Left-right
Asymmetry:An Examination of Factors
That May Influence Leftward Bias in the Viewing of Emotive Faces
H. G. McFadden
Northeastern
Illinois University
Left-right Asymmetry:An Examination of Factors That May Influence Leftward Bias in the Viewing of Emotive Faces
Cerebral
hemispheric lateralization of function is a very well researched area of
inquiry. Evidence suggests that there are function-specific brain mechanisms
which govern tasks such as facial recognition, interpretation of emotion,
and language skills. These phenomena have been examined in regard to many
brain structures and within a wide variety of contexts.It
is the hemispheric lateralization and mechanistic specialization of functions
relating to faces and emotive facial expression as reflected by perceptual
bias that this paper will examine.
In
general, a great body of previous research has demonstrated that the right
hemisphere of the brain is dominant in tasks involving face processing
and the evaluation of emotional expression, while the left hemisphere shows
superiority in functions involving language and object component processing.Beyond
this broad cloth, however,there
remain questions.We do not entirely
understand the role played by specific neuroanatomical structures in processing
faces and emotional facial expression, or how they relate to the cognitive
mechanisms which underlie these processes and which must be distributed
over numerous anatomical structures.Observation
of populations that display atypical hemilateral specialization patterns
may contribute to our understanding of these issues.Individuals
with Williams Syndrome ( WMS ), for example, show very different patterns
of brain activity from normal persons when viewing faces ( Mills, et al.,
2000 ).In their 1998 study, Mandal
and Asthana showed that patients with damage to only the left hemisphere
of the brain demonstrated marked superiority over right hemisphere damaged
patients in matching performance for line drawings of faces.Schizophrenic
persons also show significant differences in perceptual bias patterns when
viewing faces in comparison to nonschizophrenic controls(
Phillips & David, 1997 ).Another
group in which differences from the general population may be observed,
but who lack the structural brain damage of the above examples, is nonimpaired
left-handed adults.
There are differences in the degree of hemispheric lateralization of brain function between dextrals and sinistrals. Previous research has demonstrated that when viewing a pair of chimeric faces, right-handers show a significant bias toward evaluating the face with the smile to the viewer’s left as the happier of the two ( Levy, Heller, Banich, & Burton, 1983 ).While also manifesting this leftward bias, left-handers do so to a significantly lesser extent than do right-handers ( Levy, et al., 1983 ).Similar findings have been reported in regard to left hemispheric dominance of language functions.In work with bilingual students who had been forced to switch their preferred hand from left to right during childhood, Hoosain ( 1991 ) showed that those subjects who had experienced forced handedness switching performed significantly more similar to sinistrals than to dextrals on tasks which measured accuracy of word identification.This suggests that the dissimilarity of lateralization patterns between right and left-handers is of genetic rather than developmental origin.
While
it is well established that the right cerebral hemisphere of right-handers
is primarily responsible for evaluating faces, there is also evidence to
suggest that the brain’s face processing system may have certain inherently
bilateral facets.Parkin and Williamson
( 1987 ) demonstrated that the left-hemisphere may be dominant in the discrimination
of individual facial features.Thus
the question of holistic vs. componential processing mechanisms is raised,
which may in turn bear on developmental facets of lateral specialization.Alvarez,
Mills, and Neville ( as cited in Mills, et al.,2000
), found that adults utilize different brain systems to mediate processing
of upright and inverted faces.In
comparison, children appear to utilize a similar mechanism when viewing
both upright and inverted faces.Furthermore,
“ … the mature pattern of right-greater-than-left asymmetry to upright
faces is not evident until the late teens”(
Alvarez & Neville, as cited in Mills, et al. ).The
juxtaposition of this developmental neurologic event with the typical onset
time of schizophrenia is intriguing.
