(Stroke. 1995;26:70-73.)
© 1995 American Heart Association, Inc.
Articles |
Determination of Cognitive Hemispheric Dominance by "Stereo" Transcranial Doppler Sonography
From the Departments of Neurology (F.R., M.S., H.P.M.), Neuropsychologic Rehabilitation (K.G., B.G.), and Neurosurgery (H.-J.S.), University of Berne, Inselspital, Berne, Switzerland.
Correspondence to Frank Rihs, MD, Department of Neurology, University of Berne, Inselspital, CH-3010 Berne, Switzerland.
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Abstract |
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Background and Purpose Transcranial Doppler sonography (TCD) can assess blood flow velocity changes induced by focal brain activation. Therefore, TCD may have the potential to identify hemispheric dominance for cognitive tasks.
Methods Using a system with two TCD probes ("stereo" TCD), we monitored simultaneously both middle cerebral arteries (MCAs) of 14 healthy right-handed volunteers while they performed cognitive tasks. The averaged blood flow velocity ratio of the two MCAs and the hemispheric blood flow velocity shift induced by the cognitive task were calculated.
Results In every subject, language tasks resulted in blood flow velocity shift to the left compared with visuospatial tasks. Mean MCA blood flow velocity shift to the left was 1.67%, 2.01%, and 2.31% in three language tasks. Mean blood flow velocity shift to the right was 1.67% and 2.31% in two visuospatial tasks.
Conclusions Bilateral simultaneous MCA blood flow velocity monitoring and averaging during cognitive tasks can help to identify hemispheric dominance for cognitive tasks in individuals.
Key Words: cerebral blood flow • dominance • cerebral • cognition • ultrasonics
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Transcranial Doppler sonography (TCD) is a noninvasive tool for assessment of blood flow velocities in the basal cerebral arteries.1 It can assess velocity changes at a high temporal resolution and therefore can assess autoregulatory changes also.2 3 4 Because velocity alterations relate to changes in cerebral blood flow in the corresponding territory, TCD can be used for monitoring cerebral blood flow changes under the condition that the vessel diameter and its perfusion area do not change during measurement. Several studies have shown that this condition is most likely fulfilled.5 6 7 8
The present study was performed to assess changes in blood flow velocity of the middle cerebral artery (VMCA) during cognitive tasks and to find out if these changes occur asymmetrically during tasks known to activate predominantly one hemisphere.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Subjects
Fourteen healthy right-handed volunteers (7 men, 7 women) were examined. None had a history of neurological or psychiatric disorders or a family history of left-handedness. The ages of the men (median, 31 years; range, 28 to 40 years) and women (median, 29; range, 24 to 41 years) were not significantly different (P=.4, Mann-Whitney U test).
Doppler Technique
Velocity was simultaneously recorded from both MCAs using an MDX
TCD-7 system (DWL GmbH, German Vasculab). Both 2-MHz Doppler probes
were attached with elastic over the temporal ultrasound window. Depth
for registration of the MCA Doppler signal was chosen 1 cm proximal to
the depth where the reversed signal of the anterior cerebral artery was
recorded. Velocity was determined as the mean of the on-line
measurements of maximum blood flow velocity during 10 task cycles. The
ratio curve of left to right VMCA was called blood flow
velocity ratio (Vratio). Hemispheric blood flow
velocity shift (Vshift) was defined as percent change of
the Vratio during cognitive activity and rest (Fig 1
).
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Cognitive Tasks
Subjects were tested in a baseline condition, a simple reading
condition, and during four cognitive tasks. Tasks given to the subjects
included two left-hemisphere (LH) tasks, referred to as "syntax"
and "synonyms," and two right-hemisphere (RH) visuospatial tasks,
referred to as "faces" and "designs."9 10 Each
of these tasks consisted of 80 items requiring either a "yes" (40
items) or a "no" (40 items) answer. Each item involved the
simultaneous (synonyms, syntax, faces) or successive (designs)
presentation of two stimuli. Stimuli were presented on
29.5x21-cm white cards. Presentation time of the items varied from 2
to 9 seconds depending on task demands and speed in answering.
Successive items were shown with the minimal possible delay.
