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Alice Turk and Stefanie Shattuck-Hufnagel (2020). Speech timing:
implications for theories of phonology, phonetics, and speech motor control.
(Oxford Studies in Phonology and Phonetics 5.) Oxford: Oxford
University Press. Pp. xv + 370.
Jason A. Shaw *
Yale University
1 Overview and structure of the volume
There is increasing awareness that the temporal dimension of speech, in particular
the relative timing of speech movements, contains rich information about phono-
logical structure. Relating abstract phonological structure to the temporal unfold-
ing of realistically variable speech data remains a major interdisciplinary challenge.
It is this challenge that is taken up in Speech timing: implications for theories of
phonology, phonetics, and speech motor control, henceforth Speech timing.
The book has eleven chapters, including a short introduction and a conclusion.
The main proposal –a sketch of a model mapping phonological representations to
continuous movements of articulators –comes in the second half of the book, par-
ticularly in Chapters 7 and 10. The second half also includes chapters on optimisa-
tion (Ch. 8: ‘Optimization’) and general mechanisms for timing (Ch. 9: ‘How do
timing mechanisms work?’). These provide a unique synthesis of speech and non-
speech literature, which is highly accessible for linguists, and serves to motivate
aspects of the main proposal.
The first half of the book provides a description and critique of the theory of
Articulatory Phonology, developed in the Task Dynamics framework (AP/TD)
(e.g. Browman & Goldstein 1986, Saltzman & Munhall 1989). On the view of
the authors:
AP/TD currently provides the most comprehensive account of systematic
spatiotemporal variability in speech …it represents the standard which any
alternative theory must match or surpass, and provides a clear advantage over
traditional phonological theories as a model of the speech production process
(pp. 8–9).
The review of AP/TD in Chapter 2 of Speech timing highlights some key charac-
teristics of the theory, particularly those related to prosodic modulation of timing
and those implemented in the Task Dynamics Application, which simulates
articulatory trajectories and the resulting acoustics from specifications of phono-
logical representations. The AP/TD overview sets the stage for an exposition of
empirical phenomena in Chapters 3–6 that the authors interpret as a challenge
to AP/TD and as motivation for an alternative approach.
Thus the basic rhetorical strategy is to first problematise an existing theory,
AP/TD, and then to present an alternative approach.
* E-mail: JASON.SHAW@YALE.EDU.
Phonology 38 (2021) 165–171. © The Author(s), 2021. Published by Cambridge University Press
doi:10.1017/S0952675721000099
165
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2 Major arguments in the volume
Speech timing describes the ‘Phonology-Extrinsic-Timing-Based Three-Component
approach’, abbreviated as ‘XT/3C’. Although, as described above, this approach is
presented as an alternative to AP/TD, it has many features that may be more
familiar to readers of Phonology than AP/TD. AP/TD represents phonological
contrasts in terms of gestures, dynamically defined speech production tasks.
Gestures have temporal extent and are coordinated in time. The dynamical
approach of AP/TD defines at once discrete phonological tasks and the changes
in articulator position over time that achieve them. From this standpoint, pho-
nology and phonetics are not clearly distinct –rather they are different levels of
description of the same system. In contrast, XT/3C is characterised as having
three components: (i) phonological planning, (ii) phonetic planning and (iii)
motor-sensory implementation.
The phonology-extrinsic timing aspect of the framework, the ‘XT’of ‘XT/3C’,
highlights a key contrast with AP/TD –phonological representations in XT/3C do
not specify time beyond the linear order of segments. Moreover, in XT/3C,
phonological representations do not commit to the particular phonetic dimensions
that will serve as targets for the speech production process or how progress
towards those goals will unfold in time. Such specifications come only at later
stages in the XT/3C speech production process, when the phonological represen-
tation is input into the phonetic planning component. Representations in the
phonological planning component of XT/3C consist of phonemes, hierarchical
prosodic structure, metrical prominence and abstract characterisations of speech
rate and style. Fully specified utterances in the phonological planning component
project linearly ordered acoustic cues, which are given quantitative targets in the
phonetic planning module. Also in the phonetic planning module, articulatory tra-
jectories are computed to achieve the acoustic targets in the linear order specified
by the phonological component. The cost of deviating from acoustic targets is
balanced against the cost of movement (movement effort and movement time),
both of which could be sensitive to any aspect of the phonological representation.
