2011 Mutual Exclusivity at 16 Months

7/27/2019 2011 Mutual Exclusivity at 16 Months

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Mutual exclusivity and phonological novelty constrain

word learning at 16 months*

E M I L Y M A T H E R A N D K I M P L U N K E T T

Department of Experimental Psychology, University of Oxford

(Received 11 May 2009 Revised 22 January 2010 Accepted 9 July 2010

First published online 22 November 2010)

A B S T R A C T

Studies report that infants as young as 1;3 to 1;5 will seek out a novel

object in response to hearing a novel label (e.g. Halberda, 2003;Markman, Wasow & Hansen, 2003). This behaviour is commonly

known as the mutual exclusivity response (Markman, 1989; 1990).

However, evidence for mutual exclusivity does not imply that the infant

has associated a novel label with a novel object. We used an intermodal

preferential looking task to investigate whether infants aged 1;4 could

use mutual exclusivity to guide their association of novel labels with

novel objects. The results show that infants can successfully map a novel

label onto a novel object, provided that the novel label has no familiar

phonological neighbours. Therefore, as early as 1 ; 4, infants can usemutual exclusivity to form novel wordobject associations, although

this process is constrained by the phonological novelty of a label.

I N T R O D U C T I O N

During the second year of life, infants make considerable progress in

acquiring the lexicon of their language community. Because language is a

shared system of communication, the infant has to learn from other

speakers how words refer to the world. However, it is unclear how much thespeaker has to explicitly guide the infant towards forming the correct word

mappings. A considerable number of studies have shown that infants word

learning is sensitive to social cues such as pointing and eye gaze (e.g.

Baldwin, Markman, Bill, Desjardins, Irwin & Tidball, 1996 ; Baldwin,

1993). Nonetheless, many other researchers (e.g. Markman, 1989 ; 1990 ;

[*] This research was supported by a Biotechnology and Biological Sciences ResearchCouncil grant (BBE0074061). We thank Lucy Holdstock, Rosie Minnigin and EmilyRuzich for their assistance, and all the parents and infants who made this research

possible. Address for correspondence: Emily Mather, Department of ExperimentalPsychology, South Parks Road, University of Oxford, Oxford, UK. OX1 3UD. tel:+44(0)1865 271522; e-mail: [email protected]

J. Child Lang. 38 (2011), 933950. f Cambridge University Press 2010

doi:10.1017/S0305000910000401

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Golinkoff, Mervis & Hirsh-Pasek, 1994; Hollich, Hirsh-Pasek & Golinkoff,

2000) have argued that the infant is biased to form particular word

mappings, even in the absence of guidance from the speaker.

One proposal is that infants will map novel labels onto novel, name-

unknown objects, rather than familiar, name-known objects (e.g. Hutchinson,1986; Markman & Wachtel, 1988; Merriman & Bowman, 1989). Following

Markman (1989; 1990), we refer to this behaviour as MUTUAL EXCLUSIVITY.

For example, an infant might see an object they know the name for, such as

a ball, and an object they have not seen before. Upon hearing a novel label,

such as wug, an infant using mutual exclusivity may associate the novel label

with the novel object, rather than the ball. In principle, mutual exclusivity

enables an infant to learn the meaning of new words even when the speaker

does not provide overt cues, such as eye gaze or pointing towards a referent.

A number of explanations for this behaviour have been proposed.Markman (1989 ; 1990) has argued that infants reject second labels for

objects. Other explanations include the NOVEL-NAME-NAMELESS-CATEGORY

principle (Golinkoffet al., 1994; Mervis & Bertrand, 1994), and the PRINCIPLE

OF CONTRAST (Clark, 1987). What all these accounts have in common is that

the infant has to distinguish name-known and name-unknown objects.

Alternatively, Merriman, Marazita & Jarvis (1995) have proposed a FEELING

OF NOVELTY principle, a strategy of mapping novel labels to objects which

FEEL novel. Hence, even without knowing or retrieving the name for the

familiar object, the infant maps the novel label to the novel object. Someinvestigators have focused on deciding between these different theories

(e.g. Merriman & Schuster, 1991; Markman et al., 2003). However, in this

article we explore the question of whether evidence for mutual exclusivity is

also evidence of word learning.

To understand the impact of mutual exclusivity on vocabulary develop-

ment, it is important to examine carefully what has been demonstrated

in prior studies. The methods used to study mutual exclusivity have varied.

