Semantic priming supports infants’ ability to learn names of unseen objects

Elena Luchkina; Sandra Waxman

doi:10.1371/journal.pone.0321775

Abstract

Human language permits us to call to mind representations of objects, events, and ideas that we cannot witness directly, enabling us to learn about the world far beyond our immediate surroundings. When and how does this capacity emerge? To address this question, we evaluated infants at 12 and 15 months, asking whether they establish a representation of a novel noun’s meaning in the absence of any visible referents, and use this representation to identify a candidate referent when it later becomes available. During training, infants (67 12-month-olds; 67 15-month-olds) were primed with words and images of objects from a particular semantic neighborhood (e.g., fruits) and were also introduced to a novel noun (e.g., “a modi”), used to name a hidden object. During test, infants heard that noun again, this time with two unfamiliar objects present—one from the primed neighborhood (e.g., a dragon fruit) and the other from an unrelated semantic neighborhood (e.g., an ottoman). If infants can represent something about the meaning of the novel noun in the absence of a visible referent and then use such a representation when a candidate referent appears, then at test, they should prefer the object from the primed semantic neighborhood. At 15 months, infants succeeded. In contrast, 12-month-olds did not succeed on this task even after a full week of vocabulary training designed to boost the effect of priming. It is possible then that 12-month-olds’ representations of novel nouns’ meaning are not yet sufficiently rich (if any at all) to guide their choice of referent when one does appear. Together, these findings suggest that the capacity to establish a representation of a novel noun’s meaning in the absence of any visible referent and use this representation later to identify a candidate referent object emerges between 12 and 15 months.

Citation: Luchkina E, Waxman S (2025) Semantic priming supports infants’ ability to learn names of unseen objects. PLoS ONE 20(4): e0321775. https://doi.org/10.1371/journal.pone.0321775

Editor: Laura Morett, University of Missouri Columbia, UNITED STATES OF AMERICA

Received: May 16, 2024; Accepted: March 10, 2025; Published: April 23, 2025

Copyright: © 2025 Luchkina. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: Data can be found in a public repository hosted by Open Science Framework: https://osf.io/g6bvd/files/osfstorage?view_only=afa325c8a8c44c2aa63d819f14757d87. We include all study data, except participants' race/ethnicity, area of residence, family income, family size, and primary caregiver education. These data are considered personally identifying and are not approved for public sharing by the IRB. Primary caregiver education was include in our analyses (we found non-significant effects within preliminary analyses for both Experiment 1 and Experiment 2). However, requests for anonymized data can be transmitted to the Northwestern University Institutional Review Board via [sbsirb@northwestern.edu], and anonymized data may be provided to eligible parties on reasonable request and with the completion of any required prerequisites (such as a Data Use Agreement).

Funding: This research was funded by an NIH NRSA F32 Postdoctoral Fellowship awarded to Dr. Elena Luchkina (GRANT13251277) by Eunice Kennedy Shriver National Institute of Child Health and Human Development. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Language is a powerful tool for learning and communication. It allows us to learn information that is not perceptually available at the time of learning [1], such as historic figures (‘Napoleon’), hypothetical scenarios (‘I might go out tomorrow’), or abstract scientific constructs (‘subatomic force’). For toddlers hearing words for absent or unknown objects (e.g., “Mommy is outside fixing the trellis”) and physicists conversing about quantum entanglement alike, language enables conveying new information across space and time without requiring direct perceptual access to the objects or phenomena in question. Here, we investigate the developmental origins of this capacity. We ask whether infants can, in principle, represent something about the meaning of a novel noun (e.g., its semantic neighborhood, its plausible referents) in the absence of any visible referents and rely on those representations to identify those referents when they become visible. This is a crucial first step in the undoubtedly protracted developmental process of establishing, retrieving, and updating representations of phenomena based on language input alone.