Movement control is proposed to be guided by instantaneous awareness of the
location of articulators and the distance that they would have to move to achieve
acoustic targets, i.e. the movement endpoints; it is suggested that this idea can
be appropriately formalised in tau theory, following Lee (1998). Finally, the
motor-sensory implementation component issues the motor commands at appro-
priate times to achieve the planned acoustic cues to phonological structure.
Some of the key contrasts between AP/TD and XT/3C discussed at length in
Speech timing are: (i) spatio-temporal (AP/TD) vs. symbolic (XT/3C) phono-
logical representations; (ii) articulatory (AP/TD) vs. acoustic (XT/3C) phonetic
targets; (iii) phonology-intrinsic (AP/TD) vs. phonology-extrinsic (XT/3C)
timing; (iv) onset-triggered (AP/TD) vs. endpoint-triggered (XT/3C) move-
ments. The XT/3C assumption of symbolic phonological representations, (i),
appears to dictate much of how the rest of the XT/3C architecture, i.e. (ii)–(iv),
differs from AP/TD. In AP/TD, the mapping from phonological representations
to observable speech behaviour is compositional. Since gestures are specifications
of how phonological contrasts shape speech production over time, any contextual
deviation must be attributed to another overlapping gesture. This includes the
π- and μ-gestures involved in prosodic modulation (Byrd & Saltzman 2003,
Katsika et al. 2014). Positing symbolic phonological representations liberates
XT/3C from having to account for contextual variation, including prosodic
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effects, compositionally, as spatio-temporal modifications. Rather, in XT/3C,
since phonological representations lack spatio-temporal content, there is no
‘default’to modify. Any aspect of the phonological representation, including
combinations of segmental contrast and phonological position can condition
unique, context-specific phonetic targets. Since phonological representations
lack spatio-temporal content, this needs to be provided in other components,
which relate to (ii)–(iv).
The above aspects of XT/3C that differ from AP/TD are each motivated with
some discussion of empirical data. Most of the data that is key to the discussion
is already published. One exception is a study presenting some proof of concept
of the applicability of Lee’s theory to speech, cited as Lee & Turk (in preparation)
and presented briefly in §9.2.1 (pp. 259–261). Although the relevant data is gen-
erally not new, it is uniquely synthesised as motivation for XT/3C.
The empirical motivation for loosening the strict compositionality of AP/TD
comes in part from the observation that phrase-final lengthening in Finnish is at-
tenuated when a vowel-length contrast is at stake (Nakai et al. 2009, Nakai et al.
2012). The Finnish data is interpreted as an argument against phonology-intrinsic
timing, because the lengthening effect of the prosodic boundary is not uniform
across short and long vowels, an apparent violation of the compositionality inher-
ent in AP/TD. The XT/3C alternative is that surface durations result from an
optimisation of cues to both phonological contrast and prosodic structure. Since
the symbolic representations of XT/3C are phonetically unconstrained, they can
reflect sensitivity to contrast maintenance that flouts the cues to prosodic bound-
aries just when contrast is at stake.
Key evidence for acoustic phonetic targets include well-known cases in which it
appears that different combinations of articulatory constrictions vary in order to
maintain relatively stable acoustic targets, such as the trade-offbetween tongue-
dorsum retraction and lip rounding to maintain a relatively stable F2 for /u/ in
American English. In addition to examples from speech, Speech timing also
reviews empirical studies documenting other motor behaviours, such as typing
and tapping, which tend to show greater temporal variability at movement
onsets than at movement endpoints. One speech production study is cited as evi-
dence for this claim (Perkell & Matties 1992). Another possible line of evidence for
endpoint-triggered movements comes from spatially conditioned gestural timing
(for discussion, see Shaw & Chen 2019), although there may be alternative expla-
nations for these observations that are also consistent with onset-triggered
movement.
3 Critical discussion of data and claims
Although Speech timing raises some important issues that will guide future
research on the topic, to really adjudicate between approaches it will be necessary
to consider a wider range of data and to work out XT/3C in greater detail, ideally
to the extent that it can make quantitative predictions. I elaborate on these points
in this section.
