1. INTRODUCTIONTop

The question of how sounds change over time has fascinated speakers and linguists for centuries. Knowledge about sound change helps to reconstruct our linguistic past and it has always been a central part of historical linguistics. But sound change is an especially popular research topic at present with a biannual workshop series established in 2010 and at least five volumes dedicated to sound change appearing since that time (Recasens, Sánchez Miret, & Wireback, 2010; Solé & Recasens, 2012; Sánchez Miret & Recasens, 2013; Yu, 2013; Harrington & Stevens, 2014). The contributions in these volumes show that sound change research now incorporates aspects of many areas of linguistics and neighbouring disciplines including cognitive psychology, computational science, experimental phonetics, laboratory phonology, language acquisition, sociolinguistics, phonology and physics.

Sound change can be defined as change to the shared perception and production target for a speech sound within a speech community, a definition that encompasses changes that directly impact the number of categorical contrasts between sounds (e.g. neutralization) as well as changes that involve a shift in the pronunciation target for a speech segment without loss or introduction of a phonemic contrast (e.g. vowel chain shifts). The conditions that give rise to sound change are typically distinguished from those that have to do with its diffusion through a speech community (e.g. Janda & Joseph, 2003; Ohala, 1993). Broadly speaking, phonetic models tend to concentrate on identifying the perceptual and articulatory forces that provide the pre-conditions for sound change and that drive it in a particular direction (e.g. Ohala, 1993). In order to identify these forces, experimental phonetic studies on sound change often factor out between-participant variability. On the other hand, individual speaker-listeners are crucial to the origins of sound change because, for sound change to occur, it is individual production and perception targets for speech sounds that must change. Or, as Milroy and Milroy (1985) noted “linguistic change must presumably originate in speakers rather than in languages” (p. 347). Recent phonetic studies have shown that people with similar linguistic backgrounds can differ in terms of their cognitive mapping between the auditory signal and perceptual categories (Beddor, 2009, 2012) and in their production of speech sounds (Johnson, 2006; Koenig, Lucero, & Perlman, 2008). This article explores the idea that systematic differences between individual members of a speech community may play an important role in the early stages of sound change. Key importance has been given to heterogeneity between individuals in approaches to the diffusion of sound change. Labov’s (1963) pioneering research with residents of Martha’s Vineyard linked speaker attitude to participation in a sound change in progress and Milroy and Milroy’s (1985) social network theory suggests certain individuals play a more crucial role in spreading sound change than others depending on their social position in the community. An ongoing challenge in sound change research is to link the initiation of sound change within individual cognitive grammars with the diffusion of novel variants through the community.

This paper first addresses the way that sound changes can originate in the everyday variability of spoken interactions in Section 2. Section 3 then focuses on the actuation of sound change and shows convergence between recent approaches on the idea that variability between individuals may be the key to understanding how some synchronic variation can become sound change. Expanding on this theme, Section 4 considers the evidence for individual differences based on results from four areas of phonetic research and the potential role of such individual differences in driving sound change.

