Israeli sign language

ISL is the major sign language of the Israeli deaf community. It emerged in the 1930s and 1940s with the formation of the deaf community in Israel and the language is now in its fourth generation. Many immigrants from all around the world brought with them sign languages and home sign systems from their countries of origin. These languages influenced the lexicon of ISL. Today, ISL is widely used by about 10,000 signers and, like any language, it is used across all aspects of life (Sandler, Reference Sandler2018). Linguistic characteristics of ISL are documented in numerous publications (e.g., Dachkovsky & Sandler, Reference Dachkovsky and Sandler2009; I. Meir, Reference Meir2002, Reference Meir2003, Reference Meir2006). As in the case of other sign languages, the lexicon of ISL is independent from the societal spoken language, Hebrew, and, in many instances, it reflects semantic distinctions that do not appear in the societal spoken language. For example, the Hebrew word “ahava” meaning love has two ISL signs, one sign denotes love for inanimate objects and the other sign denotes love for animate beings. We adapted the ISL-CDI from the Hebrew-CDI (Gendler-Shalev & Dromi, Reference Gendler-Shalev and Dromi2020) based on the assumption that the experience and cultural background of Hebrew toddlers would be the closest to the toddlers using ISL. Similarly, previous studies developing adaptations for a national sign language were based on the dominant spoken language. For example, the CDI adaptation for Spanish Sign Language was developed based on Spanish (Rodríguez-Ortiz et al., Reference Rodríguez-Ortiz, Pérez, Valmaseda, Cantillo, Díez, Montero and Saldaña2020). Signers of ISL are integrated within the society of Hebrew speakers and many of them are bimodal-bilingual. Support for the similarities between the vocabularies of the spoken and sign languages can be found in Anderson and Reilly (Reference Anderson and Reilly2002) for American Sign Language of deaf signers and in Rinaldi et al. (Reference Rinaldi, Caselli, Di Renzo, Gulli and Volterra2014) for Italian Sign Language of deaf signers. In both studies, the vocabulary of the CDI version for sign languages was comparable to that of the spoken language of children speaking the society language: English and Italian.

The adaptation in the current study took into consideration modality and language-specific properties of Hebrew and ISL, and these are described in the “Methods” subsection. For example, we excluded some signs for body parts that are represented by pointing.Footnote ² Furthermore, the nonsign gestures section was excluded from the CDI to focus exclusively on the lexicon and reduce confusion between signs and gestures by the parents who fill in the questionnaire (for a detailed review on adapting sign language tests from a spoken language see Haug, Reference Haug2011).

Sign language acquisition

Direct measures of vocabulary assessment and parental questionnaires tapping into vocabulary development in sign languages show positive correlations between chronological age and vocabulary size (e.g., for British Sign Language [Thompson et al., Reference Thompson, Vinson, Woll and Vigliocco2013]; for American Sign Language [Caselli & Pyers, Reference Caselli and Pyers2017]; and for Turkish Sign Language [Sümer et al., Reference Sümer, Grabitz and Küntay2017]). As children grow older, they score higher on vocabulary size measures, similarly to children who acquire spoken languages (Frank et al., Reference Frank, Braginsky, Yurovsky and Marchman2017). Following these studies, our study also aimed to investigate whether vocabulary size in ISL of bimodal-bilingual children is associated with age (Study 2), which was also used to support the validation of the ISL-CDI.

Deaf children of deaf parents acquire two languages, some of them become bilingual during school age; others become bimodal-bilingual. Bilingual children use sign language as their home language and acquire the written modality of the spoken language later during school years. Yet, today with the advancement in technology many of the deaf children of deaf parents are bimodal-bilingual at preschool age. By means of hearing aids and cochlear implants, deaf children born to deaf parents have access to auditory input that allows them to acquire the spoken language at younger ages in its spoken modality. In previous studies, native signing toddlers are defined as monolinguals because they were exposed only to a sign language (e.g., Woolfe et al., Reference Woolfe, Herman, Roy and Woll2010). The children in the current study were early bimodal-bilinguals. They were exposed to both ISL and Hebrew and had partial auditory capacity to perceive the spoken language in addition to their native ISL (as described in more details in the “Methods” subsection of Study 2).