It appears that the processes of facial recognition and the interpretation of emotional expression may be more broadly distributed and bilaterally interactive than we understand them to be.Such systems are inherently difficult to study, largely confined as we are to the observation of the emergent properties of these complex mechanisms.This study attempts to add a piece to the puzzle by purposefully stimulating such interactivity and observing its effect on leftward bias.It was hypothesized that by giving subjects instructions which directed them to pay careful and considered attention to individual facial characteristics ( and thereby stimulating activation of left hemisphere processes ), the typical leftward bias should be significantly reduced in comparison to subjects who received traditional, more intuitively focused instructions.
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Insert Figure
1 here.
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While
a strong trend was exhibited ( p = .051 ), no significant difference
was found between the LQ of male and female subjects, t ( 23 ) =
2.062, p > .05.Further analysis
also indicated that no relation existed between age and LQ, r (
25 ) = -.021, p > .05.Interestingly,
a significant positive correlation was shown between LQ and the difficulty
with which participants reported making their decisions, r ( 23
) = .396, p = .05.As illustrated
in Figure 2, as leftward bias decreased, subjects rated the task more difficult.
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Insert Figure 2 here.
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The
significant correlation between decreasing leftward bias and increasing
perception of task difficulty may be indicative of an increase in bilateral
cognitive processing among those subjects.That
is to say, those subjects who more fully utilized the evaluative strategy
outlined in the detailed instructions ( and thereby increased bilateral
processing ) found the task more arousing ( i.e. more difficult ).Although
the conducted analyses do not provide adequate evidence to conclusively
support such an assertion, the data is tantalizingly suggestive.
There are a number of ways in which this experiment might be partially redesigned in future research to more fully explore the various phenomena involved.There was, in actuality, very little difference in the instruction set delivered to both groups of subjects.Given that the independent variable was instruction difference, it would appear that in future trials the detailed instruction set could be much more strongly worded to stimulate greater strategy compliance among subjects without compromising internal validity.Observation time might also be moderately increased across both experimental conditions.Delivery of stimulus and response via an apparatus which could measure response time is recommended, as presumably a longer response time would indicate greater care and attention to detail.
Given
the volume of research in this area, new combinations of previously utilized
methodologies would appear to be in order.This
could provide data for futuremeta-analysis
which may help clarify more specifically how the lateral specialization
of
component
processes of face recognition, visual hemifield attention, and the evaluation
of emotional expression interact and how these factors may differ in left
and right-handers.Moreover, if
a set of conditions may be developed under which dextrals would reliably
return responses similar to those of sinistrals rather than dextral controls,
we might more readily understand the mechanistic differences of hemilateral
specialization between right and left-handers.
Hoosain,
R.,( 1991 ).Cerebral
lateralization of bilingual functions after handedness switch in childhood.Journal
of Genetic Psychology, 152, 263-268.
Levy,
J., Heller, W., Banich, M., & Burton, L.(
1983 ).Asymmetry of perception in
free viewing of chimeric faces.Brain
and Cognition, 2, 404-419.
Mandal,
M. K., & Asthana, H. S.,( 1998
).Right hemisphere damage impairs
the ability to process emotional expressions of unusual faces.Behavior
Modification, 22, 167-177.
Mills,
D. L., Alvarez, T. D., St. George, M., Appelbaum, L. G., Bellugi, U., &
Neville, H.( 2000 ).Electrophysiological
studies of face processing in Williams Syndrome.Journal
of Cognitive Neuroscience, 12 ( Suppl. 2000 ), 47-64.
Parkin,
A. J., and Williamson, P.( 1987
).Cerebral lateralization at different
stages of facial processing.Cortex,
23, 99-110.
Phillips,
M. L., & David, A. S.,( 1997
).Viewing strategies for simple
and chimeric faces:An investigation
of perceptual bias in normal and schizophrenic patients using visual scan
paths.Brain and Cognition, 35,
225-238.
Figure
2.As
subject perception of task difficulty increases, mean laterality quotient
decreases ( n = 25 ).