2. SYNCHRONIC VARIATION AND THE ORIGINS OF SOUND CHANGETop

Variability is an inherent part of the transmission of language between speakers and listeners, and can occur due to a range of linguistic and non-linguistic factors. To illustrate with just one example, local speakers of an Australian English variety typically pronounce Melbourne as something like ['mæ^əbṃ], with a vocalized or completely elided /l/ alongside reduction of the second unstressed syllable (see e.g. Cox & Palethorpe, 2007 on Australian English). Although deviating from the spelling and causing some comment among visitors, this pronunciation is not unusual in terms of typical patterns of synchronic variation. /l/ is more prone to vocalization in syllable-coda position than elsewhere (e.g. Recasens, 2012) and segments in unstressed syllables are more likely to be reduced due to gestural overlap and blending (de Jong, Beckman, & Edwards, 1993). Frequency of use also influences pronunciation (e.g. Bybee, 2002), so that one might expect reduction processes to affect the local pronunciation of Melbourne but not e.g. (peach) melba, a phonologically similar but comparatively rare item for this particular speech community. The link with historical sound change is that synchronic tendencies in the way speech is produced and perceived can, over time, cause permanent categorical change (e.g. Beckman, De Jong, Jun, & Lee, 1992; Hansson, 2008; Harrington, 2012; Hura, Lindblom, & Diehl, 1992). This can be seen by comparing the pronunciation of Melbourne with English words like salmon and talk, for example, in which /l/ is no longer pronounced, or by comparing standard Italian words (e.g. caldo ‘hot’) with cognates from other varieties also descended from Latin in which /l/ has been modified or lost e.g. caudo, cardo, cada (examples from Rohlfs, 1966, p. 342). The similar way that pre-consonantal /l/ is subject to change in unrelated languages may have its origin in phonetic bias factors that are common to spoken languages. These biases mean that the phonetic variation that provides the input to historical sound change is both non-random and directional (Garrett & Johnson, 2013). In a recent detailed overview, Garrett and Johnson (2013) group these bias factors into four main areas: motor planning, aerodynamic constraints, gestural mechanics, and perceptual parsing.

Not all instances of synchronic phonetic variation lead to permanent sound change, of course. Instead, sound change can be seen as a two-step process of variation and selection (Lindblom, Guion, Hura, Moon, & Willerman, 1995; Ohala, 1981, 1993) or, similarly, of channel and analytic biases (e.g. Moreton, 2008). For Ohala, novel variants arise constantly in production, and their eventual selection for sound change depends entirely on listener perception. Lindblom et al. (1995) instead argue that language users—as listeners and speakers—evaluate novel forms according to articulatory as well as perceptual and other criteria and that selection happens when listeners pay close attention to how something is being said and actively choose to reproduce that variant in their own speech.

Building on a long tradition of linking sound change to listener perception (e.g. Baudouin de Courtenay, 1972; Paul, 1888), Ohala’s model was the first to provide a framework to test the evident parallels between synchronic variation and diachronic change in the laboratory. In speech perception, listeners normally compensate for the contextual effects on segments due to coarticulation (e.g. Fowler, 2005). For example listeners typically take account of context effects such as the more fronted tongue position in loot than in loop in mapping the auditory signal to their cognitive representation for /u/. Ohala (1981) suggests that on some rare occasions the listener may be unable for whatever reason to attribute the contextual variation (in this case /u/-fronting) to its source (the /t/ in loot). Here the listener would interpret the high f₂ as an inherent part of the vowel /u/, would update their cognitive model with this new ‘hypo-corrected’ variant accordingly (which may then also be introduced into the listener’s speech production). Ohala argues that mini-sound changes of this kind within an individual’s grammar happen randomly and frequently (2012, p. 23, note 2), and that only a sub-set of these may eventually proceed to become a sound change at the community level. Sound change is rare; this is because, Ohala argues, listeners are normally very good at adjusting for contextual variation [1].

Sound changes like /u/-fronting that involve the uncoupling of coarticulatory variation from its source are most apparent in the case where the source of the coarticulatory effect is eventually lost. A familiar example is historical vowel nasalization in French (e.g. Italian pane v. French pain), a sound change that has been extensively studied by Beddor (2009, 2012). Beddor’s experiments suggest that the phonologization (Hyman, 1976, 2013) of vowel nasalization comes about via a stage where coarticulatory information for nasalization on the vowel gradually becomes a sufficient cue for listeners, while the information provided by the source (the nasal consonant) becomes less important and may eventually be lost. Alternatively, Kirby (2013) describes how the waning of one cue can drive the enhancement and phonologization of another. Kirby models this relationship with an agent-based simulation of a sound change underway in Seoul Korean involving the phonologization of f₀ and dephonologization of voicing.

