The Perception of Syntactic Disambiguation in Infant-Directed Speech

Suzanne Curtin

Stephen Winters

October 12, 2011

Acoustics Week in Canada

Infant-Directed Speech: Acoustic Properties

• Utterances directed towards infants generally have (Fernald & Mazzie, 1991):

1. A higher pitch

2. A greater pitch range

3. Slower rate of speech

4. Longer pauses

5. Are shorter than utterances directed towards adults.

Infant-Directed Speech: Perceptual Benefits

• Infant-directed speech (IDS) is thought to serve several functions:

1. Marking turn-taking episodes (Snow, 1977)

2. Helping infants segment the speech stream (Fernald & Simon, 1984)

3. Highlighting new linguistic information (Fernald, 1984)

• Typically developing infants demonstrate a preference for IDS as young as 4 months of age (Werker & McLeod, 1989)

Hmm.• If IDS helps infants learn how to take turns, segment the speech stream, etc….

• does it also help them interpret higher-level syntactic structures in speech?

• Specifically: can the acoustic information in IDS cue listeners in to the correct interpretation of a syntactically ambiguous string of words?

• For instance: does IDS disambiguate whether a string of words belongs to a single phrase or crosses a phrase boundary:

• ...[the giant]np [hands over the candy]vp (noun - verb)

• ...[[the giant hands][over the candy]]np (adj - noun)

Experimental Design1. Six syntactically ambiguous phrases were

constructed:

• all with similar structure to “giant hands over the candy”

• Note: some of these phrases were only potentially ambiguous within a fantasy context:

• “the peach colours on the wall”

2. For each phrase, two children’s stories were written to provide a meaningful context (one A-N and one N-V)

• And pictures were drawn to help bring them to life.

Stimulus Production3. 12 new mothers read these stories to two target

audiences:

1. their infant children (sitting in their laps)

2. an adult experimenter (not sitting in their laps)

• Mothers paged through the storybooks as they read the stories

• Stimuli were recorded via a wireless lapel microphone

• Adult-directed reading always followed the infant-directed reading.

• All readers were native speakers of English.

Example Story• “George is a very large man. He is the tallest man in the whole town. When the children come over to play, he hides toys and treats for them.”

Example Story• “Laughing with delight, the children discover the toy cars behind George’s big feet, and see the giant hands over the candy. George shares the toys and candy with the children.”

Perception Experiment• Ambiguous phrases were excised from their

contexts

• amplitude normalized (70 dB)

• presented to listeners in a perception experiment

• Note: some infant-directed productions were unusable, due to background noise.

• 25 native English-speaking listeners participated in the perception experiment

• (each received course credit)

Perception Experiment: Tasks

1. Read through series of children’s stories.

2. Listen to 144 syntactically ambiguous phrases:

• 12 from each speaker

• 24 repetitions of each phrase

• (12 for each syntactic context)

3. After each stimulus, determine:

• Which picture best depicted the context the utterance was read in. (syntax identification)

• Whether the phrase was read to an adult or an infant. (target identification)

Perception Experiment: Demo

• Listeners could replay stimuli as many times as they liked.

• Order of pictures was counterbalanced across presentations.

• Target identification significantly better than chance (p < .001)

• No significant difference in accuracy between IDS and ADS utterances.

• For raw % correct:

• Syntax identification also significantly better than chance (p < .001)

• Significant difference in accuracy between IDS and ADS utterances.

• but…

Further Analysis• Responses to the syntactic identification task were converted into scores of sensitivity and bias for each listener, for each type of stimuli (IDS and ADS) (Wickens, 2002)

• Sensitivity (d’):

• quantifies how easily listeners can distinguish between AN and NV response options

• Bias ():

• quantifies how much listeners favor one response option over another

• In terms of sensitivity (d’):

• Syntax identification still significantly better than chance (p < .001)

• No significant difference in accuracy between IDS and ADS utterances. (p = .467)

• In terms of bias ():

•Listeners significantly biased towards NV responses (p < .001)

• Greater bias towards NV responses for IDS utterances. (p = .002)

Frequency FactsTarget Phrase(Brown corpus)

Adj-Noun Frequency (pl)

Noun-Verb Frequency (pl)

giant hands 284 0

elephant rides 6 4

orange drinks 20 2

candy sticks 19 3

orange colours 46 3

princess dresses 9 0

• There was a nearly significant negative correlation between (raw) correct syntax identification percent and frequency count (p = .055; r = -.118)

Syllable DurationPhrase Syntax IDS ADS

candy sticks AN .20 (.03) .17 (.01)

NV .21 (.03) .17 (.02)

elephant rides AN .21 (.05) .17 (.02)

NV .20 (.03) .17 (.01)

giant hands AN .22 (.03) .19 (.03)

NV .22 (.03) .17 (.03)

orange drinks AN .22 (.04) .19 (.03)

NV .24 (.04) .20 (.03)

peach colours AN .23 (.03) .20 (.02)

NV .25 (.06) .21 (.03)

princess dresses AN .21 (.03) .19 (.04)

NV .20 (.04) .18 (.02)

• Significant correlation between target identification accuracy and the average syllable duration of the phrase (p = .008; r = .228)

Duration and Syntax• There also seemed to be a positive correlation between the length of syllables in the target sequence and the percentage of noun-verb responses:

r Long (NV%) Short (NV%)

candy sticks .626(92.3)

(36.4)princess dresses .576

peach colours .542

orange drinks .470

elephant rides .141

giant hands -.423 (25.0)

(77.8)

Maybe Intonation Helps?

candy sticks (adj-noun)

candy sticks (noun-verb)

Less Expected Intonation

candy sticks (adj-noun)

candy sticks (noun-verb)

Intonation Implications• The intonation patterns produced by our speakers were highly variable;

• it is not clear to what extent they might have influenced the syntactic identification task.

• Mean F0 ranges (in Hz):

• IDS: 167 - 379

• ADS: 153 - 288

• A variety of factors (articulatory gaffes, baby cries, voice quality shifts, etc.) makes a comprehensive acoustic analysis of F0 patterns difficult.

Conclusions• It’s relatively easy to tell the difference between infant-directed and adult-directed speech

• acoustic cues like duration and F0 range provide strong cues

• It is not as easy to identify the syntactic structure of ambiguous phrases

• and embedding ambiguous phrases in IDS doesn’t seem to make it any easier.

• Experimental context may have biased listeners towards noun-verb responses

• IDS biased them even further in this direction;

• Longer syllables seem to play a role…

Future Directions• More in-depth acoustic analysis

• Digital manipulation of cues

• or possibly using trained speakers to produce the relevant intonational patterns…

• Eye-tracking study (with both infants and adults)

Acknowledgments• This research was funded by an NSF grant awarded to S. Curtin.

• Thanks to Stephanie Archer, Jennifer Campbell, and Jennifer Ference for data collection and stimuli creation.

• Tara Dainton helped run listeners, and Paige Leishman and Breanna Isted helped code the stimuli for acoustic analysis.

• Special thanks to Maryellen MacDonald and Robert Thornton for contribution to the conceptualization of the study.

Duration and Syntax• There also seemed to be a positive correlation between the length of syllables in the target sequence and the percentage of noun-verb responses:

r Long Short

candy sticks .626

princess dresses .576

peach colours .542

orange drinks .470

elephant rides .141

giant hands -.423

Correlation Graph?