Conditioned allophony in speech perception: An MEG study

Mary Ann Walter & Valentine Hacquard

walterma@mit.edu hacquard@mit.edu

Dept. of Linguistics & Philosophy, MIT

KIT/MIT MEG Lab

2nd Old World Conference in Phonology

CASTL-Tromsø, January 20-22, 2005

Questions about (conditioned) allophony:

• Structural properties of a language’s phonology significantly affect the perception of speech sounds (particularly well-documented wrt phonemic inventory (Kuhl 1993, Best 1995))

• How perceptible are differences between allophones to speakers?

• How does the perceived similarity of allophones compare to that of phonemes, free variants, and other kinds of contrast?

• What implications follow for models of phonology and similarity computation?

Overview:

• Subphonemic perceptibility

• MEG and the MMF

• Where allophones fit in:

• Russian/Korean

• French/Spanish

• Quebecois French

• Results: Equivocal, but ours suggest that allophones affiliate with structural, phonemic contrasts, rather than within-category free variants

Allophone perceptibility:

• Speakers produce consistent and finely controlled distinctions between allophones

• Suggests they must be able to perceptually distinguish them, at least proprioceptively

• Anecdotally this often seems not to be the case

• When measured experimentally, subjects typically distinguish allophones at above chance levels, but much less easily and well than phoneme pairs (for aspiration in English, see Pegg & Werker 1997, Whalen et al. 1997, Utman et al. 2000, Jones 2001)

• For the segment pair we will discuss (e and ε), evidence is contradictory:

• Pallier et al (1997) find that the pair is indistinguishable for Spanish speakers for whom they are conditioned allophones, even when bilingual in a language in which they are phonemic (Catalan)

• Escudero and Boersma (2002) find that discrimination does improve for such speakers, however

Magnetoencephalography (MEG):

• Measures brain responses as indexed by magnetic activity

• Millisecond-by-millisecond temporal resolution

• Gradient responses

• Avoids problems and task effects of offline similarity judgment tasks

• Consistency of rating scale

• Understanding of task

156 channels

Origin of signal in auditory cortex

The mismatch field response (MMF):

• An non-attentive auditory response that indexes perceived (cf tone experiments) (dis)similarity

• One of a number of language-sensitive neural responses

• Peaks between 180-250 ms post stimulus onset

The mismatch field response (MMF):

• Each line represents averaged sensors

• The mismatch response is the difference between the standard and deviant responses at peak/in the relevant time window

The mismatch field response (MMF):Voicing (Sharma & Dorman 1999)

VOT (ms)

0 10 20 30 40 50 60 70

d t

d t

t1 t2

• Significant MMF for both pairs ( within-category differences perceptible)

• Significantly greater MMF for the cross-category pair, despite equal acoustic distance

The mismatch field response (MMF):Vowel backness (Näätänen et al. 1997)

F2 (Hz)

• In general, larger frequency deviation larger MMF

• For Finns, significantly greater MMF for deviant ö than õ, despite greater acoustic distance of latter from standard (e)

e1940

o851

õ1311

ö1533

Finnish+

Estonian

Finnish+

Estonian Estonian

Finnish+

Estonian

• Studies have examined contrasts of:

• phonemic pairs versus within-category free variants (Sharma & Dorman)

• phonemic pairs versus non-prototypical segments (Näätänen et al.)

Phonemic contrasts have a special status in speech processing

• Are they the only ones?

• Allophonic contrasts are also encoded in a language’s phonology

• Allophones show a bimodal distribution on the surface, like phonemes

Contrast types and the MMF:

Predictions:

Phonemes a-e a-e a-e

Allophones e-ε e-ε e-ε

Free variants ε1- ε2 ε1 - ε2 ε1 - ε2

MMF Amplitude

Allophonic mismatches:Russian/Korean Voicing (Kazanina & Phillips 2004)

Russian:

d/t phonemically distinct

• Phonemes elicit a significant MMF response; allophones not at all

Korean:

d/t distributed allophonically (intervocalic voicing)

?

Predictions:

Phonemes a-e a-e a-e

Allophones e-ε e-ε e-ε

Free variants ε1- ε2 ε1 - ε2 ε1 - ε2

Kazanina & Phillips 2004

MMF Amplitude

vs.

• French: phonemic contrast

• Spanish (Buenos Aires): allophonic contrast

e / __ C ]σ

• Spanish (Puerto Rican): free variation

Allophonic mismatches:French/Spanish vowel tenseness – a three-way comparison

e e e e e e e a e e e e e e e e e e e e e e

• Standard: /e/ 1050

• Deviants: /a/ 100 random order

// 100

Non-attentive paradigm (silent movie)

French (n=10), Spanish-Argentinian (n=9), Spanish-Puerto Rican (n=4)

Method:

Results:

0

1E-14

2E-14

3E-14

4E-14

5E-14

6E-14

F A PR

T

E

e

a

• Middle bars represent baseline response to standard /e/, flanking bars are responses to deviants // (left) and /a/ (right)

• Mismatch response is significant for both deviants for all language groups (p<.02) (contrast with Kazanina and Phillips’ findings)

E-e Mismatch Wave Peaks

0

5

10

15

20

25

30

35

fT x

10^5

French

Argentinian

Puerto Rican

Results:

• A downward progression is observed in the MMF response, according to language group

• However, these differences do not reach statistical significance

• A trend differentiating French and Argentinian Spanish from Puerto Rican Spanish appears (p=.11)

Predictions:

Phonemes a-e a-e a-e

Allophones e-ε e-ε e-ε

Free variants ε1- ε2 ε1 - ε2 ε1 - ε2

Kazanina & Phillips 2004

MMF Amplitude

?

0

10

20

30

40

50

60

French Argentinian Puerto Rican

e-ae-E

*

*

T x

10^

-13

Results:

• a-e contrast elicits greater MMF for all language groups

• greater acoustic distance • consistently phonemic

• Significant for French and Puerto Rican, not Argentinian (p<.001, p=.043 vs p=.095)

Results:

• For Argentinian speakers the a-e and e-ε contrasts are both phonologically relevant, and therefore qualitatively of the same type

MMF amplitude not significantly different

• For Puerto Rican speakers one is phonologically relevant (a-e) and one is not (free variants e-ε) – the contrasts are of qualitatively different types

MMF amplitudes are significantly different

• For French the responses also differ, though both contrasts are phonemic

• French MMF responses greater across the board (inventory size effect) (Hacquard & Walter 2003)

• Differences also spread wider and reach significance sooner

Results:

• Why the inconsistencies between these two studies?

• K&P manipulate VOT in stimuli

not a primary cue for voicing in Korean

• K&P include multiple tokens in each category

biases subjects toward focusing on categorically phonemic distinctions

• Use of consonants versus vowels

different timing of acoustic information results in different processing

Experiment 2 (in progress):

Quebecois French – a within-language three-way comparison

High vowels alternate between long/tense and short/lax (Dumas 1976, Dechaine 1991, Martin 2002)

word-final word-initial word-medialopen syllable long long/short short/devoiced/deletedclosed syllable short short short/devoiced

i u

I U

ε

free variants

phonemic phonemic

allophonic allophonic

allophonic

Stay tuned!

Conclusions:

• Allophones are distinguishable in early speech processing

• Allophones appear to pattern with other structural, phonological contrasts, in contrast to within-category free variants

• In conjunction with behavioral research on the relative (im)perceptibility of allophone pairs, these results necessitate a model of phonology in which similarity may be computed over at least three domains: phonemic, allophonic, and acoustic

Thank you – now let’s look at the sun!