"Ventral" area in the rat auditory cortex: a major auditory field connected with the dorsal division of the medial geniculate body.

The rat auditory cortex is made up of multiple auditory fields. A precise correlation between anatomical and physiological areal extents of auditory fields, however, is not yet fully established, mainly because non-primary auditory fields remain undetermined. In the present study, based on thalamocortical connection, electrical stimulation and auditory response, we delineated a non-primary auditory field in the cortical region ventral to the primary auditory area and anterior auditory field. We designated it as "ventral" area after its relative location. At first, based on anterograde labeling of thalamocortical projection with biocytin, ventral auditory area was delineated as a main cortical terminal field of thalamic afferents that arise from the dorsal division of the medial geniculate body. Cortical terminal field (ventral auditory area) extended into the ventral margin of temporal cortex area 1 (Te1) and the dorsal part of temporal cortex area 3, ventral (Te3V), from 3.2-4.6 mm posterior to bregma. Electrical stimulation of the dorsal division of the medial geniculate body; evoked epicortical field potentials confined to the comparable cortical region. On the basis of epicortical field potentials evoked by pure tones, best frequencies were further estimated at and around the cortical region where electrical stimulation of the dorsal division of the medial geniculate body evoked field potentials. Ventral auditory area was found to represent frequencies primarily below 15 kHz, which contrasts with our previous finding that the posterodorsal area, the other major recipient of the dorsal division of the medial geniculate body; projection, represents primarily high frequencies (>15 kHz). The posterodorsal area is thought to play a pivotal role in auditory spatial processing [Kimura A, Donishi T, Okamoto K, Tamai Y (2004) Efferent connections of "posterodorsal" auditory area in the rat cortex: implications for auditory spatial processing. Neuroscience 128:399-419]. The ventral auditory area, as the other main cortical region that would relay auditory input from the dorsal division of the medial geniculate body to higher cortical information processing, could serve an important extralemniscal function in tandem with the posterodorsal area. The results provide insight into structural and functional organization of the rat auditory cortex.