the nature of lexical representations stored by children as they develop a mental lexicon is a widely studied aspect of language acquisition .
whether they are holistic and undifferentiated or , rather , adult - like in their detailedness .
recent findings show that children s lexical processing is modulated by featural manipulations made to words ( e.g. , dog 
tog ) , suggesting that early words must be sufficiently specified so as to enable establishing a match to a given label ( fikkert , 2010 , swingley and aslin , 2000 , yoshida et al . , 2009 ) .
however , the precise degree of this specificity in early words requires further investigation . whereas some studies have found evidence for children s ability to detect differing degrees of mismatch ( mani et al . , 2012 ,
ren and morgan , 2011 , white and morgan , 2008 ) , others have not ( bailey and plunkett , 2002 , swingley and aslin , 2002 ) .
this study sought to determine children s sensitivity to degree of mispronunciation with a tool that minimizes task demands  pupillometry .
numerous studies attempting to uncover the nature of early lexical knowledge have probed infants perceptual abilities .
it has been widely recognized that infants are excellent discriminators of phonetic detail  be it native or non - native  during their first months of life and are able to form phonetic categories ( curtin & archer , 2015 ) .
furthermore , infants are able to categorize consonant  vowel sequences by disregarding irrelevant acoustic differences ( eimas et al . , 1971 , jusczyk et al . , 1977 ) .
such skills have been sometimes interpreted as an index of the infants ability to detect phonological features .
however , these discrimination skills are not necessarily revealed during word processing because children might not distinguish newly learned words from phonological neighbors ( stager & werker , 1997 ) .
these discrepancies between discrimination and word recognition have raised the question : what details are stored in the developing lexicon ? to address this issue , studies may take infants ability to detect mispronunciations of words as a measure of specificity and detailedness of early words ( e.g. , swingley & aslin , 2002 ) . in such studies ,
17- to 19-month - olds have demonstrated sensitivity to a range of contrasts effected through featural changes , including voicing ( e.g. , dog  tog ) , manner of articulation ( e.g. , swing  twing ) ( swingley & aslin , 2002 ) , and height and backness in vowels ( e.g. , bed  bid , brush  brash ) ( mani et al . , 2012 ) .
moreover , children as young as 14  months have the ability to detect mispronunciations involving place of articulation ( e.g. , bin  din ) ( swingley & aslin , 2000 ) . given this body of research in mispronunciation detection , it can be concluded that infants are able to detect the difference between correct and featurally manipulated word forms .
a natural step forward is to ask how far their lexical knowledge extends : are infants sensitive to the degree of mispronunciation ( i.e. , to the degree of featural distance between the correct and incorrect forms ) ?
such gradient sensitivity would suggest that lexical representations are not only specific but also fine - grained enough to encode the degree of overlap with other minimally different words .
so far , only a handful of studies , all using the preferential looking paradigm , have considered the question of whether children younger than 3  years are sensitive to differing degrees of mismatch between a target word and its mispronounced variant .
these studies have obtained mixed results ; some demonstrate sensitivity to degree of mismatch ( mani and plunkett , 2011 , ren and morgan , 2011 , white and morgan , 2008 ) , whereas others do not ( bailey and plunkett , 2002 , swingley and aslin , 2002 ) . by including a greater range of contrasts than previous studies and , crucially , unfamiliar distractors in a preferential looking paradigm ,
the results indicated a gradual decline in the proportion of children s target looking time as the number of feature changes increased ( corrected for looking times in the salience phase ) .
for example , children s looking time toward the picture of  keys  was greater than that toward an unfamiliar object ( e.g. , an abacus ) when presented with the correct label  keys  . with labels exhibiting a one - feature change (  teys  ) ,
infants still preferred to look toward the target , but to a lesser extent than in the correct condition .
when two - feature changes were introduced (  deys  ) , infants exhibited a nonsignificant target preference , and when three - feature changes were introduced (  zeys  ) , they exhibited a nonsignificant distractor preference ( the two- and three - feature change conditions overlapped ) .
