Two thalamic pathways to primary auditory cortex.

Neurons in the center of cat primary auditory cortex (AI) respond to a narrow range of sound frequencies and the preferred frequencies in local neuron clusters are closely aligned in this central narrow bandwidth region (cNB). Response preferences to other input parameters, such as sound intensity and binaural interaction, vary within cNB; however, the source of this variability is unknown. Here we examined whether input to the cNB could arise from multiple, anatomically independent subregions in the ventral nucleus of the medial geniculate body (MGBv). Retrograde tracers injected into cNB labeled discontinuous clusters of neurons in the superior (sMGBv) and inferior (iMGBv) halves of the MGBv. Most labeled neurons were in the sMGBv and their density was greater, iMGBv somata were significantly larger. These findings suggest that cNB projection neurons in superior and iMGBv have distinct anatomic and possibly physiologic organization.