Speech Perception 4/6/00

Acoustic-Perceptual Invariance in Speech Perceptual Constancy or Perceptual Invariance: –Perpetual constancy is necessary, however, acoustic properties of individual phonetic events vary across speakers, across conditions (speaking rate, repetition of the same sound) Five theories have been proposed to explain variance and perception

Prototypes or Templates “Ideal Forms” of the acoustic pattern for a phoneme Incoming stimuli is compared to the ideal and the closest match is selected Store of Templates Comparator Speech Output Output Decision

Prototypes or Templates Template serves as a reference pattern for incoming sound Incoming stimuli may be normalized or preprocessed to compare with prototype –Can process intertalker differences (age & gender) Perceptual magnet effect –Sounds similar to the prototype are identified with it –Perceptual magnet “warps” the perceptual space, so acoustic patterns close to “magnet” are perceived as similar

Features Features: –Various attributes that compose stimuli are identified through abstraction –Defining attributes of elements to be recognized –Nonspeech example: young girl (female, human, young) –Speech sounds can be analyzed in terms of characteristic features [b] bay: stop, voiced, bilabial –Change in features can make the difference in how a sound is recognized

Features Drawbacks: features are difficult to describe and define Features in many systems are composites of many features: articulatory, acoustic, & perceptual Speakers analyze sounds in terms of features –Confusions are feature based [p] instead of [b]

Higher Order Variables Invokes higher order variables as the means for perceptual constancy Acoustic events are uniquely specified by time varying properties Listener learns that patterns of energy arise from a consistent sound source. Problem: How are variables identified for complex stimuli?

Innate Systems Invariant cues are identified by innate mechanisms Simply born with ability to achieve perceptual constancy Problem: How do you account for effects of sensory experience? Innate mechanisms are the result of evolutionary adaptations by which sensory systems are equipped to reliably perceive the environment

Connectionst Networks Parallel distributed processing (PDP) –Behavior can be modeled with networks that involve large numbers of interconnected units –Modeled from computer theory –Learn patterns that represent the structure of phonemes –Network learns the structure through various adjustments until the input matches the output –“learning” language patterns sometimes make the same kind of developmental errors committed by children

Experimental Phenomena in Speech perception Categorical Perception Laterality Effects Adaptation Effects Trading Effects

Categorical Perception Listener categorized stimuli so that the stimuli within a category sound alike The continuum of a stimuli must vary along some parameter (VOT) and has one or more boundaries that divide the stimuli into perceptual categories- Identification Experiment (label stimuli) –ex. VOT differences with voiced & voiceless phonemes –ex. same categorical perception with a color spectrum –Speech categories: voice onset time, place of articulation, rise time

Categorical Perception Discrimination Experiment- –Stimuli within a category be poorly discriminated compared to stimuli from different categories ex. VOT values of ms should be hard to tell apart Same category- hard to tell apart Different category- easy to tell apart –Categorical perception has been studied using VOT- VOT stimuli are heard as 1 of 2 categories- Voiced (b) or voiceless (p). –[b] is chosen with values less than 20 ms –[p] is chosen with values higher than 30 ms

Categorical Perception Categorical perception of VOT for nonspeech –Tones are distinguished the same as speech VOT (perception may be with acoustic stimuli) Categorical perception of VOT in infants- –Show adult like discrimination (heart rate deceleration) Categorical perception of VOT in Animals- –Chinchillas can discriminate VOT differences (acoustic discrimination based on the mammalian auditory system)

Responses of [b] and [p] to Voice-Onset Stimuli Phonetic Boundary at 20 ms

Laterality Effects Right or left ear advantage Dichotic perception (delivery of different stimuli to different ears) The advantage or most heard words will go to one ear or the other –REA (Right ear advantage)- for stops mostly –LEA (Left ear advantage)- for tones & steady state vowels Some sounds are processed better in one hemisphere than the other REA is a left hemisphere advantage

Adaptation Repeated exposure to an extreme member of some acoustic continuum causes a shift in a category boundary Auditory system adapts to repeated stimuli Stimulus becomes less effective in evoking the phonetic property –ex. voiceless sound becomes harder to distinguish after repetition

Trading Relations Sounds associated with two or more acoustic cues –ex. stop consonant (brief silence, burst, transition) Integration of cues into single phonetic event & recognizes the different cues as belonging to that sound –Phonetic contrasts: Witch and Wish Differ in final consonant Distinguishing difference is duration of silence & frication –The listener evaluates 2 or more cues and makes a relative decision based on their properties

Trading Relations How does this phenomenon take place? –The cochlea functions as a sensitive detector and spectral analyzer of brief transient events

Segmentation What about distinguishing between words? –Listener uses a variety of cues- rhythmical pattern- stress & prosody –I.e. strong syllable most likely to initiate word onset Silence or Pausing- often pause between words Phonotactics- phoneme arrangement is set –I.e. velar nasal [n] as in [thing] cannot occur in word initial position