3.4 Categorical Perception: Definition & Explanation
An Example of Categorical Perception
When someone speaks a language you don’t speak, you of course don’t understand the meaning of what they say. But did you know that you actually hear different sounds than what a native speaker of that language hears? Take for example the /l/ and /r/ sounds of the English language. While native English speakers easily distinguish these two sounds, Japanese speakers are generally unable to hear the difference. The English words ‘lag’ and ‘rag’ would sound the same to them. Likewise, there are sounds that English speakers are unable to distinguish, such as the /t/ and /T/ sounds in Hindi. Why is that? The reason has to do, in part, with a trick of the brain called categorical perception.
Categorical perception helps us to detect the differences between things when we need to be able to, and it masks the differences when we need to treat things as the same. The Japanese language doesn’t differentiate between the /l/ and /r/ sounds, so through categorical perception, the brains of native Japanese speakers have learned to treat the two sounds as the same by actually hearing the same sound when each is spoken. However, because those sounds are differentiated in English (changing the l in ‘lag’ to an r will change the meaning of the word), native English speakers have learned to hear a difference between the two.
Definition of Categorical Perception
Categorical perception occurs when items that range along a continuum are perceived as being either more or less similar to each other than they really are because of the way they are categorized. For example, if items falling within a certain range along that continuum belong to a single category, they will be perceived as being more similar to each other than items outside of that range.
Categorical Perception of Speech Sounds
Categorical perception was first demonstrated with speech sounds (such as /ga/, /pa/, and /du/). Researchers were trying to make sense of the fact that humans understand speech at all when there is so much variability in how different speakers pronounce speech.
For example, when your friend says the word ‘dog’, it is going to be pronounced slightly differently from how your mom says the word ‘dog’; yet each time, you will still hear the same ‘dog’ despite the variability. What’s even more interesting is that you likely never even notice the variability.
Some speech sounds differ only by voice onset time (VOT). VOT is the time it takes for the vocal cords to start vibrating after the release of certain consonant sounds. For example, the sound /da/ differs from /ta/ only in the amount of time it takes for the vocal chords to vibrate after making the consonant sound. Give it a try. Make the sound /da/ and then the sound /ta/. Did you notice how it takes longer for the ‘ahh’ sound to come out with /ta/ than it does with /da/? In fact, the VOT for /da/ ranges from about 0 to 30 milliseconds (msec), and the VOT for /ta/ runs from about 50 to 80 msec. Take a look at Figure 1. You can see that if the VOT is between 0 to 30 msec, people report that they hear /da/, but if the VOT is 50 to 80 msec, they report hearing /ta/.
Here in Figure 1, notice how there is a range of msec for each speech sound. What researchers find is that it doesn’t really matter what the VOT is within that range; listeners will hear the same sound regardless. /da/ with a VOT of 10 msec is going to sound like the same /da/ with a VOT of 30 msec. But, increase that VOT to 50 msec, and suddenly you have /ta/.
Isn’t that strange? You can have two sounds that differ in VOT by exactly 20 msec, but if they both fall within the 0 – 30 msec range, they will sound the same (they will both sound like /da/). However, if those two sounds fall within different category ranges, say 30 msec and 50 msec, they will sound different (one like /da/ and the other like /ta/). This is referred to as crossing the phonetic boundary, which is the point along the continuum in which the perception of speech sound changes from one category to another (as you can see in Figure 2 here).
Categorical Perception and Experience
Categorical perception occurs through experience. For example, infants are born with the ability to hear every distinguishing sound in every language. However, as they grow and learn to identify the sounds of their native language, their brains begin to enhance the distinguishing features of their native language so that those sounds that change the meaning of a word actually sound different to them. Categorical perception also causes different sounds that are not distinguished in their language to sound the same so that they can no longer hear the differences.
How It Affects Perception
Categorical perception isn’t limited to the sounds of speech. Researchers have found evidence of categorical perception in how we identify faces, recognize emotion, see different colors, hear musical chords, and more. Researchers can even create artificial categories and train participants to begin to perceive items differently depending on which category they come from.
Categorical perception occurs when a person begins to perceive items belonging to a category as being more similar to each other and less similar to items from other categories. This change in perception occurs with experience. Categorical perception was first demonstrated with the sounds of speech, and through the study of it over the years.
Researchers identified voice onset time (VOT),which is the time it takes for the vocal cords to start vibrating after the release of certain consonant sounds, and the phonetic boundary, which is the point along the continuum in which the perception of speech sound changes from one category to another. For example, if two sounds fall on either side of the phonetic boundary, you will perceive them as different sounds.
Because categorical perception causes sounds to be arranged along a continuum, they can be perceived as being more similar than they actually are, such as the blending of the /r/ and /l/ sounds in the Japanese language or the /t/ and /T/ sounds of the Hindi language that English speakers can’t understand.
However, the reverse can be true as well. Categorical perception has also been shown to occur with the identification of faces, how we recognize emotion, see different colors, hear musical chords, and even perceive artificially created categories.