Surgery – Diagnostic testing – Ear or testing by auditory stimulus
Reexamination Certificate
2000-12-01
2002-06-18
Shaver, Kevin (Department: 3736)
Surgery
Diagnostic testing
Ear or testing by auditory stimulus
C381S015000
Reexamination Certificate
active
06406439
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved evoked response audiometer for use in the diagnosis of deafness.
BACKGROUND OF THE INVENTION
The diagnosis of deafness at an early stage in paediatrics is important to enable the early fitting of hearing aids and/or cochlear implants in order to assist language development in a hearing-impaired child. It is also important to be able to diagnose deafness in adults who are unable, due to mental illness or disability, or unwilling, for various reasons, to participate in conventional behavioural deafness testing.
In our U.S. Pat. Nos 4,462,411 (Rickards) and 5,023,783 (Cohen and Rickards), we have described evoked response audiometers which use a continuous auditory tone that is frequency or amplitude modulated, the auditory tone being presented for a sufficiently extended period of time to enable phase-locked steady-state potentials to be evoked in the brain of the person being tested. An electro-encephalograph (EEG) signal from the scalp of the person is manipulated such that the components due to the modulation carried by the auditory stimulus is extracted and detected.
The modulated auditory stimulus produces separate and distinct evoked potentials in the brain depending on the nature of the modulation. These evoked potentials can be difficult to detect, particularly for low sound levels which are less audible to the person being tested.
SUMMARY OF THE INVENTION AND OBJECT
It is therefore an object of the present invention to provide an improved evoked response audiometer incorporating an improved modulation technique which produces stronger evoked potentials using low sound level auditory stimulus signals.
The invention provides an evoked response audiometer comprising means for supplying to a patient an auditory stimulus signal consisting of a carrier frequency which is modulated by at least two different forms of modulation such that the stimulus is at least substantially frequency specific, said auditory signal being presented for a sufficiently extended period of time to enable phase-locked steady-state potentials to be evoked in the brain of the patient, means for sampling the brain potential signals evoked by said auditory signal, and means for analysing said brain potentials to determine whether phase-locking of said brain potentials to the modulated auditory signal has occurred, said means for supplying said auditory signal being selectively controlled to advance or delay one modulation with respect to the other modulation to cause enhancement of the evoked response to the auditory stimulus.
Research has indicated that the combined modulation of the auditory stimulus enables significant improvements in the detection of the evoked potentials whereby evoked potentials of amplitude large enough for detection will be produced by auditory stimuli of lower sound level, and hence lower subjective loudness.
The invention also provides a method of testing for hearing impairment, comprising the steps of supplying to the patient an auditory stimulus signal consisting of a carrier frequency which is modulated by at least two different forms of modulation so that the stimulus is at least substantially frequency specific, presenting the auditory signal for a sufficiently extended period of time to enable phase-locked steady-state potentials to be evoked in the brain of the patient, sampling the brain potential signals evoked by said auditory signal, analysing said brain potentials to determine whether phase-locking of said brain potentials to the modulated auditory signal has occurred, and selectively controlling said auditory signal to advance or delay one modulation with respect to the other modulation to cause enhancement of the evoked response to the auditory stimulus.
In a preferred form of the invention, the auditory stimulus signal is amplitude modulated and frequency modulated, preferably in a periodic manner, such as sinusoidal. The potentials evoked in the brain by amplitude modulation and frequency modulation have been found to differ in phase, indicating different delays in the processing by the auditory system to amplitude modulation and frequency modulation. By compensating for the delay in perception of the amplitude and frequency modulation, the auditory signal compensates for the auditory system process by artificially advancing or retarding in time the amplitude modulation or the frequency modulation relative to each other, resulting in the equalisation of the phase delays occurring in the evoked brain potentials.
Without the necessary equalisation, the response to the amplitude modulated signal and the response to the frequency modulated signal can have a phase relationship which results in response cancellation when the responses are vectorially summed. By compensating for the delays in the actual auditory stimulus, the phase of the two responses can be altered so that the vectorial sum is significantly enhanced beyond the stimulus achieved by the use of amplitude modulation or frequency modulation alone. This enhancement results in a higher detection sensitivity to the stimulus by virtue of an improved signal to noise ratio, and consequently, the hearing threshold determined when using the evoked response audiometer much closer to the true behavioural hearing threshold of the patient under test. As a result, estimations of the true behavioural thresholds from the patients evoked response thresholds are improved.
Depending on the frequency of the carrier, the modulation frequency and the corresponding modulation indexes of the auditory signal, the measured physiological delays will vary. All such delays can be compensated for by adjusting the phase relationship between the AM modulation and the FM modulation of the stimulus signal.
In terms of hearing perception, AM is produced by modulating a pure tone (or sinusoid) whose amplitude is varied in a sinusoidal manner by another sine wave at the modulation frequency. FM is produced by modulating a pure tone whose frequency is varied in a sinusoidal manner by a sine wave at the modulation frequency. When both forms of modulation are combined, the frequency and amplitude can be varied together in a number of subtly different ways. For example, the frequency can be high when the amplitude is high; the frequency can be low when the amplitude is high; the frequency can be midway when the amplitude is high, or the frequency can be midway when the amplitude is low.
The relative phase between the AM signal and the FM signal can be given any value between +/− about 60°, depending on the signal parameters, to produce enhanced evoked potentials in the brain of the patient.
The responses to AM and FM stimuli, detected in the overall EEG activity, differ. To improve the detection process both modulation methods are used together and the phase difference between the AM and FM modulations is selected to result in constructive addition of the AM and FM response components. In a preferred form, this occurs when the phase difference between the AM and FM modulations is about 30°. If the modulation components are about 210° apart, cancellation will occur. The AM/FM stimulus in this case would produce no or very little detected response to the stimulus.
The phase relationship between the AM and FM detection processes depends on the mechanics of the ear and brain physiology. It also depends on the modulation indices used. The modulation indices determine how much the carrier amplitude is changed by amplitude modulation and how much the frequency of the carrier is changed by the frequency modulation.
It is expected that different relative phase delays will be required depending on the patient tested, the carrier and modulation frequencies, and the AM and FM modulation indices used. Norms for different age groups and conscious states are determined experimentally. To this extent the solution of the more appropriate phase difference is initially determined empirically. However, once an appropriate phase difference is determined, it can be used for similar patient typ
Cohen Lawrence Thomas
Parker John Charles
Rickards Field Winston
Dennison, Scheiner & Schultz
Shaver Kevin
The University of Melbourne
Wingood Pamela
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