Demonstrations: Psychophysics and Signal Detection Theory
1. Weber's Law. Take the penlight and aim it at a white
wall. Note that as you move the pen further from the wall, the
spot gets larger and dimmer (less light per unit area, actually).
Get a feel for how far from the wall you need to move until you can
barely make out the light as you turn it on and off (this is 'method of
adjustment'!). Next, turn the room lights down. Notice how
much further you now need to move to reach the threshold for
discriminating the intensity of light reflected from the wall from the
ambient illumination as opposed to the amount from the ambient light
plus the flashlight (this is a 'difference threshold'). If you
were to measure these amounts of light with a light meter (e.g. a
photometer), you would find that the ratio would be approximately the
same for the bright and for the dim room. This is Weber's law.
2. Psychophysical Measurement Procedures: On the CD that came with the textbook there are sample experiments using all of the basic psychophysical procedures that you should try. These include magnitude estimation, forced choice (also known as the yes-no or signal-detection task), and two-interval forced choice, including both method of constant stimuli and staircase procedures.