Auditory representation within principal division of cat medial geniculate body: An electrophysiological study

Extracellular responses to auditory stimuli of single units in the principal division of the medial geniculate body were examined. Two distinct areas were identified on the basis of these responses. These positions correspond to the dorsocaudal (DC) and ventral (MGV) divisions of the principal division, as differentiated cytoarchitecturally. Within MGV, the vast majority of units discharged transiently to the onset of the stimuli with a short latency. These units were also highly frequency specific, having sharp threshold tuning curves. In contrast, the majority of DC units were broadly tuned, responded at a long latency, and often habituated quickly to repetitive stimulation. A small subset of DC units responded similarly to MGV units having sharp tuning curves and exhibiting short latencies to tonal stimuli. Unlike MGV, there was no tonotopic organization within DC. Two separate tonotopically organized regions were found in MGV corresponding to the differentiation of this region into two separate but adjacent laminated nuclei. The tonotopic organization in three dimensions of pars lateralis (PL) revealed low frequencies ventrocaudolaterally with higher frequencies being added progressively dorsorostromedially. The frequency organization within pars ovoidea (OV) was not so easily discernible. However, penetrations in a dorsocaudal to ventrorostral orientation encountered an abrupt change from low to high frequencies at the presumed PL-OV border, followed by an increasing sequence of best frequencies within OV. The differences in auditory response characteristics between MGV and DC are discussed in relation to the known anatomy and connections of the cat medial geniculate body.