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Toddlers activate lexical semantic knowledge in the absence of visual referents: evidence from auditory primingInfancy, 1–23, 2013Copyright International Society on Infant Studies (ISIS)ISSN: 1525-0008 print / 1532-7078 onlineDOI: 10.1111/infa.12026 Toddlers Activate Lexical Semantic Knowledge in the Absence of Visual Referents: Evidence from Auditory Jon A. Willits, Erica H. Wojcik, Mark S. Seidenberg, and Jenny R. Saﬀran Department of Psychology University of Wisconsin-Madison Language learners rapidly acquire extensive semantic knowledge, but thedevelopment of this knowledge is diﬃcult to study, in part because it isdiﬃcult to assess young children's lexical semantic representations. In ourstudies, we solved this problem by investigating lexical semantic knowledgein 24-month-olds using the Head-turn Preference Procedure. In Experiment1, looking times to a repeating spoken word stimulus (e.g., kitty-kitty-kitty)were shorter for trials preceded by a semantically related word (e.g., dog-dog-dog) than trials preceded by an unrelated word (e.g., juice-juice-juice).
Experiment 2 yielded similar results using a method in which pairs of wordswere presented on the same trial. The studies provide evidence that youngchildren activate of lexical semantic knowledge, and critically, that they doso in the absence of visual referents or sentence contexts. Auditory lexicalpriming is a promising technique for studying the development and structureof semantic knowledge in young children.
Understanding the structure and origins of semantic knowledge is one ofthe central problems in the study of cognition. Researchers have madeconsiderable progress toward characterizing semantic knowledge structure Correspondence should be sent to Jon A. Willits, Department of Psychology, University of Wisconsin-Madison, 1202 W. Johnson St., Madison, WI 53706. E-mail: email@example.com WILLITS ET AL.
and its brain bases (see, e.g., Bloom, 2000; Jackendoﬀ, 2010; Mandler,2000; Martin, 2007; Rogers & McClelland, 2004). However, the origins ofthis knowledge and how it develops remain controversial. Early investiga-tions used methods that involved explicitly querying children about theirknowledge of word meanings (e.g., Gelman & Wellman, 1991; Keil, 1983),and these methods have been useful in understanding young children'sexplicit semantic knowledge. Only recently, however, have researchersbegun to use implicit methods to examine early lexical semantic knowledgein younger children (e.g., Meints, Plunkett, & Harris, 1999; Styles &Plunkett, 2009). In this article, we report two studies using an auditoryparadigm adapted from adult lexical priming studies (e.g., Meyer &Schvaneveldt, 1971; Moss, Ostrin, Tyler, & Marslen-Wilson, 1995) toexamine the semantic knowledge of very young children, in the absence ofimages of the named referents or useful sentence contexts. Together, thesestudies test whether early lexical knowledge includes an understanding ofthe relationship between the meanings of various words, and whether by2 years of age, this knowledge is activated in the absence of visual refer-ents and even when single words are uttered in isolation.
Our research built on a recent study using implicit measures to explore infants' sensitivity to lexical semantic information. Arias-Trejo and Plunk-ett (2009) used an intermodal preferential looking paradigm (IPL) toexamine 18- and 21-month-olds' responses to related prime–target pairssuch as cat-dog, compared with unrelated pairs such as plate-dog. Arias-Trejo and Plunkett chose their related pairs to be both strongly associated(according to adult associative norms) and highly imageable. Infants ﬁrstheard a phrase such as "I saw a cat," followed by a target word ("dog").
They then concurrently saw two images, one related (e.g., a dog) and oneunrelated (e.g., a door). The dependent measure was looking time to thepictures. Their primary manipulation was whether the initial phrase con-tained a prime word related to the target word and picture (e.g., "I saw acat…dog") or an unrelated word (e.g., "I saw a plate…dog"). Eighteen-month-olds looked signiﬁcantly longer to the picture named by the targetword (dog), regardless of whether it had followed a related or an unrelatedprime phrase. In contrast, 21-month-olds looked signiﬁcantly longer to thenamed picture in the related prime–target condition ("I saw a cat…dog")but not in the neutral prime–target condition ("I saw a plate….dog").
Styles and Plunkett (2009) used the same paradigm, again with word pairsthat were highly associated based on adult norms, and found a primingeﬀect in 24-month-olds, but not 18-month-olds. The older children's sensi-tivity to the semantic relatedness of the previous sentence providesevidence for lexical semantic organization in infants, and the age-relateddiﬀerences suggest that semantic relationships between words are learned TODDLERS ACTIVATE LEXICAL SEMANTIC KNOWLEDGE between 18 and 21 months of age (see Mani & Plunkett, 2010, for relatedevidence concerning phonological priming). The studies by Arias-Trejo,Plunkett, and colleagues demonstrate that by 2 years of age, children areacquiring semantic knowledge that involves similarity structure and thatthis similarity structure leads to behavioral consequences, such as aﬀectingeye movements to related visual images.
However, these studies also leave open a number of interesting questions.
