1. Laboratory of Cognitive PhysiologyInstituto de Biofísica Carlos Chagas Filho, UFRJRio de Janeiro RJ21941‐900 Brazil
2. Laboratory of Brain and Cognition, National Institute of Mental HealthNational Institutes of HealthBethesda Maryland20892 USA
3. Laboratory of Neuropsychology, National Institute of Mental HealthNational Institutes of HealthBethesda Maryland20892 USA
4. McGovern InstituteMITCambridge Massachusetts02139‐4307 USA