The first reports of human functional brain mapping using MRI scanners appeared 20 years ago [1
]. Functional magnetic resonance imaging (fMRI) in its most common form, using endogenous blood oxygen-level-dependent (BOLD) contrast, is now practiced routinely at most medical centers. Despite extensive research and clinical experience, uncertainty persists over the use of fMRI in the presurgical evaluation for epilepsy. This article reviews evidence supporting the use of fMRI for predicting postoperative language and verbal memory deficits in patients undergoing elective anterior temporal lobe (ATL) surgery. This clinical setting continues to be the most common indication for the intracarotid amobarbital (Wada) test. Recent studies suggest that fMRI provides a valid noninvasive alternative to the Wada test for most patients.
Although the focus of this review is on lateralization of language and verbal memory functions, it should be noted that fMRI also provides detailed activation maps that can in some cases be used to guide surgical resections. For example, fMRI can localize primary and secondary motor areas, even in some patients whose brain anatomy has been profoundly distorted by developmental anomalies or mass lesions [3
]. Together with diffusion tensor imaging (DTI) localization of corticospinal white matter pathways, these maps can be valuable in helping surgeons maximize a resection zone while avoiding critical motor areas [9
]. Similarly, fMRI can localize primary auditory, somatosensory, and visual cortex. In the case of visual cortex, the fMRI experiment can be designed to generate a retinotopic map showing the precise cortical representation of each region in the visual field [10
], allowing the surgeon to know with reasonable certainty what pattern of visual field loss will result from resection of a particular cortical zone. The utility of these sensory and motor activation maps, however, rests on well-established lesiondeficit relationships. The discovery of retinotopic maps, for example, was based on decades of careful observation in patients with focal occipital lesions [12
], thus there is no doubt about the effects that can be expected from focal damage to these regions.
Compared to these relatively straightforward relationships, the relationships between focal lesions and specific language deficits are complex and incompletely understood. The traditional emphasis on Broca and Wernicke areas has given way in recent decades to a much more complex picture of the language system, with recognition that both production and comprehension of language involve widely distributed brain networks, including many regions outside the traditional Broca and Wernicke zones [13
]. In addition, evidence suggests that the exact location of language areas varies from person to person [22
], perhaps accounting for some of the wide variation in aphasia outcome after focal lesions [24
]. Given this uncertainty and the highly distributed nature of language processes, important concerns have been raised about the meaning of activation foci identified by fMRI language experiments. Unlike with primary motor and sensory areas, the effect of removing an fMRI-defined “language area” is simply not known. Some areas identified by fMRI could participate in language functions but play a nonessential role. Because language tasks engage a variety of general cognitive processes, such as attention and working memory, some of the areas “activated” in an fMRI language study may represent these general cognitive processes rather than the language components of interest. Adding to the uncertainty surrounding fMRI language maps is the fact that there are many different task paradigms available, which produce markedly different patterns of activation (see [25
] for examples of comparisons between paradigms). Thus, a brain area declared “not active” using one paradigm might very well turn out to be “active” using another. These legitimate concerns about the specificity and sensitivity of fMRI-defined language maps currently limit the usefulness of such maps for detailed surgical planning. Those who would use fMRI language maps in this way run two risks: sparing of “active” regions that are actually not critical for language, resulting in sub-optimal seizure control; and resection of critical language zones that are “not active” merely due to insensitivity of the particular fMRI protocol employed, resulting in post-operative language deficits. Only through carefully designed and systematic studies – in which resections are performed blind to the fMRI data, standardized procedures are used for assessing outcome, and quantitative measures are made of the anatomical and functional lesion – will the usefulness of fMRI language maps for planning surgical resections be determined.
Although in the author's opinion fMRI language maps should not yet be routinely used for planning resection boundaries, fMRI already has a clearly established role to play as an alternative to the Wada test. Temporal lobectomy is highly effective for seizure control [29
], yet roughly half of patients undergoing dominant ATL resection experience postoperative language [32
] or verbal memory decline [38
]. The traditional role of the Wada test is to estimate the risk of decline by determining the patient's hemispheric dominance for language and memory. Risk assessment provides the patient and physician with additional information that can be useful in deciding whether to proceed with treatment in elective situations. This information can also be used to select high-risk patients for more invasive procedures such as electrical stimulation mapping.