How strong, rich, and accessible are children's semantic similarity represen-tations, and how much context do children need to retrieve and use theserepresentations? It is well established that the richer the cue, the easier it isto activate a representation (Craik & Lockhart, 1972; Fernald & Hurado,2006). In Arias-Trejo and Plunkett's study, the words' retrieval cues werequite rich, including an enriching sentence context for the prime and a visualimage of the target. How critical were the visual images to the activation ofthat target word? Because the IPL method involves hearing language,attending to a visual stimulus, and responding to the visual stimulus as aresult of the linguistic input, the results from this paradigm do not mediatebetween a number of diﬀerent theories that underlie semantic memory andsemantic priming (see Hutchison, 2003; for a review). For example, the chil-dren could have been demonstrating word–word semantic priming; alterna-tively, children could also have been making a similarity judgment betweenthe auditory prime and a combined cue of the auditory target and the visualimage of the target. In a follow-up experiment, Arias-Trejo and Plunkettshowed that there is no priming eﬀect if there is no auditory presentation ofthe target word. For example, if children heard the sentence, "I saw a cat…look!", they did not show a diﬀerence in looking time between a subse-quently presented related picture (a dog) and an unrelated picture (a plate).
This suggests that hearing the target word was necessary for activating thesemantic relationship between the prime word's concept and the visual tar-get's concept. However, they did not test a condition where children heardthe target word but did not see the picture (as this would be impossible inthe IPL paradigm, which relies on children's looks to visual targets). Thus, itis unclear whether 21-month-olds only need to hear the target word to deter-mine its similarity to the prime, or if they need a richer, audiovisual cue.
Equally of interest, how important was the prime's sentence context for activating a complete representation of the prime? Some of the verbs inArias-Trejo and Plunkett's carrier phrases included a considerable amountof semantic information; some trials presented primes like "I ate abiscuit", followed by the targets "cheese" and "chair". Even the more neu-tral verbs from the study, like buy and see, are probably not truly neutral,as the aﬀordances of those verbs to diﬀerent nouns will vary. Someobjects are more likely to be bought than others, a fact that children may WILLITS ET AL.
know conceptually or linguistically (due to word cooccurrence statistics ormore complex lexical semantic knowledge). We know that 2-year-olds acti-vate semantically related nouns when processing verbs (Fernald, 2004). Asa consequence, the exact locus of the semantic facilitation eﬀects in Arias-Trejo and Plunkett's study is ambiguous. The eﬀect could have been dueto semantic relationships between the prime and target nouns that wereeasily accessible regardless of sentence context. But, the eﬀects could alsohave been dependent on the infant having the full sentence and using it toactivate a richer representation of the noun's semantics.
The goal of our studies was to ask whether children's knowledge of the meaningful relationships between words is accessible and usable in theabsence of any additional context, including visual images and sententialcontextual support. Speciﬁcally, we were interested developing a paradigmthat more similar to adult semantic priming methodologies, to investigatewhether this semantic knowledge can be activated in the absence ofpictures of the named referents. In contrast to the previous IPL studies,the stimuli in our current experiments were decontextualized single words.
Thus, with our auditory paradigm, we can directly ask whether toddler'slexical knowledge, in the absence of other contextual cues, includes thesemantic relationships between words.
There is previous work that has examined the semantic relationships that young children activate when they hear auditory words in isolation.
Torkildsen, Syversen, Simonsen, Moen and Lindgren (2007) used evokedpotentials to examine the processing diﬀerences between semanticallyrelated and unrelated pairs of auditorily presented words to 24-month-olds(see also Friedrich & Friederici, 2005). They deﬁned relatedness in termsof membership to the same superordinate category. Torkildsen et al.
found a broadly distributed N400 eﬀect for semantically unrelated wordpairs, similar to eﬀects in adults (see Kutas & Federmeier, 2011 for areview). Unfortunately, the basis of the eﬀect is again somewhat uncleardue to the nature of their experimental design. Semantic priming experi-ments with adults use counterbalanced designs in which the same stimuluswords appear in both related and unrelated conditions, to control forother lexical factors that aﬀect performance, such as word frequency,familiarity, and imageablity (McNamara, 2005). Torkildsen et al. did notfollow this procedure and used diﬀerent words for related and unrelatedtrials (e.g., they compared the results of trials like dog-horse to trials likecar-apple, rather than comparing trials like dog-horse to trials likecar-horse). Hence, the observed eﬀects could be due to the relatednessmanipulation, but they could also be due to other properties of the stimulinested within condition (i.e., diﬀerences in the frequency or imageablity ofthe target words that happened to be in the two conditions). We chose TODDLERS ACTIVATE LEXICAL SEMANTIC KNOWLEDGE our stimuli such that each word served as its own control, participating inboth a related and unrelated pair, to address this methodological issue.
The present studies used a simple auditory paradigm, inspired by semantic priming studies, to examine semantic knowledge in 24-month-olds in the absence of related visual stimuli. The two experimentsemployed a modiﬁed version of the Head-turn Preference Procedure(HPP; Kemler Nelson et al., 1995). In Experiment 1, each trial consistedof repetitions of a single word (e.g., kitty-kitty-kitty). The word presentedon the preceding trial was either semantically related (e.g., dog-dog-dog) orunrelated (e.g., shoe-shoe-shoe). In Experiment 2, each trial consisted ofrepetitions of a pair of words that were either semantically related (dog-kitty) or unrelated (shoe-kitty). The principal question in both studies waswhether responses to the words would be modulated by the semantic relat-edness of other words, either across trials (Experiment 1) or within trials(Experiment 2). Together, the two studies allow us to perform two diﬀer-ent tests of the important theoretical question of whether there is behav-ioral evidence that toddlers activate lexical semantic knowledge in theabsence of visual referents or sentence contexts.
