Lateralization Characteristics
in Learning Disabled Children
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John E. Obrzut, PhD and Carol A. Boliek, BS
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As the authors of this month's Topical Review point out, interest in explaining specificlearning problems in children by reference to underlying neurological deficits has a
very long history in our field. Certainly one of the oldest neurological hypotheses is
that abnormalities in the way the brain is lateralized for language are the cause of
reading disabilities in many chidlren. In this Topical Review, the authors consider
recent research on brain lateralization in learning disabled children that has used
three different experimental paradigms. These studies have advanced our knowledge
about the kinds of factors that can affect performance on the tasks that measure
lateralizaton, but they do not produce a conclusive picture of abnormal lateralization
in LD children. The authors conclude with an important statement about the limitations
of our knowledge in this area as it applies to remedial procedures—JKT
I
nterest in the relationship between learning disabilities and neurological factors
has existed since the late ISOO's (James,
1890; Morgan, 1896), but systematic
study of this relationship has only taken
place since the 196(Vs. One of the major
hypotheses was put forth by Orton (1937)
who suggested that incomplete cerebral
dominance resulted in cognitive deficits
causing the syndrome variously known
today as 'learning disability." This hypothesis has been tested in numerous
studies, most of which have not shown
conclusive evidence to substantiate its
claim. However, these studies have usually contained serious methodological
flaws such as inadequate sampling procedures and improper use of statistical designs, lack of sophisticated research techniques and equipment, and inadequate or
unreliable operational definitions of the
learning disability syndrome. Before any
intelligent communication can take place
on the topic of laterality in relation to
learning disabilities, definitions of both
the construct and the population characteristics need to be made explicit.
studied extensively through indirect methods or behavioral techniques. One behavioral technique involves lateral preference as measured by handedness, eyedness, and footedness. The other primary
behavioral technique involves the measurement of perceptual asymmetries via
dichotic listening, visual half-field presentation, and verbal-manual timesharing. By using these measures of lateral preference and perceptual asymmetries, inferences about cerebral dominance for language function can be made.
Second, diverse subtypes of learning
disabled populations have been utilized
in research allowing for little generalization to occur across studies. Although
there is an enormous volume of literature
available on the topic of brain asymmetry
and lateralization of function, relatively
few of these studies have used well-defined samples of learning disabled children. Researchers should be as stringent
as possible in selecting children on the
basis of their placement in learning disabled programs according to federal and
state guidelines. A multidisciplinary team
First, laterality, as used in the present including a certified school psychologist
research, refers to the voluntary use of should participate in the child's diagnosis
the peripheral nervous system in execut- and resulting special education placeing some motor, spatial, or verbal func- ment. In order to qualify, the student
tion. It is distinct from cerebral domi- must have demonstrated average intellecnance which refers to the involuntary tual potential (Wechsler Intelligence Scale
brain functioning of the left and/or right for Children-Revised, WISC-R, IQ >
cerebral hemispheres. Laterality has been 80); give evidence of a processing deficit
in reception, discrimination, association,
organization/integration, retention, or application of information; and exhibit a
two-year achievement deficit on one or
more standardized individual achievement
tests such as the Woodcock Reading Mastery Test and the Key Math Diagnostic
Test. In interpreting data from various
studies, researchers should consider
whether samples were chosen from school
or clinic/hospital settings. Children chosen from the latter institutions are often
very different (i.e., more severe) in their
presenting symptomotology. Thus, the results from these populations are inconsistent with the results of studies that utilize
school populations. Consequently, this
paper will review behavioral laterality
techniques, hypotheses, and research
findings relative to learning disabled
groups drawn from school populations.
Lateral Preference Research
Early studies of lateral asymmetries
related deficits in academic achievement
to poorly established eyedness, footedness, and handedness in learning disabled children, but only handedness has
received more than a cursory examination. It is often assumed that deficits
found in learning disabled children on
measures of handedness are reflective of
poorly established functional laterality.
Thus, if a child is left-handed and learning disabled, it is thought that the left—
handedness reflects the fact that language
abilities have been lateralized to the right
cerebral hemisphere. The relation between handedness and cerebral organization is very weak, and it is difficult to
interpret a difference between left- and
right-handers in terms of differences in
cerebral organization. For example, it has
been shown from the study of aphasic
patients that approximately two-thirds of
all left-handers are lateralized to the left
cerebral hemisphere for speech function,
as are the majority of right-handed individuals (Annett, 1975).