Speech timing does not review cases in which articulation maintains stability
across contexts even at the expense of acoustic cues to phonemic contrast, or
cases where articulatory differences corresponding to distinct phonemic contrasts
are masked in the acoustics. Extreme examples include gestural hiding and covert
contrasts. In gestural hiding, gestures are produced that have little or no acoustic
consequences, because of how the gestures overlap in time, such as the tongue-tip
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gesture for /t/ movement after lip closure for /m/ in the English sequence perfec/t/
memory (Browman & Goldstein 1989). Covert contrasts are phonemic contrasts
that are produced distinctly in articulation but still sound indistinct to listeners,
and are thought by some researchers to be widespread in both typical and atypical
language development. These observations appear to me to present a challenge for
XT/3C or any theory in which articulatory trajectories are computed as optimal
means to achieve acoustic targets. Quite specifically, the challenge situated
within the XT/3C framework is to identify the combinations of costs such that
optimisation will result in a large articulatory movement with no acoustic
consequences.
There are also cases in which the same articulatory posture appears to be reused
across segments. For example, the articulatory posture observed for fricative
vowels in Suzhou Chinese, including both alveolar fricative vowels, which are
restricted in their distribution to follow alveolar fricatives, and postalveolar frica-
tive vowels, which are not subject to such restriction, have the same articulatory
posture as corresponding fricative consonants in the language (Faytak 2018).
Faytak argues that this and other observations of articulatory uniformity, i.e.
reusing articulatory postures across different contrasting segments, may have its
basis in learning –speakers may reuse already practiced motor routines if they
accomplish ‘merely good enough’rather than optimal acoustic outcomes
(Faytak 2018: 29). This reasoning appears to me to be incompatible with
XT/3C, although, again, the precise predictions depend much on how the
interacting costs function in the optimisation process.
In addition to such cases of spatial uniformity, there are also cases of articulatory
timing uniformity in the literature. For example, Shaw & Davidson (2011)
pursued the hypothesis that native English speakers would produce Russian con-
sonant clusters, e.g. word-initial /zb dg/, etc., in ways that minimise the perceptual
distance between what they hear and what they produce –this hypothesis is con-
sistent with XT/3C, but it was not supported by the data. Rather, computational
simulations deriving the acoustic data from the articulatory timing of consonant
gestures suggested that speakers imposed uniformity in timing –producing frica-
tive–stop clusters like /zb/ with the same pattern of relative timing as stop–stop
clusters like /dg/, even though this results in different acoustic deviations from
Russian: [zb] vs. [dᵊg]. Several studies have reported systematicity in the timing
between articulatory movements such that different consonants enter into the
same temporal relations (despite different spatial targets). Key empirical observa-
tions include: (i) timing is language-specific, in that similar sequences of segments
can show different patterns of articulatory timing across languages (e.g. Hermes
et al. 2017), and (ii) within languages, different sequences of segments, such as
rising vs. falling sonority syllable onsets, can sometimes show similar patterns of
relative timing (e.g. Shaw et al. 2011). These patterns can be derived from
AP/TD straightforwardly, but it remains to be seen if they might also emerge
from surface optimisation of acoustic targets and articulatory costs, as in XT/3C.
Of course, the timing between consonant gestures is not always consistent across
consonants of different identities. For example, initial /kn/ clusters in German
have longer temporal lag than initial /kl/ clusters (Bombien et al. 2013) and
Italian /sC/ clusters pattern together in their timing to the exclusion of rising
sonority clusters (Hermes et al. 2013). In some cases, the differences in
gestural timing correspond to differences in syllable structure, providing some
cross-linguistic support for the observation that higher levels of linguistic
structure, such as syllables, may be found in characteristic patterns of temporal
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organisation between articulatory gestures (Browman & Goldstein 1988). This
points to a broader issue –how exactly phonological structure conditions variation
in articulatory coordination. Within the XT/3C framework, any such
correspondence is indirect, in that it is mediated by the linearisation of acoustic
cues to phonological structure, although the precise consequences of this
architecture depend on the cost function at play in optimisation.
The last ten years have produced a wide empirical base of studies reporting high
temporal resolution articulatory data that can constrain theorising about principles
underlying speech timing, within and across languages. Establishing whether
XT/3C can account for this range of patterns in a principled way requires
further development of the model. In my view, two further developments are
required.
One step is to converge on a precise characterisation of how cost enters into the
optimisation of surface timing, including the integration of costs for time, energy/
effort and spatial variation at the endpoint of movement, since, within the XT/3C
framework, optimisation is required to generate quantitative predictions. Speech
timing provides an extensive discussion of the issues involved in treating speech
production as a complex optimisation problem, including relevant non-speech
motor literature, and why it is difficult to identify with precision what the relevant
costs are for speech and how these costs might interact in the optimisation process.
This discussion, mostly localised in Chapter 8, includes insightful critiques of
optimisation approaches that posit durational targets for linguistic units with
penalties for deviating from targets. Speech timing argues that the durations of
linguistic units should not be specified in underlying representations, because
they can emerge instead from optimisation. In XT/3C, phonemes, syllables,
etc., gain temporal extent only through the joint optimisation of surface durations
between acoustic landmarks and the articulatory movements that give rise to them.
One major challenge for XT/3C is therefore to derive the range of temporal
patterns, including cases of apparent systematicity in articulatory timing, from
the proposed optimisation procedure.
A second step for XT/3C, in my view, is to develop the theory of how
phonological representations project linearly ordered acoustic/auditory cues.
This may be as simple as adopting existing phonological proposals, or aspects of
them. The mapping from abstract phonemic representations to context-specific
acoustic/auditory cues in XT/3C resembles the mapping from underlying to
surface representations in generative phonology, particularly versions of
generative phonology that offer an optimisation-based unification of phonetics
and phonology (e.g. Boersma 1998, Flemming 2001). Speech timing makes it
clear that the phonological component of XT/3C is not intended to be isomorphic
with the phonological grammar, even though they make use of similar abstract
linguistic units (p. 268). However, it seems to me that there is potential to do a
lot of what phonological grammars are designed to do within the XT/3C frame-
work, including mappings that can be described in terms of phonological rules.
This is because, unless further constrained, the framework leaves open the possi-
bility that the same abstract phoneme could be mapped to radically different
acoustic realisations in different phonological contexts. One constraint on
phoneme to acoustic landmark projection proposed in XT/3C is that this
mapping balances language redundancy and acoustic/auditory information so as
to maintain consistent recognition probability over time. It would be interesting
to consider how language redundancy relates to grammatical rules, particularly
in cases in which they make potentially conflicting predictions. Working out the
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mapping from phonological representations to acoustic/auditory cues would
reveal the degree to which XT/3C, as a model of speech production, can be
integrated with existing models of phonological grammar.
4 Conclusion
There are a number of properties which make Speech timing essential reading for
students and researchers interested in relating abstract phonological structure to
time-dependent articulatory and acoustic properties. From start to finish, the
book offers a balanced review of a significant amount of relevant research, some
of which is not succinctly reviewed elsewhere. Perhaps even more valuable is
that Speech timing keeps consistent tabs on the empirical facts that motivate
each component of the model, maintaining contact between data and theory.
The book can also be read with an eye to even broader conceptual issues. These
include how speech motor control relates to other aspects of motor control, such
as limb movement –‘is speech special?’–and whether speech is better conceptua-
lised as a complex computation, as in the optimisation problem characterised by
XT/3C, or as the interplay of forces tending toward equilibrium, as in the
dynamical systems of AP/TD.
The general style of the book, reasoning from theoretical predictions
(of AP/TD) to empirical data and back to theoretical alternatives, encourages situ-
ating speech data as evidence for competing theoretical hypotheses. Although the
theoretical dichotomies in (i)–(iv) are possibly too sharp –it seems unlikely that
speech production targets are entirely articulatory or entirely acoustic/auditory,
or that articulatory timing refers only to movement onsets or only to movement
endpoints –they serve to highlight important and unresolved issues, which can
be clarified by further experimental work. Data seemingly supporting one
approach or the other could ultimately be specific to particular language varieties,
phonological contexts, experimental tasks, individual speakers or stages of lan-
guage development. For example, in AP/TD it is generally assumed that the pro-
duction goals of tone gestures are acoustic in nature (e.g. Gao 2008) and some
gesture-based models consider both movement onsets and offsets (endpoints), as
well as other landmarks, to be available for coordination (Gafos 2002).
Nevertheless, the theoretical issues raised in Speech timing can focus much
future research on computational and experimental aspects of relating phono-
logical structure to the speech signal, including approaches that hybridise
aspects of AP/TD and aspects of XT/3C (see e.g. Parrell & Lammert 2019).
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