It is well-documented in the literature that language skills of bilingual children who are exposed to two spoken languages are often unevenly distributed across and within the languages spoken by the child (Kohnert, Reference Kohnert2010). In the same vein, bimodal-bilingual children’s language proficiency and language dominance can vary across the two modalities: the sign and the spoken. Vocabulary size of bimodal-bilingual children should be assessed in both languages, similarly to bilingual children who acquire two spoken languages (e.g., Armon-Lotem et al., Reference Armon-Lotem, de Jong and Meir2015). Yet, currently, there are no available tools to assess children’s language skills in ISL, while in Hebrew there is a range of standardized assessment measures (e.g., the Hebrew-CDI [Gendler-Shalev & Dromi, Reference Gendler-Shalev and Dromi2020]; Goralnik screening test for Hebrew [Goralnik, Reference Goralnik1995]). Thus, the current study aimed to address this gap by developing ISL-CDI.

There is a scarcity of research on vocabulary acquisition of bimodal-bilingual children. These existing studies support the importance of testing both languages of bimodal-bilingual due to the specific trajectories of each language. For example, Rinaldi et al. (Reference Rinaldi, Caselli, Di Renzo, Gulli and Volterra2014) showed that acquisition trajectories of lexical categories differ between sign and spoken languages. Using a naming-picture naming, the authors showed that 2–3-year-old signers scored higher on signs that describe actions (predicates/verbs) compared to signs that describe objects (nominal/nouns) (Rinaldi et al., Reference Rinaldi, Caselli, Di Renzo, Gulli and Volterra2014). The toddlers acquiring spoken language showed an opposite pattern: they scored higher on nouns compared to verbs. Another reason to test both languages of bimodal-bilinguals is their smaller vocabulary size shown when being tested in only one language compared to hearing peers. The smaller vocabulary size of signing toddlers compared to hearing children (Rinaldi et al., Reference Rinaldi, Caselli, Di Renzo, Gulli and Volterra2014; Woll, Reference Woll, Meurant, Sinte, Van Herreweghe and Vermeerbergen2013) can be explained by their bilingual status (Rinaldi et al., Reference Rinaldi, Caselli, Di Renzo, Gulli and Volterra2014). Previous studies demonstrated that bilingual children who acquire two spoken languages might have smaller vocabularies in one of their languages when compared to their monolingual peers, especially in the societal language (for an overview see Gatt et al., Reference Gatt, O’Toole and Haman2015). Thus, it is highly important to assess the vocabulary size in the sign language of bimodal-bilingual toddlers.

Iconicity of signs and sign language acquisition

Word learning requires mapping information about its form onto its meaning. Iconicity is a relationship of similarity between the two aspects of a word or a sign: its form and its meaning. An iconic sign/word is one whose form represents its meaning in some way (Dingemanse et al., Reference Dingemanse, Blasi, Lupyan, Christiansen and Monaghan2015; I. Meir & Tkachman, Reference Meir and Tkachman2018). Iconicity has different features (Winter et al., Reference Winter, Perlman, Perry and Lupyan2017), for example, in spoken languages it can be the sound of the form (e.g., onomatopoeic words like buzz in English, representing the sound of the concept) or the repetition of the form (e.g., ideophonic words like “kibikibi” [“energetic” in Japanese], which includes repeated forms representing repeated events) (for an overview of iconicity in language, see Dingemanse et al., Reference Dingemanse, Blasi, Lupyan, Christiansen and Monaghan2015). In sign languages it is the visual structure of the form (e.g., the sign TO EAT in ISL mimics the action of eating) (following convention, all English glosses of ISL signs are written in capital letters). This means that iconicity represents the visual similarity between the sign form and its meaning.

