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Running head: NORM
-
BASED CODING OF FACE IDENTITY
IN FOUR YEAR
-
OLDS 1

Four year
-
olds use norm
-
based coding for face identity Linda
Jeffery, Ainsley Read, Gillian Rhodes

ARC Centre of Excellence
in Cognition and its Disorders School of Psychology, The
Univer
sity of Western Australia

Accepted for Cognition 29 January, 2013

Word Count: 2998 (including abstract)

Author Note

We thank the staff, students and parents who participated, Mayu
Nishimura and Daphne Maurer for co
-
creating the “Robbers
Task”, Elinor McKon
e, Elizabeth Pellicano, Kate Crookes and
Elizabeth Taylor for helpful discussions. This research was
supported by Australian Research

Running head: NORM
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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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OLDS 2

Council Discovery Grants DP0770923, DP0877379 and by
ARC
Centre of Excellence Grant CE110001021.

Correspondence concerning this article should be addressed to
Linda Jeffery, School of Psychology M304, The University of
Western Australia, 35 Stirling Highway, Crawley, W.A., 6009,
AUSTRALIA. Email: linda.jeffe
ry@uwa.edu.au

Running head: NORM
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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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OLDS 3

Abstract

Norm
-
based coding, in which faces are coded as deviations from
an average face, is an efficient way of coding visual patterns that
share a common structure and must

be distinguished by subtle
variations that define individuals. Adults and school
-
aged children
use norm
-
based coding for face identity but it is not yet known if
pre
-
school aged children also use norm
-
based coding. We reasoned
that the transition to schoo
l could be critical in developing a norm
-
based system because school places new demands on children’s
face identification skills and substantially increases experience
with faces. Consistent with this view, face identification
performance improves steeply
between ages four and seven. We
used face identity aftereffects to test whether norm
-
based coding
emerges between these ages. We found that four year
-
old children,
like adults, showed larger face identity aftereffects for adaptors far
from the average than

for adaptors closer to the average, consistent
with use of norm
-
based coding. We conclude that experience prior
to age four is sufficient to develop a norm
-
based face
-
space and
that failure to use norm
-

based coding cannot explain four year
-
old
children’s

poor face identification skills.

Words 182

Keywords:
face perception, development, norm
-
based
coding, aftereffects,

face
-
space

Running head: NORM
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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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OLDS 4

Four year
-
olds use norm
-
based coding for face identity

Faces convey a wealth of information t
hat we use to guide our
social interactions. As adults we swiftly extract information about
identity, gender, ethnicity, age and emotional state from faces.
Face identification, in particular, requires exquisite sensitivity to
very subtle differences betwe
en highly similar visual patterns.
Norm
-
based coding represents an efficient and elegant solution to
this problem of representing visual patterns that share a common
structure and must be distinguished by subtle variations that define
individuals. A norm
-
b
ased system represents what is distinctive
about each face by coding how faces deviate from a perceptual
norm or prototype (see Figure 1). This system may be more
efficient than one that codes a complete structural description,
most elements of which are s
hared by all faces and therefore
redundant. Moreover, face norms are updated by experience, fine
-
tuning our face perception coding mechanisms to our diet of faces
(e.g., Rhodes, Jeffery, Watson, Clifford, & Nakayama, 2003). A
variety of behavioural and neu
rophysiological evidence supports
use of norm
-
based coding for facial identity in adults (see Rhodes
& Leopold, 2011, for a review). The functional importance of
adaptive norm
-
based coding is suggested by evidence that stronger
adaptive norm
-

based coding
of faces is associated with better face
identification performance (Dennett, McKone, Edwards, & Susilo,
2012) and that groups with known face perception difficulties
show reduced adaptive norm
-
based coding of facial identity
(Congenital Prosopagnosia
-

Pal
ermo, Rivolta, Wilson, & Jeffery,
2011; Autism Spectrum Disorders
-

Pellicano, Jeffery, Burr, &
Rhodes, 2007).