Key to Ohala’s model is the idea that many sound changes have a perceptual origin without involving a change to speech production. However as Beddor (2012) points out “perceptual grammars only contribute to sound change […] if they are publicly manifested” (p. 51). Browman and Goldstein (1991) suggested that interactions between perception and production over time at the level of the individual language user cause the shared target for a speech sound to move in a particular direction. The idea that sound change, at its origins, involves articulatory factors in combination with perceptual considerations is supported by the results of a recent physiological study on /l/-vocalization in American English. Lin, Beddor, and Coetzee (2014) find that a small degree of articulatory reduction in the apical gesture for /l/ in words like milk and help (measured as tongue tip aperture on ultrasound displays) can have major acoustic consequences, causing the first two formants to merge. Lin et al. point out that this small articulatory shift in tongue aperture could make a quantal difference in perception and raise the idea that this might be a driving factor in causing /l/-vocalization. In other words some articulatory changes may only be slight on the part of the speaker, but nonetheless enough to cause the listener to reinterpret the sound category à la Ohala’s model.

Ohala (2012, p. 23, note 2) explicitly distinguishes initiation from actuation and chooses to account only for the former within his model. Indeed, bridging the gap between models of grammatical change at the level of the individual listener/speaker and change at the community level is difficult [2]. The articulatory and perceptual phonetic forces that bring about the pre-conditions for sound change and that explain its directionality are always present in the transmission of spoken language, yet sound systems are remarkably stable over time. How is it that sound change does not always happen whenever the necessary pre-conditions are met? What causes sound change actuation?

3. THE ACTUATION PROBLEMTop

The link between innovation within an individual’s grammar, on the one hand, and widespread change for most members of a speech community on the other, is sound change actuation. There is no clear consensus in the sound change literature on the precise meaning of the term actuation and how it should be distinguished from initiation and spread. Ohala draws a distinction between the initiation of sound change versus its actuation/spread, whereas in Yu’s account (2013, p. 201) sound change actuation is seen as comprising variation and selection (i.e. as comprising initiation). The terms initiation and actuation are used synonymously in some sources, for example Hansson (2008) who refers to the “initiation (actuation) phase of sound change” (p. 8), whereas the terms actuation and spread are used synonymously in others. Baker, Archangeli, and Mielke (2011), for example, suggest that a sound change is actuated when an individual (listener) reproduces a perceived novel variant in their own speech; they claim at this point “the sound change has begun to propagate around the community” (p. 348). Hansson and Baker et al. agree, then, that a sound change is not only initiated but also actuated in the mind of the individual listener. Elsewhere in the literature this stage of listener-turned-speaker production is not considered to be sound change but rather to be an innovative “act of an individual speaker, regardless of whether or not it later catches on in a speech community” (Janda and Joseph, 2003, pp.17–18). Janda and Joseph argue that sound change should be “strictly defined as an innovation that has been widely adopted by members of […] a community” (cf. also Milroy & Milroy, 1985).

Weinreich, Labov, and Herzog (1968) famously identified the actuation problem by asking “what factors can account for the actuation of changes? Why do changes in a structural feature take place in a particular language at a given time, but not in other languages with the same feature, or in the same language at other times?” For these authors and others since (e.g. Campbell, 2013, p. 193), actuation is key to explaining linguistic change because it relates to all other sociolinguistic and structural factors. Ohala (2012) is instead of the view that attempting to explain actuation, i.e. whether, where or when change may actually happen can be likened to “asking why a coin flip results in ‘heads’ and not ‘tails’” (p. 23, note 2).

