constantstimulus

The method of constant stimuli, sometimes referred to as the constant-stimulus method, is a psychophysical procedure used to determine sensory thresholds. It relies on presenting a predefined set of stimulus intensities in a randomized order and recording observer responses.

In a typical experiment, several intensities spanning well below to well above the expected threshold are chosen. Each intensity is presented many times, and responses are collected. Because the intensities are fixed, the observer's probability of detection (or of reporting a particular percept) as a function of stimulus strength can be directly estimated.

The results are used to construct a psychometric function, often a sigmoid or cumulative Gaussian, showing performance (for example, proportion detected) as a function of stimulus strength. A threshold is defined at a chosen performance criterion, commonly 50% detection, though other criteria may be used. The slope of the function reflects sensory sensitivity, and the data can be modeled to extract thresholds and other parameters.

Advantages of the constant-stimulus method include precise control over the stimulus levels and straightforward interpretation of the resulting psychometric function, with reduced influence of response bias when appropriate trials are used. Disadvantages include that it is time-consuming and resource-intensive, since many trials must be run across multiple intensities, and it is less efficient than adaptive methods for rapid threshold estimation. It can also be affected by factors such as participant fatigue and learning.

Applications span vision, hearing, taste, and touch, and the method is used in basic research and clinical testing. It is commonly contrasted with staircase or adaptive procedures, which adjust stimulus levels based on responses to converge on thresholds more quickly.