For example, some researchers have used object selection procedures

(e.g. Merriman & Bowman, 1989; Merriman & Schuster, 1991; Horst

& Samuelson, 2008), while other researchers have used an intermodal

preferential looking (IPL) procedure (e.g. Halberda, 2003; White & Morgan,

2008; Mather & Plunkett, 2009). What these studies have in common is that

the infant is required to select or attend to a novel object, rather than a

familiar object, in response to a novel label. Thus, even though the novel

label is presented in an ambiguous context, i.e. the presence of multiple

objects, the infant is biased towards the novel object. What can be

concluded from these studies about infants ability to use mutual exclusivity

to learn new words? We argue that while an infant might attend to the novel

object, this does not guarantee that the infant has formed, or will form, an

association between the novel label and novel object.

M A T H E R & P L U N K E T T

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Our reasoning is as follows: evidence for an association between word and

object cannot be established if anything OTHER than knowledge of the

association could bias attention to the correct referent of a label. The typical

test of mutual exclusivity presents objects which differ in both familiarity

and lexical status (i.e. whether name-known or name-unknown). Thecognitive processes underlying the mutual exclusivity response need only

exploit these differences to guide attention to the novel object. Furthermore,

whether or not this leads the infant to form an association between a

novel word and a novel object could depend on a host of factors, such as

the processing load of the task. Thus, while a newly formed association

might be the cause or outcome of the mutual exclusivity response, other

explanations are possible.

Consequently, while there have been many demonstrations of the

mutual exclusivity response (e.g. Merriman & Bowman, 1989; Merriman &Schuster, 1991; Halberda, 2003; Markman et al., 2003) this response is not

clear evidence of word learning. Yet, to understand the impact of mutual

exclusivity on vocabulary development we need to know when it causes

infants to learn new word mappings. Currently, the earliest age at which

infants display the mutual exclusivity response is 1 ; 3 to 1 ; 5 (Halberda,

2003; Markman et al., 2003). However, this might not correspond to the

age at which infants use mutual exclusivity to form new word mappings.

Indeed, even infants aged 0;10 are reported to display a precursor form

of the mutual exclusivity response (Mather & Plunkett, 2010; see alsoDewar & Xu, 2007). Yet, the very limited vocabularies of infants aged 0;10

suggests that this behaviour is not contributing significantly to word

learning. Markman et al. (2003) have argued that the onset of mutual

exclusivity occurs prior to the purported vocabulary explosion, rather

than as a consequence of increased vocabulary size. Yet, it remains to be

seen whether infants as young as 1;3 to 1;5 can associate novel labels with

novel objects when mutual exclusivity is the only cue to a word mapping.

Therefore, our understanding of how mutual exclusivity impacts vo-

cabulary development can be strengthened by testing whether infants retain

novel word mappings FOLLOWING the mutual exclusivity response. We know

of two studies which have tested for novel wordobject associations fol-

lowing trials in which infants can use mutual exclusivity to disambiguate

the referent of a novel label. Mervis & Bertrand (1994) tested infants aged

1;4 to 1;8 on such a task, and Horst & Samuelson (2008) tested infants aged

2;0. However, in each case, test objects were not equated for familiarity

or prior labelling of the object. Hence, it is difficult to interpret whether

infants responses were guided by their knowledge of the word mapping or

some other process. For example, in Horst & Samuelson (2008) a distractor

object was included during test trials, which was familiarized but not

labelled. The two-year-olds in this study did not display evidence of

M U T U A L E X C L U S I V I T Y A N D W O R D L E A R N I N G

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exploiting mutual exclusivity to form word mappings, yet the inclusion of a

distractor object could have made the task unnecessarily difficult. We also

note that the use of object selection procedures and other task demands

(such as a delay between training and test trials) in Horst & Samuelson

(2008) and Mervis & Bertrand (1994) may have placed a limit on theyoungest age of successful task performance.

We have argued that it is critical to test whether mutual exclusivity is

used to learn words at the youngest age at which they display the response.

To establish evidence of word learning, it is necessary to use an exper-

imental procedure appropriate for the young age of the infants. In contrast

to object selection paradigms, the IPL procedure is a sensitive test of

word learning which does not require an overt behavioural response.

