**1. Introduction**

The majority of the world's languages distinguish between the grammatical categories of noun and verb1. Indeed, the grammatical distinction between nouns and verbs has long been considered a fundamental feature of human language (Hockett 1977; Hopper and Thompson 1985; Jackendoff 2002), thought to emerge early on in the evolution of language (Bickerton 1990; Heine and Kuteva 2007).

More recently, studies of emergen<sup>t</sup> linguistic systems offer support for the fundamental nature of the noun–verb distinction. Research on homesign systems, communication systems created by deaf children without a language model, sugges<sup>t</sup> that such children distinguish between nouns and verbs (Goldin-Meadow et al. 1994). Furthermore, research on emerging sign languages suggests that differences in how noun and verb forms are produced can emerge even in the earliest stages of language creation (Abner et al. 2019; see also Goldin-Meadow et al. 2014). However, this body of work has also noted that systematic noun–verb distinctions do not emerge fully formed, but become increasingly systematised and conventionalised through the development of a linguistic community.

**Citation:** Motamedi, Yasamin, Kathryn Montemurro, Natasha Abner, Molly Flaherty, Simon Kirby, and Susan Goldin-Meadow. 2022. The Seeds of the Noun–Verb Distinction in the Manual Modality: Improvisation and Interaction in the Emergence of Grammatical Categories. *Languages* 7: 95. https://doi.org/10.3390/ languages7020095

 Academic Editors: Wendy Sandler, Mark Aronoff and Carol Padden

Received: 1 October 2021 Accepted: 22 March 2022 Published: 11 April 2022

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It is difficult to study the factors that lead to systematization and conventionalization in a naturally emerging language, simply because we have no control over the conditions under which the language is developing. We turn here to an experimental paradigm that has the potential to allow us to model the emergence of noun–verb distinctions in the manual modality. To the extent that our experimental paradigms reveal a picture of language emergence that resembles the picture we ge<sup>t</sup> from naturally evolving languages, we will have evidence that these artificial language experiments are good models for the study of naturally emerging languages. We can then use the paradigms to experimentally explore factors that influence emergence. We focus on two processes that have the potential to shape the evolution of noun–verb categories in emerging communication systems: (i) improvisation and (ii) interaction. We examine how individual participants improvise and innovate novel gestural signals for events in which the target object is the focus (noun context) and events in which the target action is the focus (verb context), and then investigate how those gestures change when used in interaction.

#### *1.1. Noun–Verb Distinctions in Natural Sign Languages*

The grammatical categories of nouns and verbs are almost universally present across languages and modalities, and are thought to be based on pre-linguistic conceptual categories (Hurford 2007)—namely, the need to communicate about objects in the world (i.e., nouns) and the properties/actions of those objects or the relations between them (i.e., verbs). Nouns and verbs differ in how they are used in a simple proposition, with nouns typically heading subjects and objects (or other participant roles), and verbs typically heading predicates in a proposition. In addition to this functional distinction, nouns and verbs can also differ in form, either in the base form itself (e.g., *sing/song*, *procéss/prócess*), or in other formal properties that can map onto these functional contrasts. In spoken languages, for example, constituent order can distinguish verbs from nouns, and nouns and verbs can carry different inflections—nouns are often marked for gender, number and person, and verbs for tense, aspect and mood.

