What is Fechner’s Law in psychology?

Fechner’s Law is a generalization of Weber’s theory. According to him, there is a broader relationship between sensory and physical intensity. Fechner’s law states that the strength of the sensation becomes greater as the logarithm of stimulus intensity increases. In Fechner’s formulation JNDs provide the basic unit of perceived intensity. In other words, one JND is perceptually equal to another JND. With these assumptions Weber’s Law is valid and Fechner’s Law can be stated as follows:

  • S = K log1Q I, where
  • S is the subjective, perceived, psychological intensity
  • K is a constant associated with a specific sensory modality
  • I is the physical intensity as measured by an appropriate measuring device.

This formulation states that if physical intensity is increased in a logarithmic fashion the resultant psychological intensity will follow with equal steps. The equation transforms paired comparisons of physical intensity into subjective values on a ratio scale. The “log10” term shows that the physiological information processing system compresses large physical intensity ranges into smaller physiological response ranges.

Fechner’s Law emphasizes the compressed representation of the physical stimuli in the psychological domain. This compression implies that when varying signals or stimuli it may be necessary to make relatively large changes in the physical stimuli to render it psychologically perceptible.

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