“If a lion could speak …”: Online sensitivity to propositional truth-value of unrealistic counterfactual sentences

Abstract

People can establish whether a sentence is hypothetically true even if what it describes can never be literally true given the laws of the natural world. Two event-related potential (ERP) experiments examined electrophysiological responses to sentences about unrealistic counterfactual worlds that require people to construct novel conceptual combinations and infer their consequences as the sentence unfolds in time (e.g., “If dogs had gills…”). Experiment 1 established that without this premise, described consequences (e.g., “Dobermans would breathe under water …”) elicited larger N400 responses than real-world true sentences. Incorporation of the counterfactual premise in Experiment 2 generated similar N400 effects of propositional truth-value in counterfactual and real-world sentences, suggesting that the counterfactual context eliminated the interpretive problems posed by locally anomalous sentences. This result did not depend on cloze probability of the sentences. In contrast to earlier findings regarding online comprehension of logical operators and counterfactuals, these results show that ongoing processing can be directly impacted by propositional truth-value, even that of unrealistic counterfactuals.

Introduction

Wittgenstein (1958) famously argued that because language use is fundamentally grounded in common patterns of human behavior and cognition “If a lion could speak, we could not understand him”. His thought experiment illustrates the capacity to reason counterfactually about events that can never realistically take place: we can consider whether his abstruse conclusion is hypothetically true despite knowing that what it describes can never be literally true given the biological or physical laws of the world. But does propositional truth-value impact our comprehension of sentences, including ‘unrealistic’ counterfactuals, as they unfold in time? This paper reports two event-related potential (ERP) studies that investigated whether the language processing system is immediately sensitive to the truth-value of unrealistic counterfactual conditionals such as “If dogs had gills, Dobermans would breathe under water”.

Balancing our knowledge of what is true and possible in the world with what we can experience in thought and express in language may be particularly taxing when we process language with unrealistic content (as compared to, let’s say, fictional yet realistic content). Understanding an unrealistic counterfactual sentence requires the creation of an unusual conceptual combination (‘dogs’ and ‘gills’) with potential consequences that go beyond what is literally stated (e.g., the hobby of keeping dogs as pets would drastically change if they had gills instead of lungs). Whereas the sentence “Dobermans would breathe under water” by itself semantically anomalous, it may describe a hypothetically true consequence of the counterfactual premise. However, is our language comprehension system immediately sensitive to its truth-value, as compared to that of a more realistic sentence that draws upon our pre-existing real-world knowledge (e.g., “Because fish have gills, tuna breathe under water”)? And does a plausible and considered-to-be-true relationship between counterfactual consequence and premise mitigate processing costs associated with a sentence that is false with respect to knowledge of the real-world?

This latter question resonates with classic empirical questions regarding the interaction between local and global levels of language comprehension (e.g., word- or phrase-level versus discourse-level). Experimental results suggest that supportive discourse context can mitigate processing costs associated with phrases or sentences that describe unexpected events (e.g., “the electrician taught herself”; Cook and Myers, 2004, Duffy and Keir, 2004, Hess et al., 1995). According to the lexical reinterpretation model (e.g., Hess et al., 1995), discourse context modifies the representation of a concept to form the basis for further interpretation (see also Zwaan & Radvansky, 1998). Such results testify to an interactive view of language processing in which lexical, structural, and contextual information interact efficiently as a message unfolds (e.g., Marslen-Wilson, 1975), and are consistent with contextually-driven resolution (or preclusion) of syntactic ambiguity (e.g., Altmann & Steedman, 1988) and lexical ambiguity (e.g., Rayner, Cook, Juhasz, & Frazier, 2006), and with the absence of additional processing costs for enriched expressions in supportive context (e.g., Gerrig and Bortfeld, 1999, Gibbs, 1979, Traxler et al., 2005). However, it is clear neither how much discourse context is needed to modify an initial – or ‘default’ – interpretation, nor how explicit a context must be. The current study aims to establish whether a single supportive counterfactual clause can suffice.

