Have you ever wondered how auditory information is transmitted from the inner ear to the brain? What about where exactly in the inner ear this takes place? Thanks to work by a research collaboration between Uppsala University and Western University, this is now possible.

Using a novel imaging technique called synchrotron radiation phase-contrast imaging (SR-PCI), the team has performed the first three-dimensional frequency analysis of the human cochlea, showing where the various sound frequencies are captured.

The human cochlea is a spiral structure of the inner ear. Sound vibrations are transmitted to the cochlea and then transduced into electrical activity along the basilar membrane (BM). The BM is a soft-tissue structure that categorizes different acoustic vibrations based on their frequency and produces a spatial frequency map in the cochlea.

Since the late 1990s, researchers have attempted to image the fine structures of the human inner ear using synchrotron radiation, but the technique could not resolve the boundaries between the BM and the rest of the cochlea. To overcome this, one solution was to use contrast agents for better soft-tissue visualization; however, non-uniform distribution of contrast and tissue shrinkage caused problems. And while other researchers have used SR-PCI, they could not develop complex 3D frequency maps of the cochlea. Although some attempted 3D reconstruction from two-dimensional histological sections, the process was laborious and prone to artefacts.

The working principle

Now, the research collaboration – led by Helge Rask-Andersen at Uppsala University and by Hanif Ladak and Sumit Agrawal at Western University – has successfully created a three-dimensional representation of sound-frequency mapping in the human cochlea, using SR-PCI to image adult human cadaveric cochlea. The team performed the SR-PCI study at the Canadian Light Source in Saskatoon, publishing the results in Scientific Reports.

SR-PCI is unique because it can enhance soft-tissue contrast while minimizing artefacts that may be introduced through staining, sectioning and decalcification in histopathology. In SR-PCI, varying material properties within the sample cause phase shifts that are then transformed into detectable variations in X-ray intensity. These variations can help to provide edge contrast to highlight soft tissues.

The new 3D cochlear model shows where the various frequencies of sound are captured and reveals the detailed anatomical structure of the intact cochlea. This offers many advantages. First, accurate tonotopic frequency distributions could result in improved surgical outcomes for cochlear implant recipients. In addition, this new knowledge could help to better individualize the programming of cochlear implants for future patients, so that each area in the ear can be stimulated with the correct frequency. This will help to improve the sound quality for cochlear implant users.