A digital heterodyne laser interferometer for studying cochlear mechanics

Abstract

Laser interferometry is the technique of choice for studying the smallest displacements of the hearing organ. For low intensity sound stimulation, these displacements may be below 1 nm. This cannot be reliably measured with other presently available techniques in an intact organ of Corti. In a heterodyne interferometer, light is projected against an object of study and motion of the target along the optical axis causes phase and frequency modulations of the back-reflected light. To recover object motion, the reflected light is made to interfere with a reference beam of artificially altered frequency, producing a beating signal. In conventional interferometers, this carrier signal is demodulated with analog electronics. In this paper, we describe a digital implementation of the technique, using direct carrier sampling. In order to obtain the necessary reference signal for demodulation we introduce an additional third light path. Together, this results in lower noise and reduces the cost of the system.

Within the hearing organ, different structures may move in different directions. It is therefore necessary to precisely measure the angle of incidence of the laser light, and to precisely localize the anatomical structure where the measurement is performed. Therefore, the interferometer is integrated with a laser scanning confocal microscope that permits us to map crucial morphometric parameters in each experiment. We provide key construction parameters and a detailed performance characterization. We also show that the system accurately measures the diminutive vibrations present in the apical turn of the cochlea during low-level sound stimulation.

Introduction

The ear is a remarkably sensitive detector of minute pressure fluctuations. Such pressure fluctuations, sound, cause vibration of the detector cells, the inner and outer hair cells (OHCs). The hair cells convert the sound-evoked vibration into electric potentials that can be relayed to the brain through the auditory nerve.

The sensory cells are not merely passive detectors. The outer hair cells boost the vibration of the cochlear partition by supplying energy to counteract the viscous damping caused by the fluids surrounding the hearing organ (reviewed by Ulfendahl, 1997, Robles and Ruggero, 2001). The underlying mechanisms are controversial, but are believed to depend on the electrochemical gradient between outer hair cells and scala media, the endocochlear potential. To study this process, it is necessary to make precise measurements of vibrations from structures with very low optical reflectivity (Khanna et al., 1989a) that are highly sensitive to various forms of trauma. Furthermore, monitoring the electrochemical as well as electrophysiological parameters is important for understanding the function of the outer hair cells.

Previous studies showed that the vibration pattern of the hearing organ is complex. The main axis of vibration may differ for the various anatomical structures (Tomo et al., 2007, Fridberger et al., 2004). Therefore the angle of incidence of the laser beam must be precisely determined, since the interferometer will only detect the projection of the motion on the optical axis of the objective lens. A second important issue is the localization of the point from where vibration measurements are taken in the organ of Corti. Structures close by may have the same motion amplitude but a completely different phase response (Nowotny and Gummer, 2006). These factors may make interpretation of the results difficult.

The laser interferometer described here was designed to tackle the problems mentioned above. Hence the current system has novel components not found in its predecessors (Cooper, 1999, Willemin et al., 1989, Nuttall et al., 1991). To ameliorate common problems in analog electronics and to reduce overall system cost, we chose not to use conventional analog demodulation, but rather to use digital signal processing techniques based on direct sampling of the carrier waveform. To permit concurrent morphological studies and to allow precise control of the measurement location and angle, the interferometer is integrated with a laser scanning confocal microscope. Furthermore, up to two microelectrodes can monitor or modify electrochemical parameters.