Prior literature suggests that infants first begin to link familiar words to familiar perceptually unavailable items around their first birthday. For example, by 12 months, they comprehend “absent reference” – reference to objects that were previously seen but are currently hidden. When asked to locate a recently seen but now hidden object, 12-month-olds respond by pointing to its previous location, approaching that location, or looking towards it upon hearing its name [2–10]. By 14 months, infants use words to retrieve memories of past events [11] and properties of named, but currently absent, objects [12]. At first, around 12–14 months, infants rely on visual “anchors”– perceptually present reminders that scaffold their representation of the now-absent referent [13]. By 16 months, however, they comprehend absent reference in the absence of anchors [14], can update their existing representations based on language [15], and rely on category knowledge (e.g., the object is an apple) to interpret requests for unseen objects [16].

Although infants’ comprehension of absent reference for familiar words emerges around 12 months, it is not until 19–24 months that they successfully form a mental representation of the meaning of a novel word, introduced in absence of any visible referent [17–21]. For example, Ferguson et al. (2014; [22]) presented 15- and 19-month-olds with novel nouns that were arguments of known verbs, either in animacy-selecting (“The dax is crying”) or in animacy-neutral frames (“The dax is right here”) in the absence of any visible referent. When the novel word was presented in animacy-selecting frames, 19-month-olds looked to an animate object when it later became visible. Fifteen-month-olds in the same paradigm did not show this preference. This pattern presents a puzzle. If 15- to 16-month-old infants can retrieve and update an existing mental representation of familiar word referents based on language alone, then why do they not infer the absent referent of a novel word when they have been provided with semantic cues?

One possibility is that when presented with semantic cues to meaning, but no visible referent of the novel word during the naming episode, 15-month-olds are not able to establish a mental representation, however sparse, of its possible referent due to representational constraints [23]. This seems unlikely; even younger infants can infer the presence of a hidden object when provided with linguistic and contextual cues [24]. Another possibility is that infants’ relatively sparse vocabularies limit their success in using linguistic cues to infer the meaning of the novel word. That is, perhaps because 15-month-olds comprehend so few verbs (M=4.3; MacArthur Level II Short Form), they struggled to take advantage of the animacy-selecting known verb presented in Ferguson’s task. Thus, it remains an open question when infants first succeed in mapping a novel word onto a representation that is sufficiently robust to permit them to identify a referent object when it later becomes available.

To address this question, we introduced a new experimental paradigm that (1) uses ostensive noun labeling to help infants infer the presence of an unseen object and (2) relies on semantic priming to convey something about candidate referents of that novel noun. In this paradigm we build on infants’ relatively robust existing lexicon of nouns, their sensitivity to ostension [25–27], and their emerging sensitivity to semantic neighborhoods [28,29]. Prior literature documents that by 12 months, when shown images of two objects (e.g., “sock” and “juice”) and prompted to look at an object not depicted (e.g., “foot”), infants look more to the image that is semantically related to the prompt (e.g., “sock”; [30]). The literature also documents that by 12 months, infants successfully form object categories, especially in the context of naming events, and extend these names to novel exemplars of the same categories [30–33]. Grounded in this evidence, we leverage infants’ conceptual capacities to invoke semantic neighborhoods.

Prior to conducting Experiments 1 and 2, we obtained pilot data from 18 infants in the Semantic Priming condition (described in detail in Experiment 1), using a Tobii X60 corneal reflection eyetracker. These infants revealed a significant looking preference for the target object at test, M=68.43%, SD=13.99%, t(17)=5.87, p <.001, and a reliable increase in their looking to the target object each time the target novel word (one taught during training) was mentioned. These data provided assurances that the paradigm is well-suited for testing 15-month-olds’ ability to represent something about the word’s meaning, in the absence of a referent, and to use that representation when a candidate referent of the novel word appears.

