First Language · Tim Beyer, University of Puget Sound Carla L. Hudson Kam, University of California, Berkeley ABSTRACT This article describes two experiments examining how 6- and

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First Language

DOI: 10.1177/0142723708101678 2009; 29; 208 First Language

Tim Beyer and Carla L. Hudson Kam person present �s as a tense marker by 6- and 7-year-olds

Some cues are stronger than others: The (non)interpretation of 3rd

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Copyright © 2009 SAGE Publications (Los Angeles, London, New Delhi, Singapore and Washington DC)www.sagepublications.com Vol 29(2): 208–227 (200905)DOI: 10.1177/0142723708101678

Some cues are stronger than others: The (non)interpretation of 3rd person present –s as a tense marker by 6- and 7-year-olds

Tim Beyer, University of Puget SoundCarla L. Hudson Kam, University of California, Berkeley

ABSTRACTThis article describes two experiments examining how 6- and 7-year-old Standard American English-speaking children interpret 3rd person present –s as a tense marker, as compared to lexical items and past tense –ed. Because –s corresponds to multiple meanings, unlike –ed, it may result in later acquisition. Using an offline picture-choice task (Experiment 1), the study found that while all children successfully comprehended –ed, only the 7-year-olds successfully comprehended –s. Eye-tracking measures (Experiment 2) revealed that the 6-year-olds are actually sensitive to –s, but that it is not yet a particularly strong cue for them. The article argues that offline tasks may underestimate children’s developing knowledge.

KEYWORDSComprehension; cue strength; eye-tracking; tense morphology; 3rd person present –s

The acquisition of individual grammatical morphemes is affected by a number of factors that appear to influence how easy they are to learn. For example, cross-linguistic evidence indicates that, all else being equal, morphemes are acquired earlier if they show a one-to-one correspondence between meaning and form

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(Brown, 1973; Slobin, 1985), or are made phonologically salient by being word final (Slobin, 1971, 1985) or syllabic (Brown, 1973; Slobin, 1971, 1985), as compared to morphemes that do not have these characteristics (Brown, 1973; de Villiers & de Villiers, 1973).

From this perspective, 3rd person present –s should be a relatively late acquired grammatical morpheme for children learning Standard American English (SAE). Specifically, the same phonological form, –s, corresponds to three distinct morphemes: on nouns, –s marks possession (or genitive case, e.g., Jonathan’s batman watch) and plurality (e.g., Jonathan likes crayons), and on verbs, –s marks 3rd person present tense (e.g., Jonathan jumps on beds). Adding to this complexity, the verbal inflection encodes several different meanings simultaneously, indicating subject number, tense, and aspect. Although 3rd person present –s is word final (and so should be easier to learn), its surface form differs according to the nature of the stem to which it is affixed (–s, –z, and –ız are all variants), and only one of the allomorphs is syllabic. All of these factors conspire to make 3rd person present –s a difficult morpheme to learn. Indeed, Brown (1973) found that it was not reliably produced until 2;3–3;8 (MLU range 3.5–4.0), which is relatively late in comparison to the other morphemes he examined. This pattern was confirmed by de Villiers and de Villiers (1973) with a much larger sample.

Although –s is produced relatively late (Brown, 1973; de Villiers & de Villiers, 1973), research has shown that successful comprehension occurs later still. Keeney and Wolfe (1972) investigated the acquisition of –s as a number agreement marker by children aged 3–4. The children they studied not only produced –s when obligatory in their spontaneous speech, but also corrected sentences that did not contain –s but required it to be grammatically correct. However, when these same children were asked to select a picture based on linguistically-presented number information, they did not reliably use –s, whether it occurred as part of a single verb (e.g., fly vs. flies) or a complete sentence (e.g., The bird flies). In a more recent study, Johnson, de Villiers and Seymour (2005) presented 3- to 6-year-old SAE-speaking children with pictures that could only be differentiated by the number information encoded in –s (e.g., the duck swims vs. the ducks swim).1 They also found that 3- to 4-year-olds were insensitive to –s as a number agreement marker. It was not until 5–6 years of age that children appeared to use –s as a cue to number during comprehension (Johnson et al., 2005).

Importantly, –s not only indicates number, it also encodes tense information.2 It is therefore quite possible that an understanding of the correspondences between –s and number agreement on the one hand, and –s and tense on the other, develop at different times. This indeed seems to be the case. In a recent study, de Villiers and Johnson (2007) investigated whether 4- to 6-year-old SAE-speaking children comprehend –s as a tense marker using a picture-choice task.3 In contrast to their study on –s as a number marker, they found that the children were not sensitive to the tense information in –s. That is, they were still not interpreting –s in adult-like ways by 6 years of age. These results show that even children who are quite old have difficulty with –s as a cue to tense (de Villiers & Johnson, 2007).

