Patent No. 5023783 Evoked response audiometer for testing sleeping subjects

 

Patent No. 5023783

Evoked response audiometer for testing sleeping subjects (Cohen, et al., Jun 11, 1991)

Abstract

An evoked response audiometer for sleeping subjects comprising a function generator for supplying to a sleeping patient an auditory signal consisting of a carrier frequency which is periodically modulated at frequencies in excess of 60 Hz, the frequency of modulation being varied in a generally increasing manner for auditory signals of higher frequencies such that the stimulus is frequency specific, a detector for sampling and analyzing brain potentials evoked by said signal, a low-pass filter providing a time window which samples the brain potentials for a predetermined interval to provide sets of fourier analysis samples containing amplitude and phase data in narrow bands centered on the modulation frequency and its second harmonic, a computer for analyzing the Fourier analysis samples to extract means values of the amplitudes and phase angles of the signals, and for extracting from the means values of the phase angles the probabilities that the distributions of the phase angles could have occurred by change, whereby the existence of phase locking of the brain potential signals can be determined.

Notes:

Evoked response audiometer for testing sleeping subjects. Filed June 1990, granted June 1991. Shows monitoring capability while sleeping. Says can use sound to cause evoked potentials while someone is sleeping. (Falling asleep with radio or television on is not a good idea). This type of thing could be used early in the attacks to get some evoked responses to use for other attacks.

SUMMARY OF INVENTION AND OBJECTS

It is therefore an object of the present invention to provide an improved evoked response audiometer which will make use of the optimum modulation frequencies for steady-state evoked potential testing during sleep, to allow the efficient assessment of hearing of a variety of difficult-to-test patients. These include neonates, infants, young children and mentally retarded patients

The invention provides an evoked response audiometer comprising means of supplying to the patient an auditory signal consisting of a carrier frequency which is periodically modulated (for example amplitude modulation, frequently repeating tone bursts or tone pips, frequency modulation or beats) 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, means for sampling the brain potential signals evoked by said 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 auditory signal means being controlled so that said auditory signals are periodically modulated at frequencies in excess of 60 Hz, the frequency of modulation being varied in a generally increasing manner for auditory signals of higher frequencies.

In a preferred form, said frequency of modulation is about 60-115 Hz for auditory signals having frequencies less than or equal to 1.5 kHz, and said frequency of modulation is about 65-200 Hz (or more) for auditory signals having frequencies in excess of 1.5 kHz.

It should be appreciated that the modulation frequency used will depend on the frequency of the auditory signal as well as on the subject being tested As an indication of the range of modulation frequencies which may be used, the following table is provided:

______________________________________ Auditory Signal Modulation Frequency ______________________________________ (a) Normal sleeping neonates 500 Hz: about 60-140 Hz, preferably 65-95 Hz, and most preferably about 72 Hz 1.5 kHz: about 60-165 Hz, preferably 75-110 Hz, