## 1 Introduction

A speaker of (1) can use both Mickey Mouse in the main clause and he in the embedded clause to refer to Mickey Mouse.

 (1) Mickey Mouse talked to Minnie Mouse before he ate.

To understand this use of the name Mickey Mouse, it might be enough for a listener to access its representation in his or her mental lexicon, as this might be linked to the singular concept MICKEY MOUSE. But understanding he requires more than lexical access, since the pronoun itself does not lexically encode concepts like MICKEY MOUSE. Instead, interpreting he also requires consultation of other representations in memory, such as a representation of the discourse in which the pronoun occurs, of the syntax of the local sentence, of the speaker’s interests and intent, and so on.

Much work in psycholinguistics has therefore been directed at the online processing of pronouns and other anaphoric expressions, since its study promises to illuminate the mechanisms by which disparate information sources are integrated in language comprehension (Ehrlich & Rayner 1983; Blanchard 1987; Arnold et al. 2000; Stewart, Pickering & Sanford 2000; Kehler & Rohde 2013). Given the potential complexity of this task and the difficulties of measuring it experimentally, our initial questions only concern timing: How long does it take to resolve reference? And in general, is resolution initiated immediately upon perception of the anaphoric expression? Or do we, often enough, resolve reference only when our practical goals demand it? There is active pursuit of these questions in the literature (see, e.g. Stewart Holler & Kidd 2007; Karimi & Ferreira 2016; for a discussion on other instances of shallow processing, see Ferreira, Bailey & Ferraro 2002).

Seminal work by Arnold et al. (2000) using the visual-world paradigm has shown that pronouns can be rapidly interpreted to infer the referent intended by the speaker, especially when gender and number features on the pronoun agree with a single entity in the discourse. In their experiments, participants viewed pictures on a screen and listened to descriptions in which a pronoun referred to one of the two characters in the picture. Results showed that when the two characters mismatched in gender, participants looked to the correct character within approximately 200 ms after the offset of the pronoun, suggesting that they were able to use the gender features of the pronoun to successfully resolve reference by that time.

However, such studies target only one specific type of anaphoric dependency, in which the cue to anaphora is an audible noun phrase, and its resolution is guided mainly by properties of the discourse—like what its topic is (Crawley, Stevenson & Kleinman 1990; Grosz, Weinstein & Joshi 1995; Kehler & Rohde 2013) or how it could continue most coherently (Hobbs 1979; Kehler et al. 2008; Kehler & Rohde 2013)—or perhaps also by the attentional state of the comprehender (Arnold & Lao 2015). But there are other anaphoric dependencies that share neither property. The referential expression may be inaudible (such as the null pro that is said to be present as the subject of sentences like the Spanish Te amo), or reference may be resolved mainly by the syntax (as with English himself). In this paper, we investigate a case of both, where no referential expression is audible and the anaphoric dependency is resolved by the syntax. We ask if it matters to the timing of anaphora resolution whether the surface cue to anaphora is an audible pronoun, like he in (1), or instead a non-finite participial verb in a control structure, like eating in (2).

 (2) Mickey Mouse talked to Minnie Mouse before eating.

A speaker can use (2) just like (1), to say that Mickey talked to Minnie before he, Mickey, ate. But now the process of understanding the speaker is different in two important ways. First, understanding that it was Mickey who ate depends on different sorts of information in the two cases. For (2) but not (1) the sentence itself determines the interpretation, due to its structure and meaning.1 Only (1) can be used to say something else—for example, that Mickey talked to Minnie before Donald ate—since he may be used to refer to any male salient in the discourse. Second, only (1) contains an overt and unambiguous sign of reference to a particular eater, namely he. For (2), in contrast, the signal of such reference is both implicit and temporarily ambiguous. It is implicit, because it consists not in the lexical semantic properties of any one word, but in the fact that eating is here the predicate of a non-finite clause; and such clauses, when lacking an audible subject, generally have their entailed subject role filled anaphorically. And it is temporarily ambiguous, because at the point in which it is encountered, it is in principle uncertain whether eating is a signal to anaphora, as (3) makes plain. Here eating is not the predicate of any clause, but rather the gerundival subject of one, and on this use it does not signal anaphoric reference to any particular eater.

 (3) Mickey Mouse talked to Minnie Mouse before eating was forbidden.

Determining whether these differences in understanding synonymous uses of (1) versus (2) impact the timing of reference resolution is an important step toward understanding which aspects of processing anaphora are fully general, and which are instead specific to the kind of anaphoric dependency involved—for example, whether the referential expression is audible or inaudible, and whether its resolution is guided mainly by discourse or syntax.

In contrast to pronouns, there has been relatively little work on the timing of the resolution of control structure anaphora. And the majority of what we do know comes from studies of complement control as in (4), rather than adjunct control (5).

 (4) a. Mickeyi managed [___i to eat]. b. Mickeyi promised Minnie [___i to eat]. c. Mickey persuaded Minniei [___i to eat].
 (5) Mickeyi talked to Minnie [before ___i eating].

