According to the text, our senses automatically adjust to the level of stimulation they are receiving so that they can be as sensitive as possible without getting overloaded. As a result, our senses become less sensitive when the overall level of stimulation is high, but more sensitive when the overall level of stimulation is low. This explains, for example, why the tick of a watch is more annoying in a quiet room than on a busy street. This phenomenon of sensory adaptation can be readily illustrated in class with a variety of senses, including touch, taste, and vision. Depending on your class size (e.g., if you have fewer than 30 or 35 students), you could allow all students to participate in the first two exercises or for larger classes you might prefer to select a subset of volunteers.
Fred Whitford suggests a simple exercise for demonstrating sensory adaptation with touch. Bring to class a number of samples of very coarse sandpaper and distribute them to students. After rubbing their index fingers gently over the paper a few times, they should rate its coarseness on a scale from 1 (very soft) to 7 (very course). After a minute or two, have them rub the same finger over the paper and again rate its coarseness. Their senses should have adapted to the coarseness and thus the ratings for the second time should be lower.
A different exercise (suggested by John Fisher) can be used to demonstrate sensory adaptation with taste. You'll need to bring to class (a) a pitcher containing a strong solution of water and sugar, (b) a pitcher containing fresh water, and (c) several Dixie cups. Distribute two Dixie cups to each student and fill one with sugar water and one with fresh water. Instruct students to take a sip of the sugar water and to swish it around in their mouths for several seconds without swallowing it; gradually it should taste less sweet. After swallowing it (or spitting it back into the cup), students should then taste from the cup containing fresh water. Students will be shocked at how incredibly salty the water tastes and will wonder if you didn't spike it with salt when they weren't looking! Explain that when the overstimulated taste buds responsible for sweetness became temporarily less sensitive, the taste buds responsible for salt became more prominent as a result.
A final exercise (reprinted from Bill Hill) requires a little more effort but powerfully illustrates sensory adaptation in vision. Davis and Grover (1987) first described this activity, a modified version of a procedure developed by Hochberg et al. (1951), that uses a Ganzfeld (a homogenous visual field) to demonstrate that the visual system requires varied stimulation to prevent sensory receptor adaptation. To conduct this demonstration you will need to make a Ganzfeld and have a red light source, such as that on a stereo or coffee maker. The Ganzfeld is constructed using a ping pong ball. Cut the ping pong ball in half and discard the side with the writing on it. Then attach cotton around the rim of the remaining half in order to protect the student's eye. Instruct a student volunteer to place the Ganzfeld on one eye, touch the Ganzfeld on the red light, close their other eye, and continue to stare at the red light, reporting any experience that occurs. After a minute or so, although the light is still on, the student will state that you have turned the red light off. Explain to your students that this effect is the result of receptor adaptation because of the Ganzfeld.
Davis, S. F., & Grover, C. A. (1987). And then the lights went out: Constructing a simple Ganzfeld. In V. P. Makosky, L. G. Whittemore, & A. M. Rogers (Eds.), Activities handbook for the teaching of psychology: Vol. 2 (pp. 49-50). Washington, DC: American Psychological Association.
Fisher, J. (1979). Body Magic. Briarcliff Manor, NY: Stein and Day.
Hill, W. G. (1995). Instructor's resource manual for Psychology by S. F. Davis and J. J. Palladino. Englewood Cliffs, NJ: Prentice Hall.
Hochberg, J. E., Triebel, W., & Seaman, G. (1951). Color adaptation under conditions of homogeneous visual stimulation (Ganzfeld). Journal of Experimental Psychology, 41, 153-159.
Whitford, F. W. (1995). Instructor's resource manual for Psychology: An Introduction by C. G. Morris (8th ed.). Englewood Cliffs, NJ: Prentice Hall.