The assessment of hemispheric lateralization in functional MRI--robustness and reproducibility.

Various methods have been proposed to calculate a lateralization index (LI) on the basis of functional magnetic resonance imaging (fMRI) data. Most of them are either based on the extent of the activated brain region (i.e., the number of "active" voxels) or the magnitude of the fMRI signal change. The purpose of the present study was to investigate the characteristics of various variants of these approaches and to identify the one that yields the most robust and reproducible results. Robustness was assessed by evaluating the dependence on arbitrary external parameters, reproducibility was assessed by Pearson's correlation coefficient. LIs based on active voxels counts at one single fixed statistical threshold as well as LIs based on unthresholded signal intensity changes (i.e., based on all voxels in a region of interest) yielded neither robust nor reproducible laterality results. Instead, the lateralization of a cognitive function was best described by "thresholded" signal intensity changes where the activity measure was based on signal intensity changes in those voxels in a region of interest that exceeded a predefined activation level. However, not all other approaches should be discarded completely since they have their own specific application fields. First, LIs based on active voxel counts in the form of p-value-dependent lateralization plots (LI=LI(p)) can be used as a straightforward measure to describe hemispheric dominance. Second, LIs based on active voxel counts at variable thresholds (standardized by the total number of active voxels) are a good alternative for big regions of interest since LIs based on signal intensity changes are restricted to small ROIs.

How does the brain accommodate to increased task difficulty in word finding? A functional MRI study.

In functional imaging of the brain, the difficulty of a task may be critical for the pattern of activation. Increased task difficulty could lead to increased activation in task-specific regions or to activation of additional, "compensatory" regions. A previous study with functional transcranial Doppler sonography (fTCD) showed no evidence that increased difficulty in word retrieval leads to a recruitment of areas homologous to language-related regions. The question remains how the brain accommodates increasing task difficulty. Because of limitations of fTCD method, we used functional magnetic resonance imaging (fMRI) in this study. We manipulated word retrieval difficulty in healthy subjects (n = 14) to determine whether the classical language-related brain regions are activated with increasing difficulty in word retrieval. fMRI demonstrated that with increased task difficulty (I) the lateralization of language-associated brain activation remained constant, (II) no additional activation of language-related regions of the dominant hemisphere, nor of homologous regions of the subdominant hemisphere, was evident, (III) additional activation was found in right posterior parietal cortex--typically associated with sustained attention and executive control. Thus, increased difficulty in word retrieval leads to coactivation of distinct brain areas, working together in a large cognitive network, rather than to increased activation of typically language-related areas.

[Visual cortex in the Tolosa-Hunt syndrome. Functional imaging for the detection of a psychogenic disorder--a case report]. / Visusstörungen beim Tolosa-Hunt-Syndrom. Untersuchung mit der funktionellen Magnetresonanztomographie zur Abgrenzung psychogener Störungen--Ein Fallbericht.

Besides painful ophthalmoplegia, patients suffering from Tolosa-Hunt syndrome often present increasing loss of visual perception. The impairment of the optic nerve leads to a delay of the VEP (visual evoked potentials) responses. Using the method of magnetic resonance imaging (MRI), some patients present unspecific alterations in the vicinity of the optic nerve. However, both methods (VEP and MRI) are unsuitable to assess the effect of an impaired optic nerve function on neuronal processing in the visual cortex. We report one patient suffering from Tolosa-Hunt syndrome affecting the optic nerve. We used fMRI (functional magnetic resonance imaging) to show how this impairment of the optic nerve alters cortical processing of visual information. The activity of the unaffected visual cortex was bilaterally reduced when compared to healthy volunteers but greater that obtained from patients suffering from bilateral occipital infarction. Our results offer new opportunities to assess the efficiency of therapy in patients with increasing loss of visual perception due to the Tolosa-Hunt syndrome. Further studies are necessary to investigate, whether fMRI also provides the possibility to assess the efficiency of drug therapy on optic nerve function.