In the present article, we continue to investigate the comprehension of –s as a tense marker in SAE-speaking children. In particular, we ask whether children at



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and beyond the age range studied by de Villiers and Johnson (2007), namely 6- and 7-year-olds, can successfully interpret –s, in an attempt to uncover when it is that children can actually interpret –s. Results from two experiments are presented. Experiment 1 uses a standard picture-choice task to establish global comprehension patterns, and Experiment 2 uses eye-tracking to examine comprehension patterns in real-time.

In both experiments, participants were presented with a set of pictures and listened to a spoken sentence. They were then asked to select the picture that best matched the sentence they heard. We compared the children’s performance on –s to control sentences containing overt temporal words, as well as past tense –ed, a morpheme that is much less complex, and is comprehended before this age (Harner, 1980; Herriot, 1969).4 The stimuli were constructed in such a way that correct comprehension for the –s and –ed sentences crucially hinged on an understanding of these morphemes. The control items served to ensure that incorrect responses were not due to overall task difficulty, and the –ed items ensured that problems were not with interpreting tense morphology in this task. Participants also heard filler items in a variety of tenses/aspects. These items also served an important role: they reinforced the temporal nature of the task without highlighting the particular morphemes of interest. We expected that the 6-year-olds would only have difficulty with the –s items, but not the –ed or control items. The predictions for the 7-year-olds were less clear, but one might expect that they would be able to comprehend the tense information contained in –s, given their extra year of experience with the language.

EXPERIMENT 1: PICTURE-CHOICE

Method

Participants

Seventeen 6-year-olds and 15 7-year-olds participated in this experiment. (One additional 6-year-old was excluded because he could not successfully complete the training task.) The 6-year-olds (6 male; 11 female) had an average age of 6;4 (range 6;0–6;7), and the 7-year-olds (6 male; 9 female) had an average age of 7;3 (range 7;0–7;6). Participants included 24 European Americans, four Latino/as, two children of African heritage, one African American, and one child of mixed ethnicity. The children were recruited from public schools in the San Francisco Bay Area and each received a small toy for participating.

Because the school districts serving the San Francisco Bay Area are quite diverse, we confirmed each child’s language background and skills prior to inclusion to ensure that the child was actually a typically developing SAE-speaker. Teachers were asked to rate the production and comprehension of SAE for each participating student. These ratings identified all participants as typically developing, with no history of hearing or language problems. Teacher ratings were confirmed with an audiotaped story-telling task, in which participants were found to produce all grammatical morphemes required by the adult model, including the tense




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morphology under investigation here. The children also completed a sentence repetition task designed to assess the production of nonstandard dialect features (Piestrup, 1973). Results showed that all children were SAE-speakers.

Procedure

Consent forms were sent home with the students in participating classrooms. Only children with parental consent were asked to participate. The tasks were run individually in a quiet location at their school in a single session lasting 35–40 min. Prior to participation, the study was explained to the child and she or he was asked to assent. Only assenting children continued. During the session we collected various measures as part of a larger project, but here we only report the relevant task. Tests were administered by a frog puppet displayed on a stage that hid the experimenter. The experimenter spoke slowly and deliberately so that all relevant morphology was as perceptible as possible.

Participants were presented with two panoramas of pictures, such as those in Fig. 1, and heard a spoken sentence. Participants were asked to select the picture that best matched the sentence. The panoramas were constructed (and paired) such that only one picture matched the content of the sentence entirely. Participants’ responses to this task were hand-scored at the time of presentation.

Individual stimulus items consisted of two panoramas of three picture frames each in which the same character was engaged in different actions. For each panorama, frames 1 and 3 showed different actions (e.g., running and playing ball). Importantly, the actions in frames 1 and 3 were the exact opposite in the two picture series, with the result that the actions in frames 1 and 3 of panorama 1

Picture series 2

Figure 1 Example set of picture series

Picture series 1



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corresponded to the actions in frames 3 and 1 of panorama 2, respectively. Thus, the verb being tested always matched two frames, requiring the use of other information in the sentence (i.e., the adverb or the tense morphology) to correctly interpret the sentence. Frame 2 in both panoramas always showed the act of sleeping, which served as the temporal boundary marker that identified the actions in the 1st and 3rd frames as past or present.

Participants received explicit training in how to utilize ‘sleeping’ as a temporal boundary marker. They were told the actions in frame 1 occurred yesterday (past) and the actions in frame 3 occurred today (present) because ‘sleeping (frame 2) always separates yesterday (frame 1) from today (frame 3).’ Although this procedure might seem difficult, participants understood the task, as evidenced by their performance on the control items.

After training, the experiment began. Participants heard three types of sentences: (1) control sentences containing overt temporal markers (e.g., Today he jogs from the store); (2) test sentences that contained no overt temporal markers, but rather only tense morphology (e.g., He jogs from the store); and (3) filler sentences.

Each participant heard 10 test sentences (four testing –ed, six testing –s), four control sentences, and 17 fillers. This distribution resulted in about half of the sentences being relevant for the contrasts we were testing (four –ed, six –s, and four controls testing the same times), intended to prevent the children from figuring out what we were interested in. The sentences were arranged in a list in the following way: first participants received control sentences (testing whether participants had been trained successfully and could use the ‘sleep’ indicator correctly), followed by the test and filler sentences. The test and filler sentences were arranged so that no more than two sentences containing the same contrast appeared in a row.