The studies also make an important methodological contribution by showing that the HPP can be used to adapt auditory semantic primingprocedures for use with young children. Hundreds of studies have usedHPP to test infants' and toddlers' ability to discriminate between stimulibased on both preexisting knowledge as well as what they can learn duringan experimental session. While HPP is typically used with younger infants(between 6 months and 18 months), this is largely due to the fact that ithas been employed most often to test hypotheses involving perceptualdiscrimination at those ages. Indeed, HPP has also been used with olderchildren in the 20–36 month age range to investigate toddlers' knowledgeof meaning, syntax, and the relationship between the two (H€ & Santelmann, 2006; Nazzi, Barriere, Goyet, Kresh, & Legendre, 2011;Santelmann & Jusczyk, 1998; Soderstrom & Morgan, 2007; Willits, J. A.,Lany, J., & Saﬀran, J. R., 2012, in review). HPP, unlike the IPL proce-dure, allows us to present purely auditory stimuli without relying on tod-dlers' looks to speciﬁc images as a dependent measure. Moreover, HPP isconsiderably less expensive and easier to run with young children thanEEG and other methodologies that rely on neurophysiological responses.
In Experiment 1, we adapted a classic priming method used with adultsfor use with young children. In the simplest priming procedures (such as WILLITS ET AL.
in Meyer & Schvaneveldt, 1971), participants are presented with a primeword followed by a target word and are asked to make a behavioralresponse to the target word, such as reading the word aloud or making alexical (word/nonword) decision. Other research has employed a continu-ous variant of this task in which participants make a response to eachstimulus, with semantic relatedness varied across trials (McRae & Bois-vert, 1998; Moss et al., 1995; Nation & Snowling, 1999; Shelton & Martin,1992). The semantic priming eﬀect refers to faster responses when theprime and target are semantically related (e.g., bread-cake) compared withunrelated controls (e.g., chair-cake). Priming methods have been used toinvestigate questions about the structure and processes of semantic mem-ory in adults (Ferretti, McRae, & Hatherell, 2001; McNamara, 1992;Moss et al., 1995; Neely, 1991) and older children (Nation & Snowling,1999; Plaut & Booth, 2000).
Extending this approach to toddlers and infants has clear beneﬁts for understanding the early development of semantic knowledge. Experiment1 used the HPP, and on each trial, the child listened to repetitions of ahighly familiar word. Starting with trial 2, the word the child heard on thepreceding trial was either meaningfully related (e.g., "kitty") or unrelated(e.g., "shoe") to the word on the current trial (e.g., "dog"). If by24 months, toddlers have begun to develop representations of word mean-ing that are suﬃciently speciﬁc to encode relatedness, listening timesshould diﬀer for trials on which a word was preceded by a related word,compared with trials on which the same word was preceded by an unre-lated word.
One important issue surrounding any study testing for knowledge of semantic relationships is how one deﬁnes a semantic relationship. In theadult priming literature, there is considerable debate about the diﬀerentways in which words can be related and which types of relatedness lead topriming. The types of relationships explored have typically been brokendown along two dimensions. The ﬁrst is when words' referents are similarin some way (i.e., share semantic features or belong to the same taxo-nomic category). When words that are related in these ways prime eachother, this is typically referred to as "semantic" priming (McRae & Bois-vert, 1998; Neely, 1977).1 Semantic relationships also encompass situationswhere words or their referents cooccur with high frequency, participate inthe same thematic relations, or elicit each other in word association 1This label is perhaps unfortunate, given the more general and precedential usage of "semantic" to refer to meaningful relations more broadly, or to distinguish knowledge aboutwords' meanings from knowledge about words' referential concepts (Bloomﬁeld, 1933;Osgood, 1952).
TODDLERS ACTIVATE LEXICAL SEMANTIC KNOWLEDGE norms. When words related in these ways prime each other, this is typi-cally referred to as "associative" priming (for reviews of the diﬀerencesbetween these types of priming in the adult literature, see Hutchison,2003; Lucas, 2000; and McNamara, 2005). The infant study by Arias-Trejo & Plunkett (2009) chose their pairs based on adult word associationnorms, and thus can be thought of as a study of "associative" priming,whereas the study by Torkildsen et al. chose words from the same cate-gory, and thus would be considered an example of semantic priming.
Our goal in these initial studies with 2-year-olds is not to address the diﬀerences between types of semantic relationships. Instead, we were inter-ested in determining whether young children show priming eﬀects at all, inpurely auditory single-word contexts. As such, we chose our related pairsso that they were highly related along multiple dimensions (e.g., taxo-nomic overlap, thematic relatedness, shared semantic features, high asso-ciativity) to maximize the potential for observing priming eﬀects in thisage range. The primary goal of these studies was to test for semanticeﬀects in the absence of visual imagery or sentence contexts and to estab-lish the viability of the Head-turn methodology for testing these questions.
