Categorical perception (CP)

Categorical perception (CP) refers to the tendency to sort some form of stimuli into groups or categories by the ways they relate to each other. Categorical perception is most commonly associated with sounds and colors, but it has been identified in the way other forms of stimuli are sorted as well. The tendency has also been identified in animals. Researchers have also determined it can be influenced by how a person or a culture views a specific type of stimuli.

Background

The concept of categorical perception was first proposed by American psychologist Alvin M. Liberman. Liberman was especially interested in the psychology of speech development and linguistics, or how the structure of language develops. Much of his research had to do with understanding how people learned to read.

During the 1960s, Liberman and some colleagues were working on a device that could read text aloud to people who were blind. Their initial concept was a machine that made a specific sound for each phoneme. Phonemes are the building blocks of speech and refer to the possible sounds made by each letter or combination of letters, such as the various ways the letter a can be pronounced or the unique sounds that are made by letter combinations like ch and sh. Liberman assumed that taped recordings of professionals such as radio announcers could be patched together to form an audible version of the written text. Instead, they discovered that the resulting audio recording could not be understood.

This was because people do not hear phonemes as unique sounds. They are automatically blended from one to the next. Liberman and his fellow researchers determined that attempting to duplicate this blending with recordings would require so many changes between each phoneme that the resulting recording would be a buzzing sound that did not resemble speech.

After further research, Liberman determined that humans have the ability to differentiate between different sounds that allow speech to be intelligible. For instance, the Ba and Pa sounds are similar but can be clearly distinguished. If people hear these sounds played in a range that starts with Ba and gradually changes into Pa, they will most likely categorize whatever sound they hear as either a Ba or a Pa. They are not likely to say it is “sort of a Ba and sort of a Pa.” This is categorical perception in action, as the listener makes sense of the sounds and categorizes each.

It was originally thought that people categorized sounds in the order in which humans are capable of producing them. This was known as the motor theory of speech development. The tongue and other oral structures of a baby’s mouth are different from those of an older child and adult, limiting the sounds a baby can make. As they grow, the oral structure changes, and babies become capable of making new sounds. Researchers once believed that humans categorized sounds into groups based on this progression of physical and verbal development. The motor theory has since been disproven.

Overview

People have a tendency to group things by the ways they are alike and different. How things are grouped can be influenced by a number of variables. For example, someone looking at an assortment of animals that includes a house cat, a lion, a dog, a crocodile, an iguana, an eagle, and a canary might group the cat with the lion if asked to group the animals that are most alike. If asked to group them by animals with the most legs, however, the cat would be grouped with the lion, dog, crocodile, and iguana. If asked to group together the animals that are most likely to be house pets, the cat would be grouped with the dog, iguana, and canary. If asked to group the animals by size, the cat would be in the smaller animal group, while the lion would be in the larger animal group. If asked to group the animals by biological classification, the cat, lion, and dog would be mammals, the crocodile and iguana would be reptiles, and the eagle and canary would be birds.

These groups are also called categories. Categories are sets of things with something in common. Individual items can be fully part of the set, not part of the set, or partially in the set. For instance, in the list above, most people would not consider a crocodile to be a house pet, but some people do keep small crocodiles as pets. Having mixed categories like this is particularly common with sensory elements. Things that can be seen, touched, smelled, heard, or tasted are often registered in levels. For example, a pretzel might taste saltier than a piece of cheese, and the cheese may taste saltier than an apple.

Things that register in levels or degrees such as this are said to be on a continuum of perception. This is in contrast to the set levels seen with categorical perception. In reviewing the list of animals, for example, the house pet category could be considered continuous because some of the animals are more or less likely to be house pets, while the biological classification would be categorical because there are hard and fast rules that define whether an animal is a mammal, reptile, or bird. Color is often considered the classic example of demonstrating categorical versus continuous perception. Colors such as blue and yellow can be distinct and defined categorically, but colors are actually continuous when viewed in a spectrum. Blue and yellow blend together to form shades of each individual color, with green resulting where the colors come together fully.

While it has applications in other fields, the concept of categorical perception originated with the study of speech and remains an interesting area of study for experts in the field. One reason for this is that how people categorize things influences how they speak and think about them. For example, an artist might refer to the color of an object as cobalt, azure, sky blue, or steel blue, while someone else might simply identify the object as blue. This tendency applies to other categories on a cultural level as well. For example, someone from a warm climate might refer to white frozen precipitation as snow, while a person from the Inuit culture that lives in Alaska will have many words that refer to different types of snow. Understanding categorical perception helps researchers learn more about how people think.

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