Brain (2001), 124, 1657–1665
Behavioural relevance of atypical language
lateralization in healthy subjects
S. Knecht, B. Dräger, A. Flöel, H. Lohmann, C. Breitenstein, M. Deppe, H. Henningsen and
E.-B. Ringelstein
Department of Neurology, University of Münster, Germany
Correspondence to: Stefan Knecht, Department of
Neurology, University of Münster, Albert-Schweitzer-Strasse
33, D-48129 Münster, Germany
E-mail: [email protected]
Summary
In most humans, language is lateralized to the left side
of the brain. It has been speculated that this hemispheric
specialization is a prerequisite for the full realization of
linguistic potential. Using standardized questionnaires
and performance measures, we attempted to determine
if there are behavioural correlates of atypical, i.e. righthemispheric and bilateral, language lateralization. The
side and degree of language lateralization were
determined by measuring the hemispheric perfusion
differences by functional transcranial Doppler ultrasonography during a word generation task in healthy
volunteers. Subjects with left (n ⍧ 264), bilateral (n ⍧
31) or right (n ⍧ 31) hemisphere language representation
did not differ significantly with respect to mastery of
foreign languages, academic achievement, artistic talents,
verbal fluency or (as assessed in a representative
subgroup) in intelligence or speed of linguistic processing.
These findings suggest that atypical hemispheric
specialization for language, i.e. right-hemisphere or
bilateral specialization, is not associated with major
impairments of linguistic faculties in otherwise healthy
subjects.
Keywords: functional Doppler ultrasonography; language lateralization; neuroscience; hemispheric dominance; behaviour
Abbreviations: CBFV ⫽ cerebral blood flow velocity; fTCD ⫽ functional transcranial Doppler sonography; MCA ⫽ middle
cerebral arteries
Introduction
Hemispheric specialization is epitomized by the frequent
disruption of language after left-sided brain lesions. Although
the reason for this specialization is unknown, it is often
assumed that the functional lateralization of the human brain
has an adaptive value and may even present a prerequisite
for the full realization of the linguistic potential (Luria, 1973;
Geschwind and Galaburda, 1985; Hiscock, 1998). It has
been speculated that functional neuronal clustering in one
hemisphere during language development allows faster
linguistic processing because transition times are shorter than
in transhemispheric operations (Kupferman, 1995; Miller,
1996). Thus, gradually developing lateralization in children
has been proposed as an explanation of why susceptibility
to aphasia after unihemispheric lesions seems to increase
with advancing age and language capabilities (Lenneberg,
1967; Vargha-Khadem et al., 1991). Disturbances in the
development of lateralization have even been linked to the
pathogenesis of dyslexia (Orton, 1937; Lenneberg, 1967;
Annett et al., 1996). In a recent study of relative hand
skills, Crow and colleagues reported deficits in a number of
© Oxford University Press 2001
intellectual measures in subjects at the point of ‘hemispheric
indecision’, i.e. in subjects who had equal skills in both
hands on checking an array of squares on a printed sheet
(Crow et al., 1998). Others have advertised ‘enhanced
lateralization’ techniques for the treatment of dyslexia,
attempting to force children to use their left hemisphere (van
den Honert, 1977).
The theoretical simplicity of the hypothesis that language
lateralization conveys an evolutionary advantage is intriguing.
But there is little empirical evidence to support this view other
than observations of brain-damaged patients with atypical
lateralization. In these patients, however, dysfunction of
language results from the lesion itself, and atypical
lateralization is often a secondary, reorganizational
phenomenon (Vargha-Khadem et al., 1985).
Assessment of language lateralization is now possible by
various non-invasive functional imaging techniques which
allow non-invasive studies in large groups of subjects without
brain damage. Functional transcranial Doppler sonography
(fTCD) measures changes in event-related cerebral perfusion
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S. Knecht et al.
that are related to neuronal activation in a way comparable
with functional MRI (Deppe et al. 2000a). Because fTCD
integrates and averages repeated activations within the whole
territory of the insonated middle cerebral arteries, it provides a
reliable measure of the hemispheric lateralization of language
(Knecht et al., 1996, 1998a 2000a, b; Deppe et al., 1997).
