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A visible cue for CVC-final consonant voicing

Dear List,

I'm wondering if anyone can point me to references concerning a
particular visual speechreading phenomenon that I've encountered in my
research, developing a speech aid the Deaf and hearing-impaired.

As part of the study, my subjects performed a pairwise consonant
discrimination task under two sensory conditions:  lipreading-alone
and lipreading+speech-aid.  As expected, in the lipreading-alone
condition, corresponding voiced and unvoiced consonants (e.g., /b/-/p/
or /d/-/t/) are virtually indistinguishable in CVC-initial context.

However, in CVC-final context, most subjects discriminate consonant
voicing well above chance, just through lipreading (unaided).
Performance is far from perfect, but d' > 1 is the norm, and a few
subjects (both hearing-impaired and normal-hearing) consistently score
d' â 2.

My working theory is that the duration of the vowel (or of the CVC
utterance as a whole) is providing a visible cue that is correlated
with CVC-final consonant voicing.  Vowel duration as an acoustic cue
for final consonant voicing is well-known (e.g., House and Fairbanks,
1953; Denes, 1955; House, 1961; Raphael, 1972; Mack, 1982; Luce and
Charles-Luce, 1985).  But I cannot seem to find a paper that describes
or even refers to the visual correlate.

I assume that visual cues for final consonant voicing are
well-characterized somewhere in the lipreading literature.  Perhaps
I'm using the wrong search terms.  In any case, I'd appreciate the
benefit of your collective expertise.

Thanks in advance!



Theodore Moallem, Ph.D.

Sensory Communication Group
Research Laboratory of Electronics @ MIT
Harvard-MIT Division of Health Sciences and Technology