Taking time with sound

Nagoya University physiologists have furthered understanding of the bird neural circuitry that allows them to distinguish where a specific sound is coming from. Their findings, published in the journal Science Advances, could help scientists understand the basics of how mammalian brains compute the time difference between a single sound arriving at each individual ear, known as ‘interaural time difference’. This ability is an integral component of sound localization.

“Animals can perform accurate interaural time difference detection for sounds of a wide range of frequencies,” explains Rei Yamada, who specializes in cell physiology at Nagoya University’s Graduate School of Medicine. The nerve circuitry for this process is so specialized that the many branches extending from a single nerve cell, called dendrites, receive a specific sound frequency from one or the other ear. But it’s not yet clear exactly how all of this works together to enable interaural time difference detection.

Yamada and his colleague Hiroshi Kuba wanted to understand more about this process. They conducted laser experiments on chicken brain slices by stimulating excitatory receptors on a part of the brain responsible for sound localization. This was followed by simulation experiments to clarify the meaning of their initial findings.

They discovered that nerve junctions, called synapses, were particularly clustered at the ends of specialized long dendrites dedicated to conducting signals from low-frequency sounds. Counterintuitively, this clustering reduced the strength of signal transmission along the length of the dendrite so that it was smaller by the time it reached the nerve cell. This process, however, enabled the nerve cell to tolerate intense inputs arriving through dendrites dedicated to each ear, thereby maintaining its ability to conduct the necessary time difference and location computing activities.

“Many animals, including humans, use the time difference of a sound reaching both ears as a clue for sound source localization,” says Yamada. “We would like to examine whether the association we found between neural function and structure is universally common in other species. Expanding our research to mammalian brains will be important to understand the basic principle of interaural time difference detection that birds and animals have in common with humans.”

The study, "Dendritic synapse geometry optimizes binaural computation in a sound localization circuit," was published in the journal Science Advances on November 24, 2021 at https://www.science.org/doi/10.1126/sciadv.abh0024.

About Nagoya University, Japan

Nagoya University has a history of about 150 years, with its roots in a temporary medical school and hospital established in 1871, and was formally instituted as the last Imperial University of Japan in 1939. Although modest in size compared to the largest universities in Japan, Nagoya University has been pursuing excellence since its founding. Six of the 18 Japanese Nobel Prize-winners since 2000 did all or part of their Nobel Prize-winning work at Nagoya University: four in Physics - Toshihide Maskawa and Makoto Kobayashi in 2008, and Isamu Akasaki and Hiroshi Amano in 2014; and two in Chemistry - Ryoji Noyori in 2001 and Osamu Shimomura in 2008. In mathematics, Shigefumi Mori did his Fields Medal-winning work at the University. A number of other important discoveries have also been made at the University, including the Okazaki DNA Fragments by Reiji and Tsuneko Okazaki in the 1960s; and depletion forces by Sho Asakura and Fumio Oosawa in 1954.

Website: https://en.nagoya-u.ac.jp/