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Scenario 7 – Examining the Theory

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).

What would be the stimulus value (I) corresponding to the 1st JND above absolute threshold? Remember that 1 JND corresponds to the difference threshold.

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.

Next we explore the perception of light in a nocturnal animal – the owl. Despite their relatively small size, they have eyes nearly the same size as humans. Here we know that the Weber fraction is 0.2 and the absolute threshold for perceiving any light is 1.2 candelas.

Following the same process as above, determine the stimulus intensities (I) corresponding to the first 10 JND's above absolute threshold and plot them on the same graph.

Which species has the strongest perceptual experiences at 50 candelas of light? Explain your answer.

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.