Thalamocortical patches in auditory neocortex.

Thalamocortical afferents to the primary auditory cortex of the rabbit were labeled by the iontophoretic injection of the anterograde tracers PHA-L or biocytin into the ventral division of the medial geniculate body (vMGB). Single injections of either tracer into the vMGB labeled multiple "patches" of afferent axons in lamina III/IV of the ipsilateral auditory cortex. Serial section analysis revealed that single patches were elongated in the rostral-caudal axis forming bands of approximately 2 mm in length. The orientation of the bands was similar to the isofrequency contours of the tonotopic maps derived from prior electrophysiological experiments. Within the coronal plane, the topography of the patches is remarkably similar to the intermittent distribution of binaural interaction subclasses described in physiological studies. Our results are consistent with a model of vMGB organization containing functionally distinct, parallel anatomical pathways to AI.