My son got a new toy. It’s a yellow stick of foam that he can bend into different shapes. The other day, he bent it into the letter C and asked me, “Is this a C?” I said yes.
He bent the stick a little further and asked again. “Is this still a C?” I said yes.
He repeated this procedure a total of seven times. Each time, he bent the stick a little more, such that it eventually became a full-fledged O. Each time, he asked me if it looked like a C. In other words, without knowing it, my son conducted a classical categorical perception experiment.
Scientists use categorical perception experiments to investigate how people create mental categories for the things that they see or hear. To use my son’s game as an example, I’ll say that he investigated how I create mental categories for letters. To do this, he presented me with a set of seven shapes that varied between the letters C and O, and asked me to judge each one.
Crucially, the yellow stick of foam did not do anything special. The experimenter just started with a letter C and bent it, little by little, in the direction of O. The results show, however, that the mind did do something special. The subject (moi) judged shapes 1 through 4 to be the letter C, but judged shapes 5 through 7 to be the letter O. That is, despite the fact that the yellow stick changed only gradually, the mind’s judgment changed abruptly. This is categorical perception: the tendency to place things into discrete mental categories even when the stimulus changes gradually.
Categorical perception experiments have played an important role in speech and language research. In the 1950s, scientists at Haskins Laboratories in Connecticut investigated how people create mental categories for spoken consonants. They presented subjects with a set of thirteen sounds that varied continuously between “b”, “d”, and “g” and asked them to judge each one.
Crucially, just like the yellow stick, the speech sounds did not do anything special. The experimenters just started with the sound “b” and changed it little by little, in the direction of “d” and then a little further, in the direction of “g”. But results showed that the mind did do something special. Subjects judgments shifted abruptly from “b” to “d” to “g” despite the fact that the sound itself changed only gradually. You can perform this experiment on yourself, and learn how the experimenters created the speech sounds, by visiting this page from Haskins Labs.
The yellow stick experiment was fun because the experimenter was a child. What happens when the subject is a child? For speech sounds, Peter Eimas and colleagues demonstrated that children do indeed exhibit categorical perception (Eimas, Miller & Jusczyk 1987). For written letters, I plan to turn the tables and conduct the yellow stick experiment on my son. In the meanwhile, I offer the following pre-school worksheet and ask you, the reader, to be the judge: does my son possess the appropriate mental category for the letter C?