Iconicity in sign languages is usually present in one or more of the phonological parameters of the sign: the shape of the hands, the location of the hands in space or on the body, and/or the movement of the hand (Perniss et al., Reference Perniss, Thompson and Vigliocco2010). Iconic mapping goes beyond a simple relationship of form to meaning, and different factors affect it, such as age, linguistic experience, and cultural background (Ortega, Reference Ortega2017). This mapping is commonly present across languages, specifically in sign languages (Dingemanse et al., Reference Dingemanse, Blasi, Lupyan, Christiansen and Monaghan2015; Perniss et al., Reference Perniss, Thompson and Vigliocco2010). However, not all signs are iconic, in line with the agreed assumption that in many cases there is an arbitrary link between form and meaning (de Saussure, Reference de Saussure1916).

In acquisition of spoken languages, iconic words, are common in children’s early words (Laing et al., Reference Laing, Vihman and Keren-Portnoy2017; Perry et al., Reference Perry, Perlman, Winter, Massaro and Lupyan2018). For example, Massaro and Perlman (Reference Massaro and Perlman2017) showed that words learned earlier tended to be more iconic, suggesting that iconicity in early vocabulary may facilitate word learning. With increases in vocabulary size, the effect of iconicity diminishes dramatically. Yet, it should be noted that the effect of iconicity is independent from the influence of concreteness, difficulty of articulation, and parental input frequency. Another more specific iconicity effect is reported in the study by Maurer et al.’s study (Reference Maurer, Pathman and Mondloch2006). The authors found that 2.5-year-olds were sensitive to the structure of vowels in words: similarly to adults’ performance, children consistently matched words with rounded vowels (e.g., “bouba”) to round shapes and words with unrounded vowels (e.g., “kiki”) to pointed shapes. These findings provide evidence that iconicity is an important property of spoken languages.

Early work on sign languages showed contradictory findings. For example, Orlansky and Bonvillian (Reference Orlansky and Bonvillian1984) found that only one third of early-acquired signs is iconic, reflecting the overall proportion of iconic signs in American Sign Language. Yet, recent studies on sign languages showed that iconicity of signs contributes to lexical acquisition (e.g., Caselli & Pyers, Reference Caselli and Pyers2017 for American Sign Language; Sümer et al., Reference Sümer, Grabitz and Küntay2017 for Turkish Sign Language; Woolfe et al., Reference Woolfe, Herman, Roy and Woll2010 for British Sign Language). Thompson et al. (Reference Thompson, Vinson, Woll and Vigliocco2013) showed that iconicity effects increased between age 2–4-years-old, supporting the idea that cognitive development and world knowledge allow children to rely on the iconic mapping of form to meaning (Meier et al., Reference Meier, Mauk, Cheek and Moreland2008). Together these studies suggest that iconicity of signs contributes to lexical acquisition (for an overview of the effect of iconicity on acquisition, see Ortega, Reference Ortega2017).

The effect of iconicity deserves attention of research due to conflicting evidence. On the one hand, there are studies showing the effect of iconicity diminishes with age (e.g., Massaro & Perlman, Reference Massaro and Perlman2017). On the other hand, there are studies showing that its effect increases with age (Thompson et al., Reference Thompson, Vinson, Woll and Vigliocco2013). Notably, the interaction between age and iconicity seems to be critical in our understanding the contribution of iconicity to language acquisition. If iconicity (the link between form and meaning) is mediated by cognitive development and language experience, the effect of iconicity would increase with age. This means that experience affects the child’s ability to learn the form-meaning mapping of iconic signs and this would be related to age, which is a proxy of experience. In contrast, if iconicity relies on basic sensory motor associations between the form and the meaning, and it is not related to experience with language, age will not play a role (Perniss et al., Reference Perniss, Thompson and Vigliocco2010).