The measurement of infants visual fixations can be used to assess both

the mutual exclusivity response and the formation of novel wordobjectassociations. The use of the IPL procedure as a test of novel word learning

was established by Schafer & Plunkett (1998). In their adaptation of the

IPL paradigm, infants are trained and tested on two novel wordobject

associations to provide a rigorous test of word comprehension. At test, the

two novel objects are paired together, and each object is labelled in turn.

When one of the objects is labelled, a visual preference for one object over

the other can only be caused by knowledge of the novel word mapping.

The use of the IPL procedure provides the opportunity to investigate

other aspects of the mutual exclusivity constraint. An important aspect ofmutual exclusivity is that it is a response to NOVEL words. The use of mutual

exclusivity to learn new words requires the infant to discriminate novel

words from familiar words, and map the novel word to a novel object.

Studies have found that the likelihood or strength of the mutual exclusivity

response is related to the phonological similarity between a novel word

and the name for a familiar object. Merriman & Schuster (1991) found that

two- to four-year-olds were less likely to select a novel object over a familiar

object in response to a novel label when the novel label was phonologically

similar to the familiar object name than if phonologically distinct. White &

Morgan (2008) found that infants aged 1;7 would incrementally increase

attention to a novel object over a familiar object with graded decreases in

the phonological similarity between the novel word they heard and the

familiar object name, suggesting that the perceptual distance between novel

and familiar words influences the perceived novelty of a word.

Infants judgements of novelty can also draw upon their phonological

knowledge beyond words and objects immediately present. Swingley &

Aslin (2007) found that infants aged 1;6 would not map a novel phono-

logical neighbour (i.e. a novel word varying by one phonological segment)

of a familiar word to a novel object, even though the referent of the familiar

word was absent during training. However, their study used ostensive


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naming during training; it is currently not known whether the mutual

exclusivity response would also be constrained by this form of global phono-

logical knowledge. We also note that the studies by Merriman & Schuster

(1991) and White & Morgan (2008) do not assess the impact of phonological

novelty on word learning, just as other investigations of mutual exclusivitydiscussed above have not directly assessed word learning. Hence, the IPL

procedure can be used to understand the impact of phonological novelty

on word learning via mutual exclusivity. Do judgements of phonological

novelty in a mutual exclusivity task influence the formation of novel word

mappings?

We report an IPL experiment with infants aged 1;4 to test whether the

mutual exclusivity response leads to the formation of novel word mappings.

We train and test infants on two novel word mappings. During training,

each novel object is paired with a familiar object, and the novel object isnamed with a novel label. We provide infants with multiple opportunities to

form each novel wordobject association. At test, the two novel objects are

paired together across a series of trials, and each object is labelled equally

often. A further manipulation is that one of the novel words is phono-

logically distinct from familiar words known by infants of this age, while the

other novel word is phonologically similar to several words comprehended

by infants aged 1;4. If infants phonological knowledge is important in the

use of mutual exclusivity to form word mappings, we expect to observe a

difference in their response to the novel words at test. Specifically, theabsence of familiar phonological neighbours to a novel word may facilitate

infants use of mutual exclusivity to form a word mapping. In order to

maintain infants attention across the train and test procedure, we present a

video of the speaker uttering the auditory stimuli during each trial.

M E T H O D

Participants

Thirty infants aged 1; 4 (M=

1; 4.8; Range=

1; 3.231;4.24; 15 male,15 female) contributed data to the experiment. All infants had no known

hearing or visual problems and were recruited via the maternity ward at the

local hospital. All infants were from homes where British English was the

only language spoken. Nine additional infants were tested but excluded due

to: fussiness (4), experimenter error (2), difficulty coding (1), reported

hearing problems (1), or a second language spoken at home (1).

Stimuli

Auditory stimuli were two novel object labels (Meb, Pok), four familiar

object labels (Chair, Clock, Key, Train), and the general directive phrase


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Look at that! . The novel word Meb was selected because it has no

phonological neighbours in the typical lexicon of infants aged 1;4. In con-

trast, the novel word Pok has the phonological neighbour Sock, a word

commonly understood at 1;4. (Analyses of parental vocabulary report are

presented below.) Other neighbours to Pok include Clock, Block and Park.All familiar and novel object labels were embedded in the sentence Look

at the [X]!. Another token of Look! was used as an attention stimulus

between trials. Approximate durations of labels were as follows: familiar

labels ranged between 700900 ms, Meb was 500 ms, Pok was 650 ms and

that in the directive phrase was 650 ms.