Preschool aged children perform poorly on face identification
tasks, relative to older children and adults (Bruce et al., 2000) but
the source of

their poor performance is controversial

Running head: NORM
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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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(Crookes & McKone, 2009; McKone, Crookes, Jeffery, & Dilks,
2012). Early research suggested that a key mechanism of face
perception, holistic codi
ng (the integration of information across a
face, including features and the spatial relations between them)
was either absent or immature in young children (Carey, Diamond,
& Woods, 1980; Diamond & Carey, 1977). However, more recent
studies have establish
ed that holistic coding is present by age three
(Macchi Cassia, Picozzi, Kuefner, Bricolo, & Turati, 2009) and is
mature by five (see McKone et al., 2012; Pellicano & Rhodes,
2003). Therefore poor face identification performance in young
children cannot be

attributed to immaturities in holistic coding.
Here we ask whether failure to use norm
-
based coding, another
key mechanism of face perception, could explain young children’s
poor performance.

Running head: NORM
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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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O
LDS 6


Figure 1.
A simplified face
-
space showing two dimensions, two
target identities (Ted and Rob) and the average face in the centre.
In a norm
-
based model each identity is positioned on its own
unique trajectory that passes through the average. For ea
ch face we
can create an opposite “antiface” that lies on the same trajectory as
the target but on the opposite side of the average (e.g., antiTed and
antiRob). “Weaker” versions of each target are made by morphing
each target with the average face by diff
erent amounts e.g. 60%
Ted and 30% Ted as shown here. Adapting to antiTed facilitates
recognition of Ted, so that “weaker” versions of Ted are more
accurately identified and the average face takes on the appearance
of Ted.

Norm
-
based coding of facial ident
ity has been demonstrated in
adults and 7
-
9 year
-
old children using face adaptation techniques
(e.g., Jeffery et al., 2011; Leopold, O'Toole, Vetter, & Blanz, 2001;
Rhodes & Jeffery, 2006; Robbins, McKone, & Edwards, 2007;
Webster & MacLin, 1999). Adaptati
on (exposure) to faces biases
the appearance of subsequently viewed faces so that they look
wider after seeing narrow faces, more male after seeing female
faces, and so on (e.g.,

Running head: NORM
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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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OLDS 7

Webster,

Kaping, Mizokami, & Duhamel, 2004). In a norm
-
based
account these “aftereffects” are argued to reflect opponent coding,
with pairs of neural channels tuned to above
-

and below
-

average
values on each dimension in face space and the norm signaled by
balanc
ed activity in both pools. The further the adaptor is from the
norm the larger the effect of adaptation on an average test face
because more extreme adaptors produce greater activation, and
greater subsequent suppression, in the preferred channel (Rhodes
e
t al., 2005; Robbins et al., 2007). Alternative models of face
coding that do not posit a norm, e.g., exemplar coding, neurally
instantiated by multichannel coding, predict a different pattern.
Multichannel models predict that small increases in the distan
ce of
the adaptor from the average face will increase aftereffects but
thereafter increasing adaptor distance will reduce aftereffects
because these extreme adaptors will have less effect on channels
that code the average face (Clifford, Wenderoth, & Speha
r, 2000;
Dickinson, Almeida, Bell, & Badcock, 2010). The pattern of
aftereffects predicted by norm
-
based coding has been found for
adults (Robbins et al., 2007) and 7 year
-
old children (Jeffery et al.,
2011).

Here we tested whether norm
-
based coding for fa
cial identity
emerges earlier in development, by measuring four
-
year
-
olds’ face
identity aftereffects for near and far adaptors. Four year
-
olds are
the youngest age group who do not yet to attend school but can
complete adult
-
like adaptation tasks. The tra
nsition from preschool
to primary (grade) school typically results in a substantial and
sudden increase the number of individuals that children need to
distinguish and remember. This sudden increase in experience may
drive critical changes in face
-
space or
ganisation, consistent with
the assumption that face
-
space is built via experience with faces
(Johnston & Ellis, 1995; Valentine, 1991) and proposals that
changing demands on face perception skills during development
prompt reorganisation of the face
-
perce
ption system

