Perlmutter (1978) put forth the Unaccusative Hypothesis (UH), claiming there are two different types of intransitive predicates: unaccusatives, with a theme theta role, and unergatives, with an agent theta role (Perlmutter 1978). According to the UH, arguments of unaccusative verbs start as objects (R2) and advance into subjects (R1) in the derivation, whereas arguments of unergative verbs start as subjects (R1). Importantly, the UH makes two claims: (i) there are two different types of argument structure involved in intransitive predicates, one has an agent as the sole argument of the verb and the other one has a theme as the sole argument of the verb (unaccusatives) and (ii) the syntactic derivation of unaccusatives involves one more step than that of unergatives, namely the promotion of the theme argument from object to subject. Burzio (1986) rephrased this second claim in the UH in the framework of Government and Binding (Chomsky 1981) in terms of syntactic movement: the theme argument of an unaccusative verb is generated as its complement and then moves to subject position, whereas the agent argument of an unergative verb is generated as an external argument.

Bever & Sanz (1997) were the first to experimentally test the UH; they conducted a reaction time study in Spanish to explore whether the trace left by the argument of unaccusative verbs in the complement of V position, argued to be an anaphor (Chomsky 1981), would prime semantically related nouns as overt anaphors do. Their results showed that participants who scanned the linguistic sequence to find a probe word recognized it faster in preverbal position in unaccusatives than in unergative constructions, which the authors interpreted as evidence for the UH. Since this pioneering study, many other studies have tested this hypothesis experimentally; in the last decades the UH has been tested in sentence production studies on agrammatic aphasia (Thompson 2003; M. Lee & Thompson 2004; McAllister et al. 2009; J. Lee & Thompson 2011) and in healthy adults, both in sentence production (Kim 2006; J. Lee & Thompson 2011; Momma, Slevc & Phillips 2018) and in sentence comprehension. Friedmann, Taranto, Shapiro & Swinney (2008) conducted a cross-modal lexical priming experiment in English and found that subjects of unaccusatives reactivate after the verb, whereas subjects of unergatives do not. Zeyrek & Acarturk (2014) ran an eye-tracking study and the gaze regression analysis grouped intransitive verbs in two clusters with regard to the number of regressions, revealing an unergative/unaccusative split in intransitive verbs in Turkish; Koring, Mak & Reuland (2012), ran an eye-tracking study using the visual world paradigm and found that subjects were reactivated in different time intervals in Dutch unaccusative and unergative verbs; finally, Meltzer-Asscher et al. (2015) and Thompson et al. (2007) in English, and Shetreet, Friedmann & Hadar (2009) in Hebrew, using fMRI, revealed distinct activations for unaccusatives and unergatives.

These studies found differences in the representation and processing of unergative vs. unaccusative verbs, thus converging with a large body of literature in linguistics regarding this issue (Levin & Hovav 1995). However, as far as we know, there are no studies to date exploring the electrophysiological correlates of unaccusative vs. unergative sentence processing. Here we present and discuss a pioneering study testing the UH by means of Event Related Potentials (ERPs). ERPs record time-locked electroencephalographic (EEG) activity in response to sensory, cognitive or motor stimuli, which reflect the activity of postsynaptic potentials generated by a large number of similarly oriented cortical pyramidal neurons firing in synchrony (Luck 2014). Although quite poor with regard to spatial resolution, this electrophysiological non-invasive method provides an excellent temporal resolution, as it records neuronal activity millisecond by millisecond. Thus, ERPs allow us to continuously measure brain activity and to analyze how different stimuli are represented and processed. This study seeks to uncover the electrophysiological correlates of the representation and processing of unaccusative and unergative predicates. Our study is carried in Basque, an ergative language, and it deploys the subject verb agreement violation paradigm, frequently used in ERP studies.

Subject agreement has been widely studied cross-linguistically in the ERP literature, also in Basque. As a result, subject agreement violations afford a very suitable ground to test the UH by means of this technique. In what follows, we provide a brief review of the literature related to ERP studies on subject agreement processing as a background for our study.