these findings suggest that children were able to retrieve the appropriate lexical representation and , consequently , match the label with its corresponding picture when presented with the correct label .
similarly , children were able to do so when the onset differed by one feature , but less successfully than with the correct label ( as evidenced by the drop in the proportion of looking times toward the target ) .
furthermore , children did not appear to establish a link with the item exhibiting two- and three - feature changes and either the target or the distractor .
in addition , a similar graded sensitivity in looking time has been observed when manipulating vocalic rather than consonantal featural distance ( mani & plunkett , 2011 ) . in that study , 24-month - olds ( but not 18-month - olds ) showed sensitivity to the degree of mismatch such that correctly pronounced labels and one - feature deviations resulted in a target preference , whereas larger two- and three - feature deviations yielded a weak distractor preference .
we highlight here an important shared methodological characteristic of the studies described above that provides the impetus for our study  the use of a preferential looking paradigm with two pictured objects presented simultaneously in each trial . as white and morgan ( 2008 )
point out , the presence of two potential referents for the auditorily presented ( mispronounced ) word form requires a process that determines whether the presented label is a new word that may be mapped to the distractor object or whether it can still be mapped to the target object .
therefore , the looking patterns obtained in a preferential looking paradigm are affected not only by the featural distance between the correct label and its mispronunciation but also by properties of the distractor object ( e.g. , the familiarity of the object , whether the child knows the distractor label ) . in the current study ,
degree of pupil dilation in response to a single picture was measured , avoiding the need to present a distractor .
thus , by eliminating the competition between two potential referents of the label , a potentially more sensitive measure of the effect of featural distance may be obtained .
pupillometry is a method highly suitable for assessing the performance of young children , being based on an involuntary psychosensory reflex , that is , pupil dilation ( loewenfeld , 1993 ) . in pupillometry , instead of recording the pattern of gaze fixations , the eyetracking equipment is used to measure the change in pupil size over time . increased pupil dilation in young children has been interpreted to be a proxy of surprise , novelty , and cognitive effort ( for a review , see karatekin , 2007 ) .
more recently , pupillometry has been found to be a viable tool in language research .
it was shown to be sensitive to detecting acoustic ( dis-)similarity ( hochmann & papeo , 2014 ) , semantic mismatch ( kuipers & thierry , 2011 ) , and  most important for the current study  mispronunciations ( fritzsche & hhle , 2015 ) .
a number of aspects of pupillometry make it an especially appealing tool for language acquisition research .
the passive processing of the experimental stimuli ( i.e. , watching while listening ) does not necessitate an overt behavioral response . in our study , processes unrelated to the investigated phenomenon  recognition of distractors , memory requirements , evaluation and decision processes  are greatly reduced .
second , pupil dilation is a continuous response that may provide a more appropriate way in which to investigate children s reaction to degrees of mispronunciation than a pseudo - categorical response employed by preferential looking paradigms ( i.e. , looking at either the target or distractor image ) ( klingner , 2010 ) .
although electrophysiological and brain imaging techniques generally avoid the shortcomings that looking time paradigms introduce , such techniques require specialized equipment and expertise .
the eyetracking equipment needed for pupillometry is already widely used in the child language research community , and the technical competencies required for pupillometry can be readily acquired by those already familiar with the equipment . due to these properties of pupillometry , it may be the case that pupil dilation more directly reflects the costs induced by processing a mispronounced word than a measure of looking time within a preferential looking methodology and , therefore , may provide a more fine - grained insight into the effect of the degree of mismatch between the correct and mispronounced forms . furthermore , in contrast to previous studies , our study employed a featurally balanced consonantal set , also cross - balanced for feature type and change .
words with a diverse set of initial consonants were selected and then systematically manipulated not just by the number but also by the type and direction of feature changes ( see table 1 ) .
therefore , we chose to investigate 30-month - old children whose lexicons allow for the creation of a more diverse and balanced stimulus set than would be possible for younger children ( early lexicons tending to contain predominantly labial - initial words ; cf .
the current study attempted to determine whether pupillometry can be used to obtain a gradient measure of lexico - phonological knowledge .
