1. General Concepts and Methods

Coarticulation can be characterized as changes in articulation and in the acoustic signal induced by one phonetic segment (the trigger) during another one (the target) due to overlap between their articulatory gestures. Examples of coarticulation are anticipatory velar lowering during a vowel preceding a syllable-final nasal consonant (send) and tongue body raising and fronting during a schwa placed next to the palatoalveolar consonant /ʃ/ (the shore, ashamed). Coarticulatory effects may involve several articulatory structures (velum, lips, tongue tip, tongue body, jaw, larynx) and acoustic properties (e.g., second formant frequency, segmental duration). They may result from the activation of a single articulatory gesture (as in ashamed), or from the interaction among articulatory gestures for different segments (as in the cluster /sk/ in the Spanish word asco ‘nausea,’ where /s/ is front lingual and /k/ a dorsal consonant).

Coarticulation may be measured in space and time. Thus, for example, tongue dorsum raising and fronting effects exerted by /ʃ/ on an immediately preceding schwa, which has no obvious lingual constriction, are predicted to be larger in terms of spatial displacement and start earlier than those occurring during a preceding vowel exhibiting a lingual constriction, such as /i/ or /a/. Moreover, the spatiotemporal effects in question may differ in direction: they may be anticipatory, and thus proceed leftwards toward the preceding segment(s) (as in it is a shame), or carryover, and thus proceed rightwards toward the following segment(s) (as in mash a potato). It is commonly accepted that anticipatory effects reflect phonemic planning in so far as they correspond to articulatory characteristics of phonemes which are yet to be produced (Whalen, 1990); carryover effects, on the other hand, depend on the ongoing state of the articulatory structures to a larger extent, which may account for why their offset is more variable (see sections 2.1 and 2.3.1.2). As pointed out for lingual coarticulation in section 2.3, the magnitude, temporal extent, and direction of the coarticulatory effects are conditioned by the place and manner of articulation characteristics of the triggering and target consonants and vowels, as well as by the articulatory subsystem which is involved in closure or constriction formation. Other factors affecting coarticulation are segmental position within the word and the utterance; the stress level of the segment; whether it occurs in a VCV (vowel-consonant-vowel), CC (consonant-consonant), or other sequence structure; speech rate; speaker; and language.

The study of coarticulation provides information about phonological assimilatory processes and sound change patterns. It has been traditionally assumed that coarticulatory effects are phonetic and thus gradual, variable, and universal, while assimilations are phonological and thus categorical, systematic, and language-specific. For example, tongue body raising and fronting effects from a palatal consonant during a schwa occur to a greater or lesser extent in any speech production event, but may only be labeled assimilatory if there is a sound change by which the coarticulatory effects in question give rise to a higher and fronter vowel such as /e/ or /i/ in a subset of lexical items or across the entire lexicon of a given language. Experimental evidence reveals, however, that the division between coarticulation and assimilation is not so straightforward. Indeed, coarticulatory effects may exhibit language-dependent differences (e.g., languages may differ regarding the degree of anticipatory vowel nasalization triggered by a syllable-final nasal consonant; see section 2.1), while processes which have been traditionally considered to be assimilatory may not apply categorically and systematically (e.g., in English or German, syllable-final /n/ assimilates completely or partially to the place of articulation of a following consonant, and more or less often depending on the consonant trigger, speaker, prosodic factors, and speech rate; see section 2.3.2).

There has been a substantial improvement of our knowledge of coarticulatory patterns in speech owing to an increasing number of large-scale phonetic experiments which have been carried out using several acoustic and articulatory recording and analysis techniques. Acoustic analysis provides information about spectral coarticulatory characteristics (also about coarticulatory effects in duration and intensity) associated mostly with tongue movement, nasality, and voicing. As for the articulation side, lingual coarticulatory effects in tongue-to-palate contact and tongue spatial configuration and kinematics have been investigated, respectively, with electropalatography and with ultrasound and electromagnetic articulometry. Electromagnetic articulometry also allows measuring coarticulation in lip and jaw displacement, while voicing coarticulation may be analyzed with electroglottographic data and anticipatory and carryover velar lowering with the help of a velotrace.