Auditory stimuli were produced by a native female speaker of British

English in an infant-directed manner. The speaker was filmed against a

plain, light background, with her head and the top of her shoulders in view.

Filming took place in a quiet room using a Panasonic NV-DS28 camcorder.Video was captured at a frame rate of 25 fps and a sample size of 24 bits.

Sound was captured at a sampling rate of 48 kHz in stereo and a sample size

of 16 bits. Video files were edited using Adobe Premiere Pro to create

separate sound and video files for each utterance. Corresponding sound and

video stimuli were edited to ensure the synchronous onset of vocalization.

Sound files were further edited in Goldwave v5.19 to remove background

noise and match average volume (to x18 dB) across all stimuli.

Visual stimuli were JPGs or AVIs with a display size of 32r24 cm and a

resolution of 300r300 pixels (files were horizontally compressed from 400to 300 pixels to accommodate to a widescreen display). Eight familiar object

images (ball, car, chair, clock, cup, key, shoe, train) were presented during

the experiment, four of which corresponded with the four familiar object

labels. Eight novel object images were of unusual objects that the infants

were unlikely to have seen or heard named (e.g. accordion, anchor, etc.). All

object images were in colour and set against a pale grey background. Seven

video files were of the speaker uttering one of four familiar object labels,

two novel labels and a neutral phrase. Examples of the objects and a static

image of the speaker are provided in Figures 1 and 2. An 8 cm square image

of a red cross against a white background was used as an attention stimulus

between trials.

Design

The experiment had two parts, a training phase and a test phase. During the

training phase, infants viewed trials in which they could use the mutual

exclusivity response to form two novel objectlabel associations. During the

test phase, trials were presented to test for infants learning of these novel

word mappings. During all trials, two images were presented, one on the

left and one on the right of the screen. A video of the speaker was presented


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in a central position between the two images, designed to maintain infants

interest for the duration of the experiment.

Every training trial presented one familiar object and one novel object.

There were three different types of training trials: FAMILIAR LABEL trials,

Look at that!

Look at the Pok!

Look at that!

Look at the Meb!

Fig. 1. Example of a training block.

Look at the Pok!

Look at the Meb!

Look at the Pok!

Look at the Meb!

Fig. 2. Example of a test block.


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presenting the name for the familiar object; NOVEL LABEL trials, presenting

a novel name for the novel object; and CONTROL trials, presenting a

general directive phrase. The familiar label trials were included to assess

infants responses to familiar word mappings. The novel label trials were

designed to both elicit the mutual exclusivity response and provideinfants with the opportunity to form a novel word mapping. Control

trials were included to determine whether infants responses to novel

label trials were specific to hearing a novel label, or a more general atten-

tional process (e.g. looking behaviour in the absence of hearing a familiar

label).

During the experiment there were eight unique trials: four familiar label

trials, two novel label trials and two control trials. A different pair of objects

was presented in each of these eight different trials. The two novel label

trials and two control trials were blocked together (see Figure 1), and thisblock was presented four times during training. Each presentation of this

block was preceded by one of the familiar label trials. Thus, each of the four

familiar labels was presented once, while the two novel label and two con-

trol trials were presented four times during the experiment. Within each

block, two different novel labels (Meb, Pok) were presented on the two

novel label trials, while both control trials presented the phrase Look at

that (see Figure 1). Trials within each block were presented in the same

order across the experiment.

Four familiar objects ball, car, cup, shoe were presented during novellabel or control trials. Four other familiar objects chair, clock, key,

train were presented during familiar label trials. For the two novel label

and two control trials, each infant received one of four possible sets of

object pairs. Each of these sets consisted of the same four familiar objects

and four novel objects, but with a different pairing of objects for each set.

Across these sets, every object was paired with two other objects, once as a

novel label trial and once as a control trial. Consequently, across infants,

the two novel labels were counterbalanced across the four possible novel

referents. Side of image presentation was counterbalanced for each of the

novel label and control trials across the training blocks, and the familiar and

novel images appeared an equal number of times to the left and right across

trials within a block.

