Coactivation Of Visual And Auditory Pathways Induces Changes In The Timing Of Evoked Responses In Populations Of Neurons: An Meg Study. Scott. A. Carter, Akaysha C. Tang, Barak A. Pearlmutter, Laura K. Anderson, Robert Christner. Abstr. Soc. Neurosci. 2000. 561.4 Magnetoencephalography (MEG) has millisecond temporal resolution, allowing for the observation of rapidly changing neuronal activity. Compared to EEG, which has similar temporal resolution, MEG has superior spatial sensitivity. Taking advantage of MEG, we studied Hebbian learning in humans by coactivating two sensory modalities with a controlled time delay. Presenting a visual and an auditory stimulus separated by a delay led to activation of primary and secondary visual and auditory sources, as well as frontal sources. Blind source separation isolated these sources and allowed us to measure their response amplitudes and latencies on a trial-by-trial basis. Preliminary evidence shows that coactivation in two modalities can lead not only to potentiation of the response amplitude but also to inhibition of the response, as compared to single modality stimulation. Inhibition was expressed not as a reduction in amplitude but as an increase in response latency. Coactivation of visual and auditory pathways also led to a change in the visual response latency of a frontal source in the right hemisphere when the visual stimulus was presented alone. This change in temporal dynamics induced by pairing was reversible through extinction and could be re-acquired within the same session. These results suggest that one expression of Hebbian learning may be a coactivation-induced modification in population response timing.