Speech Perception Essays (2326 words) - Phonetics, Speech Perception

Speech Perception Speech Perception Speech perception is the ability to comprehend speech through listening. Mankind is constantly being bombarded by acoustical energy. The challenge to humanity is to translate this energy into meaningful data. Speech perception is not dependent on the extraction of simple invariant acoustic patterns in the speech waveform. The sounds acoustic pattern is complex and greatly varies. It is dependent upon the preceding and following sounds (Moore, 1997). According to Fant (1973), speech perception is a process consisting of both successive and concurrent identification on a series of progressively more abstract levels of linguistic structure. Nature of Speech Sounds Phonemes are the smallest unit of sound. In any given language words are formed by combining these phonemes. English has approximately 40 different phonemes that are defined in terms of what is perceived, rather than in terms of acoustic patterns. Phonemes are abstract, subjective entities that are often specified in terms of how they are produced. Alone they have no meaning, but in combination they form words (Moore, 1997). In speech there are vowels and consonants. Consonants are produced by constricting the vocal tract at some point along its length. These sounds are classified into different types according to the degree and nature of the constriction. The types are stops, affricates, fricatives, nasals, and approximants. Vowels are usually voiced and are relatively stable over time Moore, 1997). Categorical Perception Categorical perception implies definite identification of the stimuli. The main point in this area is that the listener can only correctly distinguish speech sounds to the extent that they are identified as different phonemes. Small changes to the acoustical signal may make little difference to the way the sound is perceived, yet other changes which are equally as small may produce a distinct change, altering the phoneme identity. People do not hear changes within one phoneme category. Only changes from one phoneme to another phoneme are detected (Lobacz, 1984). Although categorical perception generally is considered to reflect the operation of a special speech decoder, there is a strong indication that categorical perception can also occur in non-speech signals. Musicians are a good example of this. The discrimination performance of musicians was better for frequency changes that revised the identity of the chord than for changes that did not alter the identity (Moore, 1997). Categorical perception is not unique to speech, however it appears more frequently with speech than with non-speech signals. There are three possible explanations for categorical perception. The first explanation suggests that consonants and vowels may be explained in terms of differences in the extent to which the acoustic patterns can be retained in auditory memory. Consonant sounds have a lower intensity than vowels, fluctuate more rapidly, and last for a shorter time than vowels. Therefore, the acoustic patterns of consonants frequently decay rapidly. Another explanation is that boundaries, which separate one speech sound from another, tend to lie at a point where discrimination is optimal. The last explanation is that it comes from experience with a persons own language. In this explanation it is believed that a person learns to attend to acoustic differences that affect the meaning of a word and ignore the differences that do not affect the meaning. The natural consequence of this is categorical perception (Moore, 1997). Brain Specialization Language functions are unilaterally represented in one of the two hemispheres. It is most commonly found in the left hemisphere. Therefore, the right ear will identify speech stimuli better than the left ear. This occurs because the neural pathways cross from the ear to the brain (Studdert-Kennedy and Shankweiler, 1970). Interestingly, the left ear will detect melodies better than the right ear. Speech is more readily decoded in the left hemisphere than in the right cerebral hemisphere. This is evident in people with brain lesions. The left hemisphere plays a primary role in speech perception (Moore, 1997). Speech Mode Speech mode is the perception of the restructured phonemes. If phonemes are encoded syllabically, they must be recovered in perception by a suitable decoder. Liberman (1996) stated that perception of phonemes that have been encoded may be expected to differ from the perception of the phonemes that have not been encoded and from non-speech. For example, the transition cues for /d/ in /di/ and