Studies on complement control (e.g. Boland, Tanenhaus & Garnsey 1990; Demestre et al. 1999) have generally found that the anaphor can be processed quickly. But complement control differs from both adjunct control and pronominal anaphora in an important way: the matrix verb gives early cues that a control structure is coming as well as about what the referent of the null subject will be. After the verb persuade in (4c), for example, it is likely that an infinitival complement is to follow, and that its null subject will be controlled by the direct object. It is therefore plausible that the rapid processing seen in complement control structures is due at least in part to information conveyed by the main-clause verb. With adjunct control, the participial clause is not selected by any element of the matrix clause, and accordingly its processing may differ from complement control. Betancort, Carreiras & Acuña-Fariña (2006) found slowdowns in the processing of one type of adjunct control when compared to complement control in an eyetracking while reading study, suggesting that verb information may have led to prediction of control and/or the referent of the null subject. Because adjunct control is less predictable than complement control, it may make for a more minimal comparison with audible pronouns, since in general the occurrence of a pronoun is also not strongly predicted by a prior verb.

This paper compares the processing of audible pronouns and the implicit anaphora in adjunct control structures to examine the speed with which structural information can be used in reference resolution. In two experiments, we use visual-word eyetracking to measure the timecourse of reference resolution in sentences such as (6).

 (6) Mickeyi ran into Daisyj in front of the school… a. …[before ___i picking up a blue ball]. b. …[before hei picked up a blue ball].

There are several differences between anaphora involving overt pronouns and adjunct control that might cause the processing and interpretation of the null subject in adjunct control structures to proceed relatively slowly, but there are also reasons to expect it to happen quickly. In what follows, we outline the reasons for both expectations.

As was already mentioned, one might expect the processing of adjunct control to proceed slowly because control structures have no overt morpheme dedicated to reference in their subject position, in contrast to when there is an overt pronoun. Instead, the first indication that a referent is needed is given indirectly by the non-finite morphology (-ing) on the embedded verb. Therefore, if processing is taking place incrementally, then at the embedded verb, comprehenders must interpret the verb to identify the event it corresponds to, notice that the verb is missing a subject,2 and (if indeed this prompts immediate resolution of its reference) determine which character the speaker intended to refer to with that missing subject.3 Furthermore, as mentioned above, the non-finite verb does not unambiguously indicate that a control relation is necessary, and this may not become clear until several words after the verb. If (7a) is spoken on its own, then an anaphoric dependency for the null subject is necessary; it must be understood that Mickey talked to Minnie before he, Mickey, ate pizza at the park. But if (7a) is continued with (7b), then the non-finite clause becomes the gerundival subject of the adjunct, and no anaphoric dependency is necessary.4 Because the structure of the adjunct will not be clear to the incremental processor at the point of the verb, comprehenders may wait to interpret the null subject until the structure is disambiguated.

 (7) a. Mickey talked to Minnie before eating pizza at the park… b. … was forbidden.

It has been demonstrated, though, that listeners often do not wait for disambiguating information before building a preferred parse (e.g. Marslen-Wilson 1975; Kutas, DeLong & Smith 2011). Therefore, hearing the verb may be enough to cause people to immediately attempt to resolve the structural ambiguity in favor of a control structure, and to quickly identify the arguments of the verb. In other words, hearing a verb makes it likely that a subject is needed, and people may therefore look for a potential subject and form an anaphoric dependency at the earliest possible point, despite the fact that that dependency may not end up being necessary.

But even if at the non-finite verb listeners immediately assume a control dependency is present, using structural information to retrieve a referent from memory may still be more difficult than using the morphological and discourse information relevant for overt pronouns. In cue-based retrieval models of sentence processing (e.g. Lewis & Vasishth 2005), structural dependencies between elements of a sentence may be difficult to use as a cue to anaphora resolution (Kush, Lidz & Phillips 2015). If this is the case, then reference resolution in (6a) may be slower than in (6b), due to the difficulty in retrieving the antecedent based on structural information. On the other hand, Parker & Phillips (2017) argued that structural information not only can be used as a cue to antecedent retrieval, but that it is weighed more heavily in the retrieval process for reflexives such as himself, which similarly require a structurally accessible antecedent. If this is the case, then using structural information may not cause a slowdown in reference resolution.

Another reason anaphora resolution in adjunct control may be fast is that words like before are often followed by control structures.5 Listeners may therefore predict a control structure before even encountering the verb, making it easier to identify the control dependency and begin the retrieval process.

Our results show that when adjunct control structures are highly frequent within the experiment, listeners are just as quick to resolve reference when they hear the non-finite verb in a sentence such as (6a) as they are when they hear the overt pronoun in a sentence like (6b). This suggests that the large differences between the two forms of anaphora—that one is implicit and relies mainly on structural information and the other is explicit and relies on mostly morphological and discourse information—are in at least some contexts irrelevant to the timing of anaphora resolution. When adjunct control structures are less frequent within the experiment, however, anaphora resolution in adjunct control slows in comparison to overt pronouns. We argue that this is not due to greater difficulty in using structural information, but rather to difficulty in identifying the presence of a control dependency.