In sign languages, constituent order, syntactic distribution, and morphosyntactic marking also distinguish between nouns and verbs. In addition, sign languages frequently signal grammatical categories by altering the sign form in nouns and verbs that have similar underlying forms (see Abner 2021 for an overview). For example, verb and noun signs can be distinguished by size or length of movement: verbs tend to be articulated with a larger movement (Kimmelman 2009; Pizzuto and Corazza 1996), or longer duration (Hunger 2006; Pizzuto and Corazza 1996) than nouns. They can also be distinguished by the manner of movement: verbs may be articulated with continuous movement while nouns are articulated with more restrained movement in both American Sign Language (ASL) and Australian Sign Language (Auslan) (Johnston 2001; Supalla and Newport 1978). In addition, nouns in both ASL and Auslan, as well as Russian Sign Language, tend to be articulated with repeated movements, whereas verbs exhibit variability based on their meaning (Johnston 2001; Kimmelman 2009; Supalla and Newport 1978). Finally, sign languages such as British Sign Language (BSL) can borrow mouthings from the ambient spoken language, and use these mouthings to distinguish nominal and verbal forms, with noun forms more likely to be accompanied by mouthing than verb forms in some languages (Hunger 2006; Johnston 2001; Kimmelman 2009; Tkachman and Sandler 2013). There are, however, cross-linguistic differences in how some of these strategies are implemented; for example, evidence from Turkish Sign Language (T˙ID), Sign Language of the Netherlands (NGT) and homesign shows that, in at least these cases, repetition is *less* likely to occur in nouns than verbs (Goldin-Meadow et al. 1994; Kubus 2008; Schreurs 2006).

Nevertheless, most documented sign languages demonstrate a set of noun–verb pairs distinguished by altering properties of a shared underlying form and display striking cross-linguistic commonalities in how these distinctions are made (Tkachman and Sandler 2013). Such distinctions have been shown primarily to operate over subsets of noun–verb pairs associated with concrete objects and instrumental actions (e.g., WINDOW/CLOSE- WINDOW) where the base form is iconically motivated (e.g., the two hands representing two panes of a window), though Abner (2017) provides evidence that this alternation is not limited to concrete object nouns in ASL but is also available to derive abstract, result-denoting nouns (e.g., ACCEPTANCE derived from ACCEPT). For practical reasons, experiments eliciting this contrast (including those detailed in the current manuscript) are limited to the concrete object portion of this paradigm, as these are easier to depict in video and pictorial stimuli.

Within sign language research, researchers have suggested that some of the strategies used to mark the noun–verb distinction—in particular, differences in the manner of movement and repetition—are based on iconic affordances of the categories (Abner et al. 2019; Aronoff et al. 2005; Johnston 2001; Kimmelman 2009; Tkachman and Sandler 2013; Wilbur 2008; Wilcox 2004). This iconic relationship has been suggested in particular to relate to the event structure of the verb (Wilbur 2003). Supalla and Newport (1978) observed that, while nouns in noun–verb form pairs are consistently distinguished in the same way, the specific form of the verb will depend on the properties of its event structure, consistent with the *Event Visibility Hypothesis*, which states that formal properties of predicates in sign languages reflect the semantics of event structure. For example, Tkachman and Sandler (2013) sugges<sup>t</sup> that the continuous/restrained mapping for the manner of movement to verbs and nouns, respectively, represents a mapping of continuous and temporal aspects of the event structure of verbal forms. Similarly, Kimmelman (2009) suggests that verbal forms exploit embodied iconicity to signal events (i.e., that differences in sign movement might signal a difference in the movement of the event itself), which is less inherent in the noun mapping. In this way, systematic noun–verb distinctions that evolve over time may be seeded by the iconic properties of the underlying event descriptions. To further understand how these grammatical distinctions emerge, we turn to the evidence offered from studies of emerging linguistic systems.

#### *1.2. The Emergence of Grammatical Categories*

If the grammatical categories of nouns and verbs are fundamental to human languages, then we might expect them to emerge early in the creation of a novel linguistic system (Bickerton 1990; Heine and Kuteva 2007). Currently, homesign systems and emerging sign languages provide some of the only natural examples of language creation and emergence, allowing us to observe novel linguistic systems through their earliest generations.

Homesign systems are gestural communication systems developed by children who do not have access to a conventional language model (i.e., profoundly deaf children born to hearing, non-signing parents). These systems are typically used within the immediate family and allow the child to communicate with other hearing family members (albeit with limited shared understanding; (Carrigan and Coppola 2017)). The homesign systems developed by children demonstrate properties found in natural languages—stable lexicons (Goldin-Meadow 2003), grammatical roles (Coppola and Newport 2005), displaced references (Morford and Goldin-Meadow 1997), and relational marking (Goldin-Meadow and Feldman 1977). Studying homesigns can inform us about the distinctions that language creators introduce into languages without the benefit of a conventional language model. For instance, Goldin-Meadow et al. (1994) studied David, a deaf homesigner, who initially distinguished nouns from verbs by using completely different sign forms, even for related meanings. For example, he used *twist* for the verb form (twist-open), and *round-shape* for the noun form (jar). Later he used *twist* for both the verb and noun form, but marked the distinction in gesture form; he placed the *twist* serving the role of verb near an object (similar to inflecting a verb), and produced only one rotation for the *twist* serving the role of the noun (abbreviating the noun).

