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The Perception of Speech

Speech is for rapid communication Speech is composed of units of sound called phonemes –examples of phonemes: /ba/ in bat, /pa/ in pat Speech

Speech can be characterized by a spectrogram Acoustic Properties of Speech

Spectrogram reveals differences between phonemes Acoustic Properties of Speech

Perceiving Speech So perceiving (interpreting) speech sounds is simply a matter of matching the spectrotemporal properties (the shape of the spectrogram) of the incoming sound waves to the appropriate phoneme right?…

Perceiving Speech So perceiving (interpreting) speech sounds is simply a matter of matching the spectrotemporal properties (the shape of the spectrogram) of the incoming sound waves to the appropriate phoneme Then specific phonemes must correspond to specific spectrograms - a property called acoustic-phonetic invariance

Acoustic - Phonetic invariance says that phonemes should match one and only one pattern in the spectrogram –This is not the case! For example /d/ followed by different vowels: Perceiving Speech

Acoustic - Phonetic invariance says that phonemes should match one and only one pattern in the spectrogram –This is not the case! For example /d/ Clearly perception and understanding of speech sounds is more elaborate than simply interpreting an internal spectrogram Perceiving Speech

The phrase “Peter buttered the burnt toast” has five /t/ phonemes. There are not 5 identical sweeps in the spectrogram Perceiving Speech

Segmentation is the perception of silence between words Often illusory Perceiving Speech

The phrase “I owe you a Yo-Yo” has no silence in it ! Perceiving Speech

So how do you perceive speech?

Perceiving Speech So how do you perceive speech? Some of the “strategies”: 1. reduce the data 2. use context clues 3. use vision

Categorical Perception is a phenomenon in which the brain assigns a stimulus into one or another category but never into an intermediate category Categorical Perception Sifts through the Incoming Sound

For example, /ba/ and /pa/ differ in their formant transitions –/ba/ is formed by stopping the flow of air from the lungs and releasing it after about 10 ms. (called voice onset time) –/pa/ is similar except that voice onset time is about 50 ms Categorical Perception

Voice onset time can range from zero to >50 ms. For example, you could synthesize a sound with a voice onset time of 30 ms but... Categorical Perception

Voice onset time can range from zero to >50 ms. For example, you could synthesize a sound with a voice onset time of 30 ms but... Listeners will hear either /ba/ or /pa/ but never something in between Categorical Perception

Categorical Perception is Part of Learning a Language Babies can discriminate /ba/ from /pa/ and can discriminate these from phonemes with intermediate voice onset times! By 10 to 12 months, babies (learning English) stop discriminating intermediate voice onset times

Categorical Perception is Part of Learning a Language Once category boundaries are learned it is impossible to unlearn them –non-native speakers can often never hear certain phonemes –as a consequence they will always have at least some slight accent

Another example: Categorical Perception

Perception (of all types) Makes Use of Context The stream of information contained in speech is usually ambiguous and incomplete Your brain makes a “best guess” based on the circumstances

Perception (of all types) Makes Use of Context Consider the following example: “The __eel fell of the cough shoe”. car”.

Perception (of all types) Makes Use of Context Consider the following example: Listeners report hearing the “appropriate” phoneme during the cough “The __eel fell of the cough shoe”. car”.

Why rely on only one sensory system when there is information in two !? Much of Speech Perception isn’t Auditory !

Why rely on only one sensory system when there is information in two !? The brain seamlessly integrates any information it is given - this is called cross- modal integration Much of Speech Perception isn’t Auditory !

Speech perception involves the synthesis of vision and hearing The McGurk effect demonstrates the critical role of vision on speech perception Cross-modal Integration

The McGurk Effect Cross-modal Integration

Next Time: Vision