During the test phase, each trial presented the two novel objects from

the novel label trials. Each novel label was presented on two test trials,

counterbalancing for side of target presentation (see Figure 2). Each infant

received a different randomization for the order of trials in training and test

blocks. All trials during the experiment lasted 6 s, with the onset of naming

(or the onset of that) at 2633 ms into the trial. Allowing for a response

latency of 367 ms (Swingley & Aslin, 2000), trials were split into equal 3 s

pre- and post-naming phases.


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Procedure

Prior to participation, all parents were requested to complete a British

adaptation of the MacArthur-Bates Communicative Development Inventory

(the British CDI; see Hamilton, Plunkett & Schafer, 2000). Infants sat on

their parents lap facing a flat widescreen display (110r40 cm) with their

eyes at a distance of approximately 80 cm, level with the vertical midpoint

of the images and at an equal horizontal distance from both object images.

Object images were positioned on screen at a distance of 67 cm centre-to-

centre. Two cameras mounted directly above the horizontal midpoints of

each image recorded infants eye movements. Synchronized signals from

the cameras were routed via a digital splitter to create a recording of two

time-locked images of the infant. Auditory stimuli were delivered via two

loudspeakers centrally positioned side-by-side above the display. Caregivers

were requested to keep their eyes closed, to wear headphones playing musicand to not point at the screen. Trials were manually launched by the

experimenter when the infant was looking towards the screen. If the infant

was looking away between trials, the auditory and/or visual attention stimuli

were presented to return the infants gaze to centre.

Scoring

A digital video off-line scoring system was used to assess infants eye

movements on a frame-by-frame basis (every 40 ms) by a skilled blindcoder. This technique enabled tracking of every fixation, coded either

as left looking, right looking, centre looking or other looking. Coding

reliability was assessed for all twenty-four trials by a second blind coder for

a random sample of 20% of infants who contributed data to the experiment

(N=6). The mean intra-class correlation coefficient was 0.978 (Range=

0.9220.998).

R E S U L T S

CDI analysis

British CDIs were completed by caregivers for 26 out the 30 infants

who participated in the experiment. Mean comprehension vocabulary was

157 words (Range=41348 words) and mean production vocabulary was

28 words (Range=0177 words). Reported comprehension was high for the

four familiar objects presented during novel label and control trials: Ball

(85%), Car (92%), Cup (73%) and Shoe (96%). Reported comprehension for

the four familiar objects presented during familiar label trials was somewhat

lower: Chair (77%), Clock (46%), Key (50%) and Train (73%). However,

Houston-Price, Mather & Sakkalou (2007) report parental underestimation

of comprehension vocabulary on the British CDI for infants aged between


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1 ; 3 and 1 ; 9, suggesting that the actual level of comprehension for the

familiar objects may be higher than reported.

The British CDI contains four words which can be classified as phono-

logical neighbours to the novel word Pok : Block, Clock, Park and Sock.

A high proportion of infants were reported to understand the word Sock(96%), with smaller numbers of infants reported to understand the words

Clock (46%), Block (42%)1 and Park (38 %). The British CDI does not

contain any words which are phonological neighbours to the novel word Meb.

Speaker video analysis

Prior to analyzing infants attention to the training and test images, we

examined how much time they spent looking at the speaker video. Averaged

across all 24 experimental trials, infants mean total looking time to thespeaker was 3543 ms (SD=664 ms) out of a maximum of 6000 ms total trial

duration. Thus, on average, the infants spent just over half the trial

duration looking at the video of the speaker. Infants total looking time

to the speaker declined significantly across the experiment (b=x0.846,

t(22)=x7.45, p


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Plunkett, Harris & Dimmock, 2004). We also note that because the infants

were typically fixating the speaker video at the onset of naming (see

Figure 4), we cannot use standard latency analyses (e.g. Swingley & Aslin,

2000) of shifts between the target and distractor images.

A longest look difference (LLD) measure (longest look to the familiarobject minus longest look to the novel object) was calculated for the pre- and

post-naming phases of all 20 training trials for each infant. Initially, we

assessed whether the LLD measure is a reliable index of infants compre-

hension within our speaker video paradigm. The LLD measure was

averaged across the four familiar label trials for each infant (one infant was

excluded from these analyses for not looking at either image during the

post-naming phase of these trials). There was a highly reliable effect of

naming, with the mean LLD increasing from x24 ms (SD=336 ms) to

497 ms (SD=565 ms) (t(28)=4.45, p


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Test analysis

The LLD measure (longest look to the target minus longest look to the

distractor) was calculated for the pre- and post-naming phases of each test

trial for every infant. These scores were averaged across trials for each of

the two novel labels, and entered into a repeated-measures ANOVA with

the factors of naming (pre-naming, post-naming) and label (Meb, Pok).

