constantstimuli

Constant stimuli, or the method of constant stimuli, is a psychophysical procedure used to measure perceptual thresholds by presenting a fixed set of stimulus intensities in a random order to an observer. The chosen intensities bracket the expected threshold, covering levels well below and above the point at which the stimulus becomes detectable or discriminable. Each intensity is presented a predetermined number of times, and responses are recorded to determine detection or discrimination performance.

In practice, observers perform a simple task on each trial, such as indicating whether they detected the stimulus (yes-no task) or identifying which of two intervals contained the stimulus (two-alternative forced-choice). The resulting data are used to construct a psychometric function that relates stimulus intensity to the probability of a correct or detected response. From this function, threshold estimates such as the intensity corresponding to a 50 percent detection rate or discrimination probability are derived. The method can also be used to estimate just noticeable differences when the task involves discriminating between stimuli of different intensities.

The strengths of the method of constant stimuli include its precision and the ability to map the full psychometric function under controlled conditions. Its main drawbacks are time consumption and practicality, since many trials are required. The approach is often contrasted with the method of limits and staircase procedures, which can be faster but may introduce different biases. When bias is a concern, forced-choice designs are commonly used to improve the reliability of threshold estimates.