Frequency of signs and sign language acquisition

Frequency represents item-specific characteristics, or how often a word/sign is used in a given language. Its influence on language acquisition is beyond cognitive abilities as it refers to the input to which a child is exposed, suggesting that higher frequency words/signs have higher baseline activation that affects acquisition (Degani et al., Reference Degani, Krizer and Novogrodsky2019; Gollan et al., Reference Gollan, Montoya, Cera and Sandoval2008).

It has been shown that the acquisition of lexical items is positively correlated with the frequency of these items in child-directed speech (e.g., Edwards et al., Reference Edwards, Beckman and Munson2015) and in language use (Perry et al., Reference Perry, Perlman, Winter, Massaro and Lupyan2018). For example, lexical items like “baby” and “ball” are acquired earlier than “coffee” and “computer,” as the frequency of the words “baby” and “ball” is higher in child-directed speech. However, not all types of words are equally affected by frequency of usage. Closed-class (function) words (e.g., pronouns, prepositions, articles) are highly frequent, but have a negative correlation between frequency and child’s lexical production (Braginsky et al., Reference Braginsky, Yurovsky, Marchman and Frank2016; Goodman et al., Reference Goodman, Dale and Li2008); that is, parents produce them frequently, but children master them later than other words.

The effects of frequency of use on the acquisition of lexical items change during child development (Goodman et al., Reference Goodman, Dale and Li2008). The effect of frequency is also mediated by children’s chronological age and vocabulary size, such that correlations with frequency become more pronounced after the first 100 words. Thus, the effects of frequency are sensitive to grammatical category and change over early language development.

Studies evaluating the effect of sign frequency in sign languages have come to contradictory conclusions. In one influential paper, Thompson et al. (Reference Thompson, Vinson, Woll and Vigliocco2013) reported no effect of sign frequency, but subsequent studies in other sign languages do report a robust effect of frequency (Caselli & Pyers, Reference Caselli and Pyers2017 for American Sign Language; Sümer et al., Reference Sümer, Grabitz and Küntay2017 for Turkish Sign Language). Whereas the term “familiarity” was used by Thompson et al. (Reference Thompson, Vinson, Woll and Vigliocco2013) and the term “frequency” was chosen by Caselli and Pyers (Reference Caselli and Pyers2017), the two terms are closely linked (Vinson et al., Reference Vinson, Cormier, Denmark, Schembri and Vigliocco2008). Both studies operationalized their terms similarly by asking native adult signers to rate the frequency of the signs. Thus, the contradictory findings cannot be explained by the methodology of collecting frequency ratings.

Previous studies on sign languages have used a subjective index of frequency in adult-directed speech using ratings by fluent signers (Caselli & Pyers, Reference Caselli and Pyers2017; Sümer et al., Reference Sümer, Grabitz and Küntay2017; Thompson et al., Reference Thompson, Vinson, Woll and Vigliocco2013). Although frequency in child-directed speech better predicts acquisition (Goodman et al., Reference Goodman, Dale and Li2008; Hansen, Reference Hansen2017), these data are not available for any sign language, including in ISL.

To sum up, previous studies on sign language bring inconclusive evidence on the effects of iconicity and frequency on vocabulary acquisition. The current study aimed to contribute to this ongoing debate by investigating a sign language, that is, ISL, for which no vocabulary tools are available and no acquisition data have been previously reported.

The current study

The current study was devised to develop an ISL-CDIFootnote ³ assessment tool and to explore vocabulary development in the sign modality of children who acquire ISL. We aimed to assess the effects of iconicity and lexical frequency on vocabulary acquisition in ISL of bimodal-bilingual children. With this aim in mind, we conducted two preliminary studies to obtain iconicity ratings from sign-naïve Hebrew-speaking adults (Study 1A) and lexical frequency ratings from native ISL signers (Study 1B).

