Those of us who taught anatomy and physiology of the auditory mechanism in the 1970s have changed our lectures on auditory physiology a number times in the past 40 years or so. As new research is conducted, new procedures invented, and knowledge becomes greater, it’s more evident that our theories of how things work require modification. Such is the lot of the scientist and especially those who study and conduct research in auditory physiology.
Once more, it looks as though we may be on the way to modifying our thoughts on how the inner ear works. Since there has been no way to directly observe how hearing actually functions, cochlear research has always been developed on theory. The cochlea is embedded in the thick temporal bone that is difficult to penetrate without causing damage to the structures under investigation.
Now, new tools have been developed and are in use by a multinational research team studying the inner ear. Anders Fridberger at the Department of Clinical and Experimental Medicine Cell Biology at Linköping University in Sweden, indicates that their research group has developed a new type of confocal microscope which makes it possible to see sound-evoked motion and allows direct observation of how hearing occurs.
The tool also enables study of how the auditory system converts sound into electrical impulses within the auditory nerve. That conversion, which is of fundamental importance for the sense of hearing, depends on small protrusions found at the tops of the sensory cells in the inner ear, called stereocilia. The new type of confocal microscope allows researchers to study the size and direction of the stereocilia movements through optical flow calculations.
But….What is a Confocal Microscope?
Throughout history, medical science discoveries have depended on new tools to facilitate the observation of processes. In the 1500s, the microscope allowed famous anatomists to directly observe the microscopic components of the inner ear and many other structures. That’s how the cochlear explorers, such as Scarpa, Reissner, Corti and many others were able to identify, describe and theorize as to the function of components of the inner ear. Other than the researchers, most audiologists have never heard of a confocal microscope, so what is this tool that has been modified to “see” how the ear works?
Pawley (2006) describes Confocal Microscopy as an optical imaging technique for increasing optical resolution and contrast of a micrograph by means of adding a spatial pinhole placed at the confocal plane of the lens to eliminate out-of-focus light. This enables the reconstruction of three-dimensional structures from the obtained images by collecting sets of images at different depths (a process known as optical sectioning) within a thick object. This technique has gained popularity in the scientific and industrial communities and typical applications are in life sciences, semiconductor inspection and materials science. A conventional microscope “sees” as far into the specimen as the light can penetrate, while a confocal microscope only “sees” images one depth level at a time. In effect, the CLSM achieves a controlled and highly limited depth of focus.
The invention of the confocal microscope began in the 1940s with the development of the slit lamp for eye examinations. The device went through a number of modifications, some European, some Japanese, until 1955 when a post doctoral student at Harvard, Marvin Minsky (1927- 2016), invented the device and patented it in 1957. Minsky went on to a distinguished career at the Massachusetts Institute of Technology (MIT) as an expert in artificial intelligence. Minsky’s confocal microscope offers a number of advantages over conventional microscopy, including shallow depth of field, elimination of out of focus glare, and the ability to collect serial optical sections from thick specimens. The scope has had a major effect on the ability to image either fixed or living cells and tissues. (Click on the video by Michelle Allen to see how the scope works).
And…..What is Optical Flow?
Another concept used by Dr. Fridberger’s team to study how the inner ear works is optical flow calculations. Physicists describe optical flow calculations as the pattern of the apparent motion of objects, surfaces and edges in a visual scene caused by the relative motion between an observer which could be a camera or an eye and the scene being viewed. The concept of optical flow was first introduced by James J. Gibson in the 1940s, as he described the visual stimuli provided to animals moving through the world. Since then, followers of Gibson have demonstrated the role of the optical flow stimulus for the perception of movement by the observer in the world; perception of the shape, distance and movement of objects in the world; and the control of locomotion. Thus, these optical flow calculations are used for image processing, motion detection, object segmentation, “time-to-contact” information, “focus of expansion” calculations, luminance, motion compensated encoding, and stereo disparity measurement. These calculations then allow for the computation of how the auditory system actually moves based upon the confocal observation.
So…..That’s How They Did It!
The aforementioned multinational team has used these concepts to actually see how the auditory system works rather than simply theorizing about the physiology of the inner ear. So far, their research suggests that the components of the inner ear that process sounds such as speech and music seem to work differently than other parts of the inner ear.
The research group has found that to perceive speech and music there must be a capability to hear low frequency sounds. The brain needs information from receptors located at the top of the cochlea which has been, in the past, difficult to study due to the thickness of the bone that covers this area. Dr. Fridberger and colleagues have been able to measure the inner ear response to sound without having to open the surrounding bone structures and have found through these new methods that the hearing organ responds in a completely different way to sounds in the voice-frequency range.
To perceive speech, the brain relies on inputs from sensory cells located near the top of the spiral-shaped cochlea. This low-frequency region of the inner ear is anatomically difficult to access, and it has not previously been possible to study its mechanical response to sound in intact preparations. In their studies, the group used optical coherence tomography to image sound-evoked vibration inside the intact cochlea. They demonstrate that low-frequency sound moves a small portion of the basilar membrane, and that the motion declines in an exponential manner across the basilar membrane. Hence, the response of the auditory system to speech-frequency sounds is different from the one evident in high-frequency cochlear regions.
The team’s revolutionary findings go against previous thoughts on inner ear physiology and are published in the current issue of the Proceedings of the National Academy of Science.
Fridberger, A. (2016). New discovery on how the inner ear works. Linköping Universitet. Science Daily./ Retrieved August 14, 2016.
Gibson, J.J. (1950). The Perception of the Visual World. Houghton Mifflin. Retrieved August 15, 2016.
Minsky, M. (1988). Memoir on Inventing the Confocal Scanning Microscope. Scanning, vol.10 pp128-138. Retrieved August 15, 2016.
Pawley, J.(Ed) (2006). Handbook of Biological Confocal Microscopy, 3rd Edition. Springer Science and Business Media: New York. Retrieved August 15, 2016.
Warren, R. Ramamoorthy, S., Ciganovic, N.,Zhang, Y., Wilson, T., Petrie, T., Wang, R., Jacques, S., Reichenbach, T., Nuttal, A. & Fridberger, A. (2016). Minimal basiliar membrane motion in low frequency hearing. Proceedings of the National Academy of Sciences, July. Retrieved August 15, 2016.
Nicon Instruments (2016). Introductory Confocal Concepts. Retrieved August 15, 2016.
Batts, S. (2007). Confocal image of cochlea wins art prize. Science Blogs. Retrieved August 16, 2016.
Allen, M. (2016). Confocal Microscope. Nanoscience. Northwest Missouri State University. Retrieved August 15, 2016.