The present article is structured as follows. Section 2.1 reviews coarticulatory data in velar lowering and lip protrusion, which are expected to exhibit comparable patterns of overlap in so far as the two articulatory structures move independently of the tongue. Section 2.2 deals with voicing coarticulation, which differs from velar and labial coarticulation in that the glottal activity is heavily influenced by the intraoral pressure and airflow volume characteristics of the phonetic segments being produced. Tone coarticulation is reviewed in this same section, since it depends on laryngeal adjustments as well. Lingual and jaw coarticulation patterns, examined in sections 2.3 and 2.4, offer a rich scenario in so far as they are heavily determined by the specific consonant-vowel and consonant-consonant combinations available. Section 3 reviews briefly coarticulatory effects in the acoustic signal. Sections 4, 5, and 6 deal with how the degree of coarticulation varies with segmental position, stress, and speech rate; with the perception of coarticulatory effects and its relevance for sound change inception; with the acquisition of coarticulation in children; and with coarticulation in speakers with speech disorders. Coarticulation theories are referred to with reference to the coarticulation data for different articulators in each relevant section and also in the closing section 7, which also considers several aspects for future research.

2. Articulatory Structures

3. Acoustics

Coarticulatory effects may also be observed in the acoustic signal by means of spectrographic analysis. Consonantal effects on vowels measured at the vowel stationary part (if any) are generally positively related to effects in tongue body fronting/raising in the case of F2 and in oral opening in the case of F1. Thus, a higher and more anterior tongue body position during the production of schwa next to an (alveolo)palatal versus a labial consonant is associated with F2 differences ranging from 1,500 Hz in the labial context to 2,000 Hz in the (alveolo)palatal context.

Regarding consonants, vowel coarticulatory effects may be observed at the endpoint and in the trajectory shape of the vowel formant transitions, as well as in the frequency characteristics of the burst for stops, the fricative noise for fricatives, and the formant structure for sonorants (Kewley-Port, 1982; Jongman et al., 2000). The prominence of these spectral cues for segment identification in perception depends greatly on their acoustic salience, which may vary from one context to another (Dorman et al., 1977). Thus, for example, the F2 vowel transitions play a major role in the identification of consonant place of articulation in sequences such as /ɲa/ and /ɲu/, since they rise by about 500 Hz from the vowel to the consonant. Whenever the vowel transitions are less noticeable, the transitions and the nasal murmur are both taken into account by listeners for the nasal place identification. As for the effect of vowel context on place identification of the frication noise for the lingual fricatives /s/ and /ʃ/, see section 5.

4. Segmental Position, Stress, Speech Rate, and Lexical Factors

The prominence of intersegmental coarticulatory effects is conditioned by nonsegmental factors, for example, variations in speech rate, the position that consonants and vowels occupy within the syllable, word, and utterance and with respect to stress, as well as the frequency of occurrence of the words involved.

Whether bearing lexical or sentence stress, stressed vowels are encroached on less by coarticulatory effects from transconsonantal vowels than are unstressed, more reduced vowels (Fowler, 1981). Regarding the effect of segmental position, consonants are expected to allow more contextual coarticulation in syllable-final than syllable-initial position in line with the fact that they generally undergo more articulatory reduction in the former position than in the latter. Exceptions to this trend may be associated with the consonant allophonic characteristics; thus, in languages/dialects where coda /l/ is darker than onset /l/, such as several English varieties and Dutch, the former variety of the alveolar lateral should resist coarticulation to a larger extent than the latter. Also, highly constrained consonants such as the lingual fricatives /s/ and /ʃ/ may resist coarticulation in coda position as much they do in onset position. It is also the case that consonants overlap less in word-initial clusters than in clusters occurring across a word boundary and exhibit maximal superposition in word-medial clusters, which is consistent with a trend for regressive place assimilations to operate word medially rather than in C#C sequences. Coarticulatory effects also vary inversely with morphological boundary strength and thus are greater across a word boundary than across an intonational phrase boundary (Cho, 2004).

