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Chapter 13: Speech

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Western University
Psychology 2115A/B
Christine Tsang

Lecture 13 Review: Speech Perception By the end of this section, you should know:  The acoustic signal of speech  How variability affects speech perception  Categorical perception  Multimodal issues in speech  Word segmentation  Brain areas involved in speech What is Speech?  Can be understood up to a rate of 50 units/second – normal speech is 12 units/second  We can compare our rapid perception of speech to non-speech sounds in which our perception is limited to 0.67 units/second  Our ability to perceive speech sounds persists even when the sounds are distorted in various ways – like cell phones filtering the voice sound considerably – we lose a lot of resolution but are still able to perceive the speech sounds and make sense of what they mean  An acoustic signal or stimulus o Air pushed up through our lungs and through our vocal chords into the vocal tract and created by the vibration of the vocal chords in the throat o Articulators change the shape of the vocal tract in some way – narrow to create higher pitch, widens for a lower pitch  A collection of ordered sounds to which meaning is attached o Phoneme sound, vowel sound, high or low pitch  Spectrograms: graphs of the amount of energy at various frequencies over time o Darkness reflects how intense the sound is o 4 dark bands/regions – called formant, referring to bands of energy in the spectrogram o We number our formants – lowest is 1, etc. o The first 2 formants we pay the most attention to – they tell us about vowels and the difference between 1 and 2 tells us about which vowel is being sounded Vowels  Produced by vibrating the vocal folds as air moves out of the lungs through the open mouth o If you produce any vowel, your mouth is fairly open and the sound we produce depends on the various positions of the articulators in your mouth  Sound produced depends on positions of structures of the vocal tract o Tongue position is a critical part of producing vowel sounds o Vowel sounds largely produced in conjunction with vocal chords moving in and out and the articulators Consonants  Produced by closing or constricting the vocal tract  Place of articulation: at what point is the vocal chord restricted?  Manner of articulation: how is the vocal chord constricted?  Voiced vs. unvoiced: reflects how the air is being pushed through the opening itself o Related to both place and manner o Voiced: constricting the airflow o Unvoiced: do not begin their vibrations in the vocal chord until a long time after constriction The Phoneme  How we measure units of speech  Phoneme: smallest unit of speech  Contains sound energy at a number of different frequencies, creating an acoustic signal  In English, we have 47 phonemes and our vocal tracts can produce 100-120 different sounds The Variability Problem – Context  Context changes the relationship between the acoustic signal and individual phoneme  Problems of variability – no 1 to 1 simple correspondence between acoustic speech signal and the actual phoneme  If we looked at an acoustic speech signal, could we pick out the phonemes in the signal?  Why is that a problem? Because the speech context changes the way the speech signal and the phoneme react to one another or the relationship between them  The context of phonemes in speech don’t always sound the same  This is a problem because of co-articulation: the overlap between articulation of 1 phoneme and a neighboring phoneme in a speech sound/word  Boot/bat articulation of the b-lips are rounded differently… the 2 b’s from a signal point of view are different from one another but from a perceptual point of view, we hear them both as the same sound but really they are variations The Variability Problem – Speaker Variation  Variability from different speakers creates a problem in terms of constancy of sounds  E.g. all our voices sound different but we can still make sense of a sentence said by different people  Can still make sense of what someone is saying if the pitch, speed, or pronunciation is changed even though each vary from person to person Categorical Perception  How do we solve the variability problem? o One solution is the fact that we can categorically perceive certain sounds o By categorizing certain sounds into one category or another, this constrains the variability that exists in the signal  Categorical perception occurs when a wide range of acoustic cues results in the perception of a limited number of sound categories o Associate colour labels to colours o Turns out consonants are categorically perceived Voice Onset Time  Manipulate VOT o Voice onset time: the time delay between when the sound begins and when we start voicing that sound  Phonemic boundaries o You can shift the phonetic boundary by giving you exposure to different phonetic categories Is Speech Special?  Its a fact that we can hear and perceive phonemic rapidly and have categorical perception that we don’t have for other sounds in the environment  This suggests that we have a special place in our perception for speech, but we do not o Phonemic boundaries are perceived by non-human species  Boundaries are perceived by non-humans who do not have language/speech (e.g. hamsters not unique to humans) o Speech may just be a case of auditory perception  May just be so practiced that we cant help but be exposed to it and have a lot of practice in perceiving the boundaries  Adaption effects: we can shift the phonemic boundaries and it is very highly practiced/experienced McGurk Effect  Speech perception is not just a product of auditory processing o Although we talk about speech as an auditory signal or as an acoustic stimulus, actually there is a lot more to the perception of speech than what is present in the acoustic stimulus  McGurk effect: suggest that speech perception is multi-modal – visual and audio processing o His lips are saying ‘ga’ and the sound is saying ‘ba’ but our brain reads ‘da’ o The accurate perception of information involves more than one stimulus  multi-modal (vision and auditory) o Not a perception of one stimulus and another, but an integration
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