Deviations from stage-1 registered report

In the Stage-1 manuscript, the Switch Word condition was referred to as “Follow-up Control” condition. We chose a more informative label “Switch Word” condition for the Stage-2 submission.
The Stage-1 manuscript stated a planned cutoff of 15% of exposure to languages other than English. However, Lookit [34], which we used for data collection due to Covid-19 pandemic, discourages exclusions based on language exposure. As a result, we tested infants who varied widely in their exposure to languages other than English. Therefore, we relaxed the language exposure cutoff to 20%; this permitted us to keep the final sample size reasonably close to the planned one (N=72; determined by power analysis),
The Stage-1 manuscript stated that human coders would manually annotate infants’ looking behavior if data were to be collected on Lookit. However, the advancement of automated gaze annotation technologies enabled us to save time and effort. Prior to implementing iCatcher+ [35] for gaze annotation, we estimated the agreement between iCatcher+ and human coders in our lab using an independent sample of N=48 12-month-olds. The average confidence of iCatcher+ was 95%. The agreement between iCatcher+ and human coders was 88%, excluding track loss (see Appendix 4). This excellent agreement, which aligns well with that reported by the developer team and with intercoder agreement for trained human coders on this experimental task in our lab (90%), was high enough to warrant adopting iCatcher+ for coding the full sample.
The originally planned window of analysis, 0–6000 ms, was based on Ferguson et al. (2014). We had initially planned to implement the same time window, but subsequently realized that the current design was closer to other word-learning tasks that involve ostensive labeling. To clarify, in Ferguson et al., to respond to the audio prompt, infants had to recall the sentence in which the name of the unfamiliar object had previously been embedded (e.g., “the dax is crying”), and use that sentence to infer the meaning of the novel noun. This incurred considerable processing time: differences between conditions in that design began to diverge around 3250 ms. In contrast, in the current design, as in most word-learning tasks used for this age, infants did not need to rely on sentence processing. Instead, they could infer the referent of the novel word from the semantic neighborhood within which it was presented. We therefore selected the window of analysis—367–2000 ms after the target word onset—that is used in most infant word learning designs (see [36]; we provide additional citations in the text). This window is based on the time it typically takes infants to process a word and initiate a gaze shift. We analyzed looking preference for each of the two mentions of the target word, averaging across them to calculate the mean percent of looking to the target for each infant on each trial. This change does not affect the direction or the significance of our results. The effect size is smaller with the original window than with the new window.
The Stage-1 manuscript stated that the minimal amount of looking to the screen during test trials would be 16.67% of the entire duration of the trial (1 s out of 6 s of image display). However, here we report a more conservative cutoff of 50% (3 s out of 6 s of total image display). We did so to accommodate the new window of analysis—367–2000 ms from the onset of each mention of the target word. If we used 1 s as our cutoff and if that 1 s occurred between the two mentions of the target word, which were about 3 s apart, then no data would be available for analyses.
No Baseline assessment for Experiment 2 was included in the plan outlined in Stage-1 manuscript. However, we reasoned that this baseline measure was essential for evaluating the effect of vocabulary training.
In the Stage-1 manuscript, we stated that the plan was to ask caregivers for 14 days of picture book reading. This requirement, however, proved too demanding; Lookit requested that we shorten the vocabulary training to 7 days. In addition, to ease caregivers’ burden, we did not ask them to report their infants’ engagement during each reading session (also planned in Stage-1 manuscript).

Experiment 1

There is ample evidence that 15-month-olds successfully retrieve representations of recently seen hidden objects upon hearing their names [37,38]. Yet, to our knowledge, there is no evidence that they build a representation for the meaning of a novel word, with no referent in sight, and use that representation to identify a candidate referent when one becomes visible. Experiment 1 was designed to test this capacity in 15-month-old infants.

Method

Participants.

Participants were 67 infants (N=23 in the Semantic Priming, N=23 in the Switch Word, and N=21 in the No Priming condition) recruited on Lookit, an online platform for developmental research. We selected Lookit because it permitted us to collect data during the Covid-19 pandemic. Thirty-one additional participants were excluded from the final sample due to technical failure (N=10), not meeting study criteria (e.g., more than 20% of exposure to languages other than English; N=17), missing MCDI or language exposure data (N=3), and data withdrawal (N=1). Caregivers completed a Qualtrics survey containing a MacArthur Short Form Vocabulary Checklist: Level II, Form A [39], augmented with words used during familiarization and with questions about infants’ exposure to languages other than English (see Appendix 1). Caregivers also completed a standard Lookit demographic survey (see Appendix 2).

All participants, regardless of their inclusion in the final sample, received a $5 Amazon.com gift card. Survey information for the final sample is summarized in Table 1.

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Table 1. Demographic and vocabulary survey summary, Experiments 1 and 2.

https://doi.org/10.1371/journal.pone.0321775.t001

Timeline.

