Speech production the larynx that then is modified by the vocal tract into different vowels and consonants. However speech production can occur without the use of the lungs and glottis in alaryngeal speech by using the upper parts of the vocal tract. An example of such alaryngeal speech is Donald Duck talk. The vocal production of speech may be associated with the production of hand gestures that act to enhance the comprehensibility of what is being said. The development of speech production throughout an individual's life starts from an infant's first babble and is transformed into fully developed speech by the age
Speech perception been called vocal tract normalization (see Figure 3 for an example). Similarly, listeners are believed to adjust the perception of duration to the current tempo of the speech they are listening to – this has been referred to as speech rate normalization. Whether or not normalization actually takes place and what is its exact nature is a matter of theoretical controversy (see theories below). Perceptual constancy is a phenomenon not specific to speech perception only; it exists in other types of perception too. Categorical perception is involved in processes of perceptual differentiation. People perceive speech sounds categorically, that is to