specifically , we tested for the following effects : effect of mispronunciation : the degree of pupil dilation is larger in the mispronounced conditions than in the correct condition . this may indicate that mispronounced labels are harder to match and process along with the activated representation than correctly produced labels ( fritzsche & hhle , 2015).effect of featural distance : if , in addition to the effect of mispronunciation , the degree of pupil dilation is predicted by the number of featural changes made , this result would provide evidence that the degree of mispronunciation modulates lexical processing .
effect of mispronunciation : the degree of pupil dilation is larger in the mispronounced conditions than in the correct condition .
this may indicate that mispronounced labels are harder to match and process along with the activated representation than correctly produced labels ( fritzsche & hhle , 2015 ) .
effect of featural distance : if , in addition to the effect of mispronunciation , the degree of pupil dilation is predicted by the number of featural changes made , this result would provide evidence that the degree of mispronunciation modulates lexical processing .
a total of 48 30-month - old monolingual german children ( 26 girls ) were recruited ( mage  =  30  months , sd  =  0.56 ) from the babylab participant pool at the university of potsdam .
we assessed children s vocabulary knowledge and familiarity with the experimental items using the parental report measure frakis ( i.e. , the german adaptation of the macarthur  bates communicative development inventories [ cdi ] ; cf .
participants were reported to be familiar with the majority of ( correct ) experimental items ( m  =  79.9% , sd  =  16.9 ) .
children s reported average vocabulary ( m  =  410 , sd  =  112 ) aligned closely with frakis norms for 30-month - old german - speaking children ( m  =  439 ) ( szagun et al . ,
five children were excluded from the analyses due to providing insufficient data ( see results ) . to identify words likely to be known by toddlers , 20 easily depictable words with cvc ( consonant  vowel  consonant ) or cvcv syllable structure and word - initial stress were selected from frakis ( szagun et al . , 2009 ) .
word onsets were manipulated to create four conditions : 20 correct ( unchanged ) items ( e.g. , 
sheep ) , 20 items with one feature change ( e.g. , taf , manner of articulation change ) , 20 items with two feature changes ( e.g. , daf , manner of articulation and voicing change ) , and 20 items with three feature changes ( e.g. , gaf , manner of articulation , voicing , and place of articulation change ) .
mispronunciations resulted in nonwords for children.1 type ( i.e. , voice , manner , place ) and direction of feature change were counterbalanced . from each word ,
these phonologically related items ( e.g. , af ,  sheep  , taf , daf , gaf ) formed an item family .
an additional 40 easily depictable words from frakis were included : 20 filler items that were always produced correctly ( listed in online supplementary material ) and 20 items related to another study ( 20 items with onset clusters  also listed in supplementary material  to be reported in tamsi , mckean , gafos , & hhle , 2016 ) .
altogether , participants were presented with 35 correctly and 25 incorrectly pronounced items in each version of the experiment .
easily recognizable color drawings depicting a referent of the original word were converted to a similar size ( 200    200 pixels displayed in a 300    300 pixel area ) . for more information on luminance , refer to the supplementary material .
four versions of the task were created , with each item family occurring once in each version and with the four conditions counterbalanced across the four versions ; children never saw the same picture or heard the same label more than once .
children were told that they were to watch a short movie , during which they should sit still and , as a reward , they could choose a book afterward .
after obtaining assent from the children and written informed consent from the caregivers , children were seated in the caregiver s lap and positioned such that their eyes were approximately 60  cm from the computer screen .
all visual stimuli were shown centrally on a 17-inch ( 1280    1024 ) tft screen with a size of 300    300 pixels forming a horizontal and vertical viewing angle of 7.4. the experiment started following the calibration period ( five screen positions ,  30  s ) . in each trial , a picture was presented and remained on the screen for the duration of the trial ( 4  s ) .
the picture appeared , and 1  s later the corresponding ( correctly or incorrectly produced ) auditory label was played .
the critical window of analysis was the 3-s interval following the onset of the auditory stimulus .
the experiment encompassed 12 blocks , with each block containing 5 trials ( altogether 60 trials , including 20 fillers and 20 unrelated ) . before each block , an  attention - getter  was presented ( i.e. , a short silent movie clip of animated cartoon characters and animals ) .
the attention - getters were played in a loop until the experimenter pressed a key to start the next block . on average ,
after the experiment , caregivers were asked to complete a questionnaire in order to estimate children s vocabulary size and children s familiarity with the experimental words .