An increase in speech rate causes phonetic segments to shorten, undergo articulatory undershoot and overlap to a greater extent with other segments. An increase in coarticulation under these circumstances has been shown to occur both in consonant-vowel sequences (Gay, 1981) and in consonant clusters (/kl/, Hardcastle, 1985; obstruent sequences, Byrd & Tan, 1996). Interestingly enough, specific aspects of VCV coarticulation do not seem to be affected by an increase in speech rate, such as differences in coarticulatory resistance among the intervocalic consonants and the patterns of coarticulatory direction of the vowel effects (Recasens, 2015).

The degree of articulatory accommodation between the coda and onset consonants is also conditioned by the frequency of occurrence of the lexical item containing the former and/or latter phonetic segments (Bybee, 2001). Thus, coronal palatalization in English /C#j/ sequences is more liable to apply when /j/ belongs to a highly frequent than a less frequent word (you versus union, unit; Shi et al., 2005).

5. Perception and Sound Change

There is much evidence that listeners compensate for coarticulation by factoring out the coarticulatory effects and attributing them to their source. Thus, for example, listeners hear more /s/ tokens when presented a /ʃ/-/s/ noise continuum in the vicinity of /u/ than next to /a/ because they have some intrinsic knowledge that the spectral frequency of the fricative should become more /ʃ/-like in the /u/ context than in the /a/ context condition (Mann & Repp, 1980). Comparable results have been reported in studies on nasal coarticulation in VCN sequences (where N = nasal) and on V-to-V coarticulation in VCV sequences (Fowler, 1981; Beddor, 2016).

Failure to compensate for coarticulation may give rise to sound changes (Ohala, 1981). A fundamental aspect of the causes of sound change is the relation between the patterns of production and perception of coarticulated speech in individual language users. In this respect, there is conflicting evidence as to whether those individuals who exhibit more coarticulation in production are also more perceptually sensitive to coarticulation (Grosvald & Corina, 2012; Beddor, 2016). Different sensitivities to perceived coarticulation depending on production differences have been shown to occur in ongoing sound changes; thus, in comparison to younger subjects, older subjects of Standard British English appear to exhibit more contextual coarticulation on /u/ and compensate more for these coarticulatory effects (Harrington et al., 2008).

6. Acquisition and Speech Disorders

A research area which has attracted increasing attention in recent years is how coarticulatory abilities develop with age. Lingual configuration data collected with ultrasound (Zharkova et al., 2011, 2012) have been taken to confirm the hypothesis formulated in earlier acoustic studies that children show greater vowel-to-consonant coarticulation than adults because they use more global, less differentiated movements of individual articulators extending over a syllable-size temporal unit (Nittrouer et al., 1996; Sussman et al., 1999). This claim has been made in spite of the fact that, as reported by Zharkova and colleagues, in comparison to adults, six- to nine-year-old children show more vowel coarticulation on /ʃ/ and less vowel coarticulation on /s/, the argument being that independence between tongue front and tongue body is necessary for the coarticulation of /s/ but not for that of /ʃ/, since contextual adaptability for the palatoalveolar fricative relies basically on shifts in the forward part of the tongue. On the other hand, using ultrasound and acoustic data, Noiray et al. (2013) have found however that four- to five-year-old children exhibit comparable degrees of vowel coarticulation on /t/ to adults, which suggests that they have already learned the motor synergy between tongue front and tongue body, as well as greater vowel effects on /p/ and /k/ than on /t/, which also parallels the coarticulation scenario for adult speakers. Along the same lines, four- to five-year-old children and adults have been reported to exhibit a similar extent of anticipatory lip rounding during the phonetic segments preceding /u:/ in the sentence (Goffman et al., 2008). In spite of these conflicting results, most studies agree in that there is more within-speaker coarticulatory variability and thus less articulatory consistency in the speech of young children than in that of older subjects.