Because six infants did not contribute data to one cell of the namingrlabel

design, a missing value analysis was computed using the Expectation-

Maximization algorithm (Dempster, Laird & Rubin, 1977), for the purpose

of analyzing the data as a repeated-measures design. These values were

entered into the main ANOVA (but not included in further analyses).

There was a significant interaction between naming and label (F(1, 29)=

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Pok trials, there was a non-significant decrease on the LLD measure from

x36 ms (SD=658 ms) to x99 ms (SD=709 ms) (t(26)=0.322, n.s.). A

binomial test revealed that a significant proportion of infants (20 out of

27 infants) had a preference for the target object during the post-naming

phase of Meb trials (p


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1;4 would use mutual exclusivity to form novel objectlabel associations.

We found that the infants did not display an overt mutual exclusivity

response during training. Yet, at test, the infants provided evidence of

learning an association for the novel label Meb, but not for the novel label

Pok. We provide further discussion of the role of phonological noveltybelow. We note here that attention to the correct referent of at least one

label can only be caused by knowledge of the objectlabel association.

Furthermore, acquisition of the objectlabel association could only have

occurred during the training phase of the experiment, where novel label

trials offered infants the opportunity to exploit mutual exclusivity to form a

new objectlabel mapping. Our results, therefore, demonstrate that infants

aged 1;4 can use mutual exclusivity as a cue to form novel word mappings.

Although the precise pattern of findings differs from previous studies of

mutual exclusivity in infants (e.g. Halberda, 2003; Markman et al., 2003),we nonetheless provide convergent evidence that infants as young as 1;4

can map a novel label to a novel object in an ambiguous context. Our

finding reinforces the argument that mutual exclusivity contributes to

infants vocabulary development during the first half of the second year, and

that infants can exploit their existing vocabulary to learn new words. Our

findings do not discriminate between different explanations of the mutual

exclusivity response (e.g. Clark, 1987 ; Markman, 1989; 1990; Mervis &

Bertrand, 1994). However, our findings show that whichever process is

responsible for this word-learning bias, it also leads infants to form novelobjectlabel associations.

The most notable difference between our findings and those of Halberda

(2003) and Markman et al. (2003) is that we do not demonstrate the mutual

exclusivity response per se. During training, the infants did not increase

attention to a novel object over a familiar object upon hearing a novel label.

Although it is not clear why they failed to do so, the presence of the speaker

video drew infants attention away from the objects, and may have conse-

quently altered their pattern of attention to the objects. However, successful

target looking during familiar label trials was not associated with greater

overall attention to the objects than during novel label trials. Thus, the

reduced attention to the objects might not itself be the reason for the

absence of the mutual exclusivity response. A related possibility is that

infants interest in the familiar objects affected their pattern of attention to

the novel objects. A bias towards familiar, nameable objects has been

reported in other studies (e.g. White & Morgan, 2008; Schafer, Plunkett &

Harris, 1999; see also Halberda, 2003). In contrast, the absence of dis-

tracting familiar objects in the test trials may have provided a more sensitive

test of comprehension.

Other investigators (e.g. Halberda, 2003; White & Morgan, 2008) report

an increase in attention to a novel object in response to a novel label.


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However, it is not clear that infants look MORE to the novel object than the

familiar object particularly if they have a preference for the familiar object

prior to naming (White & Morgan, 2008; Schafer et al., 1999). Thus,

although some processing of the novel object is necessary, infants use of

mutual exclusivity to learn new words might not require sustained attentionto a novel object. In support of this position, Mather & Plunkett (2009)

found that infants aged 1 ;10.15 did not display a mutual exclusivity re-

sponse the first time they were presented with a novel label and familiar and

novel object. However, upon re-presentation of the stimuli, the infants

looking behaviour suggested that they had formed an association between

the novel label and novel object. Therefore, processing of the relation

between the novel label and novel object occurred during the original

presentation, but this was not evidenced by any preferential fixations of the

objects.In combination with previous investigations, our findings suggest that

1;4 is a threshold age for word learning via mutual exclusivity, and as such,

an age where infants are sensitive to the learning context. Markman et al.