In Study 2, following previous literature on vocabulary development of sign languages (see the “Introduction” section), it was hypothesized that age, iconicity, and frequency will uniquely contribute to vocabulary development in ISL. Moreover, it was predicted that the effects of iconicity and frequency will increase with age, and will show interaction of age and iconicity and age and frequency.

Study 1 (A, B)

Results

Study 1A: Iconicity ratings

Mean iconicity ratings for the 563 ISL signs varied between 1.2 and 6.9 (see Figure 1 for the distribution of signs per Iconicity level). Fifty-six percent of the ISL-CDI signs were rated as highly iconic, that is, they received a score higher than a four on the 7-point scale (e.g., NO, PUSH, SLEEP, SMILE, TELEPHONE).

Figure 1. Distribution of signs per iconicity level.

Within the 17 different lexical semantic categories, the categories of pronouns (M = 5.80, SD = 0.76) and prepositions of places (M = 5.83, SD = 1.00) received the highest iconicity ratings, while the category of people (M = 2.57, SD = 1.31) received the lowest iconicity ratings (e.g., UNCLE, KINDERGARDEN-TEACHER) (Table 1). Most signs in the pronoun category are signed by pointing to the referent direction (e.g., I is signed by pointing to self). In the category of prepositions of places, most signs are signed with the two hands representing the meaning of the preposition relationship (e.g., ON is signed with open flat handshape and the dominant hand is placed on the back of the nondominant hand with palms facing down; for IN, the dominant hand is produced with fingers together touching the thumb held [in a “c” shape] and the dominant hand moves inside the nondominant hand within the neutral signing space).

Table 1. Mean iconicity and frequency rating per lexical semantic category

* n/a = not applicable.

Note: In ISL many signs for body parts are signed with pointing and were excluded from the pretested version of the test. The nonpointing signs were added to the questionnaire only after iconicity ratings were collected. Thus, this category is not tested as part of the iconicity effect on acquisition.

Study 1B: Frequency ratings

Mean frequency ratings varied between 2.3 and 6.7 (see Figure 2 for the distribution of signs per frequency level). Ninety-eight percent of the ISL-CDI signs in the ISL-CDI were rated as highly frequent (e.g., DOG, CAR, FOOD), that is, they received a score higher than four on a 7-point scale.

Figure 2. Distribution of signs per frequency level.

We further tested if there were any differences in frequency ratings across different lexical semantic categories in the ISL-CDI (see Table 1). The category of body parts received the lowest frequency ratings (M = 5.19, SD = 1.06) this category did not include body parts that were signed by using a pointing (see the “ISL-CDI Adaption Materials” section for more details) and the category of time words received the highest frequency ratings (M = 6.72, SD = 0.23). As stated previously, these ratings are based on frequency of use by adults, not in child-directed signs. Pearson correlational analysis showed that there were no significant correlations between iconicity and frequency ratings (r = .073, n = 554, p = .088).

Discussion of study 1a: iconicity

When exploring iconicity across different lexical categories, the categories of pronouns and prepositions of places received the highest iconicity ratings. In contrast, the category of people received the lowest iconicity ratings in ISL, similarly to iconicity examples from British Sign Language (Vinson et al., Reference Vinson, Cormier, Denmark, Schembri and Vigliocco2008).

An interesting difference between the current study and other studies is the pattern of iconicity ratings. In the current study participants rated the signs across the scale as can be seen in Figure 1. In Vinson et al. (Reference Vinson, Cormier, Denmark, Schembri and Vigliocco2008), iconicity rating were bimodally distributed, and in Tasci et al.’s study (Reference Tasci, Sumer and Eker2017) deaf participants gave higher iconicity ratings than hearing nonsigners. One explanation for this difference between these studies can be the profile of the participants (Occhino et al., Reference Occhino, Anible, Wilkinson and Morford2017). In Vinson et al.’s study (Reference Vinson, Cormier, Denmark, Schembri and Vigliocco2008), the participants were native signers, in the current study they were hearing sign naïve adults. These gaps suggest that knowing a language might push raters toward stricter boundaries of iconicity or to higher ratings. In the current study there were significant correlations between the sign-naïve participants and the native ISL signers (r = .71, p < .001). The different pattern of iconicity in hearing adults and native signers rating awaits future studies.