We chose to study 24-month-olds in light of previous studies examining knowledge of relations between word meanings in young children. In theirstudies using the IPL method, Styles and Plunkett (2009) found a primingeﬀect in 24-month-olds but not 18-month-olds, and Arias-Trejo andPlunkett (2009) found a priming eﬀect in 21-month-olds but not in18-month-olds. Other research has found that 24-month-olds activateproperties (such as color) or a word's meaning when they hear that word(Johnson, McQueen, & Huettig, 2011; Swingley & Fernald, 2002). Becauseof the novelty of our method, and especially due to the removal of all con-textual cues to a word's meaning, we studied 24-month-olds in order toensure that participants had the requisite knowledge for the task.
Using the HPP methodology oﬀers one further potential advantage.
The extensive body of work using the procedure may allow us to makedirectional hypotheses about toddlers' looking behavior. Infants couldshow a familiarity bias (longer looking to familiar or related stimuli) or anovelty bias (longer looking to unfamiliar or unrelated stimuli). Hunterand Ames (1988, see also Houston-Price & Nakai, 2004) have argued thatinfants' novelty and familiarity biases in HPP will follow a predictabletrajectory as a function of their familiarity with the stimuli. If infants areextremely familiar with the stimulus (either due to long exposure times orstrong preexisting knowledge of the stimuli) they tend to show a noveltybias in the experiment. In contrast, if infants have low familiarity, theytend to show a familiarity bias. In our experiments, as we used highfrequency words that children of this age were likely to know very well WILLITS ET AL.
Stimulus characteristics for individual words in Experiments 1 and 2 (see Table 1), then under Hunter and Ames hypothesis we would predictto ﬁnd novelty eﬀects; if infants are sensitive to the semantic relationshipsbetween the words, they ought to listen longer to unexpected stimuli (e.g.,have longer listening times on unrelated trials).
Participants were 32 monolingual English-learning toddlers (16 male) witha mean age of 24 months (M = 24.3, range = 22.5–25.4). All participantswere full-term, were reported to have normal vision and hearing, and werefrom households with a minimum amount of exposure to non-Englishlanguages (<4 h/week of exposure to another language). One additionaltoddler was unable to sit through the task and was excluded from theanalyses.
Stimuli and design While adult priming studies beneﬁt from the use of word associationnorms in choosing appropriate stimuli, there is no equivalent database foryoung children. Thus, we relied on Dale and Fenson's (1996) MCDIlexical development norms to choose words that at least 75% of 24-month-olds are reported to understand. We further constrained our choiceof stimuli such that the words could be arranged into highly related wordpairs. Within each pair, the words were related along a number of diﬀer-ent dimensions, including shared taxonomic category membership; similarthematic relations; high semantic feature overlap in normative evaluations TODDLERS ACTIVATE LEXICAL SEMANTIC KNOWLEDGE of semantic features (McRae, de Sa, & Seidenberg, 1997); and often highassociativity in normative measures of word association in adults (Nelson,McEvoy, & Schreiber, 1999).
The stimuli, spoken by an adult female in an infant-directed register, consisted of eight words: dog, kitty, shoe, sock, juice, milk, mouth, andnose. We limited our study to these eight words to reduce the amount ofinteritem variance. The stimuli were also equated for volume, pitch, andthe length of trials such that these factors were not confounded with relat-edness condition. In addition, we also calculated the frequency of thewords in a corpus of child-directed speech, created by combining all sam-ples of child-directed speech from the CHILDES database (MacWhinney,2000) for children up to 24 months of age. We converted these frequenciesto percentiles, ﬁnding that all of our words were in at least the 94th per-centile (e.g., of all the words in the CHILDES corpus database, our wordswere in the top 6% in terms of word frequency). The eight items and theirstimulus properties are shown in Table 1.
The words were then organized into lists consisting of 16 trials. On each trial, one word was repeated with a 750 ms pause between repeti-tions. Half the trials were preceded by a trial containing repetitions of therelated word (e.g., dog on trial n, kitty on trial n 1), and half were preceded by repetitions of an unrelated word (e.g., dog on trial n, shoe ontrial n 1). As described above, we chose the related pairs in order to maximize relatedness among many dimensions, maximizing the chance ofﬁnding an eﬀect, resulting in the related pairs "dog-kitty," "shoe-sock,""mouth-nose," and "milk-juice." The pairings and their word associationstrengths from adult behavioral norms (in both directions, e.g. dog-kittyand kitty-dog) are shown in Table 2.
The unrelated pairs were created by pseudorandomly repairing the items for each participant, with the constraint that pairings that wouldhave resulted in thematic relationships (such as mouth-juice or kitty-nose)were not allowed as unrelated items. In addition, the word associationstrength in adult norms for unrelated pairings was always zero. Thus, forsome participants, the unrelated trial that preceded the shoe trial may havebeen dog, and for others, it may have been juice. Importantly, within eachparticipant's stimulus list, they heard each word exactly twice, once as arelated trial and once as an unrelated trial. This technique of randomlyrepairing unrelated items across diﬀerent participants is common in theadult semantic priming literature (McNamara, 2005), as it leads to a betterestimate of the actual unrelated average response time, rather than reﬂect-ing idiosyncratic relationships that may exist for particular unrelated pair-ings if the same unrelated pairing is used for every participant in theexperiment. The set of related stimulus pairings, as well as their word WILLITS ET AL.