(2003) found that a group of infants aged 1;3 to 1;5 displayed the mutual

exclusivity response in an object search task. Yet, in Halberda (2003), only

infants aged 1;5 displayed evidence of mutual exclusivity. At 1;4, infants

did not display a systematic response. In contrast, we have evidence of

mutual exclusivity at 1;4 in an IPL procedure similar to Halberda (2003).

It is possible that the infants aged 1 ; 4 in Halberdas study might havedisplayed evidence of forming a novel word mapping if they had been tested

for subsequent word learning. Alternatively, the inclusion of the speaker

video in the present experiment may have facilitated the use of mutual

exclusivity, even if it reduced attention to the objects. Unlike Halberda

(2003), the speaker video adds a social dimension to the learning context,

similar to Markman et al. (2003). Thus, while mutual exclusivity enables

infants to form word mappings in the absence of social or pragmatic cues,

the presence of a social context might facilitate the use of mutual exclusivity

to learn words.3

A further experimental manipulation involved training infants on one

novel label without any known phonological neighbours (Meb) and on

another novel label with several known phonological neighbours (Pok).

[3] We ran an additional experiment in response to an anonymous reviewers suggestion, inwhich we removed the speaker video from the display. All other aspects of the exper-imental design were identical. The exclusion of the speaker video caused a substantialincrease in attention to the objects during training. Mean total looking during trainingtrials was 2046 ms in the main experiment, and 5417 ms in the control experiment. Yet,

there was no evidence of mutual exclusivity during either the training or test trials of thecontrol experiment. The control experiment provides further support for the suggestionthat the presence of the speaker video in the main experiment facilitated word learning.


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Consistent with our predictions, we found that the infants formed a novel

mapping for the label Meb, but not for the label Pok. The counterbalanced

design of the experiment precludes the possibility that infants looking

during test trials was caused by their preference for particular objects.

Across infants, four novel objects were counterbalanced as the referent ofthe two novel labels. Furthermore, the side of presentation of the target

object was alternated for both labels at test. Hence, the asymmetry in

infants word learning is most likely an outcome of the phonological

properties of the labels. An alternative explanation to the phonological

neighbourhood account is that the infants had a spurious preference for one

label over the other, affecting their ability to map the labels. Although this

may have been true for individual infants, such an effect is unlikely to hold

across a group of infants. For example, we know that words containing the

phonemic segments present in Meb and Pok are prevalent in infants earlyvocabularies, and thus unlikely to create idiosyncratic preferences.

A more parsimonious account is that systematic differences between the

labels at the lexical level influenced infants ability to map them to novel

objects. Swingley & Aslin (2007) obtained a similar effect where the

phonological neighbourhoods of novel words affected the ability of infants

aged 1;6 to map novel labels onto novel objects during an ostensive naming

procedure. We extend these findings by demonstrating that phonological

neighbourhoods can also impact the use of mutual exclusivity to form novel

word mappings. Our findings also extend the work of Merriman & Schuster(1991) and White & Morgan (2008), by demonstrating that the effect of

phonological novelty might extend beyond the immediate naming context to

draw upon the full scope of infants lexical knowledge. Although the object

sock was not seen or labelled during the experiment, the phonological

distance between the two words may nonetheless have interfered with

forming a word mapping.4 Infants ability to discriminate between novel

words and mispronunciations of familiar words may be critical to the use of

mutual exclusivity to learn new words.

From as early as 1;4, infants can map a novel label to a novel object,

using only the difference in whether or not an object is name-known as the

cue to the mapping. Even before the middle of the second year, infants can

learn words in the absence of explicit cues from the speaker, drawing upon

their previous learning experiences to guide how they form novel word

mappings. Yet, this ability is not without limits. When there is no explicit

guidance from the speaker, the infant has to accurately identify novel word

[4] While we have identified sock as the main phonological competitor to pok, otherphonological neighbours may have played a role. For example, an image of a clock was

named during the second familiar label trial of the experiment. Either the presentation ofthe label or the priming of infants lexical entry for clock could have enhanced aphonological competitor effect.


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forms which have not already been mapped onto other object categories.

One important avenue for future research will be to investigate whether

there are developmental changes in how phonological similarity constrains

the mutual exclusivity mechanism. Our findings highlight the importance of

the novelty of both the object and the label for successful use of the mutualexclusivity constraint.

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