Patent No. 4462411 Evoked response audiometer

 

Patent No. 4462411  Evoked response audiometer (Richards, Jul 31, 1984)

Abstract

An evoked response audiometer is disclosed in which the patient is presented with a continuous auditory signal which is amplitude modulated and the evoked brain potentials of the patient (EEG) are recorded. The potentials are amplified and filtered and are averaged over a number of sampling periods to improve the signal to the noise ratio. The averaged signals are then analyzed to determine the amplitude and phase of the main components of the response to determine whether phase locking has occurred. The phase of the main components is then plotted against variation of the modulating frequency and the gradient of the plot provides the latency of the response from which neurological and other factors of significance to the hearing of the patient may be determined. The sound pressure level of the auditory signal is also varied and the amplitude and phase of the response plotted against these changes to provide an indication of the sensitivity of the hearing of the patient.

 

Notes:

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an improved evoked response audiometer which should facilitate the early diagnosis of deafness in a manner which should enable implementation of deafness diagnosis procedures on a wide spread basis.

The invention provides an evoked response audiometer comprising means for supplying to the patient under investigation an auditory signal and means for recording evoked brain potentials generated by said signal, characterized in that said auditory signal is presented in a continuous modulated form for an extended period of time to evoke phase-locked steady state potentials in the brain.

The audiometer of the present invention enables the recordal of the very small electrical potentials generated by the brain in response to modulated sounds and our research has shown that these potentials can correlate with the hearing threshold level of the patient under investigation. The audiometer embodying the present invention has the advantage over the prior art techniques of brain stem audiometry in that the acoustic stimuli are frequency specific whereas in the known brain stem evoked response transient acoustic stimuli are used which have a broad acoustic spectrum.

The auditory signal is preferably amplitude modulated although other forms of modulation, such as frequency modulation and beats may be used with acceptable results.

The brain potentals are preferably recorded by means of electrodes on the vertex and on the mastoids of the patient. In the preferred embodiment, the patient is presented with a continuous sinusoidally amplitude modulated tone. The brain activity is recorded for four cycles of modulation to cover substantially all frequencies within the hearing range, and the modulated wave form is used to synchronise brain activity with the acoustic stimulus thus enabling an averaging process to take place and improving the signal to noise ratio of the brain response. The average response is preferably Fourier analysed and relevant spectral components used to measure hearing loss.

The invention also provides a method of objectively testing the hearing of a patient comprising the steps of presenting to the patient for an extended period of time an auditory signal having a continuous modulated form, recording the brain potentials evoked by said signal, and analysing said potentials to determine whether they are phase-locked steady state potentials.

Preferably the signal is amplitude modulated.

As will be described in greater detail below, the potentials are preferably further recorded for changes in the frequency of modulation and changes in sound pressure level, and recorded potentials further analysed to determine response latency and hearing sensitivity.