Association strength of related pairings in Experiments 1 and 2 Association strength Note. aThe association strength is the proportion of adult participants who, when given the ﬁrst word as a cue, generated the second word as an association (Nelson et al., 1999).
The table shows the association strength for the related pairings used in the experiment. Theassociation strength of all unrelated pairings used in both experiments (e.g. dog-juice) wasalways zero. The listed association strengths for dog and kitty are actually those for dog andcat (dog and kitty were not normed in the Nelson et al. study).
association strengths in the related and unrelated conditions, are shown inTable 2.
We created diﬀerent pseudorandomized experimental lists for the 32 participants, counterbalancing three factors: (1) each participant heardeach word in both a related and unrelated context, with the order ofwhich condition they heard ﬁrst counterbalanced across participants; (2)to control for asymmetric association eﬀects (e.g., shoe given sock has ahigher association strength than sock given shoe), half the participantsheard the pairs in one order (e.g., sock-shoe) and the other half heardthe reverse order (shoe-sock); (3) all pairs' side of presentation was coun-terbalanced; for example, one participant's shoe and sock trials wereboth presented from the left side, a second heard both from the right, athird participant heard shoe from the left and sock from the right, and afourth heard shoe from the right and sock from the left. An examplestimulus list for the experiment is shown in the Appendix, but it isimportant to note that this is a single example stimulus list, and manyothers lists were used to pseudorandomize the presentation order of theitems.
Toddlers were seated on a caregiver's lap in a sound-attenuated booth;the caregiver wore blacked-out sunglasses and listened to music overheadphones. The neutral visual stimuli were presented on three computer TODDLERS ACTIVATE LEXICAL SEMANTIC KNOWLEDGE monitors positioned at the infant's head-level approximately three feetaway. One monitor was placed directly in front of the infant, and theother two were placed 90º to the infant's left and right. The auditory stim-uli were presented on wall-mounted speakers directly below the threemonitors. Presentation of stimuli and collection of the experimenter'sbutton presses were controlled by HABIT software (Cohen, Atkinson, &Chaput, 2000).
Each trial began with a central attention-getting stimulus (a scene of a balloon and clouds) paired with music playing from the center speaker.
Once the infant oriented to the center, the experimental stimulus began toplay on either the left or right side. This stimulus consisted of a neutralvisual stimulus (a spinning pinwheel, used on every trial) and one of theeight spoken target words repeated with 750 ms of silence between repeti-tions. The word was repeated until the infant looked away for more than2-sec, or for a total of 15 sec, whichever came ﬁrst. The experimenter out-side the booth, who was blind to the stimulus presented on each trial, usedbutton presses to keep track of the infant's looking behavior. The depen-dent measure on each trial was the total time the infant spent looking atthe monitor on the side from which the experimental auditory stimuluswas presented.
After the experiment, the child's caregiver was debriefed and given a questionnaire containing the eight target words and four ﬁller words thatwere not used in the study. They were instructed to provide a 1–7 conﬁ-dence rating for whether their child did (rating 7) or did not (rating 1)know the meaning of the words. All but two infants were at ceiling (7) forall 16 words. Nearly identical results were obtained in analyses (notreported here) that excluded data from the trials for the words that thetwo infants were reported not to know.
RESULTS AND DISCUSSION The principal data consist of mean looking times for each participanton related and unrelated trials, collapsing across items. We also com-puted mean looking times for each item for related and unrelated trials,collapsing across participants. The ﬁrst trial for each participant wasdiscarded because it was not preceded by another stimulus. Ten addi-tional trials (across seven participants) out of a total of 480 trials (15trials 9 32 participants) were excluded due to participant inattention(e.g., crying, or never looking to the stimulus side during the trial).
The results for all of the included trials are presented in Figure 1. Inline with our directional hypothesis, infants looked longer on unrelated WILLITS ET AL.
Looking Time to Related or Unrelated Words
Relationship of Trial to Previous Trial Figure 1 Mean looking times for trials that followed a related word and trials thatfollowed an unrelated word.
trials (M = 9.18 sec, SE = 0.54 sec) than on related trials (M = 8.04 sec,SE = 0.44 sec.). This eﬀect was signiﬁcant both by participants [F1(1,31) = 9.69, p < 0.01, g2 = 0.24] and by items [F2(1, 7) = 6.56, p < 0.05, g2 = 0.48]. The size of this eﬀect, both in terms of the diﬀerence scorebetween conditions (approximately 1.5 sec) as well as the statisticaleﬀect size (0.24/0.48) are in line with typical eﬀect sizes using HPP totest for infants' abilities to discriminate familiar from novel stimuli(Houston-Price & Nakai, 2004).