In regard to handedness, it has often
been concluded that learning disabled
children demonstrate weak preferences
and that left-handers are often poor readers. According to a comprehensive survey conducted by Hardyck and Petrinovitch (1977), there is little evidence to
support the notion that left-handers are
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poor readers. Other well controlled studies have examined lateral preference patterns in relation to performance on measures of cognitive ability in children
(Hardyck, Petrinovitch, & Goldman,
1976; Kaufman, Zalma, & Kaufman,
1978; Ullman, 1977). Clearly, there is
little or no relationship between laterality
scores and poor achievement. The best
evidence regarding laterality and cerebral
organization is derived from studies that
have used more direct noninvasive measures of central language processing with
learning disabled children.
Dichotic listening and visual half-field
techniques have been used primarily to
investigate this hemispheric organization.
Although these research techniques have
accounted for most of the experimental/
behavioral methodology, other methods
such as hemispheric verbal -manual timesharing (Dalby & Gibson, 1981; Hughes
& Sussman, 1983; Obrzut, Hynd, Obrzut,
& Leitgeb, 1980) and the dichhaptic analog of the dichotic listening task (Witelson, 1974) have also been employed as
noninvasive techniques to examine lateralization of language.
Dichotic Listening
The dichotic listening technique has
been used extensively in the assessment
of lateralized language abilities. According to Bryden (1982), dichotic listening
techniques have provided us with some
of the most robust effects available in
contemporary neuropsychological research. This technique was originally
conceived by Broadbent (1956) as an
experimental paradigm to investigate a
mechanical model of memory. The procedure involves presenting paired stimuli
simultaneously to each ear. Stimuli used
in research include digits, words, consonant-vowel (CV) syllables, and sentences. The stimuli reported by the subject
will usually provide evidence of an "ear
effect," where a greater proportion of the
dichotic stimuli are correctly reported favoring one ear.
Working with normal subjects, Kimura
(1961a, b) demonstrated that the majority of right-handed subjects correctly
identified more stimuli presented to the
right-ear when the stimuli were verbal
and more stimuli presented to the left-ear
when the stimuli were nonverbal. Based
on studies of neurological patients in
whom cerebral dominance had been established by the sodium amytal test (Wada
& Rasmussen, I960), the dichotic listening procedure appeared to be a reliable
and stable measure of cerebral dominance for central auditory and language
related functions. Those with known left
hemisphere representation of language
function displayed the normal right-ear
advantage (REA) on verbal material, and
those with known right hemisphere representation for language function displayed a left-ear advantage (LEA). This
suggests that crossed or prepotent contralateral auditory pathways transmit information more quickly (or have an inhibitory effect on ipsilateral pathways) than
ipsilateral pathways to the auditory cortex
(Godfrey, 1974; Kimura, 1967). Therefore, the ear opposite the dominant cerebral hemisphere will perceive correctly a
greater number of the dichotically presented stimuli. However, it must be acknowledged that most dichotic studies do
not employ external validation measures
and thus make it difficult to decipher
whether results are reflective of the
individual's different information processing strategies or different patterns of cerebral organization.
While early research utilized the dichotic listening task to examine brain damaged adults (Kimura, 1961a, b; 1963,
1967), more recent focus has been directed at observing whether normal cerebral asymmetry exists in children with
disorders of learning. Most normal (righthanded) children report right-ear stimuli
more accurately than left. This REA is
thought to reflect left hemisphere representation for language. However, results
of studies using the dichotic listening
paradigm with reading/learning disabled
populations have not produced a consistent pattern of findings. The methodological problems in these studies are numerous, with perhaps the greatest difficulty
being the lack of an agreed upon operational definition of the learning disability
syndrome. In spite of this problem, however, the dichotic listening paradigm has
produced several viable hypotheses concerning the relationship between cerebral
laterality and learning disabilities. Hypotheses such as incomplete cerebral dominance, maturational lag, and attentional
bias for learning disabilities have been
advocated.
Incomplete Dominance
Hypothesis
The incomplete cerebral dominance hypothesis is similar to the verbal-nonverbal
model earlier proposed by Kimura (1967).
