Spectral interactions, but no mismatch negativity, in auditory cortex of anesthetized rat

We used an 'oddball' stimulation paradigm to evoke epidural, event-related potentials over auditory cortex in urethane/xylazine-anesthetized rats, in order to study the neural mechanisms underlying the mismatch negativity (MMN). Oddball stimulus sets included deviants (10 kHz, 10% probability) and one of six standards that ranged from 1.25 to 9.75 kHz (overall stimulus rate 2/s). Controls included 'reference' responses (10 kHz, stimulus rate 1/5 s) obtained before and after each oddball set. We found that the deviant-evoked response was modified by the preceding standards, and the degree of modification varied systematically with the spectral separation between the standard and deviant. However, the modification was apparent relative to the reference, rather than relative to the standard (as with the MMN), and did not resemble the MMN. In fact, we found no evidence for the MMN in this preparation.