## 2 Experiment 1

In a design similar to that of Arnold et al. (2000), our first experiment uses visual-world eyetracking to examine the timecourse of interpretation of the null subject in adjunct control. Several experiments have shown that upon encountering a pronoun, comprehenders often look to the image of the character in a visual-world scene that they believe the speaker is referring to with the pronoun (e.g. Arnold 1998; Arnold et al. 2000; Arnold & Lao 2015). This is taken as evidence for the timing of resolution of the pronoun, especially when the task encourages participants to attend to what the sentence is talking about by having them verify the sentence they hear against the picture it describes. We similarly expect that upon encountering the non-finite verb in a sentence like (6a), comprehenders will look to the referent evoked by the null subject if it is being interpreted immediately. Looks to the correct referent will therefore be taken as evidence that reference resolution has been successfully completed. We measure the timecourse of looks to that referent, comparing it to the timecourse of the interpretation of an overt pronoun used to refer to the same character, as in (6b), as a baseline.

Both (6a) and (6b) involve coreference with the main clause subject. Pronouns are often biased to corefer with a prior subject rather than with an object (e.g. Arnold et al. 2000). In this way, English subjects seem to have a special role in discourse (Grosz, Weinstein & Joshi 1995). As a measure of whether a subject bias was in effect, we included a control condition with a pronoun coreferring with the main clause object. If, for example, participants are faster at resolving both subject-oriented pronouns and the null subject than object-oriented pronouns, this may be due simply to a bias to look at the character corresponding to the subject. If, on the other hand, participants are just as quick to resolve both types of pronouns, then it is unlikely that reference is being strongly influenced by such biases.

### 2.1 Materials and methods

#### 3.1.2 Materials

All stimuli from Experiment 1 were included. In addition to the 20 fillers from Experiment 1, 40 new filler items were added. These consisted of 10 items similar to the original fillers, but all with a pronoun coreferring with the main clause object as the subject of the temporal adjunct, in order to reduce the overall proportion of items with subject reference in the adjunct. The remaining 30 new fillers had a subject in the adjunct referring to something or someone other than the two main characters (e.g. Look there’s Donald! Minnie found him outside of Daisy’s house after Daisy kicked him out for being rude.); often this was the third prominent element in the image. Half of these 30 fillers were true (i.e. the image matched the description), and half were false, as were the other 10 new fillers. This brought the total number of items each participant saw to 90 (the 30 critical items from Experiment 1 and 60 fillers), 45 of which should have been judged true. These new fillers reduced the within-experiment proportion of items containing control structures from 32% to 17.8%. The recording of the auditory stimuli for these fillers followed the same procedure as in Experiment 1, including splicing the temporal adjunct into the stimulus after the preposition introducing it.

#### 3.1.3 Procedure

Stimuli were presented in nine blocks of 10 items each. The procedure followed that of Experiment 1 in all other respects.

### 3.2 Results

#### 3.2.1 Main results

Twenty-seven trials were excluded for high trackloss (greater than 25%), resulting in a loss of 3% of the data. The mean trackloss per trial in the remaining data was 3.5%.

Looks to the target or competitor beginning at the onset of the critical word in each condition are plotted in Figure 7. As with Experiment 1, looks to the target diverge from the competitor in each case by around 400 ms, suggesting that reference resolution was successful by that point. However, unlike with Experiment 1, focus appears to have had a strong early effect. At least in the pronsubj condition, when focus was on the character corresponding to the main clause object, there were more fixations on the target already at the onset of the critical word. Because this divergence appears so early, it could not be due to interpretation of the pronoun. This provides even greater justification for the use of an onset-based analysis, as it takes into account looking region at the onset of the critical region, and successful interpretation is determined based on whether participants are more likely to switch their gaze toward the target than away from it, rather than the overall proportion of fixations. Any biases in looking prior to the critical region are therefore controlled for. The proportion of trials where participants switched their fixation are given in Figure 8.

Figure 7

Experiment 2: Proportion of looks to the character corresponding to the target (orange) or competitor (blue) for the PRO (left), pronsubj (middle), and pronobj (right) conditions when either the subject character (top) or object character (bottom) was given focus.

Figure 8

Experiment 2: Proportion of trials where participants switched from their initial fixation region across conditions, either toward the target (dotted line) or away from it (solid line).

Mean switch times by condition based on whether that switch was toward or away from the target are given in Figure 9. The results of a linear mixed-effects model testing the effects of switch type, cue, and focus on first switch times (with random intercepts for participants and items) are given in Table 6. As with Experiment 1, the main effect of initial fixation region was due to participants being significantly faster to look toward the target than away from it. Pairwise comparisons on the interaction between switch type and cue are given in Table 7. These comparisons reveal that although participants were more likely to look toward the target than away from it for all three cue types, there was only weak evidence for such a difference in the PRO condition. In addition, participants were faster to look away from the target and slower to look toward it in the PRO condition compared to the pronsubj condition, as supported by both the frequentist and Bayesian comparisons.

Figure 9

Experiment 2: Average time of first switch of looking region during critical window. Error bars represent standard error of the mean.

Table 6

Experiment 2: Main results.

Factor F-value p-value
Switch type 50.93 <0.001
Cue 0.91 0.40
Focus 0.90 0.34
Switch type × Cue 4.47 0.01
Switch type × Focus 2.34 0.13
Switch type × Cue × Focus 0.42 0.79
Table 7

Experiment 2: Pairwise follow-up comparisons.