Two lists of sentences were created. Each list was used with half the participants. The lists differed in test and filler sentences, but the control sentences were the same and always occurred first. This was done to ensure that participants understood the prior training; only those participants who correctly answered at least three out of the four control sentences continued to the other sentences. Participants who failed to do so were retrained. This was rare: two 6-year-olds were retrained and passed the second time. Their performance did not differ from children who succeeded the first time. One 6-year-old failed twice, and was excluded from the study.

Target words for all sentences were selected from age-appropriate lists from the Basic Reading Inventory (Johns, 2001). Sentences were constructed and spoken such that no sounds necessary for correct comprehension were obscured by the following sounds. See the Appendix for a list of the control and test sentences.

Results

Responses were scored as correct if the participant chose the picture that corres-ponded to both the correct time and action. A second coder checked these scores for accuracy from the videotape. Accuracy was very high with only six discrepancies out of 1353 total videotaped responses. Any discrepancies were resolved using the videotaped responses.5




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Control sentences

All participants showed similarly high performance on the control items; all means were over 90%. There were no significant main effects for age, F(1, 30) = 0.11, p = 0.747, �p

2 = 0.004, or item type, F(1, 30) = 0.17, p = 0.685, �p2 = 0.006. There

was also no significant interaction between age and item type, F(1, 30) = 0.17, p = 0.685, �p

2 = 0.006. We next investigated whether these responses differed from chance. Because no significant effects of age emerged in the ANOVA analyses, we combined the data from both age groups. Both types of control sentences were significantly different from chance: past, t(31) = 21.56, p < 0.001, and present, t(31) = 17.31, p < 0.001. These data show that all participants, regardless of age, performed well on the control items, establishing that they understood the task. Thus, any comprehension difficulties found for the test sentences cannot be simply due to task difficulty or misunderstanding.

Test sentences

Mean percentages correct for the –ed and –s test items are shown separately for the two age groups in Fig. 2.

In contrast to the control items, here there were significant main effects for age, F(1, 30) = 17.23, p < 0.001, �p

2 = 0.365, and item type, F(1, 30) = 10.31, p = 0.003, �p

2 = 0.256. However, there was also a significant interaction between age and item type, F(1,30) = 4.65, p = 0.039, �p

2 = 0.134. As can be seen in Fig. 2, the interaction seems to be largely driven by the 6-year-olds’ much poorer performance on test items that contain –s. This impression is borne out by further

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Figure 2 Percentage correct on –ed and –s test sentences for 6- and 7-year-olds



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analyses. Performance on the two tense morphemes was not significantly different for the 7-year-olds, t(14) = 1.10, p = 0.328, but was for the 6-year-olds: they performed significantly worse on –s items than –ed items, t(16) = 3.28, p = 0.005. Indeed, the 6-year-olds did not perform any better than chance on –s, t(16) = 0.72, p = 0.482, in contrast to their performance on –ed, t(16) = 5.49, p < 0.001, and the 7-year-olds’ performance on both –s, t(14) = 9.17, p < 0.001, and –ed, t(14) = 11.50, p < 0.001.

Discussion

This experiment investigated how 6- and 7-year-old SAE-speaking children interpret 3rd person present –s as a cue to tense. We found that the 6-year-olds did not successfully interpret –s. This result is in line with other research showing that 6-year-olds have difficulty with –s as a tense/aspect marker (de Villiers & Johnson, 2007). However, we also found that 7-year-olds did successfully interpret –s. This finding expands on previous work, in showing that by age 7, SAE-speaking children have learned enough that they can interpret the tense information in –s.

Importantly, all participants, regardless of age or contrast tested, showed a similarly high performance on the control items that contained additional lexical cues to tense, as well as the –ed test items. This indicates that the 6-year-olds’ diffi culty with 3rd person present –s is not due to task difficulty, but rather that an understanding of the meanings that it encodes is still developing during this age range.

It is possible that the behavior we see in the picture-choice task reflects, not knowledge per se, but some type of performance limitation. The Competition Model of sentence processing (Bates, Devescovi, & Wulfeck, 2001; Bates & MacWhinney, 1987) sees language comprehension as a process whereby multiple cues to meaning are integrated to arrive at an interpretation, with different cues of differing strengths contributing to varying degrees. That is, strong cues are more likely to contribute to the final interpretation than weak cues. A speaker’s knowledge of cues and the weighting those cues receive develops over time. Children, who are still in the process of learning their native language, also have immature processing systems. This includes un-adult-like cue weightings, as well as more basic differences such as reduced ability to direct or shift attentional focus and greater difficulty with auditory processing (Leech, Aydelott, Symons, Carnevale & Dick, 2007). Thus, their interpretations can be different from adult interpretations due to processing difficulties.