In the present study, we attempted to clarify the behavioural
relevance of hemispheric specialization in a cohort of subjects
without developmental or other known medical disturbances
who had been examined previously for language lateralization
by fTCD (Deppe et al., 1997). Using standardized
questionnaires, we looked for correlations of the hemispheric
lateralization of perfusion during word generation with
linguistic, academic and artistic performance. Additionally,
we employed performance tests in a representative subgroup
to examine associations of language lateralization with IQ,
verbal fluency and speed of linguistic processing.
Methods
The study was part of the Münster functional imaging project
on the variability of hemispheric specialization in health and
disease (Deppe et al., 1997, 2000b; Knecht et al., 1998a, b,
2000a, b). The project involves the building of a growing
database on functional cerebral lateralization in healthy
subjects.
Inclusion and exclusion criteria for subjects and patterns of
language lateralization with respect to gender and handedness
have been reported before (Knecht et al., 2000a, b). Briefly,
healthy volunteers were recruited by newspaper advertisement. The advertisement especially invited subjects who were
not right-handed to participate. Subjects were not reimbursed.
Subjects were excluded if information obtained from a
standardized questionnaire suggested neurological disorders,
particularly perinatal asphyxia or kernicterus, head trauma,
loss of consciousness, epileptic seizures, meningitis or
encephalitis, or delayed or disturbed language development.
Subjects were also excluded if they had failed to complete
the equivalent of a high school qualification (‘Realschul–
Abschluss’ or ‘Abitur’). Additionally, ~3% of the subjects
were excluded because fTCD could not be performed because
of inadequate sonographic penetration of the skull. Another
1% of subjects were excluded from further analysis because
the standards set for goodness of recording were not met
(see below). A total of 326 subjects were finally included in
the study; 198 were female (age range 15–49 years, mean
26 years, SD 5.1 years) and 128 were male (age range 19–
46 years, mean age 26 years, SD 4.6 years).
Determination of language lateralization
The details of fTCD and the distribution of language lateralization in the cohort under study have been reported in detail
in a preceding paper (Knecht et al., 2000b). Here we only
describe details of the technique that are relevant for the
quantification of language lateralization and are critical issues
for this study. fTCD has been validated by direct comparison
with intracarotid amobarbital injection and functional MRI
(Knecht et al., 1996, 1998a, b; Deppe et al., 1997, 1998).
Briefly, subjects silently generated as many words as possible,
starting with a particular letter displayed on a computer
monitor. Task compliance was monitored by having subjects
report the words after a second auditory signal, 15 s after
presentation of the letter. After a relaxation period of 60 s,
the next letter was presented in the same manner.
Changes in cerebral blood flow velocity (CBFV) in the
basal arteries, as an indicator of the downstream increase in
regional metabolic activity during the language task, were
measured by dual transcranial Doppler ultrasonography of
the middle cerebral arteries (MCAs). Ultrasonography was
performed with two 2-MHz transducer probes attached to a
headband and placed at the temporal windows of the skull
bilaterally. After automated rejection of artefacts, data were
integrated over the corresponding cardiac cycles, segmented
into epochs that related to the cueing tone, and then averaged.
The epochs were set to begin 15 s before and to end 35 s
after the cueing tone. The mean velocity in the 15-s precueing
interval (Vpre.mean) was taken as the baseline value. The
relative CBFV changes (dV) during cerebral activation were
calculated using the formula:
dV ⫽ (V(t) – Vpre.mean) ⫻ 100/Vpre.mean,
where V(t) is the CBFV over time. Relative CBFV changes
from repeated presentations of letters (on the average 20
runs) were averaged time-locked to the cueing tone.