This model suggests that the superiority
of each ear for verbal or nonverbal material reflects the functional specialization
of the contralateral hemisphere (see
Witelson, 1977, for a review.). Early studies with learning disabled children were
conducted by Zurif and Carson (1970)
and Witelson and Rabinovitch (1972).
These authors reported that the normal
children demonstrated a REA that bordered on statistical significance while the
dyslexic group exhibited a slight tendency to recall more material accurately
from the left ear than the right ear. Thus,
some support was provided for the incomplete cerebral dominance theory of
dyslexia.
Bryden (1970) and Sparrow and Satz
(1970) did not find significant differences in right-ear superiority between
good and poor readers. Other researchers
have demonstrated a significant REA for
both normal and learning disabled groups
using dichotic lists of numbers or words
(e.g., Leong, 1976; McKeever & Van
Deventer, 1975; Witelson, 1976; YeniKomshian, Isenberg & Goldberg, 1975).
For example, McKeever and Van Deventer
(1975) compared a small sample of dyslexic (N = 9) and normal (N = 9) male adolescents (mean age, 13.7 years) on the
dichotic task and found that while both
the dyslexic and control children demonstrated a significant right-ear effect (left
hemisphere) for language, the dyslexics
recalled less material from both sides of
space than did the normals.
The results of these studies of an
auditory nature tend to contradict the
hypothesis that learning disabled children
are not as well lateral ized for language
functioning as are normal control children. Therefore, it may be proposed that
there is little support for the hypothesis
that the nature of dysfunction in learning
disabled children is incomplete lateralization which directly affects intellectual or
cognitive processes. It is more likely that
language is lateralized in both normal
and learning disabled children, but the
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efficiency of the language processor is
less functional in learning disabled children and may represent one aspect of a
more general maturational lag (Obrzut &
Hynd, 1981).
The Maturational Lag
Hypothesis
The basic notion underlying the maturational lag hypothesis stems from the
work of Lenneberg (1967) who contended
that the left hemisphere becomes increasingly specialized for language during development. Therefore, in normal children, between-hemisphere differences in
language mediation should increase as
the child advances in age (Rourke, Bakker,
Fisk & Strang, 1983). This theory predicts a lag in the maturation of the central nervous system in learning disabled
children. In essence, learning disabled
children experience a delay of left hemispheric specialization that may have a
negative impact on their ability to acquire
normal age-related cognitive skills.
Bakker and associates (Bakker, 1973;
Bakker, Smink & Reitsma, 1973; Bakker,
Teunissen & Bosch, 1976) as well as
Satz and colleagues (Sparrow & Satz,
1970; Satz & Van Nostrand, 1972) have
argued that there is a relation between
developmental dyslexia and degree of
lateralization only among older dyslexic
children. The theory predicts that motor,
perceptual and linguistic functions become successively important in learning
to read and lateralize to the dominant left
hemisphere at different stages of development. From this theory, these researchers hypothesized that visual-motor and
auditory-visual integration skills that presumably develop earlier would be delayed in younger dyslexic readers, while
linguistic skills that develop later would
be more delayed in older dyslexic children. This theory, of course, is based on
the assumption that speech processes become more completely lateralized with
increasing age.
Satz, Rardin, and Ross (1971) used
the dichotic listening task in a crosssectional study of younger (7 to 8 years)
and older (11 to 12 years) dyslexic boys
matched for age, gender, race, and IQ,
with normal reader control groups. The
results indicated that although a significant REA (left hemisphere) was found in
both groups of dyslexic children and in
normals, the magnitude of the REA was
significantly greater in the older normal
group as compared with the older dyslexic group. No differences in magnitude
were found between younger normals and
dyslexics. This finding led the authors to
conclude that the brain becomes increasingly lateralized with age, but this process does not occur as rapidly in dyslexic
readers as it does in normal readers.
The studies in Holland conducted by
Bakker and his associates (Bakker, 1973;
Bakker et al., 1973; Bakker et al., 1976)
have also reported patterns consistent with
the theory that normal children have
well-established cerebral lateralization,
whereas dyslexics are lateralized like their
younger normal counterparts. Further support for a developmental delay in dyslexic children comes from Witelson
(1976) and Sadick and Ginsberg (1978).
