How does a descending series differ from other methods in psychophysics?

In psychophysics, a descending series specifically begins with a stimulus that is presented at an intensity that is well above the detectable threshold and then systematically decreases the intensity until the stimulus is no longer perceived by the subject. This method is particularly useful for determining the threshold at which an individual can reliably detect a stimulus and is a standard technique for measuring sensory thresholds.

Unlike methods that involve increasing stimulus intensity, the descending series avoids the potential for a response bias that might occur with increasing intensity, where participants might become more accustomed or desensitized to stimuli. Additionally, while some methods may allow subjects to control their responses or present stimuli in a random order, the defining feature of a descending series is its structured approach of starting high and gradually decreasing the intensity. This contrasts with the ascending method, which would begin at a low intensity and increase to identify the threshold; thus, it emphasizes the systematic reduction in stimulus presentation rather than variability or control by the participant.