Nuclear Accent Shape and the Perception of Syllable Pitch Rachael-Anne Knight LAGB 16 April 2003

Outline  Factors affecting perception of pitch  Contour shape  Experiment  Design  Results  Theoretical and psychoacoustic explanations  Conclusions and implications

The Perception of Pitch  The perception of the pitch of a complex sound is related to the fundamental frequency  Sounds that have a higher fundamental frequency sound higher in pitch  And in speech are associated with greater prominence

Syllable Position  In an utterance the position of a syllable also affects the perception of pitch  If two syllables have identical F0, the one later in the utterance will sound higher in pitch  Explained as the listener ‘normalising for declination’ Frequency Time

Contour Shape  Real contours are not stylised peaks and troughs  The majority of falling nuclear accents are realised as more of a flat stretch of contour Peak Plateau

Definition of the Plateau  Plateaux are defined as being 4% down from any absolute peak in F0  4% is the range of perceptual equality Peak 4% range Plateau

Segmental and Prosodic Effects on the Production of the Plateau  Plateaux take up more of syllables that have sonorant onsets and codas  Plateaux are aligned later in the syllable in polysyllabic than monosyllabic feet  Some speakers align the end of the plateau earlier in the syllable before a word boundary

The End of the Plateau  The end of the plateau is stably aligned within the syllable regardless of pitch span  More errors made with incorrect EP alignment in a true/false judgment task  The end of the plateau seems to be the real target (rather than the peak)

Experiment  How does a plateau affect the perception of pitch?  3 possible hypotheses  No effect  A longer plateau makes a syllable sound higher in pitch  A longer plateau makes a syllable sound lower in pitch

Stimuli  Test sentence  “…came with Manny” taken from “Anna came with Manny”  Resynthesised nuclear accent (12 versions)  Frequency of contour o 160, 180, 200, 210 Hz  Shape of contour o Peak, 50ms or 100 ms plateau

Examples of Different Contours AnnaManny

Procedure  7 subjects heard pairs of sentences  ‘Manny’ differed only in shape never in pitch  Question: “In which version is ‘Manny’ higher in pitch?”  Responded by pressing 1 or 2 on a keyboard 160 Hz plateau, peak210 Hz peak, plateau

Results  Percentage of times the longer stretch of contour sounds higher in pitch than the shorter stretch  Overall  50ms vs. peak  100ms vs. peak  100ms vs. 50ms  By frequency  160Hz  180Hz  200Hz  210Hz  By Position  Longer stretch in utt. 1  Longer stretch in utt. 2  By shape 1  50ms vs. peak  100ms vs. peak  By shape 2  100ms vs. 50ms

Results Overall  The shape of the contour does affect the perception of pitch  73% of responses were for “longer stretch of contour sounds higher”

Results by Frequency  The significant result holds at each of the 4 frequencies  160 Hz 67%  180 Hz 72%  200 Hz83%  210 Hz67%

Results by Position  The result is not significant when the plateau occurs in the first token  The result is significant when the plateau occurs in the second position

Results by Shape 1  Both lengths of plateau are perceived as longer when compared to a peak  50ms 77%  100ms 77%

Results by Shape 2  There is no significant difference between the two lengths of plateau

Results Summary  Overall plateaux are perceived as higher in pitch than peaks  Regardless of the frequency  This result is only significant when the plateau is in second position  Suggesting an interaction with position  There is no significant difference between the perception of the 2 plateau lengths  Suggesting a categorical rather than gradient difference

Possible Explanations  There are two possible explanations for why longer stretches of contour may sound higher in pitch  Integration  Temporal Smoothing

Integration  The listener may be integrating the area under the curve  There is a larger area under the curve of a plateau

Smoothing  The listener may be smoothing the curve  So peaks will sound lower than they actually are

Predictions  Integration  As the plateau gets longer pitch will be perceived as increasingly higher  Smoothing  There will be a cut-off point at which a longer plateau no longer sounds higher in pitch

Psychoacoustic Explanation  “Stability-sensitive weighting”  In pitch perception less weight is given to portions of the signal where frequency is changing rapidly  Due to the sluggishness of the auditory system

Pitch Perception  There are 2 mechanisms for extracting pitch from a signal  Place mechanism  Perceived pitch corresponds to the place of maximum excitation on the basilar membrane  Temporal mechanism  Pitch is derived from the frequency of nerve firings  Phase Locking

The Inner Ear

The Cochlea

Cochlea Cross Section

The Organ of Corti

Phase Locking  Neural firings firings occur at the same phase of the waveform each time  The intervals between firings will be integral multiples of the period of the wave

Sluggishness  If the pitch is changing too rapidly  The auditory system does not have enough time to phase lock  Pitch may not be perceived accurately

Conclusions  The shape of the contour does affect perceptions of pitch  Longer stretches of pitch sound higher than single turning points  There is a categorical difference between peaks and plateau  Suggesting an explanation based on smoothing  This effect can be explained as the sluggishness of the phase locking mechanism

Implications  My PhD  Plateaux may occur to enhance the prominence of the nucleus  Models of pitch span  May need to take account of contour shape  Intonational phonology  The situation is more complicated than peaks and troughs in the contour  A more perceptual approach is necessary