Homesign systems can be studied alongside emerging sign languages to further understand the impact that having a linguistic community has on marking grammatical distinctions. For example, Nicaraguan Sign Language (NSL) began to develop in the late 1970s when a new governmen<sup>t</sup> policy established a school for deaf students in Managua. The school allowed deaf children, who had developed homesign systems with their hearing families and had no access to other deaf individuals, to come together for the first time and share their homesign systems. Nicaraguan Sign Language (NSL) was born in this first cohort. As new deaf children entered the school, they learned the language (which changed in the course of learning) from the older children, thus forming a second cohort of NSL users.

Goldin-Meadow et al. (2014) analysed the consistency of handshape forms used for nominal and predicate constructions in Nicaraguan homesigners and the first and second cohorts of NSL. Overall, handshapes in nominal signs were less variable than handshapes in predicate signs, and the variability played different roles in the two types of signs. There was no variability across grammatical contexts (e.g., an agen<sup>t</sup> vs. no-agen<sup>t</sup> context) in nominals but considerable variability in predicates. Moreover, the variability in predicates was systematic across agen<sup>t</sup> vs. no-agen<sup>t</sup> contexts, suggesting that handshape functions as a productive morphological marker on predicate signs, even in homesign. In nominals, there was no variability across grammatical contexts (agent vs. no-agent). All of the groups, including homesigners, thus distinguished between forms playing nominal vs. predicate roles.

Similarly, Abner et al. (2019) analysed differences in form between nouns and verbs in three groups: ASL users, NSL users, and Nicaraguan homesigners, focusing on pairs of nouns and verbs with the same underlying form (e.g., camera vs. taking a photo). They analysed signs based on some of the properties that have been previously shown to mark noun–verb distinctions in natural sign languages (e.g., size, repetition). All three groups marked a distinction between nouns and verbs using utterance position (verbs were placed at the end of an utterance, nouns earlier in the utterance) and movement size (verbs were made with bigger movements, nouns with smaller movements).

There was, however, variation in whether a base hand and movement repetition were used to mark the noun–verb contrast. This variation offers insight into the pressures that influence the development of a linguistic system, and into cross-linguistic variation in the signed modality (ASL vs. NSL). The first cohort of NSL uses movement repetition and base hand just like homesigners do, but different from the second cohort who entered the NSL community later and learned a *pre-existing* system. This finding suggests that intergenerational transmission to new learners (not just sharing a language with other signers) plays a key role in the development of these particular devices. These results demonstrate not only the importance of the noun–verb distinction in human communication, but also how this distinction emerges and develops in a new (sign) language.

The evidence thus far suggests that distinctions between nouns and verbs are present in the earliest stages of novel linguistic systems. However, these distinctions may not initially be fully conventionalised or codified, but may instead become conventionalised through use with communication partners in the linguistic community. We present two experimental studies that aim to explore how the improvisation of novel signals by individuals, and the interaction between users of an emerging system, affect the noun–verb distinction. Using a silent gesture task in which hearing, non-signing participants are asked to communicate using only their hands, we assess whether participants spontaneously improvise distinctions between forms playing noun-like and verb-like roles, and whether those distinctions reflect those found in naturally emerging sign languages. We also introduce shared communication into our paradigm to explore whether communicative interaction affects the development of the distinctions. In this way, we investigate the extent to which distinctions between nouns and verbs in naturally emerging languages represent natural conceptual categories, and the extent to which they do and do not depend on shared communication.

