The Account

Speech perception is not explained by principles that apply to the perception of sounds in general.

Biological specialization for phonetic gestures prevents listeners from hearing the signal as ordinary sound, but enables them to use the systematic, special relation between signal sound to perceive the gestures.

e.g., listener uses systematically varying formant transitions as information about an overlapped consonant and vowel.

Adaptation of the motor system for producing gestures and gestural overlap took precedence in the evolution of speech. A perceiving system sensitive to the consequence of such overlap must have developed in parallel.

 

This accounts for:

 

Lack of Segmentability

Perceptual system is continually extracting information about overlapping consonant and vowel gestures, from the acoustic signal.

 

Categorical Perception

Although the stimuli vary along a continuous parameter, the gestures that are perceived differ categorically.

For example, in the /bdg/ continuum, the closure gesture is perceived as produced by the lips, tongue tip, or tongue body constrictors.

For a Voice-Onset time (VOT) continuum, the stimulus is perceived as having an active abduction gesture of the vocal folds (voiceless), or not (voiced).

 

Similarities to other specialized perceiving systems

Such specialized neural systems take advantage of a complex, but systematic relation between some proximal stimulation and a distal property.

Auditory Localization as an example

Information is in differences in time of arrival of sounds to the two ears.

This difference is not perceived by the auditory system as a time delay, but is used by the specialized neural system to compute location in space.

Similarly, the temporal relation between stop release and onset of periodic energy is used as information about a glottal opening gesture. Not heard as a temporal difference by the auditory system.