The cognitive tasks were as follows:
Synonyms. Pairs of monosyllabic, disyllabic, and trisyllabic abstract nouns were constructed that were either semantically similar (eg, "difficulty" and "problem") or semantically different (eg, "decision" and "condition"). All words had a high degree of abstractness.11 Subjects were asked to decide whether two simultaneously presented nouns were semantically similar or different.
Syntax. Pairs of sentences differing only in syntactic complexity (eg, active versus passive voice) were constructed. They were either semantically identical (eg, "The man kisses the woman accompanied by a dog." "The woman accompanied by a dog is kissed by the man.") or semantically different (eg, "The man kisses the woman accompanied by a dog." "The man who is accompanied by a dog is kissed by the woman."). Subjects had to indicate whether the meaning of the two simultaneously presented sentences was the same or different.
Faces. Photographs of facial expressions (7x10.5 cm; happiness, anger, fear, disgust, sorrow, surprise, neutral) were selected from the set of Ekman and Friesen.12 Pairs of faces expressed similar or different emotions. The persons in any given pair of photographs were different. Subjects had to determine whether two facial expressions were identical or different.
Designs. An example of this task is given in Fig 2. Pairs of abstract geometric designs identical or
different in a small detail were used. To enhance task demands on
memory processing, the two items of a pair were presented
successively: the first design for 5 seconds, the second until the
subject answered. Subjects had to decide whether the pairs were
identical or different.
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Reading. Subjects read prose aloud.
Baseline. Subjects were encouraged to relax, "not to think anything," and to keep their eyes open.
Procedure
The volunteers were informed about the setting and the goal of
the study. The order of the tasks (synonyms, faces, syntax, designs,
reading, and baseline) was identical for all subjects. All six tasks
were performed in one session of 1 to 1.5 hours. Two practice items
were given to explain a task. To minimize unilateral hemispheric motor
activation, subjects had to point with both index fingers to a response
card marked with "yes" and "no" to indicate matching or
mismatching answers. There were 10 successive cycles for each task
without interruption. Each cycle consisted of 20 seconds of activity as
described above and 20 seconds of resting with eyes closed. Activity
started when the examiner told the subjects, "Open your eyes,"
and ended when he said, "Close your eyes."
Statistical Analysis
Data were analyzed by means of a two-way repeated-measures ANOVA
with gender as between-subjects factor and tasks as within-subjects
factor. Significant effects in the ANOVA were further explored by the
multistage Bonferroni procedure.13
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Results |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Each task increased VMCA on both sides. The increase was in the range of 1.9% to 30% (mean, 13.36% on the left and 14.24% on the right side). The smallest VMCA increase was observed in the baseline task with a light stimulus (eyes open) but no cognitive demands (1.8% to 15.4%; mean, 5.8% on the left and 7.8% on the right). The most marked increase occurred in the reading task, with values ranging from 8.1% to 30% (mean, 18.28% on the left and 17.46% on the right side). Mean VMCA increased between 12.85% and 16.71% in the synonyms, syntax, faces, and designs tasks. Similar curves were seen in left and right MCAs. Asymmetry of VMCA usually was not obvious from inspection of VMCA curves. It was detected by the Vratio given automatically on-line by the system (Fig 1
). Blood flow velocity
increase started in the first second of activity and peaked after 5 to
7 seconds in all subjects. Mean Vratio at the start was
1.16% in favor of the right side (maximum, 36.54%; standard
deviation, 14%). Doppler sonography cannot differentiate between real
VMCA asymmetries and differences caused by slightly
different insonation angles. However, by introducing Vshift
as a relative value, this unknown factor becomes negligible in our
setting, where insonation angles are constant throughout the
examination.
Individual Vshift ranged from -8.41% to 15.38% (mean of
all tests, 0.08%; median, 0.00). The Vshift was always
smaller than VMCA changes. The VMCA,
Vratio, and Vshift for the syntax task
in one subject is given in Fig 1. VMCA curves are
remarkably similar in shape. Similar curves resulted in all tasks with
varying Vshift scores.