Data for Experiment 1 were collected between 17/11/2020 and 13/11/2021

Apparatus.

Caregivers logged into Lookit on their desktop or laptop computers to access the study and completed a consent process by video-recording their verbal consent with a web camera. Caregivers were then asked to hold their infants about 50–60 cm from their monitor, positioning them over the shoulder, to ensure that only the infants’ (and not their own) eye movements would be recorded via the web camera. First, infants participated in a calibration phase during which a colorful spinning abstract shape moved around the monitor screen, with each movement accompanied by a chime sound. Then the experiment proper began. All stimuli were presented on caregiver’s desktop or laptop computers. Infants’ gaze during the study was recorded with a web camera, similar to how it would be recorded in a lab by a camera located underneath or above the screen displaying experimental stimuli. Most videos were recorded at 30 HZ per second; all were resampled to ensure the same rate of 30 Hz per second. Video recordings were stored on Lookit’s servers.

Stimuli.

Infants viewed four video-taped vignettes, each featuring a novel word-object pairing. Each video began with an actor pointing to three familiar objects, which appeared behind her, but were visible to the infant. She named each familiar object with its basic-level count noun (e.g., “apple”, “orange”, “banana”). Next, the actor turned her head to look toward another object”), also behind her but this time not visible to the infant (it was obscured by her body), and named it (e.g., “modi; see Fig 1 for a representation of a single trial and Appendix 3 for a complete list of stimuli). All stimuli are available on the Open Science Framework: https://osf.io/g6bvd/?view_only=afa325c8a8c44c2aa63d819f14757d87.

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Fig 1. A representative example visual* and auditory information presented during the Priming and Test phases in the Semantic Priming, Switch Word, and No Priming conditions.

Pointing and ostension cues were provided to support an inference that the last episode of the Priming phase, in which the object is hidden, was a labeling episode as well. *Source of images in depicted here: https://unsplash.com. Actual stimuli may differ (all stimuli are available on OSF).

https://doi.org/10.1371/journal.pone.0321775.g001

Stimulus selection and design.

We selected visual and linguistic stimuli based on vocabulary norms [40], focusing on the four semantic neighborhoods—fruits, clothing, vehicles, and animals – that include nouns that most 15-month-olds (at least 60%) comprehend. In the Priming phase, infants viewed images of familiar objects from these semantic neighborhoods: apple, banana, orange (fruits), jacket, sock, hat (clothing), truck, car, bus (vehicles), cat, dog, horse (animals). In the test phase, infants viewed images of two unfamiliar objects for which most 15-month-olds do not have a name. One (the target object) was a member of the same semantic neighborhood as objects presented during Priming. These included a Gursky’s spectral tarsier (animals), a doctoral gown (clothing), a drone car (vehicles), and a dragon fruit (fruits). The distractor object was an unfamiliar member of a different semantic neighborhood, unrelated to objects presented during Priming, including a coil of rope (tools), a spatula (utensils), a primula coffee maker (vessels), and an ottoman (furniture). All images were approximately the same in size, resolution, and their levels of contrast and saturation.

Procedure.

Experimental sessions were conducted at participants’ homes at the time of their choosing. The procedure, conducted entirely on Lookit, was ‘asynchronous’—with no experimenter present. Before the task began, caregivers viewed instructions displayed on the screen and provided verbal consent; their consent was video recorded and then reviewed by trained research assistants.

Each infant completed 4 trials (within-subject), corresponding to the four semantic neighborhoods, each including a Priming and a Test phase (Fig 1). Infants were assigned randomly to conditions (between subjects). The target image appeared on alternating sides across four trials (e.g., left-right-left-right or right-left-right-left). We created 4 possible sequences of trials using a Latin square design. Infants were assigned randomly to one of these sequences. We implemented a between-subjects design to avoid potential carry-over effects, which could dilute the predicted advantage of semantic priming. In the Semantic Priming and Switch Word conditions, all Priming phase objects were members of the same semantic neighborhood; what varied by condition was the word presented at test. In the No Priming condition, priming objects were members of different semantic neighborhoods; the test phase was identical to that in the Semantic Priming condition.

