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Affective processes are asymmetrically represented inthe
brainUsing a variety of methods to make inferences aboutregionally
specic patterns of activation, many investi-gators have now
reported systematic asymmetries inpatterns of activation in specic
brain regions in responseto certain types of positive and negative
affective chal-lenges (for reviews, see [4,5] ). Despite the
complexitiesassociated with aggregating studies with vastly
differentexperimental designs, a recent meta-analytic review
hasalso supported the notion that certain forms of positiveand
negative emotion exhibit different patterns of func-tional brain
asymmetry, particularly in prefrontal corticalterritories [5] .
Complementing this work, Heller andcolleagues have proposed that
asymmetries in parietalcortex may be associated with arousal such
that greaterright-sided posterior activation is associated with
higherarousal emotion [6] . Our own work has also highlightedthe
importance of considering individual differences andhas
consistently found that subjects exhibit stable differ-ences in
asymmetric patterns of activation in prefrontal
brain regions that predict various features of
affectivereactivity [7] .This asymmetrical organization of affect
in the two
hemispheres of the brain forms the conceptual backdrop
toinvestigations of how emotion acts asymmetrically on thetwo sides
of the face. However, the control of the facialmusculature is
complex, with different patterns of neuralinnervation present for
the upper versus lower face(e.g. [8]). Most of the extant
literature broadly suggeststhat the lower face is predominantly
under contralateralcontrol compared with the upper face (e.g. [8,9]
; but see[10] ). We would therefore expect to nd evidence of
asymmetrical facial expression more in the lower facecompared with
the upper face, although this distinctionwas not examined by
Nicholls and co-workers. Based uponresearch that has measured
regional brain activationduring emotion, one would be led to make
the followingpredictions, particularly for the lower facial region:
(i)overall, emotional expressions should be more left-sidedcompared
with non-emotional expressions since arousalwould be assumed to be
more elevated during the formercompared with the latter states; and
(ii) that negativeemotions should exhibit the greatest
left-sidedness in lightof the role of right prefrontal regions in
negative affect. Of course, these hypotheses are predicated on the
view thatboth prefrontal and parietal activation asymmetries
willmodulate activation in premotor regions that innervatethe
facial nucleus. The extent to which this is true remainsto be
empirically veried.
The Nicholls et al. studyThere are several notable features of
the recent report byNicholls et al. (compare with [11,12] ) that
represent apotentially signicant advance over other efforts in
thepast (for reviews, see [13,14] ).
First, they used a sensitive 3D imaging technique tocapture
facial behavior. This method measures movementthat is perpendicular
to the facial surface and could detectmovement in this plane to an
accuracy of 0.16mm, relativeto a neutral face baseline. Second,
they had observers rate
facial expressions that were digitally rotated 35 8 to theright
(so that the left side of the face is featured) or 35 8 tothe left
(so that the right side of the face is featured) toexamine the
differential impact of exposure to the leftversus right side of the
face on emotion judgments. Third,to disentangle the impact of
physiognomic asymmetriesfrom perceptual biases to left and right
hemispace, theyhad observers rate both original and
mirror-reversedfacial expressions.
Sixteen models posed neutral expressions and also themost
intense displays possible of happiness and sadness.Consistent with
literature suggesting that there areasymmetric representations of
certain components of emotion in particular cortical regions (e.g.
[4,5] ), Nichollset al. found that their objective 3D measurement
pro-cedure detected greater movement on the left than on theright
side of the face, which was more pronounced forsad compared with
happy expressions (see Figure 1 ).Interestingly, greater left-sided
movement forfacial expressions has even been reported in
non-humans(e.g. chimpanzees; [15,16] ).
When observers viewed these faces in left- versus right-rotated
orientations, they rated right-rotated faces hap-pier and
left-rotated faces sadder, an effect that wasattenuated for
mirror-reversed expressions. These datacomplement recent ndings by
Jansari, Tranel andAdolphs [17] concerning the free-eld viewing of
negativeand positive expressions and suggest that head
orien-tation, or the lateralized placement of features, issufcient
to alter ones perception of an expressionsemotional intensity.
Furthermore, the impact of mirror-reversal on the interaction of
head orientation andexpression adds to a growing literature that
suggests theleft and right sides of the face convey somewhat
different
affective signals (e.g. [10,18] ).
(b)(a)
Left face
Right face
Figure 1. (a) Examples of the images used by Nicholls et al. to
determine theperceptual effectsof asymmetries in the expressionof
emotion.The upper imageisrotated 35 8 to the right so that the left
side of the face is featured, and vice versa forthe lower image.
Theimage depicts a model expressing sadness. (b) Mapsshowingthe
amount of inward and outward movement perpendicular to the surface
of theface (measured in mm, relative to the baseline condition of
no emotion). Sadnessresults in more movement of the left face.
Reproduced with permission from [3].
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However, before concluding that Nicholls and col-leagues data
lend strong support to the notion of hemispheric differences for
positive versus negative affect,several limitations of this study
are important to consider.First, the facial stimuli were posed by
actors and it islikely that the sad faces were more difcult to
producethan the happy faces. Second, the stimuli were notmatched in
intensity or systematically coded (cf. [19] ).Rather posers were
simply instructed to show the mostintense expression they could. In
light of these two facts,it is possible that at least some of the
differences betweenthe happy and sad conditions are produced by
variationsin difculty level of the production and/or in intensity
of the production. Nevertheless, the ndings are innovativeand
provocative and at the very least, underline the factthat facial
expressions are not objectively symmetric andthat the left and
right sides of the face convey somewhatdifferent emotional
information to na ve viewers.
Why should affect be asymmetrically organized?Much has been
written on the possible adaptive signi-
cance of an asymmetrically organized brain (see [20,21]
forrecent reviews). In a series of articles, Davidson [7,22,23]has
suggested, based upon a diverse corpus of literature,that a
fundamental dimension along which certainprefrontal regions are
specialized is the approachwith-drawal dimension. Approach-related
positive affect, par-ticularly those forms of positive affect that
involve theimplementation of appetitive goals, are
preferentiallyrepresented in specic left-sided dorsolateral
prefrontalterritories, whereas withdrawal-related negative
affect,particularly those forms of negative affect that
involveheightened vigilance toward threat-related cues in
theenvironment, are preferentially represented in specicright-sided
lateral prefrontal territories. Segregatingthese functions in
separate hemispheres could minimizecompetitive interaction between
these systems andfacilitate adaptive responding to biologically
salientreward and punishment stimuli. To the extent that
thegeneration of facial expressions recruits
affect-relatedcircuitry (and some recent evidence implies that this
isthe case (e.g. [24,25] ), we might expect functionalcortical
asymmetries to be produced in prefrontalterritories that could then
result in the slight butinuential facial asymmetries detected by
both theobjective methods and the observer ratings in theNicholls
et al. report. The ndings from the Nichollset al. report clearly
underline the pervasive inuenceof cerebral asymmetries in emotion
processing. It willbe important in future research to monitor
brainfunction directly, both in the individuals posing theemotional
expressions as well as in the subjects askedto judge these
expressions. In this way, we can beginto examine more directly the
brain bases of thesefascinating facial asymmetries.
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