Categorical perception

Categorical perception is a phenomenon of perception 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 [ba] and [pa], for example, [p] and [b] 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 [p] and [b] 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 perception," 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 [ba] has shorter VOT than the "p" in [pa] (i.e. the vocal folds start vibrating around the time of the release of the occlusion for [b], but tens of miliseconds later for [p]; but note that different varieties of English may implement VOT in different ways to signal contrast).