │ │ Where: S = sensation, I = intensity, k = constant │ │ │ └─────────────────────────────────────────────────────────────┘ ``` ## Fundamental Laws | Law | Formula | Meaning | |-----|---------|---------| | **Weber's Law** | ΔI/I = k | JND is constant proportion of intensity | | **Fechner's Law** | S = k·log(I) | Sensation grows logarithmically | | **Stevens' Power Law** | S = k·I^n | Sensation grows as power function | ## Key Concepts | Concept | Definition | |---------|------------| | **Absolute threshold** | Minimum detectable stimulus | | **Difference threshold (JND)** | Minimum detectable change | | **Weber fraction** | ΔI/I for a given modality | | **Point of subjective equality** | When two stimuli seem equal | | **Psychometric function** | Detection probability vs intensity | ## Classical Methods | Method | Procedure | |--------|-----------| | **Method of limits** | Ascending/descending series | | **Method of adjustment** | Observer adjusts stimulus | | **Method of constant stimuli** | Random presentation, yes/no | | **Magnitude estimation** | Assign numbers to sensations | | **Magnitude production** | Produce given sensation level | ## Weber Fractions by Modality | Modality | Weber Fraction | Interpretation | |----------|---------------|----------------| | **Brightness** | 0.08 | 8% change detectable | | **Loudness** | 0.05 | 5% change detectable | | **Weight** | 0.02 | 2% change detectable | | **Taste (salt)** | 0.08 | 8% change detectable | | **Electric shock** | 0.01 | Very sensitive | ## Stevens' Power Law Exponents | Modality | Exponent (n) | Function Shape | |----------|--------------|----------------| | **Brightness** | 0.33 | Compressive | | **Loudness** | 0.6 | Compressive | | **Line length** | 1.0 | Linear | | **Electric shock** | 3.5 | Expansive | | **Heaviness** | 1.45 | Slightly expansive | ## Key Figures | Person | Contribution | |--------|--------------| | Ernst Weber | Weber's Law (1834) | | Gustav Fechner | Founded psychophysics (1860) | | S.S. Stevens | Power law, magnitude estimation | | Georg von Békésy | Auditory psychophysics, Nobel Prize | | Stanley Smith Stevens | Measurement scales | ## Modern Applications | Domain | Application | |--------|-------------| | **Display technology** | Brightness perception, gamma correction | | **Audio engineering** | Loudness standards, compression | | **Clinical** | Hearing tests, pain measurement | | **UX design** | Perceptual thresholds in interfaces | | **Neuroscience** | Neural correlates of perception | ## Timeline | Year | Development | |------|-------------| | 1834 | Weber's Law published | | 1860 | Fechner's *Elemente der Psychophysik* | | 1879 | Wundt's lab (first psychology lab) | | 1950s | Stevens' Power Law | | 1950s-60s | [[Signal Detection Theory]] | | Modern | Neuroimaging + psychophysics | ## References - Fechner, Gustav. *Elemente der Psychophysik* (1860) - Stevens, S.S. *Psychophysics* (1975) - https://en.wikipedia.org/wiki/Psychophysics ## Related - [[Signal Detection Theory]] - [[Perception]] - [[Cognitive Psychology]] - [[Neuroscience]] - [[Weber's Law]] - [[Stevens' Power Law]] - [[Gustav Fechner]] - [[Sensory Systems]]