Correlation of neural response properties with auditory thalamus subdivisions in the awake marmoset.

As the information bottleneck of nearly all auditory input that reaches the cortex, the auditory thalamus serves as the basis for establishing auditory cortical processing streams. The functional organization of the primary and nonprimary subdivisions of the auditory thalamus is not well characterized, particularly in awake primates. We have recorded from neurons in the auditory thalamus of awake marmoset monkeys and tested their responses to tones, band-pass noise, and temporally modulated stimuli. We analyzed the spectral and temporal response properties of recorded neurons and correlated those properties with their locations in the auditory thalamus, thereby forming the basis for parallel output channels. Three medial geniculate body (MGB) subdivisions were identified and studied physiologically and anatomically, although other medial subdivisions were also identified anatomically. Neurons in the ventral subdivision (MGV) were sharply tuned for frequency, preferred narrowband stimuli, and were able to synchronize to rapid temporal modulations. Anterodorsal subdivision (MGAD) neurons appeared well suited for temporal processing, responding similarly to tone or noise stimuli but able to synchronize to the highest modulation frequencies and with the highest temporal precision among MGB subdivisions. Posterodorsal subdivision (MGPD) neurons differed substantially from the other two subdivisions, with many neurons preferring broadband stimuli and signaling changes in modulation frequency with nonsynchronized changes in firing rate. Most neurons in all subdivisions responded to increases in tone sound level with nonmonotonic changes in firing rate. MGV and MGAD neurons exhibited responses consistent with provision of thalamocortical input to core regions, whereas MGPD neurons were consistent with provision of input to belt regions.