Whereas several studies have reported that contextual relevance outweighs real-world constraints from an early moment on (e.g., a cartoon-like story about an amorous peanut can invert the relative ease of processing “the peanut was salted” as compared to “the peanut in love”, Nieuwland & Van Berkum, 2006; see also Filik, 2008, Filik and Leuthold, 2008), some results on the processing of literally false or unrealistic sentences suggest that context does not completely overrule briefly disruptive effects of local violations (e.g., Hald et al., 2007, Warren et al., 2008). For example, Warren et al. reported that despite a fantasy-context (e.g., Harry Potter practicing magic spells on food items), words that incurred semantic violations (e.g., ‘bread’ in “Harry used a book to teach the tough bread”) elicited longer fixations and gaze durations than in unproblematic sentences (e.g., “Harry used a microwave to heat the tough bread”). These brief disruptions correspond with predictions from the Bonding and Resolution framework (e.g., Garrod & Terras, 2000) and ‘memory-based’ language processing theories (e.g., Gerrig and O’Brien, 2005, Kintsch, 1988, Myers and O’Brien, 1998) that initial processing is dominated by local semantic relationships with effects of contextual relevance or propositional truth-value emerging later.

There is little reason to believe that the reported discrepancies stem from different techniques (for example, if eye-tracking during reading were sensitive to early processes that ERPs, in particular the N400 amplitude dependent measure, are not). Ferguson, Sanford, and Leuthold (2008) reported that early processing disruptions as indexed by increased first-pass reading times but not in later measures coincided with an increased N400. This is consistent with observations that first fixation durations are correlated with N400 amplitude and that these measures are sensitive to the same lexical and sentence variables (e.g., Dambacher & Kliegl, 2007; see Dimigen, Sommer, Hohlfeld, Jacobs, & Kliegl, 2011, for in-depth discussion). Hence, ERPs and eye-tracking can, in principle, be similarly sensitive to the type of processing disruptions during language comprehension under study here, despite the fact that serial visual presentation as used in ERP research on sentence reading is less natural than sentence reading eye-tracking.¹

The reported discrepancies may, however, reflect differences between studies in how smoothly interpreting incoming information is related to the context, perhaps depending on how elaborate and explicit the discourse context is (see Warren et al., 2008, for discussion). Warren et al. created a variety of fantasy contexts that each made a novel unrealistic event more plausible, but realistic continuations were nevertheless on average more expected than unrealistic continuations (for related discussion, see Matsuki et al., 2011). In contrast, Nieuwland and Van Berkum (2006) created relatively uniform cartoon-like contexts that always involved an inanimate object behaving like a human being, and that each consisted of 4–5 sentences containing repeated animacy violations. This experimental manipulation strongly constrained the interpretation of the critical phrases, but it begs the questions whether the reported results were contingent upon repeated exposure of similar anomalies on the same lexical item, as could be argued from the lexical reinterpretation account (e.g., Hess et al., 1995), or whether they reflect expectations based on the discourse genre and therefore carry over to novel entities and events (e.g., Zwaan, 1994; see also Gerrig and Murphy, 1992, Van Berkum, 2010). The current studies address this by examining comprehension of single-sentence counterfactual conditionals about events that do not correspond with the natural world and that require readers to incrementally compute the unrealistic consequences of a counterfactual premise.

To understand counterfactual language requires that people balance their factual knowledge about the world with their readiness to engage in suspension of disbelief (e.g., Searle, 1975). Counterfactual thought enables humans to decouple from reality (e.g., Cosmides & Tooby, 2000), yet may be organized along the same principles as rational thought, and some aspects of reality are more readily ‘undone’ than others (see Byrne, 2007, for review). People do not usually create “miracle-world” counterfactuals but generate plausible alternatives to real-world situations (e.g., McMullen & Markman, 2002). Nevertheless, the ability to generate and understand unrealistic counterfactuals testifies to the creative nature of our cognitive endowment (e.g., Fauconnier & Turner, 2002).