The present study aimed to determine what the behavioural and electrophysiological correlates are of the processing of unergative versus unaccusative predicates in Basque. We do this by exploring person versus number subject agreement. A 2 × 2 × 2 design was used for the experiment. We manipulated the TYPE of the predicate (2 levels: unaccusative vs. unergative), the FEATURE (2 levels: person and number) and the GRAMMATICALITY factors (2 levels: grammatical and ungrammatical).

The present study had a twofold goal: first, to determine what the processing cost and electrophysiological correlates are for unergative versus unaccusative predicates in Basque. Second, it investigated the processing of person and number phi-features in the context of these two types of predicates. Two hypotheses were tested, the Unaccusative Hypothesis (Perlmutter 1978) and the Feature Separability Hypothesis that we put forth. The UH makes two claims: first it claims that there are two types of intransitive predicates (unaccusatives and unergatives) with different argument structure and syntactic representation, and second it claims they also have different derivations, where the argument of an unaccusative verb undergoes at least one more syntactic computation than the argument of an unergative verb to become subject in the case of Perlmutter (1978) or to receive nominative case in the case of Burzio (1986). The FDH hypothesis claims that person and number are processed and represented differently yielding distinct processing signatures.

Participants were highly accurate, as expected given they were native speakers. They were also faster, as expected, when responding to ungrammatical sentences, and performed with higher accuracy in person feature than in number feature conditions overall, replicating previous findings (Mancini et al. 2011; Zawiszewski et al. 2011; 2016).

Our results reveal significant differences between unaccusatives and unergatives in Basque, both behaviourally and electrophysiologically, thus yielding a new type of evidence in support of the existence of two different types of intransitives in language: unaccusatives that carry theme arguments and unergatives that carry agent arguments. Importantly, Basque participants were faster processing unaccusatives than unergative sentences when performing an acceptability judgement task. This finding suggests that, at least in active-ergative languages like Basque, unaccusatives do not involve movement, as suggested by Levin (1983) and argued by Laka (2006a, b; 2017). Other experimental studies, like Koring et al. (2012) with Dutch, a nominative language, reported the opposite pattern where the unaccusatives took longer than unergatives to process, as a result the authors interpret as evidence that arguments of unaccusatives undergo a further step in the syntactic derivation (promotion to R1 in the case of Perlmutter 1978, working in Relational Grammar, and movement in the case of Burzio 1986, working in Generative Grammar).

Regarding the electrophysiological pattern, we found that unaccusatives and unergatives differ in their processing of phi-features: unaccusatives elicited a larger positivity for number violations than did unergatives and unergatives elicited a larger positivity for person violations than for number violations. Therefore, unaccusative and unergative predicates reveal different electrophysiological responses, specifically related to phi-features. The pattern of response obtained in unergative sentences, that is, larger positivity for person than for number violations, is the electrophysiological signature found by previous studies analyzing phi features violations in transitive sentences (Nevins et al. 2007; Zawiszewski et al. 2016). In this sense, the ERP signature elicited by unergatives is similar to what has been reported for transitives, a finding that is convergent with the predictions made by the UH regarding the subjects of unergative and transitive sentences, which are “born subjects” and do not involve the promotion of extra movement step required of unaccusatives by the UH.

It is important to bear in mind that the UH was originally formulated based on data from nominative-accusative languages, where the case morphology of all intransitive subjects is the same, namely nominative. Hence, the promotion or movement of the “object born” unaccusative argument is a plausible mechanism to account for its case. In the case of Basque, the case morphology of unergative subjects is ergative, as in transitive subjects, whereas the case morphology of unaccusative subjects is absolutive, as in transitive objects. Levin (1983) was the first to discover this correlation between case morphology and argument structure, showing that the class of unaccusative predicates in this language correlates with absolutive-marked subjects, whereas the class of unergatives has ergative-marked subjects (see also Laka 1993 for a discussion). Since unaccusative subjects do not have the same case as other subjects, Levin speculated that there might be no syntactic movement for unaccusative subjects in Basque, but refrained from claiming it because it violated the principle in Government and Binding that case could not be determined by D-Structure representations (Chomsky 1981). More recently, Laka (2006a, b; 2017) has shown that this correlation is strict, and has argued that case is inherent in Basque, not structural. This would entail no extra derivational step for the sole argument of unaccusative predicates, and thus no extra cost when processing unaccusative sentences as compared to unergative sentences.