Thinking about 3rd person –s from this perspective is particularly interesting. Cue strength is a function of cue frequency as well as cue reliability (Bates & MacWhinney, 1981). As we described in the introduction, –s is a complicated morpheme in English, and its multiple meanings and functions very much affect its reliability. Moreover, although one of its meanings is ‘present,’ speakers do not solely use –s to describe events in the present tense. Rather, speakers very often use the present progressive to talk about such events (Celce-Murcia & Larsen-Freeman, 1999). Given this, –s may not be a very highly weighted cue. If this is true, children may actually know what –s means despite not using it in comprehension. Further,




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this would explain how they could produce –s appropriately at ages when they seem not to comprehend it. To explore this possibility we conducted a second experiment.

EXPERIMENT 2: EYE-TRACKING

In Experiment 1, we found that 6-year-olds do not seem to fully comprehend –s, despite the fact that they already produce it in an adult-like way. However, picture-choice is not a very sensitive measure because the participant has to definitively select one of the options (see Keeney & Wolfe, 1972). Thus, while picture-choice is very useful for measuring overall comprehension, it can miss subtle differences in processing that might indicate weak or developing knowledge; this is a potential problem as children’s explicit responses can belie differences in their understanding of the information, especially when the child is undergoing a conceptual shift (see, e.g., Goldin-Meadow & Singer, 2003).

In order to address this, we assessed children’s knowledge of –s using a much more sensitive online measure of their processing, rather than offline com prehension. Specifically, we were interested in how 6-year-old SAE-speakers interpret –s in real-time; if 6-year-olds truly have no sensitivity to –s this should be evident in an online measure. However, if their failure to use –s is not due to a lack of knowledge, but rather because –s is a weak cue, their online processing should reflect this (weak) sensitivity. Thus, we presented a different group of parti-ci pants with the same stimuli as in Experiment 1 and eye-tracked them during comprehension and picture selection. Eye-tracking is a noninvasive technique that monitors a participant’s eye movements to a visual array as linguistic information unfolds over time (Aslin & McMurray, 2004; Eberhard, Spivey-Knowlton, Sedivy, & Tanenhaus, 1995; Tanenhaus, Spivey-Knowlton, Eberhard, & Sedivey, 1995). The patterns and timing of eye fixations to visual stimuli are closely time-locked to the speech signal as it unfolds (Tanenhaus, Magnuson, Dahan, & Chambers, 2000). Tracking the time course of eye movements to visual stimuli as linguistic information is encountered provides a continuous measure of spoken language processing, measuring how and when listeners interpret information in the speech signal (Tanenhaus et al., 2000). Importantly, this measure can be used even with young children (Hurewitz, Brown-Schmidt, Thorpe, Gleitman, & Trueswell, 2000; McMurray & Aslin, 2004; Trueswell, Sekerina, Hill, & Logrip, 1999).

We anticipated that the 6- and 7-year-olds would show patterns and timings of eye fixations that accord with the global comprehension patterns found in Experiment 1. Specifically, both 6- and 7-year-olds were expected to rapidly isolate (i.e., look toward) the target upon hearing the necessary disambiguating information for the control and –ed test sentences. In contrast, we expected differences in the eye movement patterns for –s of the 6- and 7-year-olds. We expected that the 7-year-olds would quickly isolate the target upon encountering –s information, just as in the other sentence types. If the 6-year-olds are not sensitive to –s, then they should not show systematic looking patterns in relation to –s information, that is, their eye movements should not be time-locked to –s in any way. In contrast, if the



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6-year-olds are sensitive to –s, we might expect brief, but transient, looks to the target after encountering –s information.

Methods

Participants

Twelve 6- and 12 7-year-olds participated in this experiment. The 6-year-olds (6 male; 6 female) had an average age of 6;5 (range 6;0–6;10), and the 7-year-olds (5 male; 7 female) had an average age of 7;4 (range 7;0–7;8). Participants included 22 European Americans, one Latino/a, and one Asian. Participants were recruited through an existing university database, flyers posted in the San Francisco Bay area, and personal contacts.

Prior to participation, parents were asked to fill out a language background questionnaire (for the child) along with the consent form. These ratings identified all participants as typically developing SAE-speakers, with no history of hearing or language problems. The ratings were confirmed using the same story-telling and sentence repetition tasks as in Experiment 1. All participation took place on campus. Parents received US$20 for compensation and travel. The children each received a small toy.

Equipment

Eye movements were recorded using an ASL Pan/Tilt Model 6R remote eye-tracker. The camera was located directly under the computer screen and participants were seated approximately 25” away. The camera recorded a close-up image of one of the participant’s eyes and determined eye position by continuously tracking both the center of the pupil and the corneal surface reflection at a sampling rate of 60 Hz. A computer analyzed the eye image in real-time, superimposing the horizontal and vertical eye position on the scene image displayed to the participant. The scene image and the superimposed eye position were recorded to tape once every 33 ms, using a frame-accurate digital video recorder (Sony DSR-30).

Procedure

The study was conducted in lab space on the university campus. Prior to participation, the study was explained to the children and they were asked to assent. No child refused to participate in this study.

The general procedure was identical to Experiment 1, except that participants were eye-tracked during comprehension and picture selection. After training, we calibrated the eye-tracker for each participant using a standard nine-point technique. This served to map the output of the eye-tracker onto display position. All children were successfully calibrated and continued on to the experiment.

Stimuli were presented over a computer. The sentences were prerecorded and filtered using PRAAT (Boersma & Weenink, 2003). Pictures were presented on a 12” × 15” computer screen using PowerPoint. Each panorama was 4.5” × 13”. The two panoramas were centered on the screen, 1” apart. The frames were each 4” × 4” and were 0.25” apart.




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Each trial lasted 12 s. The pictures were presented at the start and remained visible for the entire trial. The sentence began after 4 s, giving participants time to scan the pictures. Between trials, an otherwise blank screen with a centered star was presented for 4 s. The star spun during seconds 3 and 4 to draw participants’ attention (and gaze) to the center of the screen, and to indicate the start of the next trial. Children were instructed to look at the picture that best matched the sentence and then to say aloud that picture’s number. These responses were recorded by an experimenter.

Results

Offline measure (picture-choice)

The overall pattern of responses in Experiment 2 was similar to that in Experiment 1: performance on control and –ed sentences was uniformly high (all means over 80%), and the 6-year-olds, but not the 7-year-olds, had difficulty on the –s sentences. As before, for the control items there were no significant main effects for age, F(1, 22) = 0.71, p = 0.409, �p

2 = 0.031, or sentence type, F(1, 22) = 0.47, p = 0.501, �p

2 = 0.021, nor was there a significant interaction, F(1, 22) = 0.00, p = 1.00, �2 = 0.00. As in Experiment 1, we then investigated whether these responses differed from chance, using the combined data from both age groups. Both types of control items were significantly different from chance, past, t(23) = 12.69, p < 0.001, and present, t(23) = 9.35, p < 0.001. For the test items, there was a significant main effect for age, F(1, 22) = 5.70, p = 0.026, �p

2 = 0.206, and item type, F(1, 22) = 5.28, p = 0.031, �p

2 = 0.194, and a significant interaction between the two, F(1, 22) = 6.56, p = 0.018, �p

2 = 0.230. As before, performance on –ed and –s was signi-ficantly different for the 6-year-olds, t(11) = 3.38, p = 0.006, but not the 7-year-olds, t(11) = 0.19, p = 0.853. The 6-year-olds did not perform significantly better than chance on –s, t(11) = 1.38, p = 0.195, in contrast to their performance on –ed, t(11) = 6.97, p < 0.001, and the 7-year-olds’ performance on both –s, t(11) = 7.99, p < 0.001, and –ed, t(11) = 5.20, p < 0.001.

Eye-tracking measure

For each child and each response, eye position relative to the scene image was hand-coded for every 33 ms video-frame. Eye position was coded into the following categories: (1) target (correct for that sentence), (2) action competitor (or AC, picture in which the action, but not the tense, matched the sentence), (3) time competitor (or TC, picture in which the tense, but not the action, matched the sentence), (4) incorrect (picture in which neither the action nor the tense matched the sentence), (5) sleep (one of the two pictures depicting sleep), and (6) track loss.

These coded looks were then coupled with the auditory information. Using PRAAT (Boersma & Weenink, 2003), the earliest point of disambiguation (POD) was calculated for each sentence. This was when both main verb and tense information became available. The POD was used as the common feature to align all individual sentences of the same type. Once sentences were aligned in this way, all codeable data from each participant for each sentence was averaged into 100 ms bins (three 33 ms frames) starting at the POD. Using this as the anchor point, additional 100



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ms bins were calculated backwards to the beginning and forwards to the end of the sentence. Coupling the coded looks with the POD in this way allows for an assessment of how and when disambiguating information is interpreted. The data were examined separately for correct and incorrect trials, as assessed by their offline responses.

Figure 3 shows the proportion of looks to the target, AC, TC, incorrect picture, and sleep picture during correct –s trials, separately for the 6- and 7-year-olds. The graphs contain solid vertical lines indicating the average sentence onset, POD (the onset of –s), and average sentence end, respectively. They also include a dashed line that indicates the point in time at which the children are looking at the eventually-selected picture significantly more than the other pictures. The number of individual trials included in each graph is stated.6 A sample sentence of the type shown is given as a reference. Importantly, because it may take children up to 300–400 ms to program and launch an eye movement (Hurewitz et al., 2000), looking patterns were not expected to change immediately upon encountering dis ambiguating information. Thus, as in adults, the time point at which an eye movement measurement is taken reflects interpretation or integration of infor-mation from an earlier point in the speech stream. (Because the children were free-viewing the stimuli as the sentence began, very early looks are likely due to features of the pictures themselves, not the incoming speech.)

As can be seen in Fig. 3a, the 7-year-olds rapidly integrate verb and –s infor-mation. Prior to the POD, the 7-year-olds are scanning all the pictures, and as verb information is encountered, looks to both the target and AC begin to increase. After the POD, looks to the target and AC continue to increase, but within 200–300 ms post-POD, looks to the target begin to diverge from the competitors. Within 400 ms, looks to target become significantly different from the TC, t(11) = 2.55, p = 0.027, and within 600 ms from the AC, t(11) = 2.35, p = 0.039.

The 6-year-olds show a similar but prolonged looking pattern for correct –s trials (Fig. 3b). After the POD, looks to both the target and AC begin to increase and remain virtually indistinguishable until 400 ms post-POD, at which point looks to the target begin to diverge from the AC. Within 600 ms post-POD, looks to the target are significantly different from the TC, t(9) = 3.30, p = 0.009, but it is not until 1200 ms post-POD (after the sentence has ended) that looks to the target are significantly different from the AC, t(9) = 2.81, p = 0.020. This looking pattern suggests that the 6-year-olds are capable of interpreting verb and –s information, and like the 7-year-olds, do so incrementally. Their processing is not as efficient as it is for the 7-year-olds, but it does not appear that the 6-year-olds are engaging in a fundamentally different process.

Interestingly, there appears to be more interference from the AC for the 6-year-olds than the 7-year-olds, which is more consistent with a processing problem than an interpretation problem. To better understand the 6-year-olds’ difficulty with –s it is necessary to look at the patterns for the incorrect –s trials.

Figure 4a shows the data from the 6-year-olds’ incorrect –s trials.7 On these trials, they show elevated looks to the AC prior to the POD. As the sentence unfolds and verb information is encountered, looks to the target increase slightly, while looks to the AC decrease slightly; however, the AC always has more looks than the




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Figure 3 Proportion of looks for correct 3rd person present –s sentences by 7-year-olds (Fig. 3a) and 6-year-olds (Fig. 3b)

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Figure 4 Proportion of looks for incorrect 3rd person present –s sentences by 6-year-olds (Fig. 4a), and 7-year-olds (Fig. 4b)




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target, and prior to the POD, looks to the AC again increase. While there is also a slight increase in looks to the target post-POD, these are not sustained. Rather, the 6-year-olds continue with their initial interpretation and further increase their looks to AC, not the target. This suggests that the disambiguating –s information that becomes available after the POD is not a strong enough cue to drive looks away from the AC and toward the target. There are significantly more looks to the AC than the TC by 300 ms after the POD, t(7) = 2.76, p = 0.028, and the target by 500 ms, t(7) = 3.19, p = 0.015. In contrast to the incorrect trials, the 6-year-olds show very few early looks to either the target or AC during the correct –s trials. Thus, it appears that if the 6-year-olds start out looking more at the AC (which also matches verb information), the –s is not enough to sway them from their (incorrect) interpretation; but, if they do not begin the trial looking preferentially at the AC, they can, and do, use the –s in their interpretation. The poor performance of the 6-year-olds found in the offline measure therefore does not seem to be an accurate reflection of their knowledge.

Unlike the 6-year-olds, the 7-year-olds do not show a systematic looking pattern for incorrect –s trials that can explain their performance (Fig. 4b). As verb information becomes available, the 7-year-olds show increased looks to the target and AC prior to the POD; in fact the 7-year-olds have more looks to the target than the AC immediately prior to the POD. Looks to the target are not sustained, however, and only looks to the AC continue to increase after the POD. Looks to the AC continue to increase and become significantly different from the TC within 600 ms post-POD, t(7) = 2.65, p = 0.03. Although the AC is selected as the best answer during these trials, looks to the AC are not significantly higher than looks to the target during the time window shown. Given the looking patterns during correct –s trials, these infrequent incorrect trials likely represent cases when the 7-year-olds were distracted or not paying attention to the point of missing relevant information in the speech stream.

The looking patterns for the correct control and –ed items were similar to the correct –s trials, and the timing of looks was similar to those of the 7-year-olds in the correct –s trials (see Table 1).

Table 1 Timing of looks (post-POD) to target during correct trials (control and test) by age

Item Age Diverge Significant*

Past control 6 and 7 200 ms 500 msPresent control 6 and 7 300 ms 500 msPast tense –ed 6 and 7 200–300 ms 400 ms3rd person present –s 6 only 400 ms 600 ms (from TC)

1200 ms (from AC)7 only 200–300 ms 400 ms (from TC)

600 ms (from AC)

Note: * p < 0.05 for both target vs. AC and target vs. TC.



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The few incorrect control and –ed trials patterned like the 7-year-olds’ incor-rect –s trials, rather than the 6-year-olds’ incorrect –s trials. That is, the looking patterns for correct trials reflected incremental interpretation, and for incorrect trials reflected errors, likely due to occasional inattention. Thus, the 6-year-olds’ incorrect –s trials stand out from the other trials, and suggest that while 6-year-olds are sensitive to –s, it remains a weak cue at this age.

GENERAL DISCUSSION

The present set of experiments investigated how 6- and 7-year-old SAE-speaking children interpret 3rd person present –s as a cue to tense. We compared the children’s performance on –s to control sentences containing overt temporal words, as well as past tense –ed. Because –s encodes tense, aspect, and number simul taneously, it is quite opaque in its form-to-meaning mapping. Cross-linguistic evidence suggests that this makes –s a likely candidate for late acquisition. Moreover, because –s corresponds to these multiple meanings, it may not be a very reliable, and hence strong, cue. Thus, while 6- and 7-year-olds should successfully comprehend –ed and lexical cues to tense, this may not be true for –s.

This was indeed the case: offline performance (Experiment 1) showed that while both the 6- and 7-year-olds successfully comprehended –ed and lexical cues to tense, only the 7-year-olds, but not the 6-year-olds, successfully comprehended –s. This replicates previous findings that 6-year-olds do not reliably use –s as a cue to tense/aspect (de Villiers & Johnson, 2007), but also expands on this line of work by showing that 7-year-olds do reliably use –s. However, this offline measure cannot clarify why the 6-year-olds do not seem to interpret –s. Is the global pattern of responses due to the 6-year-olds not understanding –s? Or are they sensitive to it, but failing to integrate its meaning into their final interpretation?

De Villiers and Johnson (2007) suggest two possibilities for children’s trouble with –s. The first possibility is that children at this age simply do not understand the tense/aspect encoded by it. Specifically, they argue that due to its use as an agreement marker it may not survive to the level of Logical Form (Chomsky, 1995). However, because even 3-year-olds already understand some of the concepts behind tense and aspect (Valian, 2006), it seem implausible that much older and linguistically advanced children should not understand these concepts, and why only –s in particular should be affected. Moreover, our online data argue against this account. In particular, the looking patterns in correct trials by the 6-year-olds showed quite early sensitivity to –s. Looks to target increased within 400 ms after the POD, and these looks became significant between 600 and 1200 ms after the POD, suggesting that the 6-year-olds are sensitive to the meaning of –s. The second possibility suggested by de Villiers and Johnson (2007) is that successful performance on these sorts of comprehension tasks is very dependent on metalinguistic abilities, which do not fully develop until a later age. By extension, this argument also applies to –ed, however, the 6-year-olds had no difficulty with this morpheme. In addition, the eye-tracking results show that the children are not engaging in a purely metalinguistic task to successfully interpret sentences




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that contain –s; they do not wait until the end of the sentence before interpreting its content.8 Rather, the children’s eye movements showed rapid and incremental processing of both verb and –s information. The overall patterns suggest that the 6-year-olds’ difficulty with –s is not due to lack of understanding, but rather due to difficulty in integrating the multiple meanings of this particular morpheme with the rest of the sentence.

Our analysis is in line with other studies that investigated the online processing of linguistic information by young children. For example, Trueswell et al. (1999) investigated the online interpretation of temporary syntactic ambiguities resulting from reduced relative clauses by 5-year-olds and adults. In that study, the children, but not the adults, showed an inability to revise an initial interpretation. Moreover, children’s offline responses were partially correlated with early looks, i.e., those prior to disambiguating information. Hurewitz et al. (2000) report a similar pattern of results for the online processing of temporarily ambiguous pre-positional phrases by 5-year-olds. Although young children are capable of rapidly interpreting and integrating a variety of linguistic cues, their commitments to an early interpretation can, at times, make them insensitive to disambiguating information that subsequently becomes available. This appears to be the case even if the disambiguating cues that signal the need to revise an incorrect interpretation are reliably produced at a much earlier age, such as –s.

There is further evidence that aspects of –s, namely agreement marking, remain a weak cue even into adulthood. For example, MacWhinney, Bates and Kliegl (1984) investigated how adult speakers of various languages make use of different types of cues. Specifically, they presented adult English-, German-, and Italian-speakers with simple transitive sentences that systematically contrasted possible linguistic cues in these languages, such as word order, agreement, animacy, and stress in order to investigate which cues listeners relied on most heavily. In line with the Competition Model, it was found that language background strongly affected cue use, such that the adult listeners made use of the most valid, hence strong, cue given their language. Thus, English-speaking adults were found to primarily rely on word order during comprehension, although agreement marking was available (MacWhinney et al., 1984). Whether adult English-speakers rely on –s as a tense marker when competing cues of different strengths are available is not clear (see also de Villiers & Johnson, 2007). Future research on this is necessary.

Situating these results within the larger literature suggests that verbal –s morphology shows different developmental trajectories in production and in comprehension. Previous research has documented that –s is reliably produced between 2;3 and 3;8 (Brown, 1973; de Villiers & de Villiers, 1973), but the various meanings –s encodes do not appear to be comprehended until much later. Speci-fically, comprehension of –s as a number agreement marker seems to develop between 5 and 6 years (Johnson et al., 2005), and the present results, coupled with those of de Villiers and Johnson (2007), indicate that comprehension of –s as a tense marker develops between 6 and 7 years. While 6-year-olds produce –s when obligatory, and appear to be sensitive to the meanings it encodes, they do not yet appear to fully utilize all the information marked by this morpheme during comprehension. Our account helps to reconcile the seemingly conflicting findings



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from production and comprehension by shifting the source of the comprehension failures to processing.

These results underscore the fact that taking production as the indicator of acquisition, while generally accurate, may miss subtle and fine distinctions in children’s developing knowledge of language and language use. For –s in parti-cular, it is not possible to distinguish the various meanings it encodes in pro-duction as these meanings are all marked by the same morpheme. Likely, other morphemes that are relatively opaque in their form-to-meaning mapping and/or a weak cue will also show successful production prior to successful comprehension. Counterintuitively, because SAE has little morphology, with multiple meanings often conflated within the same morpheme, even some seemingly simple morphemes (as measured by production) may be harder to learn than they appear. As the current study demonstrates, 3rd person present –s is such a case.

ACKNOWLEDGMENTS

We would like to thank the children, their parents, their teachers, and their schools for their participation and help with the project. We would also like to thank Amy Finn, Whitney Goodrich, Andrew Silva, James Edwards, and Kim Carter for help collecting and scoring data. Lastly, we would like to thank the two anonymous reviewers for their helpful comments and suggestions on an earlier draft of this article. This work was partially supported by a Diebold Fellowship and a University of California, Berkeley Psychology Department Research Grant awarded to TB and a Hellman Family Faculty Fund research award to CHK.

NOTES

1. Unlike Keeney and Wolfe (1972), Johnson et al. (2005) selected verbs that began with an /s/ consonant cluster that was co-articulated with the plural /s/ on the noun in normal speech, and thus masked the presence of plural –s information.

2. In the morphological sense, tense (past, present, future) refers to inflections of finite verbs. In SAE, finite verb stems are not inflected to express future; rather future events are indicated periphrastically, by adding modals (e.g., will, shall), phrasal modals (e.g., be going to), and adverbials. SAE therefore expresses only two tenses directly on main verbs, past (remote events and past time) and nonpast (present and future time as well as ‘timeless’ cases) (Celce-Murcia & Larsen-Freeman, 1999; Comrie, 1976; Valian, 2006; cf. Enç, 1996). Accordingly, –s is actually nonpast –s, although it is often referred to as present tense –s. Importantly, nonpast –s includes the present tense interpretation investigated in the current work. –s also indicates the simple (or generic) aspect, which refers to events that are conceptualized as complete wholes (Hirtle, 1967). However, here we only manipulate time, and not aspect. Thus, we use the traditional label for the morpheme – 3rd person present tense –s – keeping in mind that this may not be the most accurate description of its function.

3. De Villiers and Johnson (2007) refer to –s primarily as the generic aspect; however, the contrast they tested was quite similar to the one we test in this article.

4. –ed can also signal a past participle that can be formed by adding –en, –ed, or by a vowel change. Although –ed corresponds to a different part of speech in such cases, it corresponds to past tense there as well (Celce-Murcia & Larsen-Freeman, 1999).




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5. Due to video equipment failure, this was impossible for four 6-year-old participants. For these four participants, the scores recorded at time of test were used. However, due to the high accuracy of scoring at the time of test, we are confident that the responses recorded at test for these children are accurate.

6. Due to track loss, data for every sentence could not be included. In particular, if there was track loss throughout the region of interest (i.e., from the POD through the end of the sentence), the trial was excluded from analyses. This affected 21 out of 144 trials for the 6-year-olds and 14 out of 144 trials for the 7-year-olds.

7. Although trials could be incorrect in a number of ways (participants could choose the AC, TC, incorrect, or sleep picture), in almost all incorrect trials participants chose the AC. For this reason, graphs for the incorrect trials are not split apart by incorrect answer type (e.g., AC, TC, incorrect, sleep).

8. During correct –s trials, the 6-year-olds first had significantly more looks to the target than the AC 100 ms after the sentence ended. However, looking patterns, i.e., increased looks to the target relative to the AC, began 400 ms after the POD, in response to –s, as the sentence was still unfolding.

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APPENDIX

Item List one List two

Past control Yesterday he jogged from the store.

Yesterday he jogged from the store.

She played baseball yesterday. She played baseball yesterday.Present control Today he carries an apple. Today he carries an apple.

She eats pizza today. She eats pizza today.Past tense –ed He jogged to the store. He walked to the store.

She played baseball. He played ball.She carried the book. He carried the apple.

She walked from the library. He walked from school.3rd person present –s He runs from school. She walks from the library.

He keeps the apple. She keeps the book.He holds the ball. She holds the baseball bat.She leaves the playground. He leaves the store.She talks loudly to her teacher. He talks with his friends.She eats pizza. He eats a sandwich.

ADDRESS FOR CORRESPONDENCE

Tim BeyerUniversity of Puget Sound, Department of Psychology, 1500 North Warner,

Tacoma, Washington 98416-1046, USAE: [email protected]



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