A Low Cost Setup for Behavioral Audiometry in Rodents

Summary

A fast and inexpensive method for the behavioral determination of hearing parameters like hearing thresholds, hearing impairments or phantom perceptions (subjective tinnitus) is described. It uses pre-pulse inhibition of the acoustic startle response and can be easily implemented in a personal computer using a programmable AD/DA-converter and a piezo sensor.

Abstract

In auditory animal research it is crucial to have precise information about basic hearing parameters of the animal subjects that are involved in the experiments. Such parameters may be physiological response characteristics of the auditory pathway, e.g. via brainstem audiometry (BERA). But these methods allow only indirect and uncertain extrapolations about the auditory percept that corresponds to these physiological parameters. To assess the perceptual level of hearing, behavioral methods have to be used. A potential problem with the use of behavioral methods for the description of perception in animal models is the fact that most of these methods involve some kind of learning paradigm before the subjects can be behaviorally tested, e.g. animals may have to learn to press a lever in response to a sound. As these learning paradigms change perception itself ^1,2 they consequently will influence any result about perception obtained with these methods and therefore have to be interpreted with caution. Exceptions are paradigms that make use of reflex responses, because here no learning paradigms have to be carried out prior to perceptual testing. One such reflex response is the acoustic startle response (ASR) that can highly reproducibly be elicited with unexpected loud sounds in naïve animals. This ASR in turn can be influenced by preceding sounds depending on the perceptibility of this preceding stimulus: Sounds well above hearing threshold will completely inhibit the amplitude of the ASR; sounds close to threshold will only slightly inhibit the ASR. This phenomenon is called pre-pulse inhibition (PPI) ^3,4, and the amount of PPI on the ASR gradually depends on the perceptibility of the pre-pulse. PPI of the ASR is therefore well suited to determine behavioral audiograms in naïve, non-trained animals, to determine hearing impairments or even to detect possible subjective tinnitus percepts in these animals. In this paper we demonstrate the use of this method in a rodent model (cf. also ref. ⁵), the Mongolian gerbil (Meriones unguiculatus), which is a well know model species for startle response research within the normal human hearing range (e.g. ⁶).

Protocol

1. Setup Assembling and Software Programming

1. Install a D/A card in a Personal Computer (e.g.: NI PCI 6229, National Instruments) and connect it to a breakout-box (e.g.: BNC-2110, National Instruments), both should support at least one input and one output channel with a sampling rate of at least 44.1 kHz each.
2. Connect the output of the breakout-box via BNC cable to a sound amplifier (e.g: AMP75 wideband power amplifier, Thomas Wulf).
3. Install an infrared webcam (e.g.: Grand IP Camera Pro, GrandTec Electronics) for animal surveillance in darkness.
4. Install an Integrated Development Environment ....

Discussion

We present a cheap and easy to build setup for audiometric measurements in rodents based on pre-pulse inhibition of acoustic startle responses that can be used to determine behavioral hearing thresholds (= audiograms ¹⁰) and auditory phantom percepts like subjective tinnitus ¹¹. Especially the latter measurements are in the focus of several recent reports ^8,12,13,14 and can be seen as one prerequisite for electrophysiological investigations of the neuronal mechanisms underlying this disea.......