Tonotopic organization in ventral nucleus of medial geniculate body in the cat

Responses of single units and clusters of units to tone burst stimulation were recorded at 100-micron intervals along vertical electrode penetrations through the medial geniculate bodies of eight barbiturate-anesthetized cats. Marking lesions were placed at two or three locations along most penetrations to aid in histological reconstruction of electrode tracks. Best frequencies and suprathreshold-response latencies were studied at each location along a penetration. The ventral nucleus is physiologically characterized as a region containing narrowly tuned, short-latency (less than or equal to 40 ms) responses and an orderly tonotopic organization. Best frequencies were plotted as a function of depth along single electrode penetrations, and the sequences from different locations in the ventral nucleus were compared. Two-dimensional best-frequency maps were obtained at different rostrocaudal levels. Each map was constructed from best frequencies encountered along several electrode penetrations in the same transverse plane in one brain. We divided the ventral nucleus into seven different rostrocaudal levels, each characterized by a different pattern of tonotopy. Caudolaterally, isofrequency contours parallel the ventrolateral border of the medial geniculate body. At middle levels, low- and mid-frequency contours course ventromedially from the dorsal border of the ventral nucleus toward its medial border, then turn sharply and continue ventrolaterally. Higher-frequency contours parallel this course, but consist of discontinuous dorsal and ventral segments. Rostrally, isofrequency contours are vertically oriented. A model of the three-dimensional tonotopic organization of the ventral nucleus is described that is consistent with the two-dimensional best-frequency maps obtained at different rostrocaudal levels and with locations of ventral nucleus neurons labeled by horseradish peroxidase injections into low-, mid-, and high-frequency representations in auditory cortex. The model includes a planar component and a concentric component. Within the planar component, low frequencies are represented laterally and high frequencies are represented rostromedially. Within the concentric component, low frequencies are represented in a central column that extends mediolaterally through a hole in the mid-frequency representation. The mid-frequency representation in turn is partially surrounded by the high-frequency representation. There is a continuous representation of a "single" frequency in both the planar and concentric components of the model.(ABSTRACT TRUNCATED AT 400 WORDS)