These studies found that good readers
displayed a consistent decrease in ambilaterality and a consistent increase in
magnitude of the REA with increasing
age. In contrast, poor readers displayed
little evidence of a shift in ambilaterality
with age. Proficiency in the early stages
of the reading process is correlated with
the absence of lateral asymmetries and
supports the theory that the side of hemispheric involvement in reading is stage
dependent.
Recently, a series of studies using
well-defined groups of learning disabled
children and normal counterparts have
been carried out by Obrzut and his associates (Hynd & Obrzut, 1981; Hynd,
Obrzut, Weed, & Hynd, 1979; Obrzut et
al., 1980; Obrzut, Hynd, Obrzut, &
Pirozzolo, 1981) in an attempt to investigate the developmental hypothesis. The
results of these studies have shown that
although both the normal and learning
disabled children demonstrated a significant REA, no significant developmental
effects or interactions were noted. The
authors concluded that no developmental
lag or delay existed between normal and
learning disabled children in terms of
lateralized language processing.
In summary, although most of the studies using older dyslexic children and normal reader controls have reported that
both groups demonstrate the REA (left
hemisphere), the dyslexic children perform at a degraded level in comparison to
their normal counterparts. These findings
were interpreted, for example, by Bakker
et al. (1976), Satz (1976), and Leong
(1976) as support for a developmental lag
theory of dyslexia. Other researchers such
as Witelson (1976) interpret their crosssectional data as indicating a disorder in
left hemispheric functioning for speech
in dyslexic children. Finally, the studies
conducted by Hynd et al. (1979), Hynd
and Obrzut (1981), Obrzut et al. (1980),
and Obrzut et al. (1981) do not support
the notion that cerebral lateralization increases with age. These studies suggest
that lateralized language capabilities exist
in normal children from ages 6 through
12. Furthermore, it appears as though
these lateralized languaged asymmetries
do not develop after age 6 nor are they
affected by gender. Because these latter
studies controlled for selective attention,
the differences in lateralized language
processes between normal and learning
disabled children may not be due to delayed cerebral dominance but rather could
be attributed to attentional deficiencies in
the learning disabled children.
Attentional Bias Hypothesis
Following this line of thinking. Kinsbourne (1970) proposed an alternative
hypothesis of functional asymmetry. His
model of lateral functioning states that
the REA results from a selective arousal
of the left hemisphere for verbal activity.
The expectation of a verbal task selectively activates the left hemisphere which
causes a shift in attention toward the
right side of space (i.e., language activity would lead to a tendency to turn the
head and eyes to the right). Kinsbourne's
model emphasizes situational variables of
selectivity, attention, planning, and related factors. Even the expectancy for
verbal input would serve to activate the
left hemisphere and bias attention to the
right side. Conversely, a set for nonverbal stimuli would activate the right hemisphere and bias attention to the left side.
In an effort to test the attentional hypothesis, Obrzut et al. (1981) employed a
prestimulus cueing paradigm in addition
to the standard free recall condition. In
the cued condition, each child was told
to listen carefully and report only stimuli
received in the one (target) ear. In the
free recall condition, both groups clearly
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demonstrated a significant REA, and no
developmental effect existed. However,
when attention was directed to the left
perceptual field (left ear), an interaction
between subject group and ear effect was
found. The normal children did not shift
their ear effect while the learning disabled subjects dramatically reversed their
ear effect, showing a strong LEA. When
attention was directed toward the stimuli
presented to the right ear, a similar interaction was found in that while both
groups of childen showed a REA, the
learning disabled children could increase
the magnitude of the between-ear difference. Although their performance generally was deficient when total accuracy
was examined, the learning disabled children were able to increase correct recognition of left- and right-ear presentations
on cue from the examiner. These results
suggest that learning disabled children
probably do not suffer from developmental delays but rather from a defect in
callosal functioning that interferes with
their ability to simultaneously process
verbal information (Obrzut et al., 1981).