Support for the effect of language knowledge and world knowledge on iconicity comes from a study that tested the ability of hearing adults and children who did not know sign language to guess the meaning of iconic signs from the ISL-CDI list (Ali & Farage, Reference Ali and Farage2017). The study included 30 video signs from the ISL-CDI with high iconicity rating of 5.8–7.0. Whereas adults guessed in average 78% of the signs, children guessed only 29% of these signs with significant difference between the groups. It suggests that having more experience with language and world knowledge may result in more pronounced effects. Thus, the effects of language patterns and world knowledge and their influence on language acquisition await future research. We further explored the relationship between iconicity and age and the contribution of iconicity to language acquisition in Study 2.

Discussion of study 1b: frequency

Most signs (98%) of the ISL-CDI were frequent signs based on the ratings of native signers. Note that all ISL-CDI signs are expected to be acquired at early age (up to 36 months). These results are in line with previous studies showing that early-acquired vocabulary is frequent in the language (Braginsky et al., Reference Braginsky, Yurovsky, Marchman and Frank2016; Diessel, Reference Diessel2007; Goodman et al., Reference Goodman, Dale and Li2008). Children are sensitive to external variables, that is, how frequently words appear in their input: frequent words are acquired earlier than rare words (Goodman et al., Reference Goodman, Dale and Li2008). Similar findings are shown also for American Sign Language (Caselli & Pyers, Reference Caselli and Pyers2017).

Finally, there was no correlation between frequency and iconicity, suggesting that each of these two factors can independently facilitate acquisition. We examined this question in Study 2.

Study 2

Study 2 tested early acquisition of ISL vocabulary based on the ISL-CDI parental questionnaire and explored the relationship between age, iconicity, and frequency of vocabulary development in the sign modality (ISL) of bimodal-bilingual children born to native signers.

Method

Participants

Thirty-four bimodal-bilingual deaf children (M _age = 39 months, SD = 25, Min–Max: 8–86), acquiring ISL as their native language, participated: 22 younger children and 12 older children (Table 2). The sample was comprised of 20 boys and 14 girls (in the younger group: 14 boys and 8 girls; in the older group: 6 boys and 6 girls). The rational for testing older children on the ISL-CDI questionnaire was motivated by the need to identify the age of ceiling scores on the instrument. All children had severe to profound hearing loss, they all had two deaf parents, and ISL was the home language. None of them had any neurological disorders based on parental report. Families were recruited across the country (north, center, and south) using personal contacts in the deaf community. The sample although small in size, represents 0.3% of the community (based on estimation of 10,000 members in the community; Sandler, Reference Sandler2018). All families were from middle socioeconomic status as measured by mother’s years of education (M = 13 years, SD = 2, Min–Max: 10–17) and father‘s education (M = 13 years, SD = 1, Min–Max: 10–16). All participants were diagnosed as having hearing impairment before age of 1 month (Table 2) due universal newborn hearing screening practice in Israel (Gilbey et al., Reference Gilbey, Kraus, Ghanayim, Sharabi-Nov and Bretler2013). Children started intervention during the first months after birth. In the younger group, 5 children had two cochlear implants, 16 children had two hearing aids, and 1 child was in the process of hearing aids fitting. In the older group two children had one cochlear implant, seven children had two cochlear implants, and three children had two hearing aids. Intervention included both languages (Hebrew and ISL) and hearing training. IT-MAIS scores (Ben-Itzhak et al., Reference Ben-Itzhak, Greenstein and Kishon-Rabin2014) (Table 2) show the range of hearing capacity of the group. As expected, hearing function scores on the IT-MAIS were higher for the older group compared with the younger group because the children in the older group had more hearing experience. This finding is in line with previous studies (e.g., Novogrodsky et al., Reference Novogrodsky, Meir and Michael2018). With age, children who use functional hearing devices (hearing aids and cochlear implants) and receive hearing intervention progress in their hearing function capacities, which contributes to spoken language acquisition. This might affect the current group’s spoken vocabulary acquisition. However, this is beyond the scope of the current study.

