[1] Univ Penn, Dept Bioengn, Philadelphia, PA 19104 USA
[2] Univ Penn, Dept Elect & Syst Engn, Philadelphia, PA 19104 USA
[3] Univ Penn, Appl Math & Computat Sci Grad Grp, Philadelphia, PA 19104 USA
[4] Univ Calif Santa Barbara, Dept Psychol & Brain Sci, Santa Barbara, CA 93106 USA
[5] Univ Calif Santa Barbara, Brain Imaging Ctr, Santa Barbara, CA 93106 USA
[6] Johns Hopkins Med Inst, Dept Phys Med & Rehabil, Human Brain Physiol & Stimulat Lab, Baltimore, MD 21205 USA