Comparison Estimate SE t-ratio p-value K-value
Switch toward target – away from target by Cue
PRO –119 44.4 –2.69 0.007 2.29
Pronsubj –364 43.6 –8.35 <0.001 >100
Pronobj –214 44.6 –4.79 <0.001 >100
PRO – Pronsubj by switch type
Toward target 81.8 33.4 2.45 0.04 6.69
Away from target –162.6 51.3 –3.17 0.004 6.08
PRO – Pronobj by switch type
Toward target 31.5 32.8 0.96 0.60 0.19
Away from target –63.0 52.7 –1.20 0.46 0.31
Pronsubject – Pronobj by switch type
Toward target –50.3 33.2 –1.52 0.28 0.50
Away from target 99.6 52.0 1.92 0.13 0.45

#### 3.2.2 Exploratory analyses

As with Experiment 1, two other linear mixed-effects models tested whether looks in the PRO and pronsubj conditions were affected by the cloze probability of reference to the subject character or of a control structure, or by trial order within the experiment. Including random effects in the pronsubj model led to non-convergence due to singular fit; therefore, for this analysis only, a general linear regression without random effects was used. Significant results are given in Table 8 for the PRO condition, and in Table 9 for the pronsubj condition (p > 0.05 for all other effects).

Table 8

Experiment 2, PRO analysis: Significant results.

Factor F-value p-value
Switch type 4.34 0.04
Switch × Focus × Clozesubj. ref. × ClozePRO 3.30 0.04
Table 9

Experiment 2, pronsubj analysis: Significant results.

Factor F-value p-value
Switch type 71.07 <0.001
Switch × Order × Clozesubj. ref. 3.10 0.047

In PRO items, there was a significant four-way interaction between switch type, focus, and the two cloze measures. This interaction was driven by the object focus condition, in which the effect of cloze probability of subject reference on switch times away from the target was significant when the cloze of PRO was high (p = 0.01), with higher cloze of subject reference leading to slower switches away from the target, but not with mid to low cloze of PRO (p > 0.05), as illustrated in Figure 10. Switches toward the target were not affected (p > 0.1). The interaction between switch type and the cloze measures did not persist when initial focus was on the subject.

Figure 10

Experiment 2, PRO, focusobj items: Residualized time of first switch of looking region during critical window, based on cloze probability of reference to the subject character, cloze probability of PRO, and switch type. Each point represents a single observation, with darker points indicating higher cloze of PRO. Regression lines represent the estimated effect of cloze of subject reference across three values of the cloze of PRO: the mean (dashed line), and one standard deviation above (solid line) or below (dotted line) the mean. Shading indicates the standard error of the estimated slope.

In pronsubj items, there was a significant three-way interaction between switch type, order, and cloze probability of subject reference. Follow-up analysis revealed that there was a significant effect of order on switches away from the target when cloze of subject reference was high (p = 0.03), but not when it was mid to low (p > 0.1), as illustrated in Figure 11; participants looked at the target for longer when cloze of subject reference was high as the experiment went on. There was no effect on switches toward the target.

Figure 11

Experiment 2, pronsubj items: Residualized time of first switch of looking region during critical window based on trial order, cloze probability of reference to the subject character, and switch type. Each point represents a single observation, with darker points indicating higher cloze of subject reference. Regression lines represent the estimated effect of order across three values of the cloze of subject reference: the mean (dashed line), and one standard deviation above (solid line) or below (dotted line) the mean. Shading indicates the standard error of the estimated slope.

### 3.3 Discussion

#### 3.3.1 Difficulty in resolving PRO

In Experiment 1, PRO appeared to be interpreted just as fast as overt pronouns. We hypothesized that this was in part due to the high proportion of control structures and/or of items that contain elements in the adjunct clause that make reference to the character named by the main-clause subject; participants may have adjusted quickly to these within-experiment frequencies and begun to predict PRO and resolve its reference earlier than would normally be possible. If that were the case, then we would expect reference resolution to be slower in Experiment 2, in which the within-experiment frequency of both control structures and subject reference was reduced. This prediction was confirmed. In Experiment 2, participants were both slower to look toward the target and faster to look away from it in the PRO condition than in the pronsubj condition. Switch times in the pronobj condition were between the other two, with no significant difference between the pronobj condition and either of the others. The fact that reference resolution in the pronobj condition may have been somewhat more difficult than in the pronsubj condition could be due a lingering subject preference for the pronouns. Importantly, despite this possible preference, the PRO condition, which also included reference to the subject character, was still more difficult than the pronsubj condition.

This result strongly suggests that in Experiment 1, the interpretation of PRO was indeed influenced by the high within-experiment frequency of a control structure. In Experiment 2, participants could no longer depend on the high frequency of control structures to predict the presence of PRO, and instead had to rely more on bottom-up input. The fact that reference resolution slowed in comparison to overt pronouns could be due to more difficulty either in using structural information as opposed to morphological/gender information to resolve reference, or simply in recognizing that a referential dependency was necessary, since the bottom-up cue to anaphora was longer in duration for the PRO versus pronoun conditions.

Additionally, although the difference in switch times toward or away from the target in the PRO condition still reached significance, the Bayes factor analysis indicates only weak evidence for that difference, as compared to both pronoun conditions, in which there is extreme evidence for faster looks toward the target than away from it. The exploratory analyses of the PRO items indicate that the small difference that was present was due to both structural and referential predictions. When focus was on the object, participants looked longer at the target only if reference to the subject character was predicted and if that reference was predicted to be realized in a control structure. This is different from what was seen in Experiment 1, in which only referential predictions affected PRO’s interpretation. This again suggests that in Experiment 1, participants were adjusting to the high frequency of control structures within the experiment; when this frequency was lowered, participants’ interpretations were still aided by structural predictions, but only based on individual item contexts.

#### 3.3.2 Pronsubj items

Turning to the exploratory analyses of the pronsubj items, in Experiment 1, looks away from the target were affected by the cloze probability of reference to the subject character when the focus was on that character. Experiment 2 saw a similar effect, in that participants looked longer at the target the more predictable reference to the subject was, but only later on in the experiment. This may be due to the lower proportion of items in Experiment 2 with reference to the subject character. Participants could not use within-experiment frequency expectations, but did rely more on referential predictions as the experiment went on.

The pronsubj condition of Experiment 1 also saw an effect of the cloze probability of a control structure in an interaction with other measures. We hypothesized that participants were faster to look toward the target when multiple factors all led to a prediction of subject reference, including the high proportion of items in the experiment with subject reference. When in Experiment 2 this proportion decreased, the effect of item-wise prediction of a control structure disappeared. If it had any effect, it was too small to detect without all the other factors involved.

## 4 General discussion

### 4.1 Summary of findings

The experiments reported here provide evidence for the rapid interpretation of PRO in temporal adjuncts, modulated by the predictability of reference to PRO’s antecedent as well as by how likely a structure containing PRO was to occur. Our results show that participants can use the structural information inherent in the control dependency to resolve PRO just as quickly as they can use gender information to resolve an overt pronoun, but only when a structure containing PRO is predicted. The strongest effect of such prediction in these experiments was due to the high within-experiment frequency of such structures seen in Experiment 1. A a weaker effect was also seen with a lower within-experiment frequency of PRO in Experiment 2 based on predictions arising from individual item contexts. When neither factor led to prediction of PRO, its interpretation slowed significantly.

### 4.2 Use of structural information in reference resolution

There are two main tasks a listener faces in interpreting referential expressions: recognition and resolution. Listeners must recognize that a speaker is attempting to refer to someone, and they must also use the information available—from the discourse context, structural and morphological features of the speaker’s utterance, etc.—to decide who that someone is, i.e. to resolve reference. The slowdown in the interpretation of PRO seen in Experiment 2, especially for items where PRO or reference to the character corresponding to the main clause subject was not predicted, could in principle be due to difficulties in either of these tasks.

First, the interpretation of PRO may be slowed due to difficulties in using structural information versus morphological/discourse information in reference resolution. However, a large body of literature has argued that structural constraints can apply at the earliest stages of processing for reflexives such as himself, which are similar to PRO in the way structure guides resolution (for an overview, see Dillon 2014). Dillon’s explanation of this fact is that for reflexives, antecedent retrieval involves serial search of the syntactic structure. Although Dillon argues that such a search is rapid, such a search may be slower due to its serial nature than the cue-based retrieval argued to be involved in pronoun resolution, which searches all potential antecedents simultaneously (Lewis & Vasishth 2005; for discussion, see Kush, Lidz & Phillips 2015). That being said, Parker & Phillips (2017) argue that structural information can be encoded as a searchable, direct-access cue, and that it may in fact have a stronger weight than morphological information. If this is true, and listeners are able to use structural information so early in the processing of reflexives, then it would be surprising if they were unable to similarly do so to resolve PRO in the current experiment.

The other possible explanation for the slow interpretation of PRO when it is not predicted is that it simply took listeners longer to recognize that reference resolution was necessary. This seems likely for a number of reasons. First, the bottom-up cue that a referential dependency is needed in the PRO sentences, namely the nonfinite verb, had a longer duration than he/she, the cue to anaphora in the pronoun conditions, by an average of 160 ms. Furthermore, it is the final syllable of this cue (the -ing) that indicates the presence of a control structure. It is therefore possible that participants simply did not realize that reference resolution was needed as quickly in unpredicted control structures as when there was an overt pronoun or when a control structure was predicted. Second, although the non-finite verb was the cue to anaphora in the PRO condition, this verb does not itself have reference to an individual as its semantic function, unlike a pronoun. Lexically, the verb expresses an event concept, or the concept of a relation to an event. The truth conditions of this concept will entail the role that would be associated with the subject, but the signal of anaphoric reference to an individual bearing that relation comes only from the verb’s grammatical context. In addition to interpreting the verb, listeners must also find one of its participants in order to establish the referential dependency necessary to interpret PRO. This is one instance where, as Van Berkum (2008: 376) put it, it is not the case that “[f]irst you recognize each of the words, then you look up their meaning in your mental dictionary, and then, using syntax to guide the combination, you simply combine the meanings so that you know what I said.” Instead, recognition of the nonfinite verb in this particular context activates both the event concept lexically expressed by the verb, and the cue to anaphoric resolution of the subject argument. The extra semantic processing required may therefore have delayed the initiation of reference resolution in PRO versus pronoun items.

There are many different kinds of referential expressions and types of anaphora. Each may rely on syntactic, discourse, and conceptual sources of information in different ways. The interpretation of PRO in adjunct control structures is heavily dependent on structural features of the sentence, while pronouns, although restricted by their “φ features” (person, gender, and number), rely more on discourse information. If the slowdown in the resolution of PRO when it was not predicted is due to difficulty in the recognition of the need for reference resolution, and not in the resolution itself, then this would mean that structural information can be used in absence of other features just as efficiently as information guiding the interpretation of overt pronouns, once the cue to anaphora has been identified.

One additional note on the use of structural information as a cue to retrieval is necessary. Although Parker & Phillips (2017) argued that structural information is a searchable cue, they did not elaborate on the nature of that cue. In the ACT-R architecture of Lewis & Vasishth (2005), searchable cues on an NP are said to be encoded in memory chunks representing the head N. For example, the pronoun he searches for an NP in memory with the feature [+masculine]. Whether a potential antecedent is in a proper structural position would be difficult to encode in such a manner.

Kush, Lidz & Phillips (2015) provide a possible solution for bound variable anaphora in sentences such as (9), in which he can only be bound by any janitor if the pronoun is c-commanded by the quantificational phrase, as in (9a).

 (9) a. Kathi didn’t think any janitori liked performing his custodial duties when hei had to clean up messes. b. Kathi didn’t think any janitori liked performing his custodial duties, but he*i had to clean up messes.

Kush, Lidz & Phillips argue that the pronoun triggers retrieval of an antecedent based in part on the cue ACCESSIBLE, which is only present on the memory chunk for any janitor in (9a). The memory chunk for any janitor in (9b) loses its ACCESSIBLE feature when the sentence reaches the end of any janitor’s scope domain, indicated by the conjunction. This does not occur for non-quantificational NPs, however. In (10), the janitor does not require c-command in order to corefer with a later pronoun, and so it does not lose its ACCESSIBLE feature. As a result, the pronoun he is able to corefer with it.

 (10) Kathi didn’t think the janitori liked performing his custodial duties, but hei had to clean up messes.

Although this allows the structural information relevant to bound variable anaphora to be encoded in a content-addressable way, this approach cannot be directly applied to the resolution of PRO. The reason is that the antecedent to PRO need not be a quantificational phrase. Because of this, there is no reason that only c-commanding antecedents would remain accessible.9 The chunk representing Minnie in (11) would still remain active, and there would be no way to distinguish c-commanding from non-c-commanding antecedents.

 (11) Mickeyi talked to Minniej before PROi/*j putting on a hat.

If, however, a similar feature to Kush, Lidz & Phillips’s ACCESSIBLE were to represent syntactic accessibility for any NP, then the structural information relevant to adjunct control could be encoded. In (11), the chunk for Minnie would lose its SYNTACCESS feature as soon as the parse required adjoining a clause higher in the structure—in this case the temporal adjunct. But because the adjunct adjoins lower than Mickey, it keeps its SYNTACCESS feature, and is therefore retrievable once the control structure is recognized if PRO triggers a search only for memory chunks that have that feature.

Regardless of whether the slowdown is due to difficulty in recognizing the referential dependency, as we have argued, or in its resolution, the results of these experiments make clear that when an adjunct control dependency is highly frequent, its resolution is just as easy as the resolution of overt pronouns.

### 4.3 Prediction in the resolution of anaphora

This experiment also has implications for the role of prediction in the processing of anaphora. Effects of prediction in reference resolution have been well documented. For example, Kehler et al. (2008) provide evidence that pronoun interpretation is incrementally influenced by predictions listeners make about what coherence relations are likely to be at play as well as what discourse entities are most likely to be next mentioned (see also Kehler & Rohde 2013). In the examples in (12) from Caramazza et al. (1977), for example, listeners predict that an explanation for the first clause will be given, and their interpretation of the pronoun is biased toward whatever interpretation will satisfy that prediction. In (12a), this explanation is likely to be a description of something Mary did, and in (12b), something Jane did. Listeners may be quick to assign those referents to the pronoun, despite the fact that the rest of the sentence may favor an alternative interpretation, as (13) does.

 (12) a. Jane hit Mary because she had stolen a tennis racket. b. Jane angered Mary because she had stolen a tennis racket.
 (13) Jane hit Mary because she reacts violently to criticism.

The present experiments give evidence that not only conceptual predictions, but also predictions about the upcoming structure of a sentence may affect anaphora resolution. How quickly a listener can resolve the reference of the null subject of an adjunct control structure is impacted by how strongly that control structure was predicted. This is in line with the arguments of Kehler (2008) that reference resolution is not a purely reactive process, with participants always waiting for cues before retrieving potential antecedents. Instead, listeners may actively predict upcoming structure and likely referents before any cue for reference is received.

There were two sources for effects of prediction in the present experiments. In Experiment 1, participants seem to have been affected by the high frequency of control structures within the experiment, which may have led them to predict PRO more often then they otherwise would have. This is in line with a large body of research demonstrating that statistical learning within an experiment can increase reaction times and lead participants to make new predictions (see, e.g. Wells et al. 2009; Misyak, Christiansen & Tomblin 2010; Dale, Duran & Morehead 2012; Karuza et al. 2014). In Experiment 2, when the frequency of control structures was lessened, the prediction of PRO affected resolution times only based on individual item contexts. In either case, the prediction of a control structure led to more rapid resolution of PRO than when such a prediction was likely to be absent.