Table 2. Demographic data per group

* For children with hearing aids, hearing age was calculated based on age of hearing aid fitting. ** For children using cochlear implants, hearing age was calculated based on cochlear implant activation. Cochlear implantation is recommended, when no sufficient gain in hearing function with the hearing aids is observed.

Table 3 presents information on the exposure to ISL and Hebrew in home setting (on a scale of 1–7). The results of the parental questionnaire demonstrate that children are exposed to both languages, yet they get more exposure in ISL as compared to Hebrew and this is especially evident in the younger group.

Table 3. Exposure to ISL and Hebrew in the home setting per group (rating range 1–7)

Looking into the patterns of exposure at educational settings based on parental reports, in the younger group, most children (20 out of 22) have access to ISL with signed Hebrew and spoken Hebrew. In the older group, most children (10 out of 12) are exposed to ISL with signed Hebrew and spoken Hebrew. However, only 10 parents out of 22 in the younger group and 7 out of 12 in the older group indicated that their children are exposed to ISL from native signers at the educational settings.

The characteristics of the current sample of native signers support that these children are early bimodal-bilinguals. With early hearing screening and the use of new technology, the bimodal-bilinguals in the current sample differ from the samples reported in earlier studies of 20 years ago where the spoken language was acquired latter by deaf children born to deaf parents during school age through print (reading and writing).

Results

The validity of the ISL-CDI

First, we analyzed the ISL-CDI data across the two groups of participants. The results showed strong positive correlations between productive ISL vocabulary (ISL-CDI) and Age (r = .852, n = 34, p <. 001) (see Figure 3). Furthermore, ISL-CDI scores significantly correlated with parental ratings of children’s ISL skills as rated in the parental background questionnaire (r = .695, n = 34, p < .001), and there were strong correlations between parental rating and ISL size even when age was controlled for (r = .549, n = 34, p = .001).

Figure 3. Scatterplots of ISL-CDI scores per child’s age in months (n = 34).

To further explore the validity of the ISL-CDI questionnaire, we correlated the ISL-CDI scores of 12 of the participants for whom we collected naturalistic data and type/token ratio scores were available. Results showed significant correlations between ISL score and type/token ratio scores (r = .603, n = 12, p = .038). These three measures support the concurrent validity of the ISL-CDI questionnaire for assessing vocabulary capacity.

Descriptive statistics

Children’s ISL-CDI scores (productive vocabulary) varied between 0 and 557 (M = 286, SD = 211), out of a total of 563 ISL signs. In the younger age group (age range: 8–36 months), children scored between 0–460 (M = 173, SD = 156) (Figure 3). For example, one child, aged 11 months, was reported to use no signs, whereas another child, aged 8 months, was reported to use one sign and a 15-month-old child was reported to use seven signs. These results reflect a variation in vocabulary size within this age group. In the older age group (age range: 46–86), most children showed a ceiling effect, meaning that they had acquired nearly all of the 563 signs of the ISL-CDI (M = 495, SD = 118, Min–Max: 129–557) (Figure 3).

Two signs were produced by above 90% of the children: LIGHT (94%) and BYE-BYE (91%), suggesting that these are the earliest acquired signs. Eleven signs were produced by 80% of the children and above. These signs represent the categories of family members (e.g., MOTHER, FATHER), food (e.g., WATER, FOOD), animals (e.g., FISH, DOG, BIRD), toys (e.g., BALL), greeting (e.g., HI), action signs (e.g., SLEEP), and situations and events (e.g., MORE).

