Categorical perception is a phenomenon of

perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous syste ...

of distinct categories when there is gradual change in a variable along a continuum. It was originally observed for auditory stimuli but now found to be applicable to other perceptual modalities.

Motor theory of speech perception

If one analyzes the sound spectrogram of aand a for example, and can be visualized as lying somewhere on an acoustic continuum based on their VOT (voice onset time). It is possible to construct a continuum of some intermediate tokens lying between the and endpoints by gradually decreasing the voice onset time. Alvin Liberman and colleagues (he did not talk about voice onset time in that paper) reported that when people listen to sounds that vary along the voicing continuum, they perceive only /ba/s and /pa/s, nothing in between. This effect—in which a perceived quality jumps abruptly from one category to another at a certain point along a continuum, instead of changing gradually—he dubbed "categorical perception" (CP). He suggested that CP was unique to speech, that CP made speech special, and, in what came to be called "the motor theory of speech

," he suggested that CP's explanation lay in the anatomy of speech production. According to the (now abandoned) motor theory of speech perception, the reason people perceive an abrupt change between /ba/ and /pa/ is that the way we hear speech sounds is influenced by how people produce them when they speak. What is varying along this continuum is voice-onset-time: the "b" in ahas shorter VOT than the "p" in a(i.e. the vocal folds start vibrating around the time of the release of the occlusion for but tens of miliseconds later for but note that different varieties of English may implement VOT in different ways to signal contrast). Apparently, unlike the synthetic "morphing" apparatus, people's natural vocal apparatus is not capable of producing anything in between ba and pa. So when one hears a

sound In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...

from the VOT continuum, their brain perceives it by trying to match it with what it would have had to do to produce it. Since the only thing they can produce is /ba/ or /pa/, they will perceive any of the synthetic stimuli along the continuum as either /ba/ or /pa/, whichever it is closer to. A similar CP effect is found with ba/da (or with any two speech sounds belonging to different categories); these too lie along a continuum acoustically, but vocally, /ba/ is formed with the two lips, /da/ with the tip of the tongue and the alveolar ridge, and our anatomy does not allow any intermediates. The motor

theory A theory is a systematic and rational form of abstract thinking about a phenomenon, or the conclusions derived from such thinking. It involves contemplative and logical reasoning, often supported by processes such as observation, experimentation, ...

of speech perception explained how speech was special and why speech-sounds are perceived categorically: sensory perception is mediated by motor production.

Acquired distinctiveness

If motor production mediates sensory

, then one assumes that this CP effect is a result of learning to produce

speech Speech is the use of the human voice as a medium for language. Spoken language combines vowel and consonant sounds to form units of meaning like words, which belong to a language's lexicon. There are many different intentional speech acts, suc ...

. Eimas et al. (1971), however, found that infants already have speech CP before they begin to speak. Perhaps, then, it is an

innate {{Short pages monitor