[From Bill Powers (931014.1500 MDT)]
Bruce Nevin (931013.1222) --
I like your idea of projecting what the "targets" must have
been from the enacted trajectory as successive phonemes foil
one another from actually attaining their "targets".
The idea wasn't so much that one phoneme prevents another from
attaining its target as the fact that phonemes follow in
sequence, and it takes time for the articulators to change from
the configuration for producing one phoneme to the different
configuration for producing the next one. If this process of
change is still under way when the reference-signal for the
phoneme changes to a different one, the articulators will
suddenly start changing in a new way as they are used to move the
phoneme from wherever it is toward a new target state. The
perceived phoneme will never actually arrive at the first target
state before that target is switched off and a new one is
switched on. Think of moving your finger toward a suddenly-
presented target, and having the target jump to a new position
when your hand is about half-way to the first target. Maybe this
is what you meant, but the way you stated it sounded like an
interaction among phonemes that exist at the same time. The
phonemes can't actually do anything to each other; it's the
control system producing them that has speed limitations. There's
only one phoneme being heard/produced at a time, as far as I
understand it.
Your prediction about the terminal phoneme runs into the fact
that, at least in English, terminal syllables tend to be
unstressed. (There is a very strong tendency, across languages,
for word margins to be deemphasized, so it is not just
English.)
Is this true even for the case I described, the terminal phoneme
in an utterance, with silence following? I wasn't talking about
margins between words, but the final sound as at the end of a
sentence. I realize that words actually run together in a
continuous stream in the middle of a sentence. But don't you
think that the reference-"t" ending "don't" would be approached
less closely in "don't you" than in a terminal "don't?" "Don't do
it, don't do it, don't!"
In playing with sound spectrograms, I noticed that my mouth tends
to shape itself for the next vowel even before the intervening
consonant is spoken -- you can see the frequencies shift as the
tongue moves toward the new position, so the consonant excites
formants typical of the next vowel. When you say "dee" the pre-
release vocalization contains more high frequencies than when you
say "dough" because the tongue is already in position for the
following vowel and the mouth cavity is differently-shaped. And
as you say, the preceding vowel also has an effect -- if you're
saying "odo" there's no frequency-change during or just after the
consonant, but if you say "odee" there an upward sweep and if you
say "eedo" a downward swoop, even during the consonant and before
the release. You can see that the middle and back of the tongue
are in motion from one configuration to the next even while the
tip is closing the cavity to make the "ud" sound. There seem to
be three or four tongue control systems acting at once,
independently: back of the tongue, middle, tip, and margins
(which contract and spread, as in saying "lee"). And they seem to
make easily-discernible differences in the perceived phoneme.
I'm bothered by how easily we perceive differences between sounds
that look identical in the spectrogram. I'm not talking about
sounds embedded in words, but just vowels uttered by themselves.
To me, the sound of "ee" is markedly different from the sound of
"ih", yet the difference is barely discernible on a spectrogram,
and is totally wiped out if I change the pitch of my voice. This
makes me more and more sure that the spectrogram is the wrong
model of the perceptual process. It's just not sensitive enough
to differences.
In the physics of sound production, you have a sound-generator
consisting of two membranes with their edges almost in contact,
under variable tension, with air being forced between them. Each
vibration launches a pressure-pulse up the throat and into the
mouth and (if open) nasal cavity. This pulse is partly reflected
at constrictions, and often partly escapes into the next cavity.
The reflected pulse must modify the pressure at the vocal cords,
changing their mode of vibration, so the underlying sound
generator is not just an independent vibrator, but part of the
whole system of cavities, resonating with them, with pressure-
waves travelling both ways at all times.
The pulses create many harmonics, but these harmonics must be
locked together in phase, or at least must be, for any phoneme,
in constant phase relationships that depend on the shapes of the
cavities. The sound spectrograph discards this phase information
because of the way the narrow-band filtered oscillations are
rectified and smoothed. Only amplitude information appears in the
output signal for any channel. This may be where the difference
between "ee" and "ih" is lost in the spectrogram.
What we desperately need are other ways of characterizing the
sounds of speech. It's easy to talk about what the "real" sound
is as opposed to the "perceived" sound, but what we call the
"real" sound is simply the output of an artificial perceptual
function. A sound spectrogram no more represents the physical
sound than the perceived phoneme does. We will not have the right
artificial perceptual function until it produces output signals
that can discriminate every sound that _we_ can discriminate, and
preferably more.
When you say that people perceive differences in sounds when
there are, in fact, no differences in the sound-waves, what do
you mean by "in fact?" You are really comparing human reports on
differences with differences detectable by an artificial input
function. If the artificial input function is incapable of
discriminating sounds with the same finesse as human perceptual
functions, then there could well be clear differences that simply
don't show up in the crude "objective" representation.
I think we need to do a lot more work on finding ways of
representing speech productions detected by a microphone, until
we can detect more differences than the human system can detect
rather than fewer. I mistrust stacking up hypotheses and
conclusions when the basic observations have such low resolution.
Even when we have an input function that can really discriminate
sounds well, we will have only a set of input signals for the
next level of perceptual functions. Here again we need to think
things through in more detail. A next-level input function can be
designed so as to treat a whole range of input signals as the
same -- produce the same output signal over this range. Yet when
the input signals change in a different pattern each next-level
input function could respond with a change in its output signal.