2009 ) plus 12 additional items relevant to an experiment not reported here . on average ,
a total of 48 30-month - old monolingual german children ( 26 girls ) were recruited ( mage  =  30  months , sd  =  0.56 ) from the babylab participant pool at the university of potsdam .
we assessed children s vocabulary knowledge and familiarity with the experimental items using the parental report measure frakis ( i.e. , the german adaptation of the macarthur  bates communicative development inventories [ cdi ] ; cf .
participants were reported to be familiar with the majority of ( correct ) experimental items ( m  =  79.9% , sd  =  16.9 ) .
children s reported average vocabulary ( m  =  410 , sd  =  112 ) aligned closely with frakis norms for 30-month - old german - speaking children ( m  =  439 ) ( szagun et al . ,
five children were excluded from the analyses due to providing insufficient data ( see results ) .
to identify words likely to be known by toddlers , 20 easily depictable words with cvc ( consonant  vowel  consonant ) or cvcv syllable structure and word - initial stress were selected from frakis ( szagun et al . , 2009 ) .
word onsets were manipulated to create four conditions : 20 correct ( unchanged ) items ( e.g. , 
sheep ) , 20 items with one feature change ( e.g. , taf , manner of articulation change ) , 20 items with two feature changes ( e.g. , daf , manner of articulation and voicing change ) , and 20 items with three feature changes ( e.g. , gaf , manner of articulation , voicing , and place of articulation change ) .
mispronunciations resulted in nonwords for children.1 type ( i.e. , voice , manner , place ) and direction of feature change were counterbalanced . from each word ,
three mispronunciations were created by changing one , two , or three features . these phonologically related items ( e.g. , af ,  sheep  , taf , daf , gaf ) formed an item family .
an additional 40 easily depictable words from frakis were included : 20 filler items that were always produced correctly ( listed in online supplementary material ) and 20 items related to another study ( 20 items with onset clusters  also listed in supplementary material  to be reported in tamsi , mckean , gafos , & hhle , 2016 ) .
altogether , participants were presented with 35 correctly and 25 incorrectly pronounced items in each version of the experiment .
easily recognizable color drawings depicting a referent of the original word were converted to a similar size ( 200    200 pixels displayed in a 300    300 pixel area ) . for more information on luminance ,
four versions of the task were created , with each item family occurring once in each version and with the four conditions counterbalanced across the four versions ; children never saw the same picture or heard the same label more than once .
children were told that they were to watch a short movie , during which they should sit still and , as a reward , they could choose a book afterward .
after obtaining assent from the children and written informed consent from the caregivers , children were seated in the caregiver s lap and positioned such that their eyes were approximately 60  cm from the computer screen .
all visual stimuli were shown centrally on a 17-inch ( 1280    1024 ) tft screen with a size of 300    300 pixels forming a horizontal and vertical viewing angle of 7.4. the experiment started following the calibration period ( five screen positions ,  30  s ) . in each trial ,
a picture was presented and remained on the screen for the duration of the trial ( 4  s ) .
the picture appeared , and 1  s later the corresponding ( correctly or incorrectly produced ) auditory label was played .
the critical window of analysis was the 3-s interval following the onset of the auditory stimulus .
the experiment encompassed 12 blocks , with each block containing 5 trials ( altogether 60 trials , including 20 fillers and 20 unrelated ) . before each block , an  attention - getter  was presented ( i.e. , a short silent movie clip of animated cartoon characters and animals ) .
the attention - getters were played in a loop until the experimenter pressed a key to start the next block . on average ,
, caregivers were asked to complete a questionnaire in order to estimate children s vocabulary size and children s familiarity with the experimental words .
