Frequency organization and cellular lamination in the medial geniculate body of the rabbit

Cellular laminae within the tonotopically organized ventral division of the medial geniculate body (MGV) of the cat have been proposed as the anatomical substrate for physiologically defined isofrequency contours. In most species, the laminae are not visible with routine Nissl stains, but are defined by the dendritic fields of principal cells and the terminal arbors of afferents arising from the inferior colliculus. In the present study, we have used the rabbit to directly examine the relationship between the laminar and tonotopic organization of the MGV. Best frequency maps of the MGV in anesthetized adult New Zealand white rabbits were generated from cluster responses recorded at 30-100 microm intervals to randomly presented tone bursts. Parallel vertical penetrations, roughly perpendicular to the laminae, revealed a low-to-high frequency gradient within the MGV. Non-laminated regions of the ventral division, generally found at the rostral or caudal poles, did not demonstrate a systematic frequency gradient. In contrast to a predicted smooth gradient, best frequencies shifted in discrete steps across the axis of the laminae. A similar step-wise frequency gradient has been shown in the central nucleus of the inferior colliculus of the cat. It is proposed that the central laminated core of the MGV represents an efficient architecture for creating narrow frequency filters involved in fine spectral analysis.