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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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(Scherf & Scott, 2012). Certainly performance appears to improve
more steeply between these ages than during later childhood
(Bruce et al., 2000; Ellis, 1992; Kinnunen, Korkm
an, Laasonen, &
Lahti
-
Nuuttila, 2012), consistent with a qualitative shift in
processing between four and seven years of age. While four year
-
old children possess at least a rudimentary face
-
space that
produces distinctiveness effects (e.g., McKone & Boyer
, 2006) and
produces atypicality biases (Tanaka, Meixner, & Kantner, 2011) it
is has not been established that their face
-
space is norm
-

based. It
remains possible that young children represent faces in an
exemplar manner (neurally instantiated by multicha
nnel coding)
until sufficient demands are placed on their face
-

recognition
system to prompt reorganisation of face
-
space to be norm
-
based.
Increased exposure to faces at school may also be crucial in
developing a representation of the average face that is

sufficiently
good to function as a norm, or in developing sufficiently good
representations of the dimensions on which to code individual
faces as deviating from this norm.

We sought evidence of norm
-
based coding of face identity in four
year
-
olds using t
he face identity aftereffect in which adaptation to
an individual face (e.g. Ted) biases perception of an identity
neutral average face so that it resembles an individual with
characteristics opposite the adapting face (e.g., antiTed, see Figure
1). Identi
ty aftereffects directly tap face identification processes
(Rhodes, Evangelista, & Jeffery, 2009) and have not previously
been demonstrated in young children. We asked whether children’s
identity aftereffects would be larger for more extreme versus less
ex
treme adaptors, consistent with norm
-
based coding.

Participants

Method

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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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Sixteen children (
M
= 4:5 years, range 4:0


5:1, 4 female) were
recruited from a preschool in Perth, Western Austra
lia. Data from
three additional children were removed from all analyses. Two
failed to learn the target faces and one was an outlier (see Results).
Seventeen undergraduates from the University of Western
Australia participated for course credit (
M
= 18 yea
rs, range 17


22 years, 16 female). Participants were primarily
Caucasian/European (16 adults, 11 children). Written consent was
obtained from adult participants and the children’s
parents.

Stimuli

The stimuli were taken from Jeffery et al., (2011). Grays
cale
photographs of two male identities (Ted and Rob) comprised the
targets. Reduced strength versions were made by morphing each
face toward an average face (constructed from 20 adult male faces)
using Gryphon Morph 2.5 (Maxwell, 1994), in varying increme
nts
to produce 40%, 60% and 80% versions of each target. The 40%
and 60% stimuli were used only in training and the 80% and
average (0%) stimuli were used as test faces (see Figure 2). Test
and training faces subtended a visual angle of 6.0° x 5.6° when
vi
ewed from approximately 45cm. For each target face, two
adapting stimuli (antifaces) were made by caricaturing the average
face away from each target (after Leopold et al., 2001) by 40%
(near adaptors) and 80% (far adaptors) using Gryphon Morph (see
Figure

2). These levels were chosen because both are sufficiently
extreme that a multichannel model would be unlikely to predict an
increase in adaptation across this range. The textures of the
average were applied to all stimuli. Adapting stimuli were larger
th
an test stimuli (7.5° x 7.4°) to rule out purely low
-
level
(retinotopic) adaptation as the source of any aftereffects (Zhao &
Chubb, 2001).

Procedure

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BASED CODING OF FACE IDENTITY
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Children and adults were tested indi
vidually in quiet rooms at their
preschool and the university, respectively. The task was presented
on a Mac Powerbook Pro (15
-
inch LCD screen with anti
-
glare
covering), using Cedrus Superlab 4.06 software (Abboud, Schultz,
& Zeitlin, 2008). The task was p
resented as a “Robbers Game” for
both children and adults. Participants began with training in
recognizing the targets (Ted and Rob) and familiarization with
lower strength versions of the targets, who were introduced as the
brothers of each target. Partic
ipants were told to respond with the
team leader’s name (target) whenever they saw him or a member
of his team (his brothers). Children responded verbally and adults
used labeled keys (“x” key/Ted, “,” key/Rob). Auditory feedback
(beeps) was provided for c
orrect and incorrect responses
throughout training.

Running head: NORM
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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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Figure 2.
The adapting stimuli for (A) near adaptors and (B) far
adaptors. (C) Shows the test (black rectangles) and training stimul
i,
with identity strength and target identity indicated below.