An fTCD laterality index LI was calculated using the
formula:
tmax⫹0.5tint
1
LI ⫽
∆V(t)dt
tint tmax–0.5tint
∫
where ∆V(t) ⫽ dV(t)left – dV(t)right is the difference between
the relative velocity changes of the left and right MCAs. The
term tmax represents the latency of the absolute maximum of
∆V(t) during an interval of 10–18 s after cueing, i.e. during
verbal processing. For integration, a time period of tint ⫽ 2 s
was chosen. The test–retest reproducibility of this procedure
(Pearson product moment correlation coefficient) was
r ⫽ 0.95, P ⬍ 0.0001 (Knecht et al., 1998b).
Goodness of recording
To control for goodness of the Doppler sonographic recording,
the root mean square of the averaged CBFV fluctuations in
the control condition (–17 to –3 s before the cueing tone)
was calculated. All recordings with root mean square values
greater than 2% of the averaged CBFV were discarded. This
applied to ~1% of recordings.
Categorization of language representation
Left-hemisphere language dominance was assumed in all
cases in which there was a positive LI that deviated by
Atypical language lateralization
⬎2 SE from 0. Right-hemisphere language dominance was
assessed in a similar way. In cases in which LI did not
deviate by ⬎2 SE from 0, lateralization was classified as
‘low’. To ensure that noisy recordings would not produce
false categorization as low, the criterion of goodness of
recording was introduced. Because the lack of lateralization
could not be explained by a lack of cooperation, which had
been controlled for, low lateralization was considered to
indicate bihemispheric language representation.
Standardized questionnaires
Handedness was assessed with the Edinburgh Handedness
Inventory (Oldfield, 1971), in which scores range from
–100 for strong left-handedness to ⫹100 for strong righthandedness. For statistical analysis, handedness was grouped
into the following categories: ⬎75; 75 to –75; ⬍–75.
A standardized questionnaire was used to screen subjects
for the number of foreign languages spoken fluently. The
number of languages was assumed to indicate linguistic
talent. Academic achievement, i.e. a university qualification,
was chosen as an additional indicator of linguistic proficiency
(Tainturier et al., 1992). Furthermore, subjects were asked
about artistic activities. They had to report whether they were
actively involved in music, painting or sculpture. Such
activities were taken to reflect artistic inclination and, by
inference, artistic ability. Some authors perceive artistic
ability as a faculty complementary to verbal ability and as
being subtended by the right hemisphere (Peretz et al., 1997).
1659
Performance measures
The average number of words per letter produced in the
word generation task during the fTCD assessment was
calculated as a direct index of verbal fluency. Verbal fluency
is assumed to reflect the ease and quantity of verbal production
and is related to the level of education (Ratcliff et al., 1998).
Additional performance measurements were assessed in a
subgroup of the cohort (n ⫽ 21) selected to represent the
whole spectrum from left to right language lateralization
(Figs 5 and 6). This group was recruited by categorizing all
subjects as having left, bilateral or right language dominance
according to the criteria outlined above. From each group,
the seven subjects who had been examined last and were
available for additional testing were subjected to IQ testing
with the Hamburg Wechsler Intelligence Test for Adults, i.e.
the complete form of the German version of the Wechsler
Adult Intelligence Scale (Hamburger–Wechsler Intelligenztest für Erwachsene Revision 1991; Tewes, 1991).
To obtain a measure of linguistic processing speed, we
had subjects perform a picture–word verification task (Fig. 1).
Thirty black-and-white drawings of concrete objects were
shown on a computer screen for 1.5 s each. They were
taken from the repertoire of drawings standardized for name
agreement and visual complexity by Snodgrass and
Vanderwart (Snodgrass and Vanderwart, 1980) and depicted
well-known objects such as animals and tools. Because each
drawing was shown once with a correct and once with an
incorrect subtitle, there was a total of 60 pictures in each block.
