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Measuring Hearing Organ Vibration Patterns with Confocal Microscopy and Optical Flow

^*Center for Hearing and Communication Research and Department of Otolaryngology, Karolinska Institutet, Stockholm, Sweden and ^**Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden

Correspondence: Address reprint requests to Anders Fridberger, Gustav V. Research Institute, Karolinska Sjukhuset M1, SE-171 76 Stockholm, Sweden. Tel.: 46-85-177-3215; Fax: 46-834-8546; E-mail: anders.fridberger{at}cfh.ki.se.

A new method for visualizing vibrating structures is described. The system provides a means to capture very fast repeating events by relatively minor modifications to a standard confocal microscope. An acousto-optic modulator was inserted in the beam path, generating brief pulses of laser light. Images were formed by summing consecutive frames until every pixel of the resulting image had been exposed to a laser pulse. Images were analyzed using a new method for optical flow computation; it was validated through introducing artificial displacements in confocal images. Displacements in the range of 0.8 to 4 pixels were measured with 5% error or better. The lower limit for reliable motion detection was 20% of the pixel size. These methods were used for investigating the motion pattern of the vibrating hearing organ. In contrast to standard theory, we show that the organ of Corti possesses several degrees of freedom during sound-evoked vibration. Outer hair cells showed motion indicative of deformation. After acoustic overstimulation, supporting cells contracted. This slowly developing structural change was visualized during simultaneous intense sound stimulation and its speed measured with the optical flow technique.

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