(Stroke. 2002;33:878.)
© 2002 American Heart Association, Inc.
Letters to the Editor |
Re: Crossed Nonaphasia in a Dextral With Left Hemispheric Lesions: A Functional Magnetic Resonance Imaging Study of Mirrored Brain Organization
Department of Neurology, University of Münster, Münster, Germany
To the Editor:
We read with great interest the article by Hund-Georgiadis et al,1 in which they presented a case report of a patient with crossed nonaphasia after left-hemispheric stroke. Functional MRI revealed "mirrored" brain organization not only for language but also for divided/selective attention, as assessed by the Stroop task. Both functions were lateralized to the right hemisphere.
The authors did not assess traditional right-hemispheric cognitive functions by means of functional MRI, such as visuospatial attention.2 They mention in their case description, however, that "the patient complained of problems in the visuospatial domain." Neuropsychological testing revealed impaired processing of nonverbal material, whereas processing of verbal material was normal. Therefore, we suspect that visuospatial attention, which normally lateralizes to the right side of the brain, was lateralized to the left hemisphere in this case, providing further support for the authors’ claim that their patient showed "completely reversed," ie, "mirrored," cognitive functions. The case by Hund-Georgiadis et al agrees with our findings from a cohort of healthy subjects who were assessed for hemispheric dominance of language and visuospatial attention by means of functional transcranial Doppler ultrasonography and functional magnetic resonance tomography.3 We found that most subjects with atypical language lateralization indeed presented with a completely reversed brain organization, ie, visuospatial attention and language being lateralized into opposite hemispheres.
Additionally, our study demonstrated that not only can completely reversed functional anatomy exist without obvious penalty to brain functions, but partially reversed functional anatomy can as well.
Four of our 10 healthy subjects with right-hemispheric language dominance did not present with a "mirrored" brain organization. Rather, both language and visuospatial attention were lateralized to the same hemisphere. Furthermore, cognitive functions can vary significantly in the extent to which they lateralize. Clinicians need to be aware of the possibility of combined linguistic and visuospatial impairments of variable degrees after left- or right-hemispheric damage.
References
1.
Hund-Georgiadis M, Zysset S, Weih K, Guthke T, von Cramon DY. Crossed nonaphasia in a dextral with left hemispheric lesions: a functional magnetic resonance imaging study of mirrored brain organization. Stroke. 2001; 32: 2703–2707.
2.
Heilman KM, Van Den Abell T. Right hemisphere dominance for attention: the mechanism underlying hemispheric asymmetries of inattention (neglect). Neurology. 1980; 30: 327–330.
3.
Flöel A, Knecht S, Lohmann H, Deppe M, Sommer J, Dräger B, Ringelstein EB, Henningsen H. Language and spatial attention can lateralize to the same hemisphere in healthy humans. Neurology. 2001; 57: 1018–1024.
Response
Max-Planck Institute of Cognitive Neuroscience, Leipzig, Germany
We thank Drs Flöel and Knecht for their interest in our study. We agree that the hemispheric dissociation of functions such as language and attention cannot be considered as a fixed pattern in each individual, although it is found in the majority of healthy subjects. This observation has important potential clinical implications, particularly in the evaluation of neuropsychological deficits after left- or right-hemispheric damage and in presurgical decision-making in patients with epilepsy, brain tumors, or vessel malformations.
Many efforts have been undertaken to establish valid imaging protocols to assess language dominance.1 In contrast, few studies have examined this issue in the field of visuospatial attention.2 Several questions rise from here: (1) Which paradigms are the most appropriate to characterize traditionally right hemisphere attentional functions? (2) Are these paradigms also applicable in patient studies? and (3) What do the commonly applied laterality indices really tell us about brain organization?
The role of the subdominant cortices in the overall network identified by imaging studies has to be further evaluated in future studies, beyond the sole determination of laterality. The recent study by Flöel et al3 used functional transcranial Doppler (TCD) for lateralization studies; while this examination technique has been shown to produce valid results, it is limited by its poor spatial resolution, especially when compared with functional MRI. It would certainly be very interesting to examine other attentional aspects in the 4 exceptional subjects who exhibited partially reversed laterality, described in the recent study by Flöel et al.3 Unfortunately, the article provided solely functional TCD data. The functional MRI technique, which is characterized by a higher spatial resolution than is achieved with TCD, would potentially enlighten the underlying physiology of partially mirrored brain organization.
References
1. Hund-Georgiadis M, Lex U, von Cramon DY. Language dominance assessment by means of fMRI: contributions from task design, performance and stimulus modality. J Magn Res Imaging. 2001; 13: 668–675.[CrossRef][Medline] [Order article via Infotrieve]
2.
Mesulam MM. Spatial attention and neglect: parietal, frontal and cingulate contributions to the mental representation and attentional targeting of salient extrapersonal events. Philos Trans R Soc Lond B Biol Sci. 1999; 354: 1325–1346.
3. Flöel A, Knecht S, Lohmann H, Deppe M, Sommer J, Dräger B, Ringelstein EB, Henningsen H. Language and spatial attention can lateralize to the same hemisphere in healthy humans. Neurology. 2001; 57: 1018–1024.
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