Psychophysics asssignment for H301, Spring 2001
The assignment is due February 12, 2001

Introduction:
Te Ebbinghaus (or Titchener circles) illusion was demonstrated in class: Two equally large target circles are surrounded by small and large inducer circles. The target circle surrounded by small inducers appears to be larger than the target circle surrounded by large inducers.
Using the method of constant stimuli, each of a fixed set of stimulus pairs (comparison & standard) were presented in a random order. The frequency with which the comparison stimulus elicited a "smaller" response was used to plot the psychometric function. The PSE (Point of Subjective Equality) is the value on the x axis where the psychometric function crosses the 50% (chance) level.

Where is PSE for the black and red psychometric curves?
If the diameter of the standard is 114 pixels, what is the amount of the illusion in pixels?
What is the amount of illusion expressed in percentage of the diameter of the standard?
What does the result mean?

You could say that there is a "perceptual inaccuracy" because the judgements were inaccurate, and PSE was substantially different than the diameter of the standard stimulus. The error gives you an estimation for the amount of illusion your subject perceives - a very subjective phenomenon assesed objectively!

Note that the Ebbinghaus illusion is only one of the size-related perceptual illusions you might want to know about.


Assignment:
Design and carry out a psychophysical experiment to test the following hypothesis:
Size-related perceptual illusions only occur when the target and inducers have the same color and luminance.
(demo on color & luminance)

You may use any other illusions where size-comparisons are made, e.g., the Muller-Lyer illusion.
You may use any other psychophysical methods to carry out the measurement, see e.g.
http://www.essex.ac.uk/psychology/experiments/muller.html.
You may use any type of procedure, either computer-based or paper-and-pencil.

Requirements:

- Use at least 5 experimental subjects
- Summarize their data by averaging and calculating the SD
        Standard Deviation: represents the scatter among observers
        SD = sqrt [ sum of  (PSEn - PSEaverage)2)/n]
- Plot the results in a graph
- Type the description of your experiment using the following sections: