Scenario 10 – From Weber's Law to Fechner

When Fechner derived his now infamous law, he made the assumption that Weber's equation was correct. When the stimulus intensity is at absolute threshold he assumed that would correspond to a zero point on his psychological sensation scale. He also assumed that all JNDs are equal, i.e. a stimulus 10 JNDs above threshold would produce a sensation twice the magnitude produced by that 5 JNDs above threshold. Here we will explore the derivation of Fechner's Law – you should end up with graphs that are representative of Fechner's logarithmic Law.

Let's say we're working with visual stimuli exploring the perception of light in humans. We can assume the Weber fraction (k) to be 0.3 and the absolute threshold of light to be 5.0 candelas (these are units of light intensity).

Using Weber's Law and the values provided above, calculate the difference threshold when the stimulus is at absolute threshold.

What would be the stimulus value (I) corresponding to the 1^st JND above absolute threshold? Recall that changing a stimulus by the difference threshold will induce a JND.

Use the same logic as you employed above and determine the values of I corresponding to the first 10 JND's above an absolute threshold of 5.0.

Make a graph of sensation magnitude, S, as a function of the stimulus intensities that you have just calculated.

Stevens Power Law was put forward as an improvement over Fechner's Law. Consider this however: If the exponent in Stevens Power Law was less than 1 for every perceptual dimension, Fechner's Law would be correct and we could disregard Stevens Power Law. Do you agree with this statement? Explain your answer.