Semantic priming condition: Priming (duration: 34–37 s; M_{across trials}=35.5 s): An actor pointed to and named three familiar objects, all from the same semantic neighborhood. These appeared one at a time, behind an actor who turned to point to each object as it appeared and name it with its familiar basic-level category name (e.g., “Look! An apple! Do you see the apple?”). The fourth object was entirely occluded by the actor’s body (seemingly unintentionally) and thus hidden from infants’ view. As in the first three trials, the actor turned, pointed, and produced a novel noun (e.g., “Look, a modi!”). The actor alternated her gaze between the camera (to emulate eye contact with the infant) and the space behind her, to indicate that the referent of the novel noun was behind her.

Test (duration: 10 s): Infants first saw an attention getter (2 s), followed by a blank white screen (2 s), during which they were prompted to look to the object corresponding to the novel label, e.g., “Now look! Where is the modi?”. Next, two unfamiliar objects appeared side-by-side—one a member of the primed semantic neighborhood and the other a member of a different semantic neighborhood. With the test objects visible, infants heard a second verbal prompt, e.g., “Can you find the modi?”.

Switch word condition: The procedure was identical to the Semantic Priming condition with one crucial exception: during Test, infants heard a different novel noun than the one introduced in Priming. This condition was designed to assess whether infants prefer the target object based on semantic priming alone, independent of the novel noun presented during Priming to name the hidden referent.

No priming condition: The procedure was identical to the Semantic Priming condition with one exception: during Priming, the three familiar word-object pairs were each selected from different semantic neighborhoods (e.g., a hat, truck, and horse) and unrelated to the test objects. The control condition was designed to assess side biases or intrinsic preferences among the test pairs.

We designed the Switch Word and No Priming conditions to maximize their similarity to the Semantic Priming condition. This was done to isolate the effects of our manipulation from any asymmetries in exposure to audio and visual information or differences in attentional depletion. For example, in the Switch Word condition, presenting infants with test images in silence instead of using an unfamiliar novel noun could also assess whether semantic priming alone explains infants’ looking preference. In the No Priming condition, instead of labeling three semantically distant objects, we could have omitted the priming phase entirely. However, the patterns of looking across conditions would have been incomparable because infants in the Semantic Priming condition would have been exposed to more information and required to distribute their attention differently than in the other two conditions.

Data preparation.

Infants’ eye-gaze was coded frame-by-frame (1 frame=33.33 ms) by iCatcher+ software. We included only those trials on which infants looked to the screen at least 80% during training and at least 50% of the time during test trials. This yielded 158 trials with each participant contributing on average 2.9 trials out 4. For each trial and each infant, we calculated two dependent measures:

Mean %LT to the target. We calculated the mean percent of looking time devoted to the target [looking to target object/(looking to target + distractor)] during the window of analysis. That window was the average of the percent of looking time devoted to the target object across two segments 367–2000 ms after the target word onset—for each of two mentions of the target word. Hereafter, “target” and “target object” refers to the object defined as the target one in the Semantic Priming condition. Although the same pairs of objects were used in test trials in all three conditions, in the Switch Word and No Priming conditions neither object could truly be considered a target.
Time course. We aggregated looking data into 200-ms bins throughout the duration of image display and calculated the percent looking to the target during each bin. This permitted us to assess the time course of infants’ looking to the target and the distractor and identify divergencies among looking trajectories across different conditions. Cluster-based permutation tests were conducted with 1000 samples for each 200-ms bin.

Planned analyses (based on Stage-1 manuscript).

We assessed the effect of Condition on the mean %LT to the target and the time course of infants’ %LT for each 200-ms bin.

1. Mean %LT to the target We used a Generalized Linear Mixed Model (GLMM) with Condition as a fixed factor and Test Item (fruits, clothing, vehicles, or animals) and Participant as random factors (grouping variables) for the intercept. This helped us reduce Type-1 error by including a grouping variable that prevented inflation of the degrees of freedom [41] and accounted for any differences in infants’ knowledge across the four semantic neighborhoods.

