Rapid Modification of Populational Neuronal Response Onset Times Via Hebbian Learning: a Non-Invasive Single-Trial Analysis of MEG Data. A.C. Tang, S.A. Carter, B.A. Pearlmutter, N.A. Malaszenko, L.K. Anderson, C.J. Aine and R. Christner. Abstr. Cog. Neur. Soc. 2001. 123C We studied Hebbian learning in humans via coactivation of the visual and auditory pathways. The experiments have three phases: baseline, learning, and extinction. During baseline (3 blocks) and extinction (2 blocks), faces and tones are presented alone in a random sequence (50 face and 50 tone presentations per block). During learning (1 block), 50 paired face-tone were added randomly to the above described standard block of single faces and tones. Using a noninvasive functional imaging method (MEG) and a blind source separation method (SOBI), we were able to monitor, during this Hebbian learning millisecond temporal resolution from localized brain regions (a few mm spatial resolution). If Hebb's rule holds, co-activation of the visual and auditory pathways should result in changes in the populational neuronal response along each pathway alone. In 6 out of 7 subjects, we found learning induced changes in the timing of the single-trial populational response onset. Majority of such neuronal populations appeared to be along the dorsal visual processing pathway in all 6 subjects. Hebbian learning is usually thought of as a change in response amplitude. Our results offer evidence for rapid induction of populational response timing change as a redult of a Hebbian learning protocol. Methodologically, these results show that it is now possible to non-invasively study repidly-occuring learning with millisecond resolution in humans.