Mean Vshift in percent for all cognitive tasks and the
baseline condition is given in Fig 3 (pooled data of 14
subjects, 10 repetitions of each task). The two-way ANOVA showed a
difference for sex (F[1,12]=4.7; P=.05) and a highly
significant effect for tasks (F[5,60]=10.62; P<.001).
There was no significant interaction between sex and tasks. Overall,
men showed Vshift more to the left (mean, 1.02%), whereas
women showed Vshift more to the right (mean, -0.86%).
Post hoc tests showed positive Vshift values for all LH
tasks (mean of synonyms, 1.67%; mean of syntax, 2.07%; mean of
reading, 2.31%) and negative values (mean of faces, -2.03%; mean of
designs, -2.67%) for RH tasks. This indicates Vshift to
the left and right, respectively. There were no statistical differences
within the group between LH and RH tasks. Vshift for the
baseline condition (mean, -0.83%) differed from the
Vshift for syntax, reading, and design tasks, but
significance level was just missed for the synonyms and faces
tasks.
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Relative Vshift for different tasks in individuals is shown
in Fig 4. In the synonyms+syntax tasks compared with the
designs+faces tasks, all 14 subjects showed relative Vshift
to the left (Fig 4, top). The reading condition alone compared with
baseline resulted in Vshift to the left in 13 of the 14
subjects (Fig 4, bottom). It is important to realize that absolute
Vshift to the right had occurred for LH tasks in 4 subjects
(negative values in Fig 4, top) but that it was smaller than that
occurring in RH tasks in the same individuals.
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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The goal of this study was to identify the dominant hemisphere for language and for visuospatial tasks in right-handed individuals using bilateral simultaneous ("stereo") TCD monitoring. In our group of volunteers, all cognitive tasks resulted in Vshift that reflected asymmetrical cerebral activation. There was Vshift in the individual volunteer for almost every task performed. Habituation was not observed. Combined results of LH tasks compared with RH tasks resulted in Vshift to the left in all subjects. Evoked blood flow velocity testing results are thus in accordance with an LH dominance in language processing for right-handed healthy subjects. Similar results were seen in previous studies using alternating left and right TCD measurements with one probe.14 15 Three of our subjects showed Vshift to the right side for all tasks. However, in these individuals Vshift was observed weakly to the right for LH tasks and markedly to the right for RH tasks. This means that absolute Vshift can be misleading in individuals; information about hemispheric dominance for a single task has to be compared with that for a baseline condition or a different task. The effect of the attention system, which is thought to be predominantly right hemispheric, might account for this. Activation by attention might be superimposed to a variable degree over the activation by our tasks.16 This might also explain why Vshift in the baseline condition had a nonsignificant tendency toward predominant RH activation. RH dominance for visuospatial tasks recently has also been reported in posterior cerebral artery–evoked flow testing.17 There was a sex difference of Vshift in accordance with earlier observations.18 In our series, women showed more RH activation compared with men.
Our results agree with previous TCD studies that used only one Doppler probe and identified hemispheric dominance for cognitive tasks in groups of subjects19 20 21 22 and in individuals.23 Confirmation of our results by other techniques would be desirable. However, validation with the Wada procedure, which is considered the gold standard,24 is not feasible in volunteers. We feel it would be unethical to perform this invasive test in healthy subjects for a scientific experiment only.
In conclusion, VMCA monitoring by bilateral simultaneous TCD is an interesting tool for the investigation of relative hemispheric activation. With the aid of a computerized data-acquisition and data-processing system and the possibility of averaging repeated measurements, this noninvasive method can be performed by a single examiner. It may be of clinical interest in the assessment of hemispheric dominance before brain surgery, and it may also become an indirect means to assess cortical activation during cognitive tasks in persons in good health and with brain disease.25
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Acknowledgments |
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This study was supported by the "Schweizerischer Nationalfonds zur Förderung der wissenschaftlichen Forschung," grant 31-32569.91. In addition, we thank Rune Aaslid, PhD, (Seattle, Wash) and Dieter Denner (DWL GmbH, Sipplingen, Germany) for technical advice and for equipment support, and Armin Schnider, MD, (Berne, Switzerland) for helpful comments.
Received July 21, 1994; revision received October 6, 1994; accepted October 7, 1994.
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