Sound Encoding in the Auditory Midbrain
By Kaiwen Shi
Our hearing relies on the auditory system, which handles sound information through various steps and pathways. One important division is between the central auditory pathway and the "higher-order" pathways. While the central pathway is known for processing and understanding sound cues, we know less about how the "higher-order" auditory circuits deal with sound information.
Importantly, the first divergence site appears at the inferior colliculus (IC), an auditory midbrain region. In the higher-order pathway, the dorsomedial “shell” region of the IC might be important for sound processing as its downstream targets play pivotal roles in perceiving vocalization sounds and regulating behavioral responses to vocalizations. Despite these meaningful connections, it remains poorly understood whether and how the shell IC represents sounds and thereby transmits these signals into high-order thalamic regions.
In our work published in Journal of Neurophysiology in March 2024, we focused on whether and how shell IC neurons encode amplitude modulation (AM), a key temporal feature in human speech and animal vocalizations. We used single-cellular two-photon Ca2+ imaging to record IC neuronal responses while awake mice were presented with different AM sounds. With this method, we can record the firing activity of hundreds of IC neurons simultaneously.
Panel A-B: Awake, head-fixed mice passively listen to AM sounds. Panel C: example field of view of mice IC during two-photon calcium imaging. Credit: Shi et al./Journal of Neurophysiology
While some neurons in the IC showed specific responses to different AM sounds, most individual neurons had broad and noisy responses to AM. This suggests that these neurons may not accurately convey information about AM features. However, when considering the collective activity of the IC neuron population, AM sounds were accurately represented, even though single neurons were not able to discriminate well.
These findings highlight the significant role of the shell IC in encoding important sounds for behavior and offer valuable insights into how vocalizations among members of the same species are processed.
Kaiwen Shi is starting a Ph.D. program in neuroscience at Harvard Medical School this fall. He is the first author on the Journal of Neurophysiology paper with Pierre Apostolides, Ph.D. (left), a 2019 Emerging Research Grants scientist and an assistant professor of otolaryngology–head and neck surgery at the University of Michigan's Kresge Hearing Research Institute. Apostolides is also an assistant professor of molecular and integrative physiology at the University of Michigan Medical School.
See prior papers funded by Apostolides’s grant here and here.