NONPRIMARY AUDITORY THALAMIC REPRESENTATION OF ACOUSTIC CHANGE

1. The mismatch response, or mismatch negativity (MMN), is a neurophys iologic response to stimulus change. In humans and other animals, the MMN may underlie the ability to discriminate acoustic differences, a f undamental aspect of auditory perception. 2. This study investigated t he role of the thalamus in the generation of a tone-evoked MMN in guin ea pigs. Electrodes were placed in the caudomedial (nonprimary) and ve ntral (primary) subdivisions of the auditory thalamus (medial genicula te nucleus). Surface epidural electrodes were placed at the midline an d over the temporal lobe. The MMN was elicited by a deviant stimulus ( 2,450-Hz tone burst) embedded in a sequence of standard stimuli (2,300 -Hz tone bursts). 3. A tone-evoked MMN was present in nonprimary thala mus but was absent in the primary thalamus. Surface-recorded MMNs were measured at the midline but not over the temporal lobe. The correspon dence between nonprimary thalamic responses and midline surface potent ials, and between primary thalamic responses and temporal surface pote ntials, is consistent with data reported for the auditory middle laten cy responses in guinea pigs. 4. The results demonstrate that the nonpr imary auditory thalamus contributes to the generation of a tone-evoked MMN in the guinea pig. Furthermore, the data indicate that the guinea pig is a feasible model for investigating central auditory processes underlying acoustic discrimination.