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.