Pictures were presented in a randomized order. Drawings
Fig. 1 Schematic of the measurement of linguistic processing speed. The mean accuracy and response times (RT) in a picture–word
verification task and a motor control task were calculated and the values for the motor control task were subtracted from those for the
picture–word verification task in order to obtain an index of linguistic processing speed. (Maus is German for mouse; Löwe is German
for lion).
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S. Knecht et al.
measured approximately 15 ⫻ 15 cm and the subtitles were
presented in 24-point Times Bold font. The distance between
the subject and the computer monitor was standardized to
0.7 m. The screen was positioned at eye level. Subjects were
instructed to press two buttons with both index fingers if the
subtitles matched the picture and two different buttons with
the two middle fingers if they did not. Each subject received
eight practice blocks (with 60 trials each) at the beginning
to overtrain reaction time performance (plateauing of reaction
times). The fastest response of either hand was used for
further analysis. Only correct picture–word verifications were
included for the calculation of mean reaction times. As a
motor control task, subjects were requested to respond as
quickly as possible upon presentation of either a screen that
was coloured red (button-press with both middle fingers) or
green (button-press with both index fingers).
Statistical analysis
Differences in behavioural measures of the categories of left,
low and right language lateralization were analysed using
the χ2 test, the Kruskal–Wallis test or the Spearman rank
correlation coefficient, as appropriate. The significance level
was P ⬍ 0.05 for all analyses.
Results
The distribution of language lateralization was roughly
bimodal, 264 subjects being left-hemispheric, 31 being lowlateralized and 31 being right-hemisphere dominant (Fig. 2).
Standardized questionnaires
As reported previously, a correlation was observed between
handedness and lateralization of language, right dominance
being more common in left-handed subjects than in righthanders (Deppe et al., 1997). No other behavioural measure
was related to language lateralization (Table 1). Also, the
three handedness groups did not differ in the numbers who
engaged in artistic activity or had received university training,
or in the number of languages spoken. Results for single
measures are displayed in Figs 3–5.
Performance measures
No correlation was found between the average number of
words produced during the word generation task and language
lateralization (r ⫽ 0.02, P ⫽ 0.69) or the degree of handedness
(r ⫽ 0.11, P ⫽ 0.04) (Fig. 6).
In the selected group (n ⫽ 21), the overall IQ did not
correlate with language lateralization (r ⫽ 0.1, P ⫽ 0.65)
(Fig. 7), nor was there a significant relationship with verbal
IQ (r ⫽ 0.004, P ⫽ 0.9) or performance IQ (r ⫽ 0.05,
P ⫽ 0.8).
In the picture–word verification task, subjects did not differ
from each other with respect to the numbers of correct and
false responses. No significant correlations emerged between
language lateralization and motor or linguistic processing
speed, as measured by response times during the motor and
linguistic tasks (Fig. 8). Response times in the picture–word
verification task did correlate negatively with IQ (r ⫽ –0.55,
P ⫽ 0.009), i.e. the higher the IQ, the faster subjects were
in picture–word verification.
Discussion
Fig. 2 Distribution of language lateralization across all subjects
(n ⫽ 326).
In a group of 326 healthy subjects, no statistically significant
differences in the number of foreign languages spoken
fluently, academic achievement, participation in artistic
activities, IQ, verbal fluency or speed of linguistic processing
Table 1 Language lateralization and behavioural measures
Language lateralization
Lateralization by fTCD (mean ⫾ SD)
Sample size (n)
No. of languages (median)
University qualification (n)
Artistic ability (n)
Statistics
Left
Low
Right
3.48 ⫾ 1.50
264
3
189/264
187/264
0.1 ⫾ 1.18
31
3
22/31
23/31
–3.58 ⫾ 1.88
31
2
21/31
26/31
Kruskal–Wallis H ⫽ 2.2, P ⫽ 0.3
χ2(2) ⫽ 0.1; P ⫽ 0.9
χ2(2)⫽ 2.22; P ⫽ 0.3
Atypical language lateralization
were found between individuals with left-hemisphere,
bilateral or right-hemisphere language representation during
word generation.
1661
Fig. 3 Lack of a systematic relationship between language
lateralization and the number of foreign languages spoken fluently
(Kruskal–Wallis test; H ⫽ 2.2, P ⫽ 0.3).
standards. This reduced the behavioural variability in our
cohort but was necessary to minimize the possibility of covert
brain lesions. (Of course, behavioural measures or imaging
techniques can never fully exclude covert lesion.) Despite
these conservative inclusion criteria, sample size was
sufficient in each group (i.e. groups with left- and righthemisphere and bilateral language processing) for statistical
comparison.
We did not evaluate language lateralization against the
‘gold standard’, the intracarotid amobarbital procedure
(Wada, 1949). Rather, we measured language task-related
increases in blood flow. The intracarotid amobarbital
procedure involves brain inactivation, whereas techniques
sensitive to blood flow involve brain activation. However, a
close correlation between the two approaches has been shown
in several studies in healthy subjects and neurological patients
with typical left-hemisphere and atypical right-hemisphere
language lateralization (Desmond et al., 1995; Binder et al.,
1996; Knecht et al., 1997; Rihs et al., 1999).
Our analysis was conceived in the tradition of lateralization
research and was designed to detect hemispheric perfusion
differences rather than bilaterality. However, because
language lateralization during word generation was found to
occur along a continuum from left to right dominance, we
were faced with a subgroup of subjects with low or absent
lateralization, although there was good cooperation during
the language task and adequate recording (Fig. 2). This
suggested that, in individuals with low lateralization indices,
both hemispheres were involved in word generation.
While the distinction between left and right language
dominance is mathematically self-evident, the definition of
Fig. 4 Lack of a systematic relationship between language
lateralization and academic achievement [χ2(2) ⫽ 0.19, P ⫽ 0.9].
Fig. 5 Lack of a systematic relationship between language
lateralization and participation in artistic activities [χ2(2) ⫽ 2.22,
P ⫽ 0.3].
Methodology
Our study was designed to detect functional impairments
associated with anomalous language lateralization in subjects
who were otherwise normal and healthy. We specifically
wanted to exclude subjects with pathological hemispheric
specialization. Therefore, our subjects had to have an
unremarkable medical history and meet minimal scholastic
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S. Knecht et al.
Fig. 6 Lack of a significant correlation between language
lateralization and the average number of words produced per
letter during the word generation task (r ⫽ 0.02, P ⫽ 0.69).
bilaterality of language is artificial and depends on the
method employed to classify lateralization. Bilateral language
representation would not become evident in stroke patients
because bihemispheric insults impair consciousness and are
fatal in most cases. The definition of bilaterality of language
varies considerably in the published work using the intracarotid amobarbital procedure. Bihemispheric or ambilateral
language representation is frequently reported during the
disruption of neural function by the intracarotid amobarbital
procedure in candidates for resective epilepsy surgery because
of brain pathology. Rasmussen and colleagues reported that
19% and Risse and colleagues reported that 20% of nonright-handed patients had bilateral speech representation
(Rasmussen and Milner, 1977; Risse et al., 1997). Benbadis
and colleagues reported evidence for speech representation
in both hemispheres in 12% of patients (Benbadis et al.,
1995). Additionally, several lines of evidence indicate that
even in otherwise typical people some components of
language reside in the subdominant hemisphere (Bottini et al.,
1994; Demonet et al., 1994; Boatman et al., 1998; Buchinger
et al., 2000). We opted to define conservatively a group of
subjects with ‘low lateralization’ when the lateralization
index was close to zero, i.e. it did not deviate from zero by
more than two standard errors of the averaged perfusion
differences. A total of 9% of our subjects (biased for lefthanders) fell into this group. A recent fMRI study focusing on
the Broca region roughly corroborates this finding, reporting
‘bilateral’ activation in 14% of 100 healthy subjects during
word generation (Pujol et al., 1999).
Atypical lateralization
Fig. 7 Lack of a systematic relationship between language
lateralization and IQ [tested with the Hamburg–Wechsler
Intelligence Test for Adults (HAWIE-R)] (r ⫽ 0.1, P ⫽ 0.65).
Fig. 8 Lack of a systematic relationship between language
lateralization and linguistic processing speed. Linguistic
processing speed was assessed as the response time (RT) in a
picture–word verification task minus the response time in a simple
motor decision task (r ⫽ 0.2, P ⫽ 0.4).
Non-left-hemisphere language dominance and non-righthandedness have time and again been associated with atypical
behaviour, be it good or bad. Inspired by the reportedly
ambidextrous Leonardo da Vinci and Michelangelo, it has
been suggested, for example, that bilaterality is associated
with superior cognitive and creative skills (Mebert and
Michel, 1980; O’Boyle and Benbow, 1990). Others hold the
opposite view. Atypical cerebral dominance is taken as an
indicator of behavioural disturbance because the reverse is
true, i.e. abnormal lateralization is frequent in brain-damaged
patients with behavioural impairments (Coren and Halpern,
1991). Additionally, there is a general tendency to associate
atypical lateralization with behavioural impediments simply
because the statistical norm is assumed to represent the
biological optimum. It has therefore been proposed that
cerebral functional lateralization is a necessity for the full
realization of linguistic and cognitive potential (Luria, 1973;
Geschwind and Galaburda, 1985; Hiscock, 1998). According
to this hypothesis, transcallosal conduction delay would make
multiple passes during language processing prohibitively
slow (Ringo et al., 1994). Although deviating from the
statistical norm of left-hemisphere language dominance, righthemisphere language dominance as a mirror organization of
left-lateralization does not challenge the conceptual
Atypical language lateralization
framework of lateralization as a precondition for higher
cognitive functions. Bilateral language representation,
however, does pose a challenge. Crow and colleagues have
taken handedness as a correlate of cerebral dominance and,
using a sample of ⬎12 000 individuals, have demonstrated
global deficits in subjects with low indices of handedness as
well as in subjects with extreme right-handedness (Crow
et al., 1998). They interpret their finding as reflecting the
operation of continuing selection on a gene involved in
language lateralization. In large population studies it is
difficult to exclude covert brain damage, particularly when
no stringent exclusion criteria have been established. Thus,
behavioural impairments and pathological non-righthandedness can both be sequels of brain injury (Dellatolas
et al., 1993). Other studies of healthy subjects were unable
to prove that non-right-handedness conveys any selective
disadvantage over right-handedness other than a possibly
increased predisposition to accidents in left-handers who
need to deal with tools and equipment designed for the safety
and convenience of right-handers (Salive et al., 1993; Coren
and Previc, 1996)
Our study was based on a smaller sample but one with
more stringent exclusion criteria than the sample studied by
Crow and colleagues. Crow and co-workers excluded from
their population the bottom 10% in terms of square ticking
ability to examine whether their results had been influenced
by a sub-group in whom hand skill in general was impaired.
