Wednesday, 6 January 2010

Inferential Models of Bilingualism

de Boer (2000)'s models showed that shared representations in a communicative system could be achieved given certain principles. Steels & Belpaeme (2005) extended this framework to the domain of colour, which is a bit more intuitive to understand: A speaker and a hearer are placed in an environment full of coloured tiles. A speaker chooses a colour and names it (with the name only the speaker knows), e.g. 'blurby'. The hearer hears the colour name, and tries to match it with a colour tile it can see. It then points to the colour tile it thinks the speaker meant, and the speaker points to the colour tile it actually meant. The hearer then adjusts its internal representations to fit what it has learned. This involves creating a new 'node' placed in a colour map at the location of the colour tile, labelled 'blurby'. From now on, the hearer can recognise any colour close to this as a 'blurby' colour. Over many exchanges like this, individuals come to share the same names for the same colour tiles.

However, no studies have looked at the additional problems produced by bilingualism. What happens when there are two languages being used? Not only can there be two labels for each type of colour tile, but the boundaries between lables can be different (for example, Welsh is one of the many languges that only has one word for the range of colours extending from green to blue).

In fact, no models even allow the possibility of having two labels for the same percept (labels are selected by calculating distances of label foci in a perceptual space). Additionally, most models implicitly assume a mutual exclusivity bias - if agents experience a new label, they assume it refers to a division of the perceptual space that they don't posses and adjust their perceptual space accordingly. However, agents could not necessarily assume this if they were learning two languages.

The solution is not as simple as creating two separate colour maps for each language, since all children have the potential to become bilingual there is no clear default. Also, being bilingual may affect the way you decide which object is being referred to. Studies into the Mutual Exclusivity bias have found that bilinguals are more likely to accept two labels for a single object, especially when they can tell the labels come from different languages (Au & Glusman, 1990; Healey & Scarabela, 2009; Byers-Heinlein & Werker, 2009).

Andrew Smith has investigated the problem of indeterminacy with computational models (Smith, 2005,2005b). The model is based on Steels' (1996) model of perceptually grounded meaning creation. Agents learn labels from each other which refer to perceptions of a simulated environment. Because agents do not have access to each other's internal representations, they need some way of overcoming indeterminacy when faced with a label they do not understand. Smith (2005) suggests that this can be achieved by relying on individual-level representation constraints (perceptual biases) or inferential tactics. One of these inferential tactics is Mutual Exclusivity. Smith models mutual exclusivity by having the agents exclude from consideration all objects for which it already understands a word when perceiving a new word.

The challenge, in light of more recent findings concerning bilinguals, is to define a model which favours mutual exclusivity within languages, but violates it between languages. It's not obvious how this could be achieved, nor how to test it. From previous studies, it's clear that stable 'bilingualism' is unlikely to emerge in such models, so perhaps two systems need to be developed in segregation and then integrated at a test stage.

First, however, there is the problem of detecting which language a word belongs to, or realising that more than one language is being spoken at all. Smith's agents perceive the world through a number of perceptual channels. Although some sort of entropy model could distinguish between two sets of words, it's unclear why this would be useful except for deciding which language was being spoken. In other words, it would only aid the fitting of the model without really explaining how it works.

One way of aiding the learning of labels in bilingual contexts would be to increase the number of perceptual channels to encode the communication context. For instance, an analogue perception of the signal to model language membership cues from prosody (although this still has the same problem as the entropy solution above). Alternatively, some channels could encode a perception of the speaker themselves. Each agent would have perceivable features, analogous to facial features perhaps, which could be used to distinguish which agent was speaking.

The channels could also be extended to include the encoding of a pragmatic cue, enabling a modeling of the experiments in Au & Glusman (1990) and Healey & Skarabela (2009).

Perhaps, however, these models already achieve such phenomena. The models begin with an inherently bilingual society - each agent begins with a minimal, random vocabulary and conceptual divisions. One question is what factors lead to a single, shared system. Is it an inevitable outcome in a cultural system with limits on resources and exposure to stimuli? Or does a shared system emerge because of biases implicit in the model which disfavour bilingualism? There appears to be a gap in the literature anyway, possibly even a PhD-shaped one.

Au TK, & Glusman M (1990). The principle of mutual exclusivity in word learning: to honor or not to honor? Child development, 61 (5), 1474-90 PMID: 2245739
Byers-Heinlein K, & Werker JF (2009). Monolingual, bilingual, trilingual: infants' language experience influences the development of a word-learning heuristic. Developmental science, 12 (5), 815-23 PMID: 19702772
DEBOER, B. (2000). Self-organization in vowel systems Journal of Phonetics, 28 (4), 441-465 DOI: 10.1006/jpho.2000.0125
Grassmann S, Stracke M, & Tomasello M (2009). Two-year-olds exclude novel objects as potential referents of novel words based on pragmatics. Cognition, 112 (3), 488-93 PMID: 19616205
Healey, E. and Scarabela, B. (2009). Are children willing to accept two labels for one object? Proceedings of the Child Language Seminar. University of Reading.
Smith, A. (2005). The Inferential Transmission of Language Adaptive Behavior, 13 (4), 311-324 DOI: 10.1177/105971230501300402
Smith, A.D.M. (2005). Mutual exclusivity: communicative success despite conceptual divergence Language Origins: Perspectives on Evolution

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