A second finding regarding processing differences between unergative and unaccusative predicates concerns phi-features. Recall that unaccusatives showed a significantly larger P600 in number violations, while unergatives showed the opposite pattern, with person violations generating a larger P600 than number violations. This is a novel finding that would have not been detected had the experimental design not considered each feature separately. As it is a novel finding, we will offer a tentative interpretation, and await further replication of similar results for a better grounded understanding. Note that unergatives show the ERP signature associated to violations in transitive sentences, which in turn reveals unergative subjects to be more similar to transitive subjects than to unaccusative subjects, as predicted by the UH. Until a corpus study is carried out in future work, we cannot discard the possibility that this differential sensitivity to the number feature displayed by unaccusatives be a frequency effect, if unaccusatives have third person subjects more frequently than unergatives. In connection to this possibility, speech act participants (speaker and addressee, the two types of arguments that involve bona fide person agreement) are more frequently subjects/agents than objects/themes in transitive clauses, a difference that might well carry over to unergatives versus unaccusatives.3

We also tested the Feature Separability Hypothesis that claims that person and number are processed and represented differently, yielding distinct processing signatures. Based on previous research (Chow et al. 2018; Zawiszewski et al. 2016) we expected a similar N400 component to be elicited by both person and number violations in both unaccusative and unergative predicates. We also expected to find a larger P600 for violations involving person than for number violations in the late time window (Zawiszewski et al. 2016). Regarding the first expectations, both person and number violations elicited a N400 in both types of predicates, as expected. However, the N400 was larger for person violations than for number violations. This difference can be due to the materials used in each study. Zawiszewski et al. (2016) employed second-person as a baseline for person and number manipulations, while here we used second-person for person manipulations and third-person for number. Hence given the high sensitivity of the methodology used in the current experiment (ERPs), we think that this difference might have led to a different electrophysiological output as the contrast between the person and the number features was already detected at early stages of processing (300–400 ms) indicating that person feature is more salient than number feature.

As concerns the late time-window (400–700 ms), our second expectations were met in that a P600 component was elicited by all violations, as commonly found in the literature on language processing (for a review see Molinaro et al. 2011). Interestingly, the positivity was larger for the ungrammatical person than for the ungrammatical number features in unergative predicates only, replicating the pattern in transitive predicates reported in Zawiszewski et al. (2016).

Altogether, both behavioural and electrophysiological results show that person and number features are processed differently, as indicated by the modulations of both N400 and P600 components.

In conclusion, this study shows differences in processing cost regarding unaccusative and unergative predicates and phi-features lending further support both to the UH and to the FDH. Our study provides behavioural and electrophysiological evidence in support of the Unaccusative Hypothesis and the Feature Separability Hypothesis. Behaviorally, we found that participants were faster processing sentences with unaccusative verbs than sentences with unergative verb. This result provides evidence for the first claim in the UH, namely, that unergatives and unaccusatives involve different thematic/structural representations. Interestingly though, it does not provide evidence for the second claim in the UH that unaccusatives involve longer derivations than unergatives, which predicts higher processing costs for unaccusatives than for unergatives. We found the opposite pattern, with unaccusatives being less costly than unergatives. This finding provides support for linguistic analyses that argue that case in Basque is inherent, a direct reflection of theta-role (Laka 2006a, b; Levin 1983) and does not support the hypothesis that case is structural in this language (Ortiz De Urbina 1989). Electrophysiological evidence also shows distinct patterns depending on predicate type, of a novel kind: number violations in the unaccusative condition elicited a larger P600 than number violations in the unergative condition and the positivity was larger for person than for number features in the unergative condition. This finding can be interpreted as a function of prototypicality: unergatives involve agent arguments, highest in animacy and very frequently human where the person feature is most salient, while unaccusatives involve theme arguments, lowest in the animacy scale, with less saliency for the person feature.

The additional file for this article can be found as follows: