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Abstract:
Classic studies on phoneme recognition for
syllable initial voiced and voiceless stop consonants
demonstrated remarkably "categorical" perception with (a)
step-like identification functions, and (b) nearly perfect
discrimination of small acoustic differences when the tokens
straddled the category boundary and near chance discrimination
when the same magnitude difference was within a category.
These results were consistent with a view of speech recognition
in which fine-grained acoustic-phonetic differences within
phonemic categories have minimal effects on lexical access.
However, recent work in articulatory phonetics suggests that
position in a prosodic domain has consistent and reliable effects
on the production of word-initial consonants. These
prosodic differences result in small but reliable variations in
word-initial VOT [1, 2]. Thus, a complementary word
recognition system should also be sensitive to fine-grained
differences in VOT.
We used an eye-tracking paradigm in which subjects
heard either a single token from a 9-step synthetic /b/-/p/ VOT
continuum (0 to 40 ms) embedded in a word, or an l- or sh-
initial filler word and responded by clicking on one of four
pictures on each trial (e.g., a picture of a peach, beach, ship
or leaf). We analyzed the proportion of fixations to the
'target' (e.g., the picture of a beach when a 0 ms VOT token was
presented), the most similar alternative or 'competitor' (e.g.,
the peach), and the two unrelated alternatives. We examined
prototypical VOTs (90 % or more of responses to one category): 0,
5 and 10 ms for /b/; 25, 30, 35 and 40 ms for /p/. We
excluded any trial in which the "incorrect" word for that
category was selected. As VOT approached the category
boundary, the duration (and proportion) of fixations to the
competitor showed gradient increases, resulting in a significant
effect of within-category VOT (/b/: F(3,48)=6.3, p=.001; /p/:
F(4,64)=8.0, p<.0001) and highly reliable linear trends.
For example, the competitor word (e.g., /peach/ when /beach/ is
the stimulus) became more active and remained active longer as
VOT increased from 0 to 5 to 10 ms. Additional experiments
demonstrated that limited sensitivity to within-category
differences in VOT results from the combined effects of using a
meta-linguistic task with non-word syllables and only two
alternatives.
These results suggest that sensitivity to
fine-grained acoustic differences, as preserved in patterns of
lexical activation, could be an important mechanism for taking
into account prosodically-conditioned acoustic differences,
integrating acoustic/phonetic features across time, and resolving
temporal ambiguities.
References
[1] Fougeron, C., & Keating, P. (1997).
Articulatory strengthening at edges of prosodic domains.
Journal of the Acoustical Society of America, 101,
3728-3740.
[2] Crosswhite, K., Masharov, M., McDonough, J.M.,
& Tanenhaus, M. K. (2001). Acoustic/phonetic
differences in onset embedded words (doll/dolphin) within and
across prosodic domains: An instrumental and perceptual
study.
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