How tightly are production and comprehension interwoven?

MacDonald’s (2013) productiondistribution-comprehension (PDC) account provides a valuable unification of three aspects of language use. It identifies some of the principles involved in production and shows how they affect both distributions of linguistic forms and the process of comprehension. Our commentary presents a complementary perspective on the relationship between production and comprehension. On the basis of extensive evidence, the PDC account proposes that speakers tend to utter easy-to-produce words early, reuse recently used structures, and avoid producing utterances in which elements interfere with each other. These principles produce distributions of utterances, and comprehenders develop processing mechanisms that are sensitive to these distributions. Thus, they find it easier to interpret utterances that accord with these principles than utterances that do not accord with them, and use these principles to guide ambiguity resolution. In other words, comprehenders are fundamentally affected by their experience of language and hence by its statistics. The relationship between production and comprehension is mediated by distribution. There is no direct (“on-line”) effect of production on comprehension. This contrasts with our integrated account of production and comprehension, in which people use production processes to guide comprehension (and in fact use comprehension processes to guide production). Pickering and Garrod (in press) argued that production and comprehension are not isolated from each other, but that instances of comprehension can involve production processes, rather than “feedback” within the comprehension system (and that instances of production can involve comprehension processes). We now propose that this interweaving in fact provides the basis for the relationships between production and comprehension identified by MacDonald. Pickering and Garrod (in press) argued that addressees construct predictions of what they are about to hear before they hear it (see also Pickering and Garrod, 2007). Such predictions make use of forward models, as assumed in many accounts of action and action perception (see Wolpert, 1997). For example, if I decide to move my hand to a particular location, I combine my intention, my hand’s position in relation to the environment, and my experience of the outcome of previous similar intentions to construct a representation of (the percept of) my predicted hand movement. For example, I might predict that a particular intention will cause my hand to move 500mm from my body and 30◦ left of my midline, in 300ms time. Importantly, the forward model output (i.e., prediction) will be ready in (say) 100ms, before the movement. In fact, I need to construct this model in order to learn to control the movement fluently (Wolpert et al., 2001). I can then compare the prediction with the actual movement when it occurs. When I see you starting to move your hand, I construct a prediction of where your hand will end up, again before you move your hand, and then compare this with your actual movement. Pickering and Garrod (in press) proposed that perceivers can do this by determining what they would do if it were their hand (using “prediction-bysimulation”). In other words, I covertly imitate your movements to determine the intention behind your movement and use that intention to predict (my percept of) your movement. The mapping from intention to prediction involves the same forward model as when I move my own hand, which constitutes part of the action system. Pickering and Garrod (in press) noted that language comprehension is a form of action perception. They therefore proposed that comprehenders covertly imitate what they hear to determine the production command (roughly, the speaker’s intended message), and then use that command to predict (their percept of) the unfolding utterance. This mapping involves a forward model that forms part of the production system. Importantly, comprehenders can predict linguistic representations concerned with meaning, grammar, or sound. For example, the comprehender hears the start of the speaker’s utterance and uses covert imitation to predict aspects of the upcoming utterance, for example that the upcoming word will refer to something edible (Altmann and Kamide, 1999) or will begin with a vowel (DeLong et al., 2005). One consequence of this framework is that ease of comprehension is affected by ease of production. For example, people utter easy-to-produce words early, so they predict that they will do so. They therefore also predict that their interlocutor will utter easy-to-produce words early. Similarly, MacDonald (2013) pointed out that this easy-first principle discourages production of utterances such as John will say that his cousins left tomorrow (as tomorrow is short), and hence that such utterances are rare. We argue that comprehenders hear the sentence up to left, and then use their production system to predict that their interlocutor will produce a local modifier. This prediction involves the rapid construction of a forward production model, which therefore tends to be ready before the speaker utters tomorrow. At this point, the