Tonotopic Map Development and Precision

Tonotopy is the spatial organization of sound in the brain, specifically, neurons are organized by the frequency they are sensitive to. The development of a tonotopic is determined by multiple factors; for example, Hoshino et al. (2021) found that ephrin-A3 is specifically important for the proper development of tonotopic maps. While ephrin-A3 is necessary for the development of tonotopic maps, Christensen et al. (2019) recently found that factors such as white noise can affect tonotopic precision without affecting the tonotopic map long term.

Tonotopic maps are developed in utero and Hoshino et al. (2021) used same-sex mice siblings, one that was wild type for ephrin-A3 and one that was an ephrin-A3 knockout, and compared the tonotopic maps after pure tone exposure at different frequencies and volumes. The study used in situ hybridization, stripe assays, NeuroVue labeling, c-fos staining, immunohistochemistry, auditory brainstem response recordings, and acoustic startle response-based assays to determine that ephrin-A3 knockout mice had significantly worse frequency discrimination due to an underdeveloped tonotopic map.

Neurons close together in a tonotopic map are specifically sensitive to a certain frequency, but are still able to perceive similar tones. Although it may seem counterintuitive, Christensen et al. (2019) found that playing white noise during tone differentiation can improve tone differentiation by decreasing the sensitivity of neurons to tones outside of their specific frequency. They came to this conclusion by recording A1 neuronal activity in the primary auditory cortex of mice with multi-electrode arrays during exposure to pure frequency tones ranging from 4 kHz to 48.7 kHz at 60 dB sound pressure level for 50 ms each. The pure tone was played during either silence or continuous white noise and the difference between A1 activity in the primary auditory cortex were used to determine which group of neurons preferred which frequencies. Then, they tested what percentage of the frequencies produced a response in a specific group of A1 primary auditory cortex neurons above a predetermined threshold with and without white noise. The study found that the percentage of above-threshold frequencies was lower when played in the presence of white noise.

Although Hoshino et al. (2021) are studying tonotopic map development and Christensen et al. (2019) are studying frequency discrimination once a tonopic map has been fully developed, they both focus on factors affecting tontopic precision.

References

Christensen, R., K., Linden, H., Nakamura, M., & Barkat, T., R., (2019). White Noise Background Improves Tone Discrimination by Supressing Cortical Tuning Curves. Cell Reports, 29(7), 2041-2053. https://doi.org/10.1016/j.celrep.2019.10.049

Hoshino, N., Altarshan, Y., Alzein, A., Fernando, A., M., Nguyen, H., T., Majewski, E., F., Chen, V., C.-F., Rochlin, M., W., & Yu, W.-M., (2021). Ephrin-A3 is required for tonotopic map precision and auditory functions in the mouse auditory brainstem. Journal of Comparative Neurology, 1-22. https://doi.org/10.1002/cne.25213