Determination of Cognitive Hemispheric Dominance by “Stereo” Transcranial Doppler Sonography
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.
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.
Subjects and Methods
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).
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⇓).
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 29.5×21-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 (7×10.5 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.
Reading. Subjects read prose aloud.
Baseline. Subjects were encouraged to relax, “not to think anything,” and to keep their eyes open.
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.”
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
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.
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.
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
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.
- Copyright © 1995 by American Heart Association
Müller HR, Casty M, Loeb J, Haefele M, Boccalini P. Prüfung der zerebralen Autoregulation mittels transkranieller Doppler-Sonographie unter lower body negative pressure. Schweiz Rundsch Med Praxis. 1992;81:1548-1554.
Aaslid R, Newell DW, Stooss R, Storteberg W, Lindegaard K-F. Assessment of cerebral autoregulation dynamics from simultaneous arterial and venous transcranial Doppler recordings in humans. Stroke. 1991;22:1148-1154.
Aaslid R. Visually evoked dynamic blood flow response of the human cerebral circulation. Stroke. 1987;18:771-775.
Dahl A, Russel D, Nyberg-Hansen R, Rootwelt K. Effect of nitroglycerin on cerebral circulation measured by transcranial Doppler and SPECT. Stroke. 1989;20:1733-1736.
Kontos HA. Validity of cerebral arterial blood flow calculations from velocity measurements. Stroke. 1989;20:1-3.
Kitterle FL, ed. Cerebral Laterality: Theory and Research. Hillsdale, NJ: Lawrence Erlbaum Associates; 1991.
Springer SP, Deutsch G. Linkes Rechtes Gehirn: Funktionelle Asymmetrien. Heidelberg, Germany: Spektrum Akademischer Verlag GmbH; 1993.
Gutbrod B. Beeinträchtigungen im Satzverständnis und im verbalen Kurzzeitgedächtnis bei Aphasikern. Konstanz, Germany: Hartung-Gorre Verlag; 1991.
Ekman E, Friesen WW. Pictures of Facial Affect. Palo Alto, Calif: Consulting Psychologists Press; 1976.
Wittich I, Klingelhöfer J, Matzander G, Sander D, Conrad B. Dynamics of visually evoked perfusion changes in the posterior cerebral artery territory. Cerebrovasc Dis. 1994;4(suppl 3):3.
Gur RC, Obrist WD, Hungerbuhler JP, Younkin D, Rosen AD, Skolnick BE, Reivich M. Sex and handedness differences in cerebral blood flow during rest and cognitive activity. Science. 1982;217:659-660.
Droste DW, Harders AG, Rastogi E. A transcranial Doppler study of blood flow velocity in the middle cerebral arteries performed at rest and during mental activities. Stroke. 1989;20:1005-1011.
Kelley RE, Chang JY, Scheinman NJ, Levin BE, Duncan RC, Lee S-C. Transcranial Doppler assessment of cerebral blood flow velocity during cognitive tasks. Stroke. 1992;23:9-14.
Thomas C, Harer C. Simultaneous bihemispherical assessment of cerebral blood flow velocity changes during a mental arithmetic task. Stroke. 1993;24:614-615.
Loring DW, Meador KJ, Lee GP, King DW. Amobarbital Effects and Lateralized Brain Function: The Wada Test. New York, NY: Springer Publishing Co; 1992.
Bruneau N, Dourneau M-C, Garreau B, Pourcelot L, Lelord G. Blood flow response to auditory stimulations in normal, mentally retarded, and autistic children: a preliminary transcranial Doppler ultrasonographic study of the middle cerebral arteries. Biol Psychiatry. 1992;32:691-699.