Inputs to combination-sensitive neurons in the medial geniculate body of the mustached bat: the missing fundamental

This study examined projections to combination-sensitive neurons in the medial geniculate body of the mustached bat. These specialized neurons respond to the combination of two temporally and spectrally distinct components of the bat's sonar pulse and echo, encoding target information. Combination-sensitive neurons respond to the bat's sonar fundamental, between 24-31 kHz, in conjunction with a higher harmonic signal. They are thought to be formed in the medial geniculate body (MGB) by convergent input from inferior colliculus representations of 24-31 kHz and higher frequencies. This study used anterograde and retrograde tract-tracing methods in conjunction with physiological recording to test this MGB convergence hypothesis. In anterograde tracing experiments, multiple deposits of two different tracers were placed in the central nucleus of the inferior colliculus (ICC), one tracer in the 24-31 kHz region and another in an ICC representation responding to a higher sonar harmonic. We found only limited overlap in the MGB labeling patterns of the two tracers, and little in many areas where combination-sensitive neurons are common. In retrograde tracing experiments, a single deposit of tracer was placed at a combination-sensitive recording site in the MGB. With the deposit mostly limited to combination-sensitive MGB areas, labeling in 24-31 kHz representations of the ICC was absent or minor. These results suggest that many combination-sensitive neurons in the MGB do not receive 24-31 kHz ICC input. The strongest inputs to combination-sensitive MGB regions originate in high-frequency representations of the ICC and combination-sensitive regions of auditory cortex. Additional projections arrive from the thalamic reticular nucleus, external nucleus of the inferior colliculus, and pericollicular tegmentum. Each projection may contribute to the 24-31 kHz sensitivity of combination-sensitive neurons in the medial geniculate body.