Testing a rational account of pragmatic reasoning: The case of spatial language

Testing a rational account of pragmatic reasoning: The case of spatial language Alexandra Carstensen (abc@berkeley.edu) Department of Psychology, University of California, Berkeley, CA 94720 USA Elizabeth Kon (ellie.kon@berkeley.edu) Cognitive Science Program, University of California, Berkeley, CA 94720 USA Terry Regier (terry.regier@berkeley.edu) Department of Linguistics, Cognitive Science Program, University of California, Berkeley, CA 94720 USA Abstract How do people recover precise meanings from ambiguous utterances? Frank and Goodman (2012) proposed that listeners do this by rationally combining evidence about word meaning and the salience of particular objects in context. They found that a Bayesian model based on this idea provided a near-perfect account of their empirical data. However, their test of the model was based on communication about simple geometrical objects that varied along only three dimensions. Here, we ask whether their proposal extends to the richer and more complex domain of spatial relations. We find that it does. While the results are not as strong as in their original study, they nonetheless demonstrate that simple formal accounts of communication may capture important aspects of pragmatic inference. Keywords: Spatial cognition, spatial language, semantics, pragmatics, communication. Language, thought, and communication A growing trend in cognitive science views language through the lens of its function: as a vehicle for informative, efficient communication (e.g. Piantadosi et al., 2011; 2012; Fedzechkina et al., 2012). One such line of work has argued that systems of word meanings in the world’s languages tend to support highly informative communication (Regier et al., 2007; Baddeley & Attewell, 2009; Kemp & Regier, 2012; Khetarpal et al., 2013). However, word meanings necessarily leave much information unspecified—thus, the use of words must be supplemented by pragmatic reasoning to allow speaker and listener to communicate effectively. What principles govern this pragmatic reasoning? Frank and Goodman (2012) proposed an account of pragmatic reasoning in language use. They argued that in conversation, listeners determine the object to which a speaker is referring by rationally combining two sorts of evidence: one concerning how well the speaker’s utterance fits each potential referent, and the other concerning how salient each potential referent is in context. Their study presented evidence that listeners combine these two sources of evidence in accord with Bayes’ rule in interpreting the speaker’s intention. However, one limitation of their study is that it was based on communication about a very simple and cleanly circumscribed semantic domain, and it is not yet known whether similar results would be obtained in a more complex domain. We seek to answer that question here, by replicating their study in the context of communication about spatial relations. Languages differ substantially in the ways they partition the spatial domain into semantic categories, and these categories sometimes involve relatively subtle features such as attachment by spiking, or being astraddle, in addition to (from a Western viewpoint) more obvious features such as containment and support (Levinson et al., 2003). Thus, the domain of spatial relations is rich enough to allow a test of Frank and Goodman’s (2012) proposal in a semantically complex domain. We first review Frank and Goodman’s study, on which ours is based. We then present our study, which tests their proposal in the spatial domain. To preview our results, we find that their account does predict pragmatic reasoning in the spatial domain, but does not do so as cleanly as in their original study in a simpler domain. We conclude that pragmatic reasoning in more complex domains is substantially but not fully accounted for by their proposal as it stands, and consider possible interpretations of this finding. Frank & Goodman (2012) How does a listener interpret a speaker’s utterance in context? Imagine that a speaker wishes to refer to a specific referent r s , which is one of several possible referents in a physical context C, and that the speaker has produced a word w to convey this to a listener. Frank and Goodman (2012) proposed that in such situations, the listener determines the speaker’s intended referent through Bayes’ rule: 1 P ( r s | w , C ) = P ( w | r s , C ) P ( r s | C ) Σ P ( w | r ʹ′ , C ) P ( r ʹ′ | C ) r ʹ′ ∈ C Here, the posterior probability P(r s |w,C) represents the listener’s subjective degree of belief that the speaker’s intended referent is r s , given word w and context C. This quantity is proportional to the product of two terms: (1) the likelihood P(w|r s ,C) of the speaker using word w given that Our notation differs slightly from that of Frank and Goodman (2012), but there is no difference in intended meaning.