For each model, we tested the residuals for normality using a Shapiro-Wilk test to ensure accurate inferences based on p-values in estimating the model coefficients. Unless explicitly stated otherwise, the distribution of residuals of our models did not deviate significantly from normal. Unless otherwise specified, Semantic Priming was the reference level for Condition (to contrast it with the other conditions).

2. Time course. We used a cluster-based permutation analysis [42] to identify any significant clusters of divergence among conditions following the mention of the target word. The threshold was established by a t-value of 1.69 for each bin, which corresponds to the alpha=.05 for an independent-sample one-tailed t-test with 36 degrees of freedom (the number of participants per group was lower than the final sample due to additional exclusions—see above).

The effects of demographic factors—age, gender, and mother’s education—vocabulary scores, and trial order were tested in a preliminary analysis, implemented by fitting a GLM to the DVs. An additional set of preliminary analyses evaluated the effects of infants’ knowledge of the words used in priming (based on caregivers’ vocabulary survey). Trial order was included to test whether infants’ looking preferences change over the duration of the procedure. In subsequent analyses, we included only those factors that yielded significant effects in the preliminary analysis. Doing so reduces the chance of multicollinearity (which increases with the number of multilevel categorical predictors included) or an overly specified model (that cannot be supported by the data given our limited sample size N=67).

Predictions.

By 15 months, infants infer the presence of a hidden object by following a speaker’s line of regard [43], understand that words communicate about mental states [44–46], appreciate referential cues [47], and leverage category membership to extend the meanings of novel nouns. Based on this evidence, we made the following predictions:

1. Mean % LT to the target.

Semantic priming condition: We predicted that the mean %LT to the target would be significantly greater than chance (50%) and higher than in the Switch Word and No Priming conditions.

Switch word condition: If infants’ looking in the Semantic Priming condition was driven by semantic priming only, unrelated to the novel noun, then the mean %LT in the Switch Word condition should not differ from that in the Semantic Priming condition. Conversely, if infants’ looking in the Semantic Priming condition reflected the representation they had formed for the meaning of the novel noun presented in Priming then their mean %LT in the Switch Word condition should be significantly lower and not different from chance. Although the principle of mutual exclusivity might predict that infants in the Switch condition would interpret the new word as referring to the distractor object, we note here that evidence concerning mutual-exclusivity inferences in infants younger than 18 months is mixed (e.g., [48] vs. [49]).

No priming condition: If infants have no intrinsic preferences for either of the test objects, then their mean %LT to the target should not deviate from chance.

2. Time course.

We predicted that in the Semantic Priming condition, if infants’ looking reflected their inferences about candidate meanings of the novel noun, they would increase their %LT to the target object after each mention of the target word (as observed in the pilot). We also predicted that their looking trajectory would significantly diverge from the looking trajectories in the Switch Word and No Priming conditions. Alternatively, if infants’ looking in the Semantic Priming condition was driven by semantic priming alone and was unrelated to the novel noun, their looking would be unrelated to the verbal prompts and they would prefer the target object consistently throughout the trial. In this latter case, we would expect the same pattern of results in the Switch Word condition and no significant divergencies in looking trajectories between the Switch Word and Semantic Priming conditions. We predicted that in the No Priming condition, the %LT to the target would not be affected by the verbal prompts and remain close the chance level throughout the trial. If we observe any consistent looking to either test object in the No Priming condition, it would suggest intrinsic looking preferences.

Results and discussion

The results provide new evidence that 15-month-olds can build a representation of the meaning of a novel noun while its referent is hidden and can subsequently draw on this representation to identify a candidate referent when it becomes visible.

Results of planned analyses

1. Mean % LT to the target.

First, we analyzed infants’ visual attention to the screen at test. Infants contributed approximately equivalent amount of looking data on each of four test trials with no significant decline in their visual attention over the duration of the experimental procedure. Preliminary analyses, conducted for each condition independently (with all predictors entered at once), revealed no significant effects of primary caregivers’ education level, or infants’ age, gender, comprehension of familiar words introduced in priming, vocabulary (MCDI), or trial order on the mean %LT to the target object in any condition. These demographic and vocabulary measures were not included in subsequent analyses.