Additional evidence for the validity of
these findings was obtained in a followup study conducted by Obrzut, Hynd
and Obrzut (1983).
the effort to identify subtypes of learning/
reading disabled children has been quite
successful (e.g., Boder, 1973; Doehring,
Hoshko & Bryans, 1979; Fisk & Rourke,
1979; Mattis, French & Rapin, 1975;
Pirozzolo & Campanella, 1981; Satz &
Morris, 1981). Most of these researchers
have obtained data to demonstrate at least
two types of disabled readers: those with
auditory-linguistic problems and those
with visuospatial problems. Perhaps not
all poor readers would show weak lateralization but only those who manifest
certain deficiencies in expressive or receptive language (Bryden, 1982).
Some support for this hypothesis was
found by Obrzut (1979), who employed
the dichotic listening task with Boder's
(1973) subtypes of dysphonetics and
dyseidetics. He found that poorer auditory lateralizaton was characteristic of
those readers with greater auditorylinguistic deficits (dysphonetics) than
those with visuospatial deficits (dyseidetics). However, no attempt was made to
examine attentional factors with these
groups of disabled readers. Clearly, the
need is to examine ear effects of lateralized
attention on carefully matched subtypes
of learning/reading disabled children.
Kinsbourne and Hiscock (1981) have
suggested that the ability of the language
dominant hemisphere to verbally mediate
linguistic material develops with a concurrent suppression of the (nondominant)
right hemisphere. The results of the last
two studies have indicated that, perhaps,
in normal children, the mechanism of
suppression of information from the
nondominant hemisphere is established.
Thus, the normal children cannot willingly attend to verbal stimuli with facility
in an incompatible perceptual field. In
contrast, the learning disabled children
have not established a mechanism for
reciprocal inhibition and thus are able to
divide or direct attention to stimuli presented in either perceptual field.
Further efforts are needed to extend
the theoretical and clinical implications
of this research. For example, the work
related to subtype analysis may shed
some light on discrepancies found with
auditory perceptual asymmetries. Until
quite recently, researchers tended to view
learning disabled populations as homogeneous entities (Benton, 1975). However,
Visual Half-Field
The visual half-field (VHF) technique
also has been employed as an experimental behavioral index of cerebral asymmetry. This technique involves the tachistoscopic presentation of verbal or spatial
stimuli to either the right- or the leftvisual field for unilateral presentations,
or both visual fields simultaneously for
bilateral presentations. Stimuli perceived
in the left VHF are processed in the right
cerebral hemisphere, while stimuli perceived in the right VHF are processed in
the left cerebral hemisphere. The typical
results of such lateralized tachistoscopic
procedures reveal that normal subjects
identify words and letters readily in the
right-VHF (left hemisphere—Kimura,
1966), while nonverbal stimuli, such as
faces and geometric forms, are recognized better in the left-VHF (right hemisphere—Kimura & Durnford, 1974).
Researchers also have utilized VHF
procedures to assess laterality in learning
disabled children. However, as with the
dichotic listening literature, results have
been inconsistent. For example, McKeever
and Huling (1970) compared the performance of ten normal children to ten poor
readers on the unilateral tachistoscopic
presentation of four letter nouns. Both
groups demonstrated a significant right
visual half-field advantage (RVHF—left
hemisphere). Thus, this study provides
little support for the theory of incomplete
or delayed cerebral dominance for speech
in the reading disabled. Although other
studies demonstrated that both reading
disabled and control children have a
RVHF advantage (Bouma & Legein,
1977; Kershner, 1977; Marcel, Katz &
Smith, 1974; Marcel & Rajan, 1975;
McKeever & Van Deventer, 1975), the
poor readers showed a lesser degree of
asymmetry in reporting words and letters.
For instance, McKeever and Van Deventer (1975) used both a unilateral and
bilateral tachistoscopic procedure with
nine dyslexic and control children ranging in age from 11 to 18 years (mean age
= 12.9 years). Based on their results,
McKeever and Van Deventer (1975) concluded that their older dyslexics possessed
clear left hemisphere language latealization. The findings of a reduced RVHF on
the unilateral task suggest these dyslexic
readers may have a dysfunction of the left
hemisphere.
Kershner (1977) also used a bilateral
presentation procedure with a group of
12 high IQ gifted readers, 11 good readers, and 10 dyslexic readers. While all
reader groups demonstrated a significant
RVHF (left hemisphere) advantage, the
gifted and good readers demonstrated a
significantly higher RVHF score than the
dyslexic group. Although the dyslexic
group reported more LVHF (right hemisphere) stimuli than both the gifted and
good readers, a significant difference was
only reached with the gifted reader
group. The author also found that when
he controlled for reading ability between
the gifted and dyslexic reader groups, the
cerebral dominance effect was eliminated.