#### *1.3. Experimentally Modelling the Noun–Verb Distinction*

Previous experimental research has demonstrated how methods such as silent gesture, artificial language learning, and experimental semiotics can be used to investigate

the pressures that shape language—specifically, pressures from the cognitive biases of individuals, and pressures from the social forces within a linguistic community (Beckner et al. 2017; Fay et al. 2010; Kirby et al. 2015; Motamedi et al. 2019; Nölle et al. 2018; Raviv et al. 2019; Silvey et al. 2019; Verhoef et al. 2014). These experiments elicit novel forms from participants across different media—gestures, drawings, non-linguistic vocalisations—to understand how participants create signals, how participants produce and interpret signals in the presence of a partner, and how signals evolve as they are used in interaction. For example, experiments investigating the creation of novel signals have shown that, in the absence of existing conventions, participants may rely on highly iconic forms to ground shared reference, but that these forms can become increasingly symbolic as they are used and conventionalised through communication (Fay et al. 2010, 2013; Garrod et al. 2007; Perlman et al. 2015; Sulik 2018; Theisen et al. 2010).

The experiments we present use the silent gesture paradigm to explore the evolution of a communication system in the manual modality. Hearing participants with no knowledge of sign language are asked to communicate using only gesture (without speech), a paradigm that has been shown to have limited influence from participants' existing linguistic knowledge (Gershkoff-Stowe and Goldin-Meadow 2002; Goldin-Meadow et al. 2008; Özçalı¸skan et al. 2016; Singleton et al. 1995). Silent gesture is a paradigm that has been widely used to understand the preferences participants have when creating novel signals. For example, a number of silent gesture studies have investigated word order in speakers of languages that exhibit different word order patterns, asking hearing participants who know no sign language to describe a series of events. Goldin-Meadow et al. (2008) found that participants with different linguistic backgrounds all produced verb-final word orders that mapped onto a Subject-Object-Verb (SOV) order when describing events in which an animate agen<sup>t</sup> acts on an inanimate patient (e.g., MAN-GUITAR-PLAYS). More recent studies sugges<sup>t</sup> that the preference for SOV may be mediated by a variety of factors, such as the semantics of the events (Schouwstra and de Swart 2014), the reversibility of the events (Gibson et al. 2013; Hall et al. 2013), or the possibility of iconic representation (Christensen et al. 2016; Meir et al. 2014).

Silent gesture is also a valuable tool to model the emergence of linguistic properties in the manual modality because it allows comparison with data from naturally emerging sign languages. By embedding silent gesture into an interactive framework in which participants use the gestures they create to communicate with a partner, we can model the processes enacted in the early emergence of sign languages—when signers bring their own homesign systems to a community of deaf individuals, each of whom also has their own homesign system. Previous experimental research that has embedded silent gesture into an interactive framework has shown that novel manual systems adapt to the pressures involved in interaction, and result in conventionalised and communicatively efficient signals (Fay et al. 2013; Motamedi et al. 2019; Nölle et al. 2018; Schouwstra et al. 2016).

Here, we model the processes of improvisation (the creation of novel signals) and interaction (use of signals with others who are also using signals) to understand how conventionalised noun–verb distinctions emerge in a manual communication system. We compare the systems resulting from these processes to two stages in the emergence of Nicaraguan Sign Language: (1) homesign, where children without a language model improvise their own communicative systems, and (2) interaction in the first cohort, where the formation of a linguistic community leads to the conventionalisation of signals from the improvised communicative systems. We asked participants to improvise gestures for a set of event scenes devised by Abner et al. (2019), and then use those gestures in interaction with a partner. We analysed the gestures participants produced using the coding system developed by Abner et al. (2019). We predict that the strategies used to distinguish nouns and verbs that have been found in the earliest stages of language creation (i.e., in homesign: the preference for verb-final ordering) will be present in the gestures that our participants create. However, the strategies that are found only in later cohorts of NSL and in ASL may be absent from the gestures that our participants create. We further predict that the

distinctions participants improvise will rely on the iconic affordances of the modality, as suggested by Wilbur (2008) and Kimmelman (2009), for example, with gesture size and repetitions iconically representing properties of individual items.

## **2. Experiment 1**

*2.1. Methods*