At least three recent studies have nevertheless addressed the actuation explicitly, namely Baker et al. (2011), Garrett and Johnson (2013) and Yu (2013). Baker et al. (2011) propose that actuation comes about via interactions between individuals whose targets for a speech sound fall at opposite ends of a production continuum, taking as a case study /s/-retraction in American English. Baker et al. classified speakers as “retractors” and “non-retractors” according to the proportion of their tokens that were judged (by the authors) to be retracted or not (62.50% and 35.48%, respectively). Baker et al. show that even speakers classified as “non-retractors” (i.e. those who pronounce e.g. stream more like [stɹ] than [ʃtɹ]) show a lower centroid frequency for /s/ in clusters, and especially so for clusters containing /r/. As such they argue that the phonetic motivation for /s/-retraction (assimilation to retroflex /r/ across the intervening /t/) can be considered to be generally present amongst English speakers. Given that /s/-retraction has been phonologized in some (e.g. New Zealand English) but not all varieties, the difficulty, as Baker et al. point out “lies in creating a theory in which sound change can plausibly occur, but without making sound change inevitable” (p. 350). Baker et al. propose that variability becomes sound change, i.e. a sound change is actuated, because individuals differ in the degree to which they coarticulate (retract). Speakers who do not tend to retract /s/ will interpret [ʃ]-like productions as a distinct target, and might imitate them accordingly. Crucial to this model is the idea that coarticulatory effects must be sufficiently perceptible if they are to be imitated by other members of the speech community (i.e. ambiguous tokens would be unlikely to be imitated because “if every speaker has essentially the same amount of coarticulation, then there is nothing to imitate in the first place” [p. 350]). This model of Baker et al. also generally predicts that coarticulatory effects that vary across speakers would be more likely to undergo sound change than those common to all speakers within a speech community. Baker et al. present only production data but it would be possible to test these predictions about the role of inter-speaker differences in sound change actuation with standard shadowing experiments. For example, one could recruit a group of speakers from a variety of English without /s/-retraction. Following Baker et al. these speakers could be classified as “retractors”/ “non-retractors” based on the proportion of str- productions judged to be [ʃ] or [s]. After exposure to words containing extreme [ʃ]-like tokens, only those speakers whose own productions typically fall at the [s]-end of the continuum should show a change in the direction of [ʃ], whereas individuals with ambiguous productions between [ʃ] and [s] should not shift their production target.

Garrett and Johnson (2013) propose that some individuals are more likely to attach social meaning to linguistic differences than others and that this is a driving force in the actuation of sound change. They draw on experimental work by Dimov (2010; also Dimov, Katseff, & Johnson, 2012) that found a link between social and personality traits and the extent to which participants compensated for altered auditory feedback (for /u/). Dimov and colleagues’ experimental work showed that the more powerful subjects judged themselves to be, based on responses to survey questionnaires, the less they compensated in the experimental task (i.e. they were either less finely attuned to phonetic variation or less willing to modify their own production in order to compensate for it). Garrett and Johnson build on this experimental research together with Giles, Coupland, and Coupland’s (1991) work on accommodation to suggest that individuals who wish to identify with a group may be more likely to interpret intrinsic phonetic variability (primarily due to coarticulation) as indexing group membership. As a result they would attach a social significance to phonetic properties that previously had none. An exemplar-based model predicts that if listeners attach social meaning to coarticulatory information then they will store both components i.e. the coarticulatory information together with the social meaning that it is assumed to convey. On the other hand, listeners who compensate for coarticulation would discard this contextual information before storing the exemplar in their cognitive grammar. This model therefore predicts that individuals who attach social significance to coarticulatory information are more likely to participate in sound change than those individuals who do not, and whose exemplar clouds (and pronunciations) should remain stable over time. Garrett and Johnson simulate their model with two groups of autonomous agents who were exposed to phonetic tokens for /z/ whose variants included a small number that were affected by an articulatory bias so that they sounded more like an approximant /r/. One group was modelled to compensate for the articulatory bias, essentially discarding the novel approximant variants, whereas the other group did not, and was thus intended to represent individuals with more sensitivity to social differences. After more than fifty iterations the two groups’ productions diverged according to how they responded to the novel variants; the group that did not compensate for the articulatory bias came to produce /r/-like tokens in their own output. In this way Garrett and Johnson model the update of novel variants according to whether or not they are cognitively stored in individuals.