Recent studies have looked at counterfactual language comprehension using online measures (de Vega et al., 2007, Ferguson, 2012, Ferguson and Sanford, 2008, Ferguson et al., 2010, Ferguson et al., 2008, Nieuwland and Martin, 2012, Stewart et al., 2009, Urrutia et al., 2012). Ferguson and Sanford (2008) showed that despite a counterfactual context (e.g., ‘‘If cats were vegetarians’’) sentences describing implausible real-world events (e.g., ‘‘Families would feed their cat a bowl of carrots’’) incurred brief disruptions during reading (i.e., longer early fixations in eye-tracking; see also Ferguson et al., 2008, for N400 evidence), suggesting that counterfactual context came into play after real-world constraints had their effect (see Ferguson, Sanford, & Leuthold, 2007, on the role of negation). However, counterfactually consistent continuations had been rated as less plausible than real-world continuations (3.6 and 4.6, respectively, out of 5-‘highly plausible’). Recent eye-tracking results on comprehension of realistic counterfactuals (e.g., “If it had rained this morning Susan would have rushed to get to work. In the end, Susan arrived at work early (inconsistent)/late (consistent)…”; Ferguson, 2012) suggest that readers do make factual inferences from counterfactuals (i.e., that it had not rained that morning and Susan had not rushed to work) yet maintain access to both counterfactual and factual interpretations (as evidenced by anomaly detection responses for counterfactual conditions compared to a factually consistent condition; see also de Vega et al., 2007, Santamaria et al., 2005).

Results from a recent ERP study by Nieuwland and Martin (2012) suggest that real-world knowledge does not modulate or delay the effect of propositional truth-value on processing counterfactuals about commonly-known historical events (e.g., “If NASA had not developed its Apollo Project, the first country to land on the moon would have been Russia/America surely”), as evidenced by reversed ERP patterns to the same lexical items as a function of the context. Perhaps that these alternative endings are easily computed because relevant information is part of our existing real-world knowledge (e.g., of the ‘Space Race’ between the USA and the USSR). Theories of counterfactual comprehension indeed assume that similarity between counterfactual worlds and the real world facilitates counterfactual reasoning (e.g., Byrne, 2007, Lewis, 1973, McCall, 1984). Thus, one unresolved issue is whether similar effects of propositional truth-value are observed when unrealistic consequences need to be computed on-the-fly. For the example sentence “If dogs had gills, Dobermans would breathe under water” the incremental interpretation involves the deduction that what holds for dogs holds for Dobermans (see Johnson-Laird, 1999), and an inductive inference regarding the potential consequences of a conceptual combination of or blend between ‘dog’ and ‘gills’ (e.g., Coulson and Fauconnier, 1999, Lakoff, 1987, Springer and Murphy, 1992, Turner and Fauconnier, 1998; see also Rips, 1975). Reasoning from this unrealistic conceptual combination within one sentence provides a strong test of the incremental contribution of counterfactual context.

The current hypotheses focused on the N400 (Kutas and Hillyard, 1980, Kutas and Hillyard, 1984), an ERP waveform whose amplitude peaks at about 400 ms post-stimulus, with smaller amplitudes indexing facilitated retrieval from semantic memory as elicited by content words or other meaningful stimuli (for reviews see Kutas and Federmeier, 2011, Kutas et al., 2006). N400 effect onset, when ERP waveforms corresponding to different conditions start to diverge, is about 200–300 ms after visual word onset (e.g., Kutas and Hillyard, 1980, Kutas and Hillyard, 1984, Van Berkum et al., 1999). With spoken words, N400 effects can start as early as 100–200 ms after word onset (e.g., Van Berkum, Zwitserlood, Hagoort, & Brown, 2003), after having heard only two or three phonemes and well before a word’s uniqueness point (e.g., Van Petten, Coulson, Rubin, Plante, & Parks, 1999). These well-established observations suggest that N400 effects that are elicited by words that are relatively unexpected (e.g., Kutas and Hillyard, 1984, Van Berkum et al., 2005) or that render sentences anomalous or false (e.g., Hagoort et al., 2004, Nieuwland and Kuperberg, 2008), reflect routine sense-making processes by which incoming words are related to the preceding context (e.g., Hagoort and van Berkum, 2007, Kutas et al., 2006, for review). This involves retrieval from semantic memory as facilitated by linguistic and non-linguistic context and potentially intensified by attentional factors (see Federmeier and Laszlo, 2009, Kutas and Federmeier, 2000, Kutas and Federmeier, 2011, Van Berkum, 2009; see also Baggio and Hagoort, 2011, Lau et al., 2008, Van Petten and Luka, 2006, for accounts of the neurobiology underlying N400 phenomena). In two separate experiments, the current study examined whether these processes are also sensitive to counterfactual context and propositional truth-value. In Experiment 1, participants read sentences that were anomalous with respect to real-world knowledge (e.g., “Dobermans would breathe under water”). In Experiment 2, new participants read these same sentences preceded by a counterfactual conditional premise that rendered the ‘locally anomalous’ sentence true (e.g., “If dogs had gills, Dobermans would breathe under water”).