To completely rule out the possibility that the diﬀerences were due to acoustic factors or other factors unrelated to meaningful diﬀerencesbetween the words, a mixed-eﬀects model was used to analyze the data(Baayen, Davidson, & Bates, 2008). In this model, we used participantand target as random factors; the ﬁxed factors were relatedness, durationof the current target (in milliseconds), diﬀerence between duration of thecurrent and previous trials, side of presentation, and whether the previous TODDLERS ACTIVATE LEXICAL SEMANTIC KNOWLEDGE trial was on the same side. In this analysis, only relatedness was a signiﬁ-cant predictor of looking time (t = 2.11, p < 0.05).2 To summarize, in Experiment 1, we show that 24-month-olds exhibit a relatedness eﬀect, similar to the priming eﬀects found in adults forspoken words: looking times were shorter on trials for which the preced-ing trial contained a semantically related word, just as reaction times foradults are faster after a semantic prime in lexical decision tasks. Ourresults are also consistent with those of Arias-Trejo and Plunkett (2009)and Styles and Plunkett (2009), who found evidence for semantic organi-zation of the infant lexicon toward the end of the second year. Ourexperiments expand upon their results by showing that by 24 months,toddlers' representations of words and the similarity structure betweenthose representations (at least for highly frequent words) are easilyretrievable. The eﬀects of semantic similarity exist even when the stimuliare purely auditory words presented in isolation, without any enrichingvisual or sentential context. These results are also consistent with Tor-kildsen et al.'s (2007) EEG results, which also found that by 24 monthsof age, toddlers show diﬀerential brain activation as a function of words'semantic relatedness. Our ﬁndings further demonstrate that semanticrelationships between words have behavioral consequences in the form ofinfants' listening times to words, and thus can be measured withoutmore expensive and diﬃcult to employ methodologies such as measuringEEGs.
A question that emerges from the results of Experiment 1 is whether the semantic relatedness eﬀect is contingent on the speciﬁc experimentalparadigm that was used. The method used in Experiment 1, in which werepeated a single word on each trial and manipulated the similaritybetween words across trials, is useful for gaining evidence concerning thedevelopment of semantic representations. However, most language inputthat infants receive does not consist of repetitions of individual words(e.g., "dog, dog, dog"), although it should not be discounted that infantsdo often hear words in isolation paired with the same words in ﬂuentspeech (e.g., "Giraﬀe! Look at the Giraﬀe!", Aslin, Woodward, LaMendo-la, & Bever, 1996; Brent & Suskind, 2001; Lew-Williams, Pelucchi, &Saﬀran, 2011). In order for the results of Experiment 1 to be generaliz-able, then, it is important to determine whether semantic relatedness 2Because mixed-eﬀects models use both the number of participants and the number of items as random factors, the degrees of freedom in these tests are not straightforward as theyare in typical signiﬁcance tests, and thus, it is not standard to report them in the samemanner (see Baayen et al., 2008 for details).
WILLITS ET AL.
eﬀects will emerge under other stimulus conditions. This issue is addressedin our second experiment.
In Experiment 2, we examined lexical knowledge using a variation on thetask used in Experiment 1. Rather than presenting a single word on eachtrial and testing the eﬀects of relatedness across trials, we repeated wordpairs on each trial and manipulated the relatedness within each trial.
Observing eﬀects of semantic relatedness within rather than across trialsallows comparisons that may constrain inferences about the cognitive pro-cesses underlying the eﬀects. Presenting two words on each trial reducesworking memory load compared with Experiment 1, in which relatedstimuli occurred several seconds apart and were separated by an atten-tion-getting stimulus. Although working memory demands did notpreclude ﬁnding an eﬀect in Experiment 1, this potential alternate expla-nation of null results could complicate the interpretation of future studiesemploying this methodology. It was therefore important to determinewhether the priming eﬀect also occurs when memory demands are mini-mized.
Additionally, for Experiment 2 we collected data concerning the partici- pants' vocabularies. There has been much discussion in the literature con-cerning the relationship between overall vocabulary level and toddlers'performance in online tasks assessing familiar word recognition (Fernald,Zangl, Portillo, & Marchman, 2008; Swingley, Pinto, & Fernald, 1999)and novel word learning (e.g., Graf Estes, Edwards, & Saﬀran, 2011;Havy & Nazzi, 2009; Lany & Saﬀran, 2011; Nazzi, 2005; Werker, Fennell,Corcoran, & Stager, 2002). Parent report measures of toddlers' vocabularylevels were therefore used to assess whether semantic relatedness eﬀectswere correlated with vocabulary development.
The participants were 32 monolingual English-learning toddlers (16 male)who had not participated in Experiment 1, with a mean age of 24 months(M = 24.5, range = 22.7–25.8). The participants were full term, werereported to have normal vision and hearing and had had a minimalamount of exposure to non-English languages (<4 h/week of exposure toanother language). Three additional toddlers were tested but excluded TODDLERS ACTIVATE LEXICAL SEMANTIC KNOWLEDGE from the analyses due to fussiness or crying during more than half of thetrials during the experiment.
Stimuli and design Each trial consisted of a word pair, repeated with a 750 ms pause betweenwords (e.g., "kitty…dog…kitty…dog…"). The words were the same asthose used in Experiment 1 (dog, kitty, shoe, sock, juice, milk, mouth, andnose), again paired to create a set of unrelated pairs (e.g., dog-shoe, juice-kitty) and a set of related pairs (e.g., dog-kitty, juice-milk). These pairswere used to create a diﬀerent counterbalanced list for each participant.
