The general objective of this dissertation was to understand how the central auditory system analyzes complex vocal signals, including biosonar signals and social communication signals such as speech. In many species, vocal signals are analyzed by combination-sensitive neurons that respond to distinct spectral elements in vocal signals occurring in a particular temporal relationship. This dissertation describes the auditory brainstem circuitry underlying combination-sensitive responses in the mustached bat, a mammal that uses complex biosonar signals and social vocalizations in acoustically guided behavior.
The first specific aim was to investigate circuitry underlying inhibitory combination-sensitive interactions, known to originate in the intermediate nucleus of the lateral lemniscus (INLL). These interactions form a type of spectral integration in which a neuron’s response to sounds at its best (high) frequency are inhibited by sounds at much lower frequencies. Previous work has shown that the low-frequency-tuned inhibition is based on the neurotransmitter glycine. I placed deposits of retrograde tracers at inhibitory combination-sensitive recording sites in INLL, and then performed anti-glycine immunohistochemistry on the brain sections containing the retrogradely labeled cells. The major finding was that inhibitory combination-sensitive INLL neurons receive predominant glycinergic input from low-frequency neurons of the medial nucleus of the trapezoid body, and high-frequency excitatory input from the ventral cochlear nucleus. I conclude that these inputs are responsible for the combination-sensitive inhibition in INLL.
The second specific aim was to investigate circuitry underlying facilitatory combination-sensitive interactions in the inferior colliculus (IC). In these interactions, a neuron’s response to sounds at its best (high) frequency is enhanced by sounds at much lower frequencies, when these two sounds occur in a specific temporal relationship. These interactions depend solely on glycinergic inputs tuned to the low-and high-frequency sounds. Using retrograde and anterograde transport methods in combination with glycine immunohistochemistry, I provide evidence that facilitatory combination-sensitive neurons in the IC receive high-frequency glycinergic input from the ipsilateral lateral lemniscal and superior olivary nuclei and low-frequency glycinergic inputs from INLL and the ventral nuclei of the lateral lemniscus. This integration of differently tuned inputs is the basis for representations of biosonar-related information in the auditory thalamus and cortex.