Rightdominant Patients

Fig. 2. Validation of fMRI as a clinical tool for assessing language lateralization and localization. Seven patients, previously Wada tested to assess hemispheric language dominance, were scanned in an fMRI experiment while they performed alternating semantic and nonsemantic tasks. fMRI lateralization was consistent with the Wada test results in all patients. The figure illustrates one coronal section for each patient, with the left side of the brain depicted on the right side of the section. Voxels depicted in the red-yellow color scale represent regions of the brain in which the semantic task produced statistically significantly greater activation than did the nonsemantic task. (From Desmond et al. [1995], ref. 91, by permission of Oxford University Press. See Color Plate 2 following p. 112.)

Fig. 2. Validation of fMRI as a clinical tool for assessing language lateralization and localization. Seven patients, previously Wada tested to assess hemispheric language dominance, were scanned in an fMRI experiment while they performed alternating semantic and nonsemantic tasks. fMRI lateralization was consistent with the Wada test results in all patients. The figure illustrates one coronal section for each patient, with the left side of the brain depicted on the right side of the section. Voxels depicted in the red-yellow color scale represent regions of the brain in which the semantic task produced statistically significantly greater activation than did the nonsemantic task. (From Desmond et al. [1995], ref. 91, by permission of Oxford University Press. See Color Plate 2 following p. 112.)

indices for the whole hemisphere vs for just Broca's area, maximum correlation values vs number of active voxels or extent) are most valid, reliable, and practical.

The few existing studies of language localization demonstrate that these paradigms are less robust than motor mapping (84) and also that the most valid results will probably be obtained with methods that combine several complementary behavioral paradigms (101). These studies include only small numbers of patients who have undergone both fMRI and intraoperative language mapping and, like the lateralization studies, demonstrate very good, but not exact, agreement with intraoperative findings. Before fMRI can replace awake craniotomy, larger patient series will have to be performed, standardized techniques developed, and systematic study of the effect of patient variables undertaken. Nevertheless, presently, fMRI of language areas can serve as an adjunct to intraoperative mapping, allowing informed surgical decisions and directed cortical mapping.

Memory

The study of memory processes with fMRI has proved quite challenging, even in healthy subjects. One reason for this difficulty is that memory-encoding systems are believed to be almost continually engaged in encoding new events into memory, thereby making the isolation of these processes with comparison to an appropriate baseline task difficult. In addition, memory is not a unified process but rather a series of processes including encoding, storage, and retrieval, each of which are necessary for normal memory performance. Failure in any one of these domains will result in a clinical deficit on memory testing.

Despite these obstacles, the determination of memory lateralization and localization is an important clinical goal. Indeed, because of the relative difficulty of testing memory processes from both a behavioral and anatomic standpoint, the development of preoperative and noninvasive methods is particularly important. Moreover, memory changes, particularly of verbal memory, in patients undergoing dominant temporal lobectomy, remain one of the most perplexing and difficult to avoid postoperative neuropsychiatry complications (102).

To date, there have only been a few studies comparing fMRI with the IAT in the assessment of memory lateralization in patients with MTL epilepsy. Using a scene-encoding task, Detre et al. (103) found that activation asymmetries in the MTL concurred with IAT-determined memory competence in all cases, including two cases in which memory was paradoxically located ipsilateral to the seizure focus. In another study, Bellgowan et al. (104) reported that MTL activation during a verbal encoding task can differentiate between patients with left (L) MTLE and right (R) MTLE. They found that RMTLE patients had LMTL activation during verbal encoding but that LMTLE patients did not. However, these findings were on the group level only and do not allow inferences to be made on an individual subject basis. Moreover, interpretation of a lack of activation, rather than an altered pattern of activation, can be problematic as there can be many technical reasons for not finding activations.

Golby et al. (105) studied the lateralization of encoding of different types of stimuli by fMRI compared with the IAT. Using a novelty encoding paradigm that investigated memory for stimuli varying in their capacity to be verbalized (faces, scenes, designs, and words), fMRI and IAT lateralization were concordant in eight of nine MTLE patients. They also demonstrated the reorganization on a group level of material-specific memory to the contralateral temporal lobe: patients with LMTLE activated the right MTL during verbal encoding (Fig. 3; see Color Plate 3 following p. 112), whereas healthy control subjects and patients with RMTLE normally activated the left MTL during verbal encoding. In the future, these types of studies may allow the quantification of surgical risk to memory and the potential tailoring of medial temporal resection. fMRI may also be useful in predicting postoperative seizure outcome. Killgore et al. (106) have found that when combined, fMRI and IAT provided complementary data that resulted in improved prediction of postoperative seizure control compared with either procedure alone.

Further studies using this and other paradigms will be necessary to determine whether the amount of functional reorganization can be quantified and whether

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