absolute threshold

Meaning: The absolute threshold is the smallest amount of a stimulus (light, sound, pressure, odor, taste) that a person can detect. Because detection is probabilistic, it is defined statistically as the intensity at which the stimulus is reported on 50% of trials. Below it, detection is less than half the time; above it, more often. Classic textbook examples include seeing a candle flame from a long distance on a dark, clear night or hearing a watch tick in a quiet room.

The threshold is not a fixed number. It shifts with attention, motivation, expectation, fatigue, adaptation and the surrounding noise. Signal detection theory explains this: the decision to say "yes, I sensed it" depends on both sensitivity and the response criterion (bias), so the same person can show different thresholds under different payoffs or expectations.

On the MCAT: Passages describe an experiment in which stimuli of varied intensity are presented and the percent detected is plotted. You may read the 50% point off a psychometric curve, or predict how a change (a noisy room, a lower motivation level, a more liberal criterion) shifts the threshold or the hit rate.

Common confusion: The absolute threshold is not the same as the difference threshold (just noticeable difference), which is the smallest change between two stimuli that is noticed and follows Weber's law. Absolute threshold concerns detecting a single stimulus versus none. Also, it is not an all-or-none cutoff; the 50% convention exists because detection near the threshold is inconsistent.