This means that "contrasts" are determined by the nature of the
input function. When we understand the way contrasts behave in
different words (a matter of experiment, as you have outlined),
we can start looking for particular forms of input functions that
will yield the same contrasts and lacks of contrast, given the
set of lower-level input signals. If this works, and it might
well work, then we can forget all the more complex hypotheses and
rules that have been thought up to explain contrast effects.
I'm convinced that we have to explain as much as possible at each
level before introducing a new level of analysis. If we do that,
then the new level will have as little as possible left to
explain. If we can explain how most sounds are discriminated into
separate sensations and configurations and transitions, then all
that the event-level would have left to do would be to recognize
that a sound that is still ambiguous doesn't match one possible
perception of a sound-event but does match another. And words
that are intended differently but are actually the same at the
event level could be discriminated by their relations to other
words, and relations that are ambiguous can be discriminated at
the category level by subject-matter, and so on.
What I'm concerned with is that if we just dive into the sea of
facts known about speech, such as you and Martin keep bringing
up, we won't have any way to know which facts are important and
which are only side-effects of the way speech recognition and
production are organized. Maybe it's true that people demonstrate
subtle and systematic differences in producing phonemes that only
a trained linguist can detect, but it may also be true that these
differences have nothing to do with word-recognition and simply
reflect mechanical or organizational side-effects. While these
differences are interesting, they may not be important. Whether I
say p'in or p`in, you still know I am talking about a small sharp
object, or winning a wrestling match, or a decorative brooch
(depending on context, not pronunciation). We may be able to
build a perfectly adequate and even skillful basic model of
speech production and recognition without specifically trying to
produce these subtle differences -- and when the model runs, many
of these differences may simply show up, unbidden, as natural
concomitants of the way the model is organized. That's if we're
lucky.
My problem with all these phenomena that you and Martin throw out
in such profusion is that I can't tell what's important and needs
to be explained first and what's only a curiosity or a
embellishment, or a minor statistical trend somebody noticed. It
seems to me that we're still unable to explain how sounds are
reliably differentiated in the easiest possible circumstances,
and how even a hyperarticulated word is recognized. If we don't
have a good working model of such simple things, how on earth can
we expect to model something like a strategy for increasing the
contrast between the vowels in "donkey" and "flunky?" We don't
even know what contrast is, not to mention how we alter it.
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I have proposed that we model pronunciation of a consonant
phoneme as control of an intended articulatory gesture
perception, which hearers reconstruct from the sounds
(behavioral outputs) because there is a well-known (publically
known) finite set of these gestures, because they are maximally
different from one another in a thoroughly familiar environment
(one's own mouth, reliably assumed to be identical in all
relevant respects to the other person's mouth), and because the
hearer assumes that both she and the speaker controls the same
number of them differentiated from each other in the same or
nearly the same ways.
All right, go ahead and model it that way. Your description above
tells us what the model has to accomplish, and why accomplishing
it might be desireable, and how it might be related to other
motivations, but it doesn't tell us what the model IS. First we
have to model an articulatory gesture, don't we? We have to
define exactly what constitutes a "gesture," how it is perceived,
and how the control systems are organized to produce perception
of a given gesture when a reference signal is set to a particular
value. I rather suspect that we will end up with what I call a
transition-level control system, but since I haven't yet got a
model of a configuration-control system for the articulators, I
can't guess what the transitions will prove to be, or how they
will be carried out.
I actually think that your proposed modeling project is out of
sequence, but if you think you can do it, go ahead. I'm still
trying to model vowel production.
Unless, of course, you mean "Let's pretend that we have actually
produced a model and that it will do the following things for the
following reasons..." In that case you're on shaky ground. How do
you know the proposed model will prove to do those things in the
way you describe? You may be quite right, but if you are it will
be a lucky guess, or an inspiration, and you still won't have an
actual model to demonstrate that you're right. Where would we go
from there?
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And when you hear win, ninny, minimum, thin, kin, gin, lint,
fin, sin, chin, din, tin, gimpy, and so on, as opposed to pin
(or bin). Then (assuming the point of view of one who doesn't
know English yet) you represent the contrast between pin and
bin by a difference in two sets of phonetic perceptions.
That is certainly a clear statement of your claim, so clear that
I feel no hestitation in doubting it. I think the same result can
be obtained without this awkward insistence on calculating the
pairwise differences among one word and all possible others. You
also have ninny, minny, manny, money, boney, bunny, burney,
burner, birder, boarder, border, badder, batter, battle, bottle,
bottler, butler, butter, mutter, matter ... it's simply endless.
A patient linguist might create a list of hundreds of thousands,
or millions, of experimentally established contrasts or lacks
thereof, but this is no indication that the brain distinguishes
words in that way. This is simply an unwieldy and unlikely model,
incredibly costly in terms of computational ability and unlikely
to be able to run in real time even given the brain's great
parallelism. And in no way does it explain why contrast exists in
the first place.
It should be possible to come up with a far simpler model that
would give the same effect -- I've mentioned a normalization
procedure as one possible method, akin to the Land model of color
reception which doesn't require any pairwise comparisons but
achieves the same result. I have no objection to your report on
the basic phenomenon. What I object to is the naive translation
of the linguist's way of investigating contrasts directly into a
proposed model of how the brain decides that THAT word is not the
same as THIS word. A judgement of contrast is the OUTCOME of a
process, not the process itself. You're begging the question.
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Best,
Bill P.