Date Published: March 22, 2017
Publisher: Public Library of Science
Author(s): Bernd Lütkenhöner, Jun Xu.
Small electrodynamic shakers are becoming increasingly popular for diagnostic investigations of the human vestibular system. More specifically, they are used as mechanical stimulators for eliciting a vestibular evoked myogenic potential (VEMP). However, it is largely unknown how shakers perform under typical measurement conditions, which considerably differ from the normal use of a shaker. Here, it is shown how the basic properties of a shaker can be determined without requiring special sensors such as accelerometers or force gauges. In essence, the mechanical parts of the shaker leave a signature in the electrical impedance, and an interpretation of this signature using a simple model allows for drawing conclusions about the properties of the shaker. The theory developed (which is quite general so that it is usable also in other contexts) is applied to experimental data obtained for the minishaker commonly used in VEMP measurements. It is shown that the experimental conditions substantially influence the properties of the shaker. Relevant factors are, in particular, the spatial orientation of the shaker (upright, horizontal or upside-down) and the static force acting on the table of the shaker (which in a real measurement corresponds to the force by which the shaker is pressed against the test person’s head). These results underline the desirability of a proper standardization of VEMP measurements. Direct measurements of displacement and acceleration prove the consistency of the conclusions derived from the electrical impedance.
Small electrodynamic shakers proved to be well suited for diagnostic investigations of the human vestibular system. Shakers used for that purpose are generally fitted with a short plastic rod, which makes it possible to apply mechanical stimuli accurately and safely to specific points on the head [1–3]. Suitable stimuli are, for example, pulses or 500-Hz tone-bursts having a duration of a few milliseconds. The skull vibrations elicited by such stimuli do, of course, not spare the inner ear, which means that they also reach the otolith organs, saccule and utricle. The latter are sensors for linear accelerations, and they play an important role in controlling posture and eye movement. The muscle reflexes elicited by stimulating the otolith organs are widely used for diagnostic testing of the vestibular system. To study these reflexes, electrodes are placed on the skin above (or in close vicinity to) an affected muscle and the so-called vestibular evoked myogenic potential (VEMP) is recorded. Depending on whether a cervical or an ocular muscle is considered, the potential is denoted as cVEMP or oVEMP [4, 5]. The two types of VEMP provide complementary diagnostic information about vestibular disorders .
Although small electrodynamic shaker have gained significant importance for VEMP investigations, their performance under typical measurement conditions, which greatly differ from the conditions referred to in the manufacturer’s specifications, is largely unknown. This study showed that the properties of an electrodynamic shaker can be determined without requiring special sensors such as accelerometers or force gauges: In essence, the shaker itself can serve as a vibration sensor, and the signature it leaves in the electrical impedance can be used to adjust the parameters of a simple model. The theory developed in this article is quite general. Thus, it is not only applicable to the special application that motivated the present study, but could be useful also in other contexts.