This is the preprint version of “ Environmental influences on infants ’ native vowel discrimination : The case of talker number in daily life ” For citation information see below

Both quality and quantity of speech from the primary caregiver have been found to impact language development. A third aspect of the input has been largely ignored: the number of talkers who provide input. Some infants spend most of their waking time with only one person; others hear many different talkers. Even if the very same words are spoken the same number of times, the pronunciations can be more variable when several talkers pronounce them. Is language acquisition affected by the number of people who provide input? To shed light on the possible link between how many people provide input in daily life and infants’ native vowel discrimination, three age groups were tested: 4-month-olds (before attunement to native vowels), 6-month-olds (at the cusp of native vowel attunement) and 12-month-olds (well attuned to the native vowel system). No relationship was found between talker number and native vowel discrimination skills in 4and 6-month-olds, who are overall able to discriminate the vowel contrast. At 12 months, we observe a small positive relationship, but further analyses reveal that the data are also compatible with the null hypothesis of no relationship. Implications in the context of infant language acquisition and cognitive development are discussed. PREPRINT OF TALKER NUMBER AND NATIVE VOWEL DISCRIMINATION 2 Forming discrete categories based on continuous sensory input is a fundamental human skill across many domains. A key instance of category formation takes place during early language acquisition, when infants tune into the sound system of their native language. There are many possibilities to divide up the acoustic space into units that can be combined to form distinct word forms. Infants have to discover which sound contrasts (such as /r/ and /l/, which change word meaning in English, but not Japanese) to pay attention to. This process has its onset early in infancy, as infants begin to show indicators of tuning into their native vowel system at the age of six months (see the meta-analysis by Tsuji & Cristia, 2014). Given this timeline, mechanisms that can account for infants’ ability to build sound categories must work from the signal, involving limited or no top-down information. Consequently, all currently proposed mechanisms of sound category acquisition rely heavily on the computation of statistics over some input representations. This includes distributional learning (Maye, Werker, & Gerken, 2002); the perceptual magnet effect (Kuhl, 2004); and proposals that allow for interactions with other sources of information, such as the developing proto-lexicon (e.g., Elsner, Goldwater, Feldman, & Wood, 2013; Feldman, Griffiths, Goldwater, & Morgan, 2013; Swingley, 2009; Yeung & Werker, 2009). Infants must both draw the lines between native sound categories, and discover how many categories actually exist, clearly a difficult joint learning problem. It is possible that talker characteristics are a hurdle during language processing, and specifically sound categorization, because talkers vary a great deal in their productions along the same acoustic dimensions that distinguish speech sounds. As a result, one person’s /a/ (as in “cot”) might be indistinguishable from another’s /O/ (as in “caught”) on the acoustic level. Talker differences can be traced to a number of factors ranging from physical differences to idiosyncratic ways of articulating sounds (Hillenbrand, Getty, Clark, & Wheeler, 1995). As a consequence, talker variation proves an interesting test case of how exactly infants begin tuning into their native vowel system, as current theories make mutually exclusive predictions on how infants must deal with variation in their input. We first lay out the three theoretical predictions, then discuss empirical evidence and finally outline our approach in the remainder of the Introduction. If infants were to track the statistics of raw acoustic realizations across salient dimensions, they might be misled as to their native category system in the presence of multiple input talkers. Talker specific variation can mask linguistically relevant information, and the overlap between categories increases dramatically when introducing multiple talkers (Hillenbrand et al., 1995; specifically Fig. 4; see also Kuhl, 2004). Alternatively to computing possibly misleading statistics over input from all talkers in the environment, infants might be able to separate talkers (Kleinschmidt & Jaeger, 2015) using contextual information, so that statistics for each talker are tracked independently. This should lead to less information being available for the formation of each talker-dependent category when being exposed to multiple people (assuming that equal amounts of speech input is available across lowand high-talker-variability scenarios). The resulting prediction in both scenarios – computing confusing input statistics and