One remaining question is the extent to which these kinds of structural predictions influence reference resolution in real-world language use. The frequency effect seen in Experiment 1 is unlikely to be seen in every-day situations outside the lab, simply because adjunct control structures are generally less frequent than what was present in the experiment. In Experiment 2, individual item contexts appeared to favor prediction of control structures. But was that due to properties unique to these items or to the simple image and discourse context? Or are control structures predicted in similar sentences more generally? Although these experiments do not answer this question, it is clear that listeners can be influenced by structural predictions in anaphora resolution.

### 4.4 Incremental sentence processing

These experiments also add to a large body of literature demonstrating incremental parsing and interpretation in sentence processing (e.g. Marslen-Wilson 1975; Altmann & Steedman 1988; Kamide, Altmann & Haywood 2003; Kutas, DeLong & Smith 2011; Poesio & Rieser 2011). Not only do these experiments demonstrate that reference resolution can be completed as soon as the cue to anaphora is heard, but also that comprehenders may initiate reference resolution before the sentence unambiguously indicates that it is needed. Although the non-finite verb in the PRO conditions was the major cue for reference resolution in the PRO items, it itself does not unambiguously indicate that a control relation is necessary. As was seen in (7), repeated in (14), at the non-finite verb, the sentence could still have a continuation that does not require a referential dependency between the null subject of the adjunct and the main clause subject. If (14a) is continued with (14b), the null subject receives an arbitrary interpretation, no anaphora being necessary, but this only becomes clear later in the sentence, long after the non-finite verb.

 (14) a. Mickey talked to Minnie before eating pizza at the park… b. … was forbidden.

The fact that participants assume a control structure when a continuation such as (14b) is possible is perhaps not surprising, however, since such continuations are likely less frequent. In addition, assuming a control structure rather than a structure where the non-finite verb is part of a gerundival subject may be preferred due to general processing strategies such as Minimal Attachment (Frazier & Rayner 1982), which favors parses with simpler structures.

Additionally, however, a non-finite verb in a temporal adjunct could also be part of a non-obligatory control structure, in which the null subject may refer to an antecedent that is not syntactically represented, as in (15), although this is far less common (Landau 2017; Green 2019a; b).

 (15) The pizza tasted better [after drinking root beer].

Although non-obligatory control in temporal adjuncts is used infrequently compared to obligatory control structures, where PRO is syntactically bound, and although all of the examples in our experiment did require control by the main clause subject, it is still possible that participants in principle would wait to interpret the null subject until it was clearly necessary. But this was not the case; participants quickly looked to the character corresponding to the main clause subject upon hearing the non-finite verb, evidently establishing the anaphoric control dependency at the earliest possible indication that it might be necessary.10

### 4.5 Additional implications and questions

The results of these experiments have several other relevant implications. First, they call into question a previous claim that the interpretation of PRO in adjuncts involves a “most-recent filler strategy.” Based on the results of an eyetracking-while-reading study, Betancort, Carreiras & Acuña-Fariña (2006; following Frazier, Clifton & Randall 1983; Nicol & Swinney 1989) suggest that upon encountering PRO, comprehenders first consider the most (linearly) local potential antecedent, even if they must later revise that initial interpretation to establish an anaphoric dependency between PRO and the main clause subject. If such a strategy were active in the current experiment, we would expect initial looks during the critical region in the PRO condition to be to the competitor (the referent of the main clause object), as it was the most recently mentioned potential antecedent. This was not the case in either experiment. At no point were participants more likely to look toward the competitor and away from the target than vice versa. Even when participants did not consistently look more toward the target than away from it, there were at least equal looks to the two characters. When the interpretation of PRO slowed, either both characters were under equal consideration during early stages of anaphora resolution, which on its own would counter the most-recent filler account, or more likely, the interpretation of PRO simply started later, with the subject being quickly recognized as the only potential antecedent thereafter.

Second, previous research on the processing of PRO (e.g. McCourt et al. 2015) has left open the possibility that its reference is not always instantaneously resolved. The current experiments give evidence that at least in some contexts, PRO is resolved quickly during incremental sentence processing. It’s still an open question for future work, however, whether the same retrieval mechanisms hold for control relations involving different cues. It may be the case that the retrieval mechanisms used in the adjunct control structures used here differ from some or all cases of complement control, for example. According to Landau (2015), complement control can either be the result of predication, which is also argued to be involved in obligatory adjunct control, or logophoric variable binding, which is only seen in non-obligatory control adjuncts. Logophoric complement control is especially likely to involve at least somewhat different retrieval mechanisms from what was used in the present experiments, since its resolution requires more than structural information.11