2009 ) plus 12 additional items relevant to an experiment not reported here . on average ,
to ensure that the words used in the experiment were part of children s lexical inventory , only those trials that included words ( and their mispronunciations ) reported to be known in the parental questionnaire were considered in the analysis of each individual child s data .
successful trials were defined as those containing pupil measures from at least half the length of the trial . based on this criterion , the proportion of successful trials was tabulated for each participant . those participants who did not reach a threshold of 50% of successful trials ( following fritzsche & hhle , 2015 )
( additional information on the pre - processing steps is given in the supplementary material . )
the mean number of successful trials was 17.19 of 20 ( sd  =  1.89 ) in the experimental trials and 17.38 of 20 ( sd  =  1.94 ) in the filler trials .
the mean number of successful trials per experimental condition was 4.30 of 5 ( sd  =  0.10 ) .
we employed linear mixed effects models with random intercepts and slopes using the lmer function ( estimates were chosen to optimize the log - likelihood criterion ) in the lme4 r package ( bates , maechler , bolker , & walker , 2014 ) .
featural distance , a within - participant factor with four levels , was entered as a fixed effect into the model : correctly pronounced as well as mispronounced with one- , two- , and three - feature changes .
we also included potentially confounding ( sub-)lexical factors as control variables ( word frequency , neighborhood density , and transitional probability , taken from the clearpond database ; marian , bartolotti , chabal , & shook , 2012 ) and children s vocabulary size estimated from the parental questionnaire .
mean change in pupil diameter ( i.e. , the mean value extracted from each 3-s window of analysis ) was used as the outcome measure .
the most parsimonious model contained featural distance , vocabulary size , and neighborhood density as fixed effects ( for model description , see supplementary material ) .
mean pupil dilation in each condition is presented in fig . 1 ( left : bar plot ; right : time - course plot ) .
visual inspection suggested that correctly pronounced words were generally associated with smaller pupil size change than mispronounced words . also for mispronounced words ,
the one - feature change condition was associated with a smaller degree of pupil dilation than the two- and three - feature change conditions .
there seemed to be no difference between the conditions with two- and three - feature changes .
statistical analysis using the mixed effects model described above confirmed these observations . in the model ,
featural distance significantly contributed to model fit , (3 )  =  10.19 , p  <  .017 .
we subsequently found the correct versus mispronounced contrast and the one - feature change versus two- and three - feature change contrast to be significant ( 1  =  0.041 , se  =  0.019 , t  =  2.21 , 2  =  0.044 , se  =  0.020 , t  =  2.39 ) .
the contrast between the two- and three - feature change conditions was not significant ( 3  =  0.005 , se  =  0.023 , t  =  0.21 ) .
further information ( including time - course analyses ) is provided in the supplementary material .
our findings indicate that pupillometry is a viable method in lexical representation research because it can capture a differential response for correctly pronounced and mispronounced items through the measurement of the degree of pupil dilation change in toddlers .
first , the significant differences in mean pupil dilation between correct and mispronounced items replicate previous findings ( fritzsche & hhle , 2015 ) and provide further evidence that the processing of mispronounced words leads to greater pupil dilation compared with correctly produced words .
second , the results indicate that the degree of mispronunciation also influenced the pupillary response ; that is , conditions that involved more than one feature change were associated with larger pupil dilation than those with only one feature change .
this result indicates that children s lexical processing is modulated by the degree of mismatch .
the fewer features shared between the correct and mispronounced forms , the harder it is to match the stimulus with the lexical representation , which is reflected by larger degrees of pupil dilation .
it is possible that the effect found here relates to general surprise caused by hearing a sound sequence that was not expected as a target label .
however , we would argue that a form - related explanation is more plausible ; that is , the pupillary response is an indicator of cognitive effort to establish a link between stimulus and lexical representation .
this argument is supported by the findings of fritzsche and hhle ( 2015 ) , a single - picture study with children of the same age , whereby pupillary responses given to correct and semantically unrelated labels were comparable . however , the results do not support complete graded sensitivity given that the three - feature change condition yielded a pupil size change that was not significantly larger than that of the two - feature change .
