Psychophysics

Psychophysics asks a measurable question about subjective experience: how much physical energy does a stimulus need before someone perceives it, and how much must it change before they notice? Gustav Fechner and Ernst Weber established the field in the nineteenth century, and their methods gave psychology its first quantitative bridge between the physical world and conscious sensation.

Two thresholds anchor the field. The absolute threshold is the minimum stimulus intensity a person detects 50% of the time — the faintest tone, dimmest light, or weakest scent that registers. The difference threshold, also called the just noticeable difference (JND), is the smallest change in a stimulus that a person can reliably detect. A stimulus below the absolute threshold is subliminal.

Weber's law describes how the difference threshold behaves: the JND is a constant proportion of the original stimulus rather than a fixed amount. If you can just detect a 2-pound difference when lifting 40 pounds, you will need a 5-pound difference at 100 pounds, because the proportion — about 5% — stays constant. Fechner extended this into a logarithmic relationship between stimulus intensity and perceived sensation, and Stevens later proposed a power function that fits some sensory dimensions better.

Classical threshold theory has an important limitation: detection is not purely sensory. Signal detection theory treats each judgment as depending both on sensitivity to the stimulus and on the observer's response criterion, which shifts with expectation, motivation, and the costs of being wrong. Its four outcomes — hits, misses, false alarms, and correct rejections — separate what the senses can discriminate from how willing the observer is to say yes. Related to this, sensory adaptation means constant stimulation reduces responsiveness over time, so thresholds themselves are not fixed.

The MCAT tests psychophysics in the Psychological, Social, and Biological Foundations of Behavior section, within sensing the environment. High-yield material includes the definitions of the two thresholds, Weber's law as a proportional rather than absolute rule, the distinction between sensation and perception, and the four outcome categories of signal detection theory.