Each counterbalanced list contained 16 trials; half unrelated and halfrelated. Within each list, the items were counterbalanced such that eachparticipant heard each word twice in a related pair and twice as an unre-lated pair and heard an equal number of related and unrelated trials onthe left and right side. Between lists, the items were constructed such thatthe order of the pairs was pseudorandomized, each pair was played oneach side for an equal number of participants, each pair occurred an equalproportion of times following a related and an unrelated trial, and theorder within the pair (i.e., which word was said ﬁrst) was balanced acrossparticipants. As in Experiment 1, the stimuli were also equated forvolume, pitch, and the length of trials such that these factors were notconfounded with relatedness condition. The stimulus characteristics forthe items in Experiment 2 are shown in Tables 1 and 2, and a samplestimulus list is shown in the Appendix.
The procedure in Experiment 2 was identical to the procedure in Experi-ment 1, with the exception that word pairs rather than individual wordswere repeated on each trial. After the experiment, the caregiver wasdebriefed and given the questionnaire used in Experiment 1. All childrenbut two were at ceiling for all of the words used in the study; near identi-cal results were obtained when trials for words that children were reportedto not know were excluded. In addition, caregivers ﬁlled out a productivevocabulary checklist (MCDI; Dale & Fenson, 1996).
RESULTS AND DISCUSSION The mean looking times for each participant on related and unrelatedtrials collapsed across items are presented in Figure 2. Again in line with
WILLITS ET AL.
Figure 2 Mean looking times for trials containing related and unrelated word pairs.
our directional predictions, looking times were longer on unrelated trials(M = 10.3 sec, SE = 0.6 sec.) than on related trials (M = 9.0 sec,SE = 0.4 sec.), an eﬀect that was signiﬁcant when analyzed by partici-pants, F1(1, 31) = 6.60, p < 0.05, g2 = 0.18, and was marginally signiﬁcantwhen analyzed by items F2(1, 15) = 4.52. p = 0.052, g2 = 0.23. The resultsreplicate the eﬀect we observed in Experiment 1: looking times wereshorter when a word was paired with a related word. Thus, relatednessaﬀects the processing of words presented together, as well as on successivetrials. As in Experiment 1, we conducted a mixed-eﬀects model to rule outnonmeaningful eﬀects on looking time, such as duration and amplitude.
Again, only relatedness was a signiﬁcant predictor of looking time(t = 2.12, p < 0.05).
We then examined correlations between a child's vocabulary size and the size of that child's semantic relatedness eﬀect. For each participant, arelatedness eﬀect was calculated by subtracting their mean on related trialsfrom their mean on unrelated trials. In this analysis, there was a weak,nonsigniﬁcant correlation between vocabulary size and priming eﬀect TODDLERS ACTIVATE LEXICAL SEMANTIC KNOWLEDGE (r = 0.18, p = 0.16). The failure to ﬁnd a signiﬁcant correlation betweenvocabulary (MCDI) and the semantic relatedness is consistent with Arias-Trejo and Plunkett (2009), Styles and Plunkett (2009), and Torkildsenet al. (2007) who also failed to ﬁnd a relationship between vocabulary andtime spent looking to a semantically primed target. An ERP study byFriedrich and Friederici (2004) did ﬁnd that high vocabulary 19-month-olds showed stronger, more adult-like eﬀects of semantic relatedness. It isthus possible that 24-month-olds perform at ceiling in our task; youngerchildren (or an experiment using lower frequency words) may show diﬀer-ences in semantic relatedness eﬀects based on vocabulary size. Futureexperiments can explore this possibility.
GENERAL DISCUSSION Our knowledge of words includes extensive information about their mean-ings and how they relate to other words. These word meanings are part ofa semantic system that also represents information about objects, individu-als, and events. This knowledge is central to language, thinking, reasoning,and other cognitive functions. Many controversies in the study of psycho-logical semantics turn on questions about the origins of this knowledgeand how it develops. However, obtaining reliable information about earlysemantic knowledge is diﬃcult. Our studies show that 24-month-olds'knowledge of word meanings is suﬃcient to produce relatedness eﬀectsboth across events (words presented on successive trials) and within anevent (word pairs presented within trials) in a purely auditory paradigmand without enriching sentential contextual information. By 24-months ofage, toddlers' representation of word meaning, as well as the associativeand similarity structures of words, are already quite rich, robust, andeasily accessible. Our studies provide convergent evidence, along withstudies by Arias-Trejo and Plunkett (2009) and by Torkildsen et al.
(2007), that by this age, children are successfully representing the semanticrelations between words when processing language.
Our ﬁndings also demonstrate that the extension of the HPP to the semantic realm has considerable promise. However, this work should notbe taken as an argument that HPP is always the best method for investi-gating semantic development. Diﬀerent methods have diﬀerent advantagesdepending upon the questions being asked (as well as the resources thatare available, in the case of studying ERPs). In principle, one importantadvantage of the IPL paradigm used by Plunkett and colleagues (as wellas EEG studies like Torkildsen et al.'s) is that it tracks behavior overtime, and thus, it provides a window into the time course of young WILLITS ET AL.
children's online processing of primed words. The IPL priming paperspublished thus far, however, have not reported time-course analyses, mak-ing it diﬃcult to ascertain whether or not the IPL procedure providesinterpretable reaction time data in practice. Additional work is needed toexplore this possible IPL advantage.