this latter result is fully consistent with previous findings from preferential looking studies ( mani and plunkett , 2011 , ren and morgan , 2011 , white and morgan , 2008 ) .
recall that these studies demonstrated graded sensitivity and no significant difference between two- and three - feature deviations .
it is possible that maximal pupil dilation was achieved with two - feature deviation and an additional feature change could no longer be reflected through a larger degree of dilation . from a physiological standpoint ,
however , larger degrees of pupil dilation have been achieved than those reported here using other non - linguistic tasks with similar age groups .
this suggests that a ceiling effect is an unlikely explanation for the findings ( for an overview on task - evoked pupillary response , see beatty & lucero - wagoner , 2000 ) .
another possibility for the lack of difference between the two- and three - feature change conditions is that individual feature changes may interact with one another and/or that the effects of featural combinations might not be linearly additive ( i.e. , the difference between the two- and three - feature changes might not be proportional to the difference between the one- and two - feature changes ) . further investigation is needed to explore the unique effect of type and direction of feature changes in consonants . in accordance with past studies ( ren and morgan , 2011 , white and morgan , 2008 ) , we defined degree of mismatch between correct and deviant forms in terms of phonological features .
it is unclear to what extent the degree of acoustic difference between the correct and incorrect forms correlates with the degree of featural distance .
it is possible that ( as argued by mani & plunkett , 2011 , for vowels ) the pattern of graded sensitivity found here may relate more to acoustic rather than phonological properties .
further research is required to address the question of whether mismatch detection is based on physical acoustic or phonological distance .
these alternatives can be tested by quantifying acoustic distance and assessing whether acoustic and/or phonological distance significantly and independently account for the results ( for an analysis with vowels , see mani & plunkett , 2011 ) .
our study is the first demonstration of children s sensitivity to the degree of mispronunciation in a passive listening task using pupillometry .
our results demonstrate that young children are sensitive to the contrast between small ( one- ) and large ( two- and three-)feature changes .
these results corroborate previous research that found toddlers word recognition to be modulated by featural changes ( fikkert , 2010 , swingley and aslin , 2000 , yoshida et al . , 2009 ) as well as by degree of mismatch caused by such changes ( mani and plunkett , 2011 , ren and morgan , 2011 , white and morgan , 2008 ) , suggesting that early lexical representations encode sub - segmental detail . in summary
, this study demonstrates that pupillometry can be used as a tool for mispronunciation detection with 30-month - old children , providing a minimally demanding alternative to other extensively applied paradigms .
therefore , it proves to be a readily available , low - cost , and reliable method with which to conduct speech processing research with infants and young children . as such , pupillometry holds promise to accelerate the rate of new discovery in this important field .
 this study introduces a method ideally suited for investigating toddlers ability to detect mispronunciations in lexical representations : pupillometry . 
 previous research has established that the magnitude of pupil dilation reflects differing levels of cognitive effort . 
 building on those findings , we use pupil dilation to study the level of detail encoded in lexical representations with 30-month - old children whose lexicons allow for a featurally balanced stimulus set . in each trial , we present a picture followed by a corresponding auditory label . by systematically manipulating the number of feature changes in the onset of the label ( e.g. , baby    daby    faby    shaby ) , we tested whether featural distance predicts the degree of pupil dilation . 
 our findings support the existence of a relationship between featural distance and pupil dilation . 
 first , mispronounced words are associated with a larger degree of dilation than correct forms . 
 second , words that deviate more from the correct form are related to a larger dilation than words that deviate less . 
 this pattern indicates that toddlers are sensitive to the degree of mispronunciation , and as such it corroborates previous work that found word recognition modulated by sub - segmental detail and by the degree of mismatch . 
 thus , we establish that pupillometry provides a viable alternative to paradigms that require overt behavioral response in increasing our understanding of the development of lexical representations . 
Introduction
Method
Participants
Stimuli
Procedure
Results
Discussion
Supplementary data