Redefining the tonotopic core of rat auditory cortex: physiological evidence for a posterior field.
In the present study we analyzed the organization of the thalamocortical projections of the specific auditory relay nucleus of the thalamus, the ventral division of the medial geniculate body (MGv), using the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin. All injections of MGv produced dense labeling of axonal fibers in temporal cortex. In all cases, labeled axons were predominantly concentrated in cortical layers III and IV and, to a lesser extent, at the junction of layers V and VI. Injections confined to the medial regions of MGv, and specifically to the ovoid nucleus of MGv (OV, pars ovoidea), resulted in anterograde labeling of TE1, with minor labeling of the ventral quarter of TE1, designated subarea TE1v. Injections placed in lateral regions of MGv and occupying the lateral ventral subnucleus (LV), or injections in the mediolateral center of MGv and occupying parts of LV and OV, also resulted in labeling of area TE1 and minor labeling of TE1v. However, these injections also produced labeling in areas TE2 and TE3. Thus, area TE1 (excluding subarea TE1v) receives heavy projections from all aspects of MGv and appears to be the core target of MGv. While regions of MGv also project to surrounding cortical belt areas, these projections tend to be lighter and to vary depending on the region of MGv examined. These results, together with other connectional findings, and cytoarchitectonic and physiological studies, suggest that TE1 (possibly excluding subarea TE1v) is the primary auditory cortex in the rat.