Assessment of verbal memory by fMRI: lateralization and functional neuroanatomy

The medial temporal lobe (MTL) is essential for declarative memory formation, but also a frequent source of seizures. To decrease the risk of amnestic impairments after temporal lobectomy, functional magnetic resonance imaging (fMRI) is increasingly used to establish pre-operative measures for a prognosis of postoperative memory performance. The present study addresses one of the major challenges in clinical fMRI, the interpretation of activation pattern in single subjects. Before investigating patients however, it must be first assessed to which extent the verbal memory paradigm can be used to determine the lateralization and the functional neuroanatomy of MTL-activity. Therefore, this study took a "step backwards" by first examining healthy subjects without known MTL pathology.

An implicit verbal encoding task was applied to a group of ten healthy volunteers using fMRI.

At the group level the MTL activation was strongly left-lateralized and separated into three distinct clusters. At the individual level, the lateralization of MTL-activity could be determined in 9 of 10 subjects. In contrast, its localization was inter-individually highly variable. In each case, only one of the three group activation clusters was detected.

The present study shows that fMRI can be used to assess the lateralization of brain activity related to verbal encoding even in individual subjects. For the routine use in a clinical setting however, the results of verbal memory paradigms must at present be treated with care if they are used to support decisions as to how far the resection of one MTL can be extended.