After doing so, they still found deficits at the point of equal
hand skill (Crow et al., 1998). More importantly still,
we were able to evaluate hemispheric language dominance
directly. Our data did not demonstrate that atypical,
particularly bilateral, language representation is associated
with any behavioural cost or peculiarity. Moreover, there was
no behaviourally relevant slowing of language processing in
subjects with bilateral language representation. Such a
slowing would have been predicted by the hypothesis that
language needs to be processed in a lateralized fashion
because transcallosal operations are prohibitively slow (Ringo
et al., 1994). We addressed this issue specifically by
measuring linguistic processing time in a picture–word
verification task corrected for variability in motor responses
(Fig. 1). This task was sufficiently sensitive to detect
differences in response times in subjects differing in IQ,
which have long been known in the psychological literature
(Sommers et al., 1970). The role of conduction delay is
further questioned by a recent finding in a patient with
agenesis of the corpus callosum, i.e. with a transcallosal
conduction stop, who had no speech impairment and had
bilateral language representation, as indicated by PET and
the Wada test (Komaba et al., 1998). Rather than a
transcallosal delay, transcallosal inhibition could have a role
in the development of lateralization. Transcallosal inhibition
is a characteristic of the adult brain but does not develop
fully before 5 years of age (Heinen et al., 1998). It appears
to play a role in the coordination of independent limb
movements. One may speculate that subjects with
1663
bihemispheric language representation are those in whom
bilateral brain regions were dedicated to language before
transcallosal inhibition became effective.
Limitations of the study
This study cannot exclude that there is a behavioural relevance
of atypical language lateralization. The number and type of
behavioural aspects we could assess were limited. Because
the focus of the study was language lateralization, academic
attainments and behavioural measures were chosen which
relate, although in some cases only indirectly, to linguistic
aptitude, i.e. the number of foreign languages spoken fluently,
academic achievement, IQ, verbal fluency and speed of
linguistic processing. Some of these qualities (skill in foreign
languages and academic achievement) could only be assessed
by subjective reports. More detailed and controlled
measurements (IQ and speed of linguistic processing) were
possible only in a subgroup of subjects reflecting the spectrum
from left- to right-hemisphere language dominance. We
therefore cannot exclude positively the possibility that there
are more subtle linguistic or other behavioural correlates of
language lateralization. For example, we did not follow
subjects to determine whether there is an increased risk of
schizophrenia in those with atypical language lateralization,
as has been suggested by Crow (Crow, 1997).
Conclusion
Our study provides strong evidence that atypical hemispheric
language lateralization results in no major behavioural
anomaly. Comparable linguistic proficiency can apparently
be achieved despite considerable variation in the underlying
neural system.
Like others, we are still inclined to believe that the
prevailing pattern of left-hemisphere language dominance,
like right-handedness, is not a random phenomenon but
has been influenced directly or indirectly by evolutionary
selection. For example, it is feasible that an adaptive
advantage of left-lateralized neural—linguistic or nonlinguistic—processing during the evolution of the human
brain may have been minimal or even undetectable at
the level of the individual. Only through amplification by
repetition over thousands of generations could this advantage
have led to a trend for lateralization of language. At present,
however, there may be one paradoxical consequence of
language lateralization. With today’s increased life
expectancy and the increased likelihood of suffering a
unihemispheric ischaemic stroke, subjects with bilateral
language representation may be less likely to become aphasic
or, at least, may recover better from language disturbances
compared with people with strong lateralization. A possible
disadvantage in the evolutionary past might thus have become
an advantage today.
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Acknowledgements
This work was supported by Nordrhein-Westfalen
(Bennigsen-Foerder-Preis IVA 6-400 302 97 and Nachwuchsgruppe: Hemispheric specialization), the Innovative
Medizinische Forschung (Kn-1-1-II/96-34) and the Deutsche
Forschungsgemeinschaft (Kn 285/4-1 and Kn 285/ 6-1).
Deppe M, Knecht S, Papke K, Fleischer H, Ringelstein EB,
Henningsen H. Correlation of cerebral blood flow velocity and
regional cerebral blood flow during word generation. Neuroimage
1998; 7 (4 Pt 2): S448.
Deppe M, Flöel A, Knecht S, Lohmann H, Konrad C, Sommer J,
et al. Temporal characteristics of cerebral blood flow velocity
changes evoked by short-term visual stimulation. Neuroimage 2000a;
11 (5 Pt 2): S790.
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Received January 25, 2001. Revised April 9, 2001.
Accepted April 19, 2001