separating input by talker – is that increased talker variation has negative consequences for sound category development. Not all researchers who work on the topic of early language acquisition agree with these negative predictions. A sizable community believes that talker variability could play PREPRINT OF TALKER NUMBER AND NATIVE VOWEL DISCRIMINATION 3 no role at all. This position rests on the assumption that, from birth, infants are able to ignore the difference between talkers, by either abstracting away features that are confusing, or computing talker-invariant representations early on during processing (Dehaene-Lambertz & Peña, 2001; Kuhl, 1979, 2004). With this ability in place, talker variation should have no impact on sound category development. Yet others argue that talker variability might be helpful during sound category acquisition. This is the case if variability leads infants to focus on linguistically informative dimensions of the speech signal (Rost & McMurray, 2009; Seidl, Onishi, & Cristia, 2014). The core assumption is that infants base their learning on a representation where talker-dependent information is not (or only weakly) correlated with linguistically important information. As a consequence, infants who are exposed to more variable input should benefit, because they will start learning which aspects of the speech signal are important for linguistic processing, and which are not, at an earlier age than peers with less variable input. Empirical support exists for all three stances and we will give a brief overview here (for a more extensive review focusing on sound discrimination, see Bergmann, Cristia, & Dupoux, 2016; for a broad discussion of talker variation refer to van Heugten, Bergmann, & Cristia, 2015). For example, Jusczyk, Pisoni, and Mullennix (1992) showed a negative effect of multiple talkers for two-month-olds in a sound discrimination task, especially when introducing a short delay that likely increases task demands. In contrast, Kuhl (1979) found that six-month-olds succeeded in distinguishing two vowels, both in the presence and absence of multiple talkers. The third, positive scenario was supported by a study showing that, when learning phonotactic rules, fourand eleven-month-old infants benefit from hearing multiple talkers during the learning phase (Seidl, Onishi, & Cristia, 2014). While the diversity in the findings just mentioned could be due to methodological and/or age differences, across the literature it is not possible to discern a clear pattern which is modulated by those factors (Bergmann et al., 2016). To shed light on the relation between early language development, specifically phonetic category learning, and input taker variability, we adopt an approach that does not rely solely on short-term learning and performance in the lab: individual variation among infants. This line of research relies on studying differences between (groups of) infants by tracking both diverging features of infants’ environments and their performance in fairly standard lab-based tasks. Correlations between the two measures are compatible with the interpretation that there may be an underlying association. This method has prominently been applied when investigating the possible link between language abilities and the quantity of input (Huttenlocher, Haight, Bryk, Seltzer, & Lyons, 1991) as well as the role of home environment quality (Melvin et al., 2017). On a more subtle level, qualitative aspects of infants’ input also shape their path into language. For example, mothers’ vowel space size when talking to their infant aged 6 to 12 months correlated with the infants’ performance in a speech perception task at the same age (Liu, Kuhl, & Tsao, 2003). In another study documenting a more specific association, differences in how distinctly /s/-/S/ (as in “sip” versus “ship”) were pronounced by caregivers predicted their respective infants’ ability to discriminate the same sound pair beyond overall speech rate or pitch (Cristia, 2011). The latter study suggests that the more confusing infants’ input, the less likely they are to perform well when having to distinguish PREPRINT OF TALKER NUMBER AND NATIVE VOWEL DISCRIMINATION 4 two similar sounds in laboratory studies. In other words, this line of research establishes that the effects of characteristics of infants’ everyday environment on language development can be measurable via laboratory tasks in the individual infant. Our study adopts the previously established approach to assessing individual variation to address the key question of whether talker variation in infants’ everyday life could potentially affect early language acquisition. If all infants received input from just one main caregiver, it would not be possible to observe any differences. However, in many cultures, different daycare models and diverse household structures (e.g., with multiple siblings or including extended family) lead to natural variation in the number of people who habitually talk to an infant. We sought to shed light on the possible link b