2AFC

Two-alternative forced choice (2AFC) is a psychophysical procedure used to measure perceptual detection and discrimination. In each trial, observers are presented with two options and must choose which one contains the target, or which interval or location held the signal. The two alternatives are presented with fixed structure, and the observer is required to select one of them, making the decision explicit on every trial.

A key feature of 2AFC is that there is no need for an internal criterion or bias judgment on the part of the observer; the task forces a choice between two alternatives, which tends to yield more objective sensitivity measures. 2AFC is widely used across sensory modalities, including vision, audition, and touch, to estimate thresholds such as detection or discrimination limits. It is commonly contrasted with yes/no tasks, where bias can influence responses.

Data from 2AFC tasks are analyzed in terms of percent correct or through signal detection theory measures. Chance performance is 50%. The sensitivity index d′ can be related to observed accuracy by the relation p(correct) = Φ(d′/√2), where Φ is the standard normal cumulative distribution function. Adaptive procedures, such as staircases, are often employed to efficiently estimate thresholds by adjusting the difficulty based on responses.

Variants include two-interval forced choice (2IFC), where a target appears in one of two time intervals rather than between two spatial options. 2AFC is widely used in experimental psychology, neuroscience, and product testing to obtain robust estimates of perceptual capability with reduced response bias.