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 processes that underlie perception of consonants and vowels are specifically phonetic, distinct from those that localize sources and assign auditory qualities to the sound from each source. This specialization, or module, increases the rate of information flow, establishes the parity between sender and receiver that every communication system must have, and provides for the natural development of phonetic structures in the species and in the individual. The phonetic module has certain properties in common with modules that are "closed" (for example, sound localization or echo ranging in bats) and, like other members of this class, is so placed in the architecture of the auditory system as to preempt information that is relevant to its special function. Accordingly, this information is not available to such "open" modules as those for pitch, loudness, and timbre.

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