Following the analysis of Frank et al. (Reference Frank, Braginsky, Marchman and Yurovsky2019), we further ran model fit curves for the proportion of each semantic category produced by each child as a function of the proportion of vocabulary size of each child (Figure 4). Figure 4 shows areas where the data deviate from the pattern of category acquisition predicted by random item sampling. The size of the region above versus below the dotted diagonal indicates over- versus undersampling for a particular semantic category. Interestingly, the categories of animals, greeting, toys, and transportation are overrepresented in early stages of vocabulary acquisition, while the categories of preposition of places, body parts, question words, and time words are underrepresented.

Figure 4. Model fit curves for proportion of each semantic category produced by each child as a function of the proportion of vocabulary size of each child.

Effects of age, iconicity, and frequency on sign acquisition

Secondly, we evaluated the effects of age, iconicity, and frequency on acquisition of vocabulary in ISL. We analyzed the data using a binominal mixed-effects logistic regression model. Given the binary nature of our dependent variable for ISL acquired signs (child produces the sign = 1, child does not produce the sign = 0), participants and items were included as random factors. The inclusion of these two variables enabled us to account simultaneously for child-specific and item-specific variability and allowed for generalization beyond both the sample of children and the set of vocabulary items. Three fixed effects were included into the model: iconicity (mean ratings), frequency (mean ratings), and age (months). We also included Age*Frequency and Age*Iconicity interactions. The analysis (Table 4) showed significant main effects of age, iconicity, and frequency. The findings showed that with increasing age children were likely to produce more signs. Children were more likely to produce signs with higher rather than low iconicity ratings, and signs with higher rather than low frequency ratings (see Figure 5). The model also yielded significant Age*Frequency and Age*Iconicity interactions, pointing at different effects of frequency and iconicity over the course of child’s language development.

Table 4. Results of the fixed effects of the binominal mixed-effects logistic regression (the results for the fixed coefficients see in Appendix 1)

Figure 5. ISL-CDI scores (% out of the total score) per (a) iconicity and (b) frequency band. Error bars present 95% CI.

For the ease of interpretation of the Age*Frequency and Age*Iconicity interactions, we divided children into several age bands (see Figure 6). Visual inspection of the Age*Iconicity interaction (Figure 6a) suggests that in the older age band group (> 55 months) there was a ceiling effect. In the younger age bands (8–24 months) only words with the highest iconicity ratings (closer to 7) were likely to be produced, whereby in the group of children of the middle band (25–54 months) the effect of iconicity is present: signs that are more iconic more likely to be produced. However, the results for the Age*Iconicity interaction must be interpreted with a degree of caution, as the interaction was not robustly significant.

Figure 6. ISL-CDI scores (% out of the total score) per (a) iconicity and (b) frequency band. Error bars present 95% CI.

Turning to the Age*Frequency interaction, visual inspection of the data (Figure 6b) suggests that the effect of frequency is robustly present after the age of 24 months, meaning that signs that are more frequent in the lexicon are more likely to be produced. Furthermore, in the younger group the effect is present yet, it is less robust as compared to older children.

General discussion

Our study was devised to develop the ISL-CDI assessment tool to explore the acquisition of sign production in ISL. The current study is the first study to investigate ISL acquisition. Secondly, it was devised to assess the role of age, frequency, and iconicity on early vocabulary acquisition in the sign modality of bimodal-bilingual children. The study aimed to add new findings to the conflicting evidence on the role of iconicity and frequency in sign language acquisition. We discuss the findings for early sign production in ISL and compare the results of the current study to previous findings of sign and spoken languages. We then discuss the relationship between age, frequency, and iconicity as shown in the current ISL-CDI data.