To evaluate the effect of Condition, we fitted a GLMM to the mean %LT. We offset the mean %LT by 50% (chance level) to improve interpretability of the effect of the intercept (β_0, the grand mean when all predictors are set to 0). The effect of the intercept was significant, β=0.18, SE=0.05, t=4.0, p=.003, indicating that infants in the Semantic Priming condition (which was the reference condition) preferred to look to the target. Moreover, their mean %LT to target was significantly higher in the Semantic Priming condition than in either the Switch Word condition, β=-0.12, SE=0.05, t=-2.5, p=.01, and No Priming condition, β=-0.18, SE=0.05, t=-3.4, p=.001 (Table 2).

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Table 2. Results of mixed-effect linear model fitting with Condition as a fixed factor and Test Item and Participant as random factors for the intercept.

https://doi.org/10.1371/journal.pone.0321775.t002

An additional GLMM with No Priming as a reference level for Condition showed no significant difference between the Switch Word and No Priming conditions, β=0.06, SE=0.05, t=1.23, p=.22. The effect of the intercept was also not significant, β=-0.01, SE=0.04, t=-0.11, p=.91, which indicates that infants had no looking preference in this condition. A GLMM with Switch Word as the reference level for condition revealed no significant effect of the intercept either, β=0.06, SE=0.04, t=-1.76, p=.21, which indicates no looking preference in the Switch Word condition either (see Fig 2).

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Fig 2. Mean %LT to the target across two windows of analysis (after each mention of the target word), by Condition.

https://doi.org/10.1371/journal.pone.0321775.g002

2. Time course.

As predicted, infants’ looking trajectories in the Semantic Priming and Switch Word conditions significantly diverged after the first mention of the target word, at 200–1200 ms, sum t-statistic=14.263, p=.026. Infants’ looking trajectories in the Semantic Priming and No Priming conditions significantly diverged after the second mention of the target word, at 3400–4200 ms, sum t-statistic=9.74, p=.04 (Fig 3).

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Fig 3. Time course of the %LT over the course of the trial for infants in each Condition.

Windows of analysis (367-2000 ms after each target word onset) are shaded grey. The onset of image display coincides with the offset of the target word.

https://doi.org/10.1371/journal.pone.0321775.g003

In the Semantic Priming condition (M=68.22%), the experimental data aligned with pilot data (M=68.43%) for this condition. Time course analyses revealed no significant clusters of divergence among looking trajectories obtained from Lookit and a Tobii automated eyetracker (Fig 4). This provides assurances of replicability of results obtained using an eyetracker, and online, using participants’ web cameras.

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Fig 4. Time course of infants’ looking during the trial in the Semantic Priming Condition obtained in lab (Tobii X60) and on Lookit.

Windows of analysis are shaded grey.

https://doi.org/10.1371/journal.pone.0321775.g004

Discussion.

We reasoned that if 15-month-olds could establish a representation, however sparse, for the meaning of the novel noun in the absence of a referent and use it to identify a candidate referent later, when one becomes visible, then infants in the Semantic Priming condition (but in neither of the control conditions) would prefer to look to the target test object. Our results are consistent with this prediction. The absence of looking preferences in either of the two control conditions—No Priming and Switch Word—suggests that infants’ success in the Semantic Priming condition cannot be attributed to semantic priming alone or to any intrinsic preferences or processing biases (e.g., object size, animacy; see [50,51]). Instead, their success reveals an ability to represent something about meaning of a novel word while the referent is hidden and subsequently draw on this link and recognize that referent when it becomes visible.

Experiment 2

The goal of Experiment 2 was to assess whether 12-month-olds, like 15-month-olds, can succeed on our task. There is reason to suspect that 12-month-olds’ comprehension of absent reference, coupled with their capacity to link words to objects and object categories [52], may succeed on this task. Notice, however, that for the semantic priming manipulation to work, infants must comprehend the words introduced during priming for each semantic neighborhood. This presents a challenge: on average, only 30% of 12-month-olds meet this requirement [53]. We therefore introduced a 7-day vocabulary training period (described below), designed to bolster 12-month-olds’ vocabularies, and measured their performance on experimental task of Experiment 1 both before and after vocabulary training.