- All subjects will be questioned prior to MRI scanning for possible occupational exposure to metal slivers or shavings, which may have become accidentally lodged in the tissues of the head or neck. Subjects with surgical clips or shrapnel in or near the brain or blood vessels, subjects with cochlear implants, subjects with any metallic body in the eye or CNS, and subjects with any form of implant wire or metal device which may concentrate radiofrequency fields will be excluded from MRI scanning experiments because of possible risks during MRI scanning. Those whose history is suggestive of such a problem will also be excluded from the MRI portion of the experiments. They may still participate in the behavioral and MEG experiments.