Yu (2013) addresses the role of individual cognitive processing style in the actuation of sound change. He compares the categorization of CV stimuli (where C is an /s…ʃ/ continuum and V = /i, u/) with personality and social traits as measured by a number of standard questionnaires. Yu reports that neurotypical listeners with fewer autistic traits (more specifically a lower Autism Quotient [AQ]) are less likely to link coarticulatory information during the fricative to its source in the following /u/ (i.e. more “ʃu” responses) than listeners with a high AQ who tend to compensate for context (and give more “su” responses accordingly). Yu argues that by failing to compensate for coarticulation, participants with a low AQ may be responsible for the creation of novel variants. Yu then relates AQ (and its sub-components of attention and social skills) to a number of personality and social traits. Ultimately Yu suggests that the same individuals who are likely to undergo mini-sound changes might also be more likely, due to their (more extroverted and agreeable) personality and social profiles, to spread such innovations within their social networks.

These three approaches together suggest that the selection of sound changes from a pool of synchronic variation depends on differences between individual members that make up a speech community. These differences are not due to chance but rather involve factors that are identifiable and generalizable to other groups of language users. For Baker et al. (2011) the systematic difference lies in production, whereas for Garrett and Johnson (2013) it involves sensitivity to social factors and for Yu (2013) it involves cognitive and social traits. Common to Garrett and Johnson and Yu is the way that individual listener interpretation provides the catalyst for sound change and speaker productions do not change—at least initially. Differences across individual speaker productions are instead crucial to Baker et al.’s model, and can only subsequently be perceived and possibly imitated by listeners. This echoes Lindblom et al. (1995) who also suggested that there must be “significant change in the phonetic pattern” (p. 16) for a variant to be noticed by listeners and to eventually undergo sound change.

The role of phonetic similarity in driving sound change actuation deserves further experimental investigation. In contrast with Baker et al. (2011), Garrett and Johnson (2013) hypothesize that slight phonetic differences in production may be more likely to lead to sound change than larger differences, because they would not be detected by listeners and would therefore be included amongst stored exemplars. Garrett and Johnson hypothesize that more dramatic phonetic differences due to e.g. production errors (here one could also consider Baker et al.’s exaggerated coarticulations) would not be automatically stored in a listener’s representation and would need to take on a socio-indexical meaning to participate in sound change. Evidence from the imitation literature (described in more detail below) favours the idea that phonetically similar variants would be more likely to undergo sound change. For example in spontaneous conversations, Kim, Horton, and Bradlow (2011) report more imitation between speaker pairs who both shared the same dialect background than between pairs with different linguistic backgrounds. Olmstead, Viswanathan, Aivar, and Manuel (2013) report that native Spanish and English listeners’ imitation of an 11-step [ba]–[pa] continuum showed less convergence outside the bounds of their native pronunciation range (i.e. English listeners converged less in the prevoiced region and Spanish listeners converged less for the long lag region).

4. SYSTEMATIC INDIVIDUAL DIFFERENCES AND SOUND CHANGETop

Labov (2006) has argued that we should not seek to code linguistic variation between individuals for its own sake. He points out that “some further justification for the description of variation is required; otherwise there will be no stop to the enterprise and we will be plunged into an endless pursuit of detail” (p. 508). Labov goes on to point out that variation is crucial to language change, in particular, and indeed the further justification here is that we do not yet have a model of how sound change is actuated. That is, we need to link the initiation of sound change in an individual’s cognitive grammar and widespread change at the group level. The approaches outlined above suggest that actuation is dependent on variation between individuals that make up speech communities. An ongoing challenge is to identify the factors responsible for variability within groups of individuals who interact on a daily basis.

5. FINAL COMMENTSTop

While phonetic research on the origins of sound change has tended to focus on group-level biases in speech production and perception, it appears that sound change should be seen as the result of interactions between individuals who have slightly different cognitive mappings, perceptual abilities, production strategies or sensitivities to particular social factors. Identifying these factors and the range of variability between individuals who make up a speech community is vital to understanding what causes certain sound categories, but not others, to become unstable over time.

ACKNOWLEDGMENTSTop

This research was supported by the European Research Council Advanced Grant n^o 295573 ‘Sound change and the acquisition of speech’(2012-2017)to Jonathan Harrington. We are very grateful to Hanna Ruch for translating our abstract into Spanish at short notice.