Tonotopic organization in lateral part of posterior group of thalamic nuclei 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 lateral part of the posterior group of thalamic nuclei (Po) in five barbiturate-anesthetized cats. Best frequencies and minimum response latencies to tone-burst stimulation were studied at each location along a penetration. Most of Po is located rostral to the medial geniculate body (MGB) and is contiguous with the ventral nucleus and medial division. Po is characterized physiologically by narrowly tuned, short-latency (less than 40 ms) responses. Considerable scatter of best frequencies occurs along electrode penetrations, although a clear tonotopic organization is apparent in the distribution of best frequencies obtained from several electrode penetrations located in the same frontal plane of an individual brain. A "single" frequency is represented as an irregularly shaped lamina. A three-dimensional "block" model of the tonotopic organization of Po is described in which the highest best frequencies are located caudally, and the lowest best frequencies are located rostrally within the nucleus. The high-frequency representation of Po is contiguous with the high-frequency representation of the ventral nucleus of the MGB. The low- and middle-frequency representations of the ventral nucleus and Po are discontinuous. The ventral nucleus and Po have similar physiological properties and together constitute the tonotopic division of the auditory thalamus in the cat. Neurons in the medial division adjacent to the medial border of Po are larger than neurons in Po, lack tonotopic organization, and respond at short latencies.