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the smallest magnitude of a stimulus that evokes a sensation.
The force of action of the stimulus must be equal to or greater than the threshold of stimulation. The smallest magnitude of a stimulus that first causes sensation is called the absolute stimulus threshold. The lower absolute stimulus threshold is the magnitude of a stimulus at which a sensation of a particular quality first arises. The upper absolute stimulus threshold is the magnitude of a stimulus at which a sensation of a particular quality does not arise or at which it changes qualitatively—for example, a powerful light flux causes in man a sensation of pain rather than light. The minimum increase in magnitude of a stimulus that causes a barely perceptible change in sensation is called the differential threshold, the limit of discriminative sensibility. The relative value of the differential threshold, according to the We-ber-Fechner law, remains constant for each analyzer within a fairly wide range of change in magnitude of the stimulus.
The stimulus threshold varies with the duration of action of the stimulus: the shorter it is, the higher the threshold. A temporal stimulus threshold is the minimum interval of time between two stimulations at which they are first perceived separately. The stimulus threshold also varies with the area of action of the stimulus; the larger the area stimulated, the lower the threshold. The double-point threshold is the smallest distance between two stimulated points at which they are first perceived as spatially separate. The stimulus threshold is inversely proportional to sensitivity, and it changes in relation to the structure of the receptors and as a result of physiological adaptation.