Much empirical and theoretical work is needed in order to ascertain whether coarticulation deficits in subjects with speech disorders and in hearing impaired subjects belong to the planning and/or execution speech components (Weismer & Green, 2015). Work on stuttering shows that stutterers coarticulate differently from normal speakers because they are motorically slower (e.g., they may show longer lip anticipation in VC_nV sequences; Almé & McAllister, 1987). A recent ultrasound study on velar stop production in different vowel contexts show less articulatory stability in a subset of adults who stutter than in fluent ones, which may be taken to indicate that the former have less accuracy of articulatory control (Frisch et al., 2016). Loss of the ability to perform movement coordination successfully in apraxic subjects results in reduced coarticulation, as shown by little or no overlap between the dorsal and apical gestures in clusters like /kt, ks, kl/ (Gibbon, 2003; Ingram & Hardcastle, 1990; Maassen et al., 2010, pp. 247–248).

7. Theoretical Implications and Future Research

The coarticulation data reviewed in this article suggest that considerations about the production requirements for individual consonants and vowels need to be invoked in order to explain lingual, jaw, and voicing coarticulatory patterns in speech. The DAC model makes some correct predictions about the patterns of lingual and jaw coarticulation based on some crucial aspects about the articulatory constraints for the phonetic segments involved. Thus, for example, gestural overlap is generally more extensive and subject to lower production constraints in consonant-vowel sequences than in consonant clusters, where assimilation and blending may occur. Moreover, the direction of the intersegmental coarticulatory effects is conditioned by the spatiotemporal characteristics of the articulatory gesture(s) both for the trigger and target segment(s) in ways which can be predicted by the model, at least for those segments which are highly resistant to coarticulation; also for these segments, coarticulatory resistance and coarticulatory aggressiveness tend to be positively correlated. In many respects, the DAC model parallels the window model proposed by Keating (1990), which uses narrower or wider articulator-specific windows for vowels and consonants depending on how resistant they are to contextual articulation (e.g., the jaw height window is high and narrow for /s/ and low and wide for /æ/). As regards other articulatory structures, anticipatory effects in lip coarticulation are somewhat adaptable to context, while those involving the velum are more temporally fixed. A robust principle which appears to hold across articulatory structures and segmental units to a large extent is that the onset of anticipatory effects is more fixed in time than the offset of carryover effects, essentially because the former reflect the planning component of speech, while the latter are more dependent on peripheral constraints.

Coarticulatory patterns vary with speech rate, stress, segmental position, speaker, lexical frequency, and language. Language-dependent differences in the number of vowels or consonants may play a role on degree of coarticulation at least in some cases, and languages have been shown to also differ regarding the direction of the vowel coarticulatory effects and as to whether assimilatory phenomena such those occurring in /nC/ sequences may apply gradually or categorically and take place more or less frequently.

As for future research, data are lacking on jaw coarticulation in consonant clusters and on lingual coarticulation for consonants produced at the back of the vocal tract such as uvulars and pharyngeals. At a less specific level, information is needed about how coarticulatory patterns arise from patterns of interarticulatory movement, and on speaker- and language-dependent differences in coarticulation and in the frequency of occurrence of related assimilatory processes. There is also a need for experiments on the perception of coarticulation and in particular on how long-distance coarticulatory effects may give rise to long-distance assimilatory processes such as vowel harmony, as well as on the coarticulatory behavior of children and of speakers exhibiting speech disorders. These and other research issues will have to be addressed using additional experimental techniques to the ones available, such as high-speed magnetic resonance imaging.