Multiple mechanisms

If we consider predictive language processing to be any pre-activation of the representational content of upcoming words, then only multiple-mechanism accounts of prediction that are even more comprehensive than P&G's can account for the multifarious phenomena documented in the language processing literature to date. Schwanenflugel and Shoben (Reference Schwanenflugel and Shoben1985), for example, have argued that more featural restrictions of upcoming words are generated in advance of the input in high as opposed to low predictive contexts. These featural (e.g., semantic, syntactic) restrictions may then constrain what words are likely to come up. In other words, pre-activation of representational content is constrained by the featural restrictions generated in particular contexts (e.g., via spreading activation in a semantic network). Whereas such online generation of featural restrictions may be compatible with prediction-by-simulation (as P&G appear to point out), it is conceivable that such pre-activation can also happen independently of prediction-by simulation and thus constitutes a third anticipatory mechanism.

Predictive language processing may also make use of certain heuristics and biases (cf. Tversky & Kahneman Reference Tversky and Kahneman1973). Borrowing the availability heuristic from decision-making research, upcoming words may be pre-activated simply because of an availability bias (certain words/representations may be very frequent or have occurred very recently). The analogy to decision-making research is not as far-fetched as one may think: Tversky and Kahneman introduced a simulation heuristic in the 1970s according to which people predict the likelihood of an upcoming event by how easy it is to simulate it. Other heuristics may well be worth exploring (in line with the affect heuristic, emotionally charged words, e.g., stupid, boring, may be predicted more rapidly). Our point is simply that predictive language processing is likely to be complex and may make use of a set of rather diverse mechanisms (with many yet unexplored).

Importance of prediction-by-simulation in development

P&G suggest that analysis of children's prediction abilities might throw light on the distinction between prediction-by-association and prediction-by-simulation and place stronger emphasis on prediction-by-association in young children: Prediction-by-association might play a more important role when listeners and speakers have little in common with each other, such as the case of children listening to adults' talking.

In a recent experiment examining 2-year-olds' prediction abilities, however, we found that, consistent with prediction-by-simulation, only toddlers in possession of a large production vocabulary are able to predict upcoming linguistic input in another speaker's utterance (Mani & Huettig Reference Mani and Huettig2012; see Melzer et al. Reference Melzer, Prinz and Daum2012, for similar results in action perception). Furthermore, if, as P&G suggest, covert imitation is the driving force of prediction-by-simulation, then 18-month-olds are equipped with the cognitive pre-requisites for covert imitation: Covert imitation can modulate infants' eye gaze behaviour around a (linguistically relevant) visual scene (Mani & Plunkett Reference Mani and Plunkett2010; Mani et al. Reference Mani, Durrant and Floccia2012) similar to adults' behaviour (Huettig & McQueen Reference Huettig and McQueen2007). Prediction-by-simulation may also be an important developmental mechanism to train the production system (Chang et al. Reference Chang, Dell and Bock2006). In sum, prediction-by-simulation appears to be crucial even early in development, and hence prediction-by-association is not necessarily the simple prediction mechanism which dominates early childhood.

Mediating factors

Finally, there are many mediating factors (e.g., literacy, working memory capacity, cross-linguistic differences) involved in predictive language processing whose interaction with anticipatory mechanisms have been little explored and whose importance, we believe, has been vastly underestimated. Mishra et al. (Reference Mishra, Singh, Pandey and Huettig2012), for instance, observed that Indian high literates, but not low literates, showed language-mediated anticipatory eye movements to concurrent target objects in a visual scene. Why literacy modulates anticipatory eye gaze remains to be resolved, though literacy-related differences in associations (including low-level word-to-word contingency statistics, McDonald & Shillcock, Reference McDonald and Shillcock2003), online generation of featural restrictions, and general processing speed are likely to be involved. Similarly, Federmeier et al. (Reference Federmeier, McLennan, De Ochoa and Kutas2002) found that older adults are less likely to show prediction-related benefits during sentence processing with a strong suggestion that differences in working memory capacity underlie differences in predictive processing. The influence of such mediating factors may greatly depend on the situation language users find themselves in: Anticipatory eye gaze in the visual world, for instance, requires the building of online models allowing for visual objects to be linked to unfolding linguistic information, places, times, and each other. Working memory capacity may be particularly important for anticipatory processing during such language-vision interactions (Huettig & Janse Reference Huettig and Janse2012).

More work is also required with regard to the specific representations which are pre-activated in particular situations. Event-related potential studies have shown that even the grammatical gender (van Berkum et al. Reference Van Berkum, Brown, Zwitserlood, Kooijman and Hagoort2005), phonological form (DeLong et al. Reference DeLong, Urbach and Kutas2005), and visual form of the referents (Rommers et al. Reference Rommers, Meyer, Praamstra and Huettig2013) of upcoming words can be anticipated. Most of these studies, however, have used highly predictive “lead-in” sentences. It also remains to be seen to which extent these specific representations are activated in weakly and moderately predictive contexts. Last but not least, languages differ dramatically in all levels of linguistic organisation (Evans & Levinson Reference Evans and Levinson2009). These cross-linguistic differences are bound to have substantial impacts on the specifics (and degree) of anticipatory processing a particular language affords.

Future work could usefully explore the cognitive reality and relative importance of the potential mechanisms and mediating factors mentioned here. Even though Occam's razor may favour single-mechanism accounts, we conjecture that multiple-mechanism accounts are required to provide a complete picture of anticipatory language processing.