Participants then completed the adaptation task. The experimenter
explained that a robber’s face (adapting antiface) would appear on
the screen, “while he was stealing things”, and then it woul
d
disappear and be followed by a very brief presentation of the face
of the team member (test stimulus) who caught the robber.
Presenting the adapting face as a threatening individual is likely to
boost attention to the faces in both children and adults (e
.g.,
Chiappe et al., 2004; Kinzler & Shutts, 2008). The participant was
asked to identify the catcher’s team. Two practice trials followed.
The task comprised 80 trials. The average face was the test face on
48 trials, which comprised 12 trials with each a
dapting face
(antiTed/antiRob) at each adaptor distance (near/far). The
remaining 32 trials featured 80% targets as test faces, with four
trials for

Running head: NORM
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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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OLDS 12

each target (Ted/Rob) with each adapt
ing face, at each adaptor
strength. The 80% test faces were included to check that
participants could accurately identify “strong” versions of the
faces. Adapting antifaces were shown for 5000ms followed by a
150ms inter
-
stimulus interval. The test stimulu
s was then displayed
for 400ms followed by a blank gray screen that remained until a
response was registered. The experimenter initiated the next trial
for children and adults did so themselves by pressing the spacebar.
The experimenter sat beside the chil
dren and ensured that they
looked at the adapting faces for the entire time they were shown.

The trials were presented in a pseudo
-
random order and divided
into eight blocks of ten trials and completed over two sessions,
conducted on different days. Each s
ession contained four blocks
and was preceded by the training and practice described above. To
maintain attention, each block featured an “escape” trial, in which
the adaptor was followed by images of the robber’s victim and the
“loot” that the robber has
escaped with, rather than a test face.
Children received stickers after each block and a confectionary
item at the end of the second session. Each session took
approximately 15 minutes.

Results

To measure adaptation we calculated the proportion of times th
e
average (0%) face was identified as the face opposite the adaptor
(i.e., the proportion of “Ted” responses after adaptation to antiTed
and the proportion of “Rob” responses after adaptation to antiRob),
in both adaptation conditions (near/far), for each
participant. An
aftereffect is indicated by greater than chance (0.5) performance.
One female child’s scores were more than three standard
deviations below the mean, for both near and far conditions, and
her data were removed.

The mean proportions are show
n in Figure 3. Performance was
significantly above chance for each age group, in each condition,
indicating that adaptation biased perception toward

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BASED CODING OF FACE IDENTITY
IN FOUR YEAR
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the opposite identity as predicted (C
hildren: near
t
(15) = 8.44,
p
<
.001,
d
= 2.25; far
t
(15) = 6.74,
p
< .001,
d
= 1.70; Adults: near
t
(16) = 3.79,
p
= .002,
d
= 0.87; far
t
(16) = 7.10,
p
< .001,
d
=
1.82).

Crucially, both children and adults showed larger aftereffects for
far than near ada
ptors, as predicted by norm
-
based coding. This
pattern was confirmed by ANOVA
1
. We found a main effect of
Adaptor Distance,
F
(1, 31) = 30.74,
p
< .001,

p
2
= 0.50 (near
M
=
0.58,
SD
= 0.07; far
M
= 0.68,
SD
= 0.11). This effect was not
moderated by age, with neither the interaction,
F
(1, 31) = 1.51,
p
=
.229,

p
2
= 0.05, nor the main effect of Age,
F
(1, 31) = 0.01,
p
=
.938,

p
2
< 0.01, being significant.

Planned t
-
tests confirmed that
aftereffects were larger for far than near adaptors for each age
group (children
t
(15) = 3.14,
p
= .007,
d
= 1.05; adults
t
(16) =
4.69,
p
< .001,
d
= 1.35). Further, the majority of participants in
each age group showed larg
er aftereffects for far than near
adaptors (11/16 children and 14/17 adults). These proportions did
not differ significantly for children versus adults (Fisher Exact Test
p
= .438).


1
Levene’s test indicated unequal variances for the near condition.
Tran
sformation of the data could not equalize the variances.
However, we note that an independent t
-
test, corrected for unequal
variances, showed no significant difference in the size of children’s
and adults’ aftereffects, t(25.31) = 0.904, p = .374.