The aim of Experiment 1 was to establish that without a counterfactual context, unrealistic consequences incurred semantic processing costs as indexed by the N400, similar to a real-world-false sentence compared to a true sentence (e.g., “Tuna breathe under poison/water”). This is not obvious due to potential processing differences related to verb tense between counterfactual real-world control sentences (conditional sentences had conditional verb tense, whereas real-world sentences did not). Conditional verb tense might mitigate the impact of propositional truth-value during processing, although earlier results suggest that this need not be the case (see Nieuwland & Martin, 2012).

Participants in Experiment 1 were native speakers of Spanish who read counterfactual-true/false control sentences and real-world-true/false control sentences (see Table 1), which were derived from the sentences from Experiment 2 (see Table 2). Critical words belonged to word pairs (e.g., ‘water’ and ‘poison’) that were matched on relevant lexical variables (see Methods section). For the full counterfactual conditional sentences (Experiment 2), one word rendered both a counterfactual and a real-world sentence true and that the other word rendered them both false (e.g., Spanish equivalents of “If dogs had gills, Dobermans would breathe under water/poison without problems” and “Because fish have gills, tuna breathe under water/poison without problems”), as established in an independent truth-value rating test. Moreover, an independent cloze completion test was conducted to ensure that the ‘true’ words were roughly equally predictable from counterfactual contexts and real-world contexts, whereas ‘false’ words received zero cloze values.

The control sentences in Experiment 1 were constructed by omitting the first clause of the sentence, and another truth-value rating test was performed to ensure that participants would consider counterfactual control sentences and real-world-false control sentences as false, but real-world-true control sentences as true. The corresponding prediction was that compared to real-world-true control sentences, larger N400 responses would be observed for counterfactual-true/false control sentences and real-world-false control sentences.

A different pattern of N400 responses was predicted for corresponding counterfactual sentences in Experiment 2 but not for real-world sentences. If counterfactual consequences are computed incrementally such that propositional truth-value impacts semantic processing without delay, a comparable N400 effect of truth-value should be observed in real-world sentences and counterfactual sentences. In the abovementioned example, the incremental interpretation involves the deduction that what holds for dogs holds for Dobermans (for review, see Johnson-Laird, 1999), and an inductive inference regarding the type of niche that could be suitable for a dog with gills (e.g., Rips, 1975; for accounts of this process in terms of conceptual blending see Coulson and Fauconnier, 1999, Lakoff, 1987, Turner and Fauconnier, 1998). Alternatively, the gradual build-up of the counterfactual context may not eliminate interpretive problems posed by semantically anomalous consequences if build-up of the counterfactual context is somehow slower and impact of truth-value therefore delayed (e.g., Fischler et al., 1983, Kounios and Holcomb, 1992, Urbach and Kutas, 2010), or because incoming statements are automatically mapped onto real-world knowledge despite the counterfactual context, in which case a smaller N400 effect of counterfactual truth-value is expected than of real-world truth-value. This would result from larger N400s to counterfactual-true sentences than real-world-true sentences, but a delayed impact of counterfactual context might also entail smaller N400s to counterfactual-false sentences compared to real-world-false sentences.