Stimulation will be conducted via LED goggles at a flash frequency of 3 Hz and 8 Hz, evenly divided among the 30 subjects, and at 0 Hz (dark condition). Statistical parametric mapping and other statistical procedures will be used to identify brain regions in which k* for AA and/or rCBF is elevated at 3 Hz compared with the dark condition; at 8 Hz compared with the dark condition; and at 8 Hz compared with 3 Hz condition.