Throughout this paper, we have remained neutral with respect to the theoretical representation of what we have been calling “PRO” and the control dependency, and theories differ with respect to how PRO is resolved. In some theories, obligatory control in adjuncts is nothing more than predication (Landau 2017; forthcoming). In others, PRO’s referent is determined through a syntactic dependency akin to binding or movement (e.g. Hornstein 1999). Still others assume that control does not involve any syntactic dependency, but instead is dependent on the semantics or on pragmatic inference (e.g. Jackendoff & Culicover 2003). Although this paper does not directly bear on this debate, future work along this line has the potential to do so. We have demonstrated that PRO has a similar processing profile to overt pronouns, but that it is somewhat less sensitive to referential predictions, consistent with its being more dependent on structural sources of information. To provide evidence on the exact nature of the control dependency, it would be fruitful to directly compare the processing of PRO with that of movement relations such as filler-gap dependencies and with the processing of (secondary) predication relations.12

Finally, an additional area for future research would be to compare the processing of obligatory control structures such as those examined in this paper with the processing of non-obligatory control adjuncts. Participants were quick to assume that an anaphoric dependency was needed in these experiments. If participants automatically attempt to retrieve an antecedent to PRO upon encountering the non-finite verb, then in sentences like (15), where the intended referent of PRO is not in the sentence, participants may experience processing difficulty due to retrieval failure. Such a difficulty has been given as one possible explanation for why obligatory control is so much more prevalent than non-obligatory control in adjuncts (Green 2019a; Landau forthcoming), but more empirical work is needed to confirm this.

## 5 Conclusion

This paper used visual-world eyetracking to investigate the processing of the null subject in adjunct control. It has shown that when adjunct control structures are predicted, the reference of the null subject can be resolved just as quickly as that of overt pronouns. Studying different forms of reference can shed light on how different sources of information are implemented during sentence processing, and this study contributes to this agenda by providing evidence that structural information can be immediately utilized in reference resolution, especially when aided by prediction of structures where such information is crucial.

## Notes

1. This is only true when “Obligatory Control” is involved. See Landau (2017; forthcoming) and Green (2019a; b) for discussions on when this is not the case. [^]
2. This is not to say, necessarily, that that missing subject is present syntactically. A major question in the study of adjunct control concerns exactly what the adjunct control dependency is. For simplicity, we will assume the presence of a null subject, and use the “PRO” label for clarity. However, our results and conclusions are not dependent on a PRO analysis of the null subject of control structures, or on the presence of a syntactically-represented subject. For a discussion on the debate over the representation of PRO, see Hornstein (2003). [^]
3. A reviewer suggests the fact that the verb provides the theta role of the null subject may counteract this potential slowdown, since it provides conceptual information about the antecedent, and the plausibility of potential arguments has been shown to affect resolution of other, similar dependencies such as in filler-gap constructions (Garnsey, Tanenhaus & Chapman 1989). [^]
4. A reviewer notes that fully interpreting the sentence still requires someone to do the eating, even if it is an arbitrary “anyone”. Even if that were to be true, though, this is crucially different from the anaphora we are describing, because it does not involve coreference with a previous expression. [^]
5. In the Corpus of Contemporary American English (COCA) (Davies 2008–), before, after, and while are followed by a gerundival verb 11.3%, 13.2%, and 16.6% of the time, respectively. [^]
6. As discussed in §1, a non-finite verb in general is ambiguous in that it can be followed by continuations that do not require a control dependency, but such continuations were absent in these experiments. [^]
7. A reviewer notes that since these fillers also included PRO or pronouns, they could in principle be analyzed with the critical items. The main reason they were not included was that in roughly half of the fillers, the discrepancy between the auditory stimulus and the visual scene occurred before the adjunct; for those items, participants could correctly answer that the scene and description did not match without having to resolve reference in the adjunct. [^]
8. See Footnote 5. [^]
9. A reviewer notes that the same is true for other instances of bound variable anaphora that do not involve quantificational antecedents, but that still require c-command (e.g. sloppy readings in ellipsis). [^]
10. Whether obligatory control would be preferred under Minimal Attachment depends on which control theory is adopted. In the Two-tiered Theory of Control (Landau 2015; forthcoming), obligatory control does indeed involve a simpler structure than non-obligatory control. In other theories, such as the Movement Theory of Control (Hornstein 1999; Green 2019a), there are no structural differences between obligatory and non-obligatory control, but obligatory control is still preferred. [^]
11. See also Jeffrey, Han & Pappas (2015), which discusses the processing of transitive subject control in complements, which under Landau’s (2015) theory would be logophoric. [^]
12. A reviewer notes that if the adjunct control dependency is processed similarly to predication or to filler-gap dependencies, then it may be fruitful to examine Gibson’s (2000) Dependency Locality Theory, which considers such dependencies together. [^]

## Ethics and Consent

This research involved human subjects, and was approved by the University of Maryland IRB (approval 00972). All subjects gave written consent prior to participating.

## Acknowledgements

Thank you to the audience at CLS 54 for helpful feedback, and to Jan Edwards and Marissa Barlaz for comments at various stages of this research.

## Funding Information

This material is based upon work supported in part by the National Science Foundation under Grants No.1449815 and No.1749407.

## Competing Interests

The authors have no competing interests to declare.

## Author Contributions

JJG and MM designed the experiments with feedback from AW and EL. JJG and MM collected the data. JJG analyzed the data and wrote the paper. All four authors participated in editing and revising the manuscript.

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