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head: NORM
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BASED CODING OF FACE IDENTITY
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Figure 3.
The mean proportion of responses opposite the adaptor
for children and adults for near and far adaptors. The dotted
horizontal line shows chance performance, the level expected if
ad
aptation failed to bias perception to the opposite identity. Errors
bars show one standard error either side of the mean.

Finally, we note that all participants performed well at identifying
the high strength test stimuli (80%), confirming that they had
le
arned the targets and maintained good performance throughout
the task (Children
M
= 0.84,
Range =
0.69


0.97; Adults
M
=
0.98,
Range
= 0.84


1.00). Adults were significantly more
accurate than children, as confirmed by Mann
-
Whitney U
-

tests
2
(Near;
U
= 1
0.00,
p
< .001,
r
=.84, Far;
U
= 48.5,
p
= .001,
r
=.58,
Overall;
U
= 13.5,
p
< .001,
r
=.79).

2
Accuracy data were highly skewed for both children and adults so
a non
-
parametric test was used.


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BASED CODING OF FACE IDENTITY
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Discussion

We have shown for the first time that four year
-
old children use
norm
-
based (two
-
pool) coding of identity in face
-
space. Face
identity aftereffects were larger for adaptors that are far from the
average than for adaptors close to the a
verage, as predicted by a
norm
-
based model. Furthermore, children did not differ from
adults in their sensitivity to the distance of the adaptor from the
average. Therefore, we conclude that experience prior to age four
is sufficient to develop a norm
-
base
d face
-
space and that failure to
use norm
-
based coding cannot explain four year
-
old children’s
poor face identification skills.

Previous research indicated that young children possess at least a
rudimentary face
-
space that produces distinctiveness effects
(4
year
-
olds, McKone & Boyer, 2006) and face identity aftereffects
(5 year
-
olds, Jeffery et al., 2011). However, an exemplar
-
based
face
-
space can also account for these effects (Robbins et al., 2007;
Valentine, 1991). One prior study provided suggestive ev
idence of
norm
-
based coding in five year
-
olds (Jeffery et al., 2010).
However this study could not establish norm
-
based coding of face
identity because the effects could equally have arisen from generic
shape coding mechanisms (see Jeffery et al., 2011). T
he present
study rules out an exemplar organization of face
-
space in young
children and demonstrates that norm
-

based coding arises from
face identity coding mechanisms.

How early in development might norm
-
based coding of face
identity emerge? Our results
show that experience with faces
gained in the first four years of life is sufficient to develop a norm
-
based face
-
space. However, it is possible that much less experience
is required. The foundations of norm
-
based coding may be present
very early. Infants
respond to an average face


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as if it is familiar within a few months of birth, suggestive of a
prototype effect (de Haan, Johnson, Maurer, & Perrett, 2001).
Five
-
to
-
eight month old infa
nts also discriminate faces differing in
averageness/distinctiveness (Rhodes, Geddes, Jeffery, Dziurawiec,
& Clark, 2002). These findings are consistent with norm
-
based
coding in infancy, although they are certainly not conclusive
because exemplar models c
an account for both distinctiveness and
prototype effects (e.g., Nosofsky, 1988).

In conclusion we have established that children’s face
-
space is
adult
-
like, in that it appears to be norm
-
based by age four.
Therefore two key mechanisms of face perception,
norm
-

based
coding and holistic coding both emerge prior to age four,
consistent with the view that face perception mechanisms are
qualitatively mature relatively early in development (Crookes &
McKone, 2009; McKone et al., 2012). Experience gained with
fa
ces prior to starting school is sufficient to develop a norm
-
based
face
-
space. Therefore changes in the fundamental organization of
face
-
space are unlikely to be the source of improvements in face
recognition beyond the preschool years. Our results leave o
pen the
possibility that there are more subtle changes in face
-
space, such as
refinement of the number and nature of the dimensions of face
-

space, that could underlie the improv
ement in face identification
performance with age.

Capacity to store more individuals and/or
speed of identification may also increase during childhood,
consistent with age
-
related increases in the size of face
-
selective
cortical regions (e.g., Golarai et
al., 2007; though see McKone et
al., 2012). Finally, general cognitive or perceptual changes that are
not specific to face perception, such as Vernier acuity or working
memory (Crookes & McKone, 2009) could account for some or all
of the improvement in fac
e identification performance during
childhood.

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