The tonotopic organization of the auditory thalamus of the squirrel monkey (Saimiri sciureus).

Single unit responses were recorded from the auditory thalamus of anesthetized squirrel monkeys. The microelectrede penetrations had an orientation perpendicular to the sagittal plane. The small cell division is tonotopically organized. The cells in the most lateral aspect of this division have their 'best frequencies' (BFs) in the low frequency range with a progressive shift in the BF toward the higher frequencies the further medially the sampling progressed. Dorsal to and extending ventromedially are large cells which are darkly stained. In addition there is a bulbous extension more ventrally of large cells less dense and perhaps less darkly stained. The large cells in these subdivisions did not always discharge spontaneously nor were they always responsive to tones. In the dorsal group, there is an initial low to high sequence of BF followed by one or more reversals in BF. Within the border region between the small cell division and both the ventromedial extension and bulbous extension of the large cells there starts a reversal, a high-to-low sequence which occurs within a comparatively short distance. Beyond the initial reversal, in both subdivisions, there are one or two additional reversals of BF.