[1] Johns Hopkins Univ, Sch Med, Dept Neurol, Baltimore, MD 21287 USA
[2] Johns Hopkins Univ, Sch Med, Dept Radiol, Baltimore, MD 21287 USA
[3] Johns Hopkins Univ, Dept Cognit Sci, Baltimore, MD 21218 USA
[4] Harvard Univ, Dept Psychol, Cambridge, MA 02138 USA