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PSYCH 2E03 (32)
Chapter 13

chapter 13.docx

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Department
Psychology
Course Code
PSYCH 2E03
Professor
Gautam Ullal

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.Psych 2E03: Sensory Processes Chapter 13: Speech Perception The Speech Stimulus - Phonemes – sounds and meanings:  The phoneme is the shortest segment of speech that, if changed, would change the meaning of a word  Phonemes refer not to letters but to speech sounds tha serve to distinguish meaning  Phonemes are defined in terms of the sounds that are used to create words in a specific language - The acoustic signal – patterns of pressure change:  Speech sounds are produced by the position or the movement of structures within the vocal apparatus, which produce patterns of pressure changes in the air that are called the acoustic stimulus  Acoustic signal for most speech sounds is created by air that is pushed up from the lungs past the vocal cords and into the vocal tract  Sound produced depends on the shape of the vocal tracts as air is pushed through it  Shape of the vocal tract is altered by moving the articulators, which include structures such as the tongue, lips, teeth, jaw and soft palate  Vowels are produced by vibration of the vocal cords, and the specific sounds of each vowel are created by changing the overall shape of the vocal tract which changes the resonant frequency of the vocal tract and produces peaks of pressure at different frequencies (formants)  Speech spectrogram indicates the pattern of frequencies and intensities over time that make up the acoustic signal  Consonants are produced by constriction, or closing, of the vocal tract  Rapid shifts in frequency preceding formants are called formant transitions and are associated with consonants The Relationship Between the Speech Stimulus and What We Hear - Relationship between the acoustic signal and the sounds we hear are extremely complex - The acoustic signal is not neatly separated into individual words - Lack of separation between the signals for each word creates the segmentation problem - The segmentation problem:  Auditory system faces the problem of separating speech stimuli into individual words  Looking at speech spectrogram, we see that the acoustic signal is continuous, with either no physical break in the signal or breaks that don’t necessarily correspond to breaks we perceive between words  The fact that there are usually no spaces between words becomes obvious when you listen to someone speaking a foreign language  The solution to the segmentation problem involves determining how we divide the continuous stream of the acoustic signal into a series of individual words - The variability problem:  Acoustic signal is so variable that there is no simple correspondence between the acoustic signal and individual phonemes  Variability from a phoneme’s context:  The acoustic signal associated with a phoneme changes depending on its context  This effect of context occurs because of the way speech is produced  The aritculators are constantly moving as we talk, so the shape of the vocal tract for a particular phoneme is influenced by the shapes for the phonemes that both precede it and follow it  This overlap between the articulation of neighbouring phonemes is called coarticualtion  The fact that we perceive the sound of a phoneme as the same ben though the acoustic signal is changed by coarticulation, is an example of perceptual constancy  We perceive the sound of a particular phoneme as constant even when the phoneme appears in different contexts that change its acoustic signal  Variability from different speakers:  Wide variations in speech mean that for different speakers a particular phoneme or word can have very different acoustic signals  The variability in the acoustic signal caused by coarticualtion, different speakers, and sloppy pronunciation creates a problem for the listener: they must somehow transform the information contained in this highly variable acoustic signal into familiar words  Variability problem and segmentation problem are the reasons that it has been so difficult to design machines that recognize speech Stimulus Dimensions of Speech Perception - Invariant acoustic cues:  Invariant acoustic cue is a feature of the acoustic signal associated with a particular phoneme that remains constant even when phonemes appear in different contexts or are spoken by different speakers  One way of displaying the acoustic signal is short-term spectrum which creates a detailed picture of the frequencies that occur within a short segment of time  The advantage od the short-term spectrum is that it provides a precise and detailed picture of the acoustic signal  A sequence of short-term spectra can be combined to create a running spectral display that shows how the frequencies in the auditory signal change over time  The invariance of these cues has been demonstrated by showing that people can identify characteristics of phonemes based on these cues, even in different contexts  It has not been possible to identify invariant cues for all of the speech sounds - Categorical perception:  Categorical perception: when a wide range of acoustic signals results in perception of a limited number of categories of sounds  Voice onset time is the time delay between when a sound begins and when the vocal cords begin vibrating  Researchers vary VOT in small steps from short to long: when they vary VOT, and ask listeners to indicate what sound they hear, the listeners report hearing only one or the other of the two phonemes  Phonetic boundary: VOT when the perception changes between two phonemes  Even though the VOT is changed continuously across a wide range, the listener perceives only two categories  Once we have demonstrated categorical perception, we can run a discrimination test, in which we present two stimuli with different VOTs and ask the listener whether they sound the same or different  When presented two stimuli on the same side of the phonetic boundary, the listener says they sound the same  When presented two stimuli on opposite sides of the phonetic boundary, the listener says they sound different  Fact that all stimuli on the same side of
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