This led Kershner (1977) to conclude
that reading impairment is related to
hemispheric differences in processing
reading material.
Recently, a study was designed to control for attentional effects to determine
the laterality difference in VHF performance between a group of carefully
matched learning disabled and normal
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controls (Obrzut, Hynd & Zellner, 1983).
This was done by employing a prestimulus
cueing paradigm, similar to that used in
dichotic listening studies. Twenty-six
learning disabled children were matched
with 26 normal children on the basis of
gender, age, and right-handedness. For
the unilateral cued condition, the subject
was told the field (left or right) in which
the word would be shown.
Control children demonstrated a superior RVHF (left hemisphere) advantage
across all conditions, whereas the learning disabled showed the expected RVHF
(left hemisphere) advantage only in the
unilateral cued condition. In both the
unilateral and bilateral conditions, the
learning disabled children recognized a
greater number of stimuli presented to
the left visual field (right hemisphere).
This result would appear to support the
incomplete lateralization hypothesis of
Orton (1937). However, evidence against
this hypothesis comes from the fact that
although learning disabled children favored the left field over the right field
under unilateral and bilateral presentations, these children did demonstrate the
RVHF superiority for word recognition under the directed cueing condition. Because of this finding, Obrzut et al. (1983)
concluded that the differences in performance do not lie solely in the functional
asymmetry of the visual-verbal processor
of the left hemisphere but in the ability
to direct attention from one field to the
other alternately. Similar shifts in perceptual asymmetries due to directed cueing
were shown using a dichotic listening
paradigm (Obrzut et al., 1981; Obrzut et
al., 1983).
It appears that structured cueing (i.e.,
by directing subjects' attention to stimuli) can modify laterality test outcome to
a greater extent with learning disabled
children than with normal subjects. From
the more recent work accomplished with
both dichotic listening and visual halffield procedures, it is likely that learning
disabled children have brain activation
patterns that are susceptible to attentional
effects that are not found in normal children. The differences in degree of lateralization often found between reading/
learning disabled children and normal
children may be strongly influenced by
individual differences in processing strategies.
Verbal-Manual Time-Sharing
Another behavioral assessment technique utilizing motor output is the timesharing paradigm. Time-sharing is a type
of experiment that contrasts the subjects'
ability to perform concurrent activities
when they are programmed in the same
hemisphere (e.g., speaking and right
manual activities) and when they are programmed in separate hemispheres (speaking and left manual activities). The consequence of this effect of "hemisphere
sharing" appears to be competition and
"cross-talk" between incompatible timing mechanisms hierarchically organized
in the brain (Kinsbourne & Cook, 1971).
Unlike the strictly auditory and visual
perceptual phenomena used in dichotic
and tachistoscopic studies, this technique
makes use of motor concomitants of
lateralized cerebral processes that may
allow for a more sensitive measure of
developmental trends in lateralized function.
In normals, researchers such as Kinsbourne and McMurray (1975), using kindergarten children, and White and Kinsbourne (1980), using children 3 to 12
years of age, have generally found that
children tap their fingers more slowly
while simultaneously verbalizing, then
when tapping without concurrent verbalization. Although the concurrent verbalization diminished the tapping rate of
both hands, the right-hand tapping rate
was lower than left-hand tapping rate.
Piazza (1977) also found that when the
concurrent tasks were divided into verbal
and nonverbal, the verbal task showed
greater interference on the right-hand tapping (left hemisphere) rate while the nonverbal task has a greater effect on the
left-hand tapping (right hemisphere) rate.
Recently, researchers have employed
this technique in the comparison of learning disabled and normal children and
have reported mixed results. Cermak,
Cermak, Drake and Kenney (1978) utilized a dichotic listening task concurrently with a manual tapping task with
three groups of learning disabled children and a normal control group. The
authors concluded that various subtypes
of learning disabled are differentially
lateralized and differentially affected by
the facilitory—inhibitory effects of concurrent hemispheric activities.
