Differenstærskelen

Differenstærskelen, also called the differential threshold or the just noticeable difference (JND), is the smallest detectable difference between two stimuli of the same modality. It is defined for a given baseline intensity and sensory system, and it reflects how finely the senses discriminate changes in stimulus properties such as brightness, weight, or loudness. In psychophysics, the differential threshold is a key measure of sensory discriminability and perceptual sensitivity.

A foundational principle is Weber's law, which states that the just noticeable difference is proportional to the initial stimulus level: ΔI = k · I. Here ΔI is the change in intensity, I is the baseline intensity, and k is the Weber fraction, a constant for many senses over a range of moderate intensities. This means detecting changes becomes harder as the baseline strength increases, even though the proportional change remains constant.

Measurement of the differential threshold employs several psychophysical methods, including the method of constant stimuli, the method of limits, and the staircase method. The threshold is often defined as the stimulus change detected on about 50% of trials, though some procedures use different criteria (for example 75%). Thresholds differ across modalities—vision, audition, touch, and taste—and can vary with context and required response.

Factors influencing differenstærskelen include attention, adaptation, fatigue, and prior experience. Individual differences such as age and overall sensory acuity also play a role. The concept has broad applications in sensory science, ergonomics, product design, and clinical testing, where understanding perceptual thresholds informs calibration, safety standards, and perceptual modeling.