A clear beneﬁt of the HPP method, on the other hand, is that it allows for the exploration of lexical knowledge without the presence of relatedvisual stimuli, providing a more direct test of purely auditory lexicalknowledge. The IPL priming methodology, along with other IPL designs,is limited to stimuli that are highly concrete and imageable, and in factArias-Trejo and Plunkett (2009) used imagability measures to choose theirstimuli. Because HPP does not require visual representations of the stim-uli, future studies can examine nonimageable, nonconcrete words that arenot well suited to eye-tracking methods.
In addition to the IPL and HPP methods, the ERP paradigm used by Torkildsen et al. (2007) is also applicable to the study of the developmentof semantic relationships. Corroborating behavioral data are often usefulto conﬁrm that neurological activity translates to expected behavioral out-comes (Picton & Taylor, 2007), and thus, the HPP provides a nice compli-ment to ERP ﬁndings. It should also not be discounted that the HPPdesign is considerably cheaper to implement and thus is accessible to moreresearchers. The greater sensitivity of infant ERP methods is important,though, and therefore the continuation of both neurological and behav-ioral research programs, and the integration of their respective ﬁndings,will lead to a deeper understanding of early semantic representations.
This extension of the priming methodology to young children has considerable potential as a tool for investigating semantic development.
Lexical priming has been widely used in studies of the organization ofsemantic information in adults (McNamara, 1992; McRae et al., 1997;Moss et al., 1995) and its breakdown in cases of brain injury or disease(Chertkow, Bub, & Seidenberg, 1989). The modiﬁed HPP used in the pres-ent studies can be extended to shed light on many other aspects of youngchildren's word and conceptual knowledge, allowing us to understand thedevelopment of the lexical system.
Future work can look at a number of diﬀerent issues alluded to above, such as how word frequency, age of acquisition, and children's vocabularysize inﬂuence semantic priming eﬀects. Future work can also explore thedevelopmental underpinnings of issues that have been explored in theadult research, such as what types of semantic relationships yield an eﬀect.
The diﬀerence between the methodologies of Experiment 1 and Experi-ment 2, which manipulates the amount of time between the prime and thetarget, may also be useful for exploring issues related to retrieval TODDLERS ACTIVATE LEXICAL SEMANTIC KNOWLEDGE mechanisms. In the adult literature, the duration between the prime andtarget has been found to aﬀect whether one ﬁnds priming between wordsthat are associatively related versus "semantically" related (see Hutchison,2003; for a review). Likewise, this method may be useful for investigatingthe developmental underpinnings of other major debates, such as whethersemantic priming eﬀects are more likely due to a mechanism like "spread-ing activation" between related words in a semantic network (Collins &Loftus, 1975; McNamara, 1992) or due to related words forming better"compound cues" to memory (McKoon & Ratcliﬀ, 1992; Ratcliﬀ & McK-oon, 1988).
In addition to issues concerning the processes and types of relations represented in toddlers' semantic memory, there are still outstanding ques-tions about how this knowledge is used. One might describe our ﬁndingsas a "purely lexical" semantic priming eﬀect, in the sense that the childrenin our study activated and made use of knowledge of word meaning in theabsence of nonlinguistic or referential information during the experiment,as well as in the absence of sentential contexts or other supportive linguis-tic information. However, this does not distinguish between diﬀerenthypotheses about the locus of these eﬀects. These priming eﬀects could bedue to knowledge about the relationships between words. Alternatively,these eﬀects could be due to conceptual knowledge about the words' refer-ents. Distinguishing between these two hypotheses is very diﬃcult, andthese two eﬀects are almost always confounded in the adult literature(Willits, J. A., Amato, M. S., & MacDonald, M. C., 2012, in review).
A ﬁnal outstanding question is the developmental trajectory of these eﬀects. We have provided evidence that children's word knowledge is quiterobust by 24 months of age and that it includes the relations betweenwords. Figuring out exactly when these abilities emerge, and how thatmight diﬀer as a function of the type of relationship (semantic versus asso-ciated) or the locus of the knowledge (word knowledge versus worldknowledge) will be an important question to pursue in future work.
With basic relatedness eﬀects in hand, future research can focus on further exploring these issues in the development of lexical semanticknowledge. Because of this methodology's simplicity and ﬂexibility, it canbe used to investigate many questions about semantic knowledge thatcould not otherwise be easily addressed.
We would like to thank the participating families and the members of theInfant Learning Laboratory. We would also like to thank Thierry Nazzi WILLITS ET AL.
and three anonymous reviewers for useful input on the study. This materialwas based on work funded in part by the National Science FoundationGraduate Research Fellowship under Grant No. DGE-0718123 to EHW.
Any opinions, ﬁndings, conclusions, or recommendations expressed in thismaterial are those of the authors and do not necessarily reﬂect the views ofthe National Science Foundation. Additional funding was provided by theNICHD to JRS (R01HD037466) and the Waisman Center (P30HD03352);and by a grant from the James F. McDonnell Foundation to JRS.
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Example stimulus lists for Experiment 1 and Experiment 2 Note. In Experiment 1, one particular toddler heard the words in the exact order shown above,and the other 31 heard pseudorandomized lists fulﬁlling the same properties (always havingeach set of two trials contain related words, with the result being that trials alternated in therelatedness of the previous trial (trial 2 was related, trial 3 unrelated, trial 4 related, etc.). InExperiment 2, all toddlers heard the items shown, and the trial order was randomized acrossparticipants.
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