Obrzut et al. (1980) also employed a
time-sharing procedure with a group of
48 learning disabled children and 48 normal children matched according to age,
gender, and handedness. The authors
found that learning disabled children were
shown to tap slower than normals and
had significantly greater magnitudes of
concurrent task interference.
Dalby and Gibson (1981) used a timesharing paradigm that involved verbal and
spatial tasks concurrently in the assessment of 45 boys classified into three
reading disabled subtypes according to
the Boder (1973) classification system. It
was hypothesized that different types of
reading disability would be associated
with different patterns of lateralized brain
function. Their results indicated that
although the control group showed the
typical left lateralization of language and
right lateralization of spatial functions,
the reading disabled groups demonstrated
atypical lateralization. Dysphonetic readers, who presumably make nonphonetic
reading/spelling errors, showed bilateral
representation of both verbal and spatial
functions; dyseidetic readers, who presumably do not respond to words as
gestalts, showed bilateral verbal representation and right lateralization of spatial
functions; and the nonspecific readers,
who make phonetically acceptable errors
and respond to gestalts, showed left
language lateralization and bilateral spatial representation. These results confirmed the authors' hypothesis that patterns of cerebral organization vary across
types of reading disability.
In a final study, Hughes and Sussman
(1983) used a time-sharing technique to
assess language lateralization in 12 language disordered children and normal
children (seven male and five female in
each group). The authors found that all
language concurrent tasks produced tapping reductions (greater interference effects) for both hands for both groups,
unlike previous studies, which showed
percentage reduction scores greater for
the right-hand concurrent conditions
compared to the left-hand concurrent
conditions (Obrzut et al., 1980; White &
Kinsbourne, 1980). For this study, the
effect of the time-sharing paradigm
worked in the opposite direction for these
language disordered children who showed
reduced verbal output in place of reduced
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finger tapping.
It appears that while the time-sharing
procedure has produced mixed results
with child populations, it is of potential
value in assessing cerebral organization.
However, experimental variables such as
the linguistic tasks employed, sample
sizes, and more control over verbal output with and without concurrent tapping
need to be implemented.
Summary and Conclusion
What conclusions can we draw from
the work with reading/learning disabled
children in relation to functional cerebral
lateralization? Results from auditory,
visual, and concurrent verbal-manual
asymmetry techniques have basically indicated that the learning disabled, like
their normal counterparts, have lefthemisphere specialization for language processing but not at the same level that is
demonstrated by normal readers. However, the research does not make it clear
whether the difference in degree of
lateralization found between the groups is
caused by incomplete cerebral dominance, developmental delay, or neurological deficits. Since the learning disabled
demonstrated deficit performance on tasks
involving all modalities, it is likely that a
more encompassing, nonspecific neuropsychological deficit is characteristic of
this syndrome.
From this review, it is clear that performance differences could be attributed
to methodological problems inherent with
the behavioral procedure employed rather
than to differences in cerebral lateralization. It is also likely that asymmetric
performance is affected by differences in
processing strategy and experimental factors, i.e., selective attention, rather than
to differences in cerebral lateralization
alone. Perhaps learning disabled children
use inefficient strategies of processing
and encoding words, which is generally
interpreted as a display of poor cerebral
organization.
Our knowledge of cerebral lateralization
and associated cognitive functions has
advanced greatly over the past several decades. However, the study of laterality
has not yet made a transition from the
discovery level to the intervention or implementation stages. Therefore, the current state of lateralization research and
cautions regarding implementation of research findings in educational settings
should be noted.
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ABOUT THE AUTHORS
Hardyck. C, Petrinwitch. L.F.. & Goldman, R.D.
(1976). Left handedness and cognitive deficit.
John E. Obrzut is a Professor and Director of the
Cortex. 12, 266-279.
School Psychology Program at the University of
Harris, L. (May. 1985). Teaching the right brain:
Arizona. Carol A. Boliek is a PhD candidate in
Some thoughts on a current educational fad.
school psychology at the University of Northern
Paper presented at the meeting of The DevelopColorado. Address: John E. Obrzut, PhD, College
mental Implications of Brain Lateralization. St.
of Education, Department of Educational PsycholCatherines, Ontario, Canada.
ogy, University of Arizona, Tucson, Arizona 85721.
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