Man could not perceive speech well if each phoneme were cued by a unit sound. In fact, many phonemes are encoded so that a single acoustic cue carries information in parallel about successive phonemic segments. This reduces the rate at which discrete sounds must be perceived, but at the price of a complex relation between cue and phoneme: cues vary greatly with context, and there are, in these cases, no commutable acoustic segments of phonemic size. Phoneme perception therefore requires a special decoder. A possible model supposes that the encoding occurs below the level of the (invariant) neuromotor commands to the articulatory muscles. The decoder may then identify phonemes by referring the incoming speech sounds to those commands.

A motor theory of speech perception, initially proposed to account for results of early experiments with synthetic speech, is now extensively revised to accommodate recent findings, and to relate the assumptions of the theory to those that might be made about other perceptual modes. According to the revised theory, phonetic information is perceived in a biologically distinct system, a 'module' specialized to detect the intended gestures of the speaker that are the basis for phonetic categories. Built into the structure of this module is the unique but lawful relationship between the gestures and the acoustic patterns in which they are variously overlapped. In consequence, the module causes perception of phonetic structure without translation from preliminary auditory impressions. Thus, it is comparable to such other modules as the one that enables an animal to localize sound. Peculiar to the phonetic module are the relation between perception and production it incorporates and the fact that it must compete with other modules for the same stimulus variations.

The phoneme is most often taken to be the smallest unit of speech that can, by itself, distinguish one utterance from another as to meaning. Thus, the existence of the two words bill and dill makes it clear that b and d are different phonemes in English. It should be emphasized that the phoneme is not a single sound, but is, rather, a class which can and usually does include a great many sounds that differ from each other in various ways without causing any change in meaning.