The C50m response: conditioned magnetocerebral activity recorded from the human brain

Recent advances in neuroimaging technology now permit a precise determination of the dynamics of specific neural activity underlying human associative learning. We used magnetoencephalography (MEG) to characterize the dynamics of conditioned responses (CRs) within auditory cortex during habituation, delay and trace conditioning training, and delay conditioning extinction. Conditioned stimuli (CS) were visually presented geometric figures, and unconditioned stimuli (US) were aversive noise bursts. CS+ stimuli were paired with the US on 50% of presentations: CS- stimuli were never paired with the US. Auditory cortex was activated following the paired CS+ at an average of 49-62 ms following US onset. Our data support the presence of a differential conditioned response (C50m) in auditory cortex following the unpaired CS+ at an average of 30-61 ms after US omission. The current source strength of the auditory C50m was subsequently quantified for the unpaired CS+ and CS- during training, the unpaired CS+ during extinction, and habituation. During delay and trace training, the C50m was stronger for the unpaired CS+ than for the CS-, and was also stronger for the unpaired CS+ during training compared to both habituation and extinction. This is the first description of magnetocerebral conditioning in normal human auditory cortex. The C50m activity in auditory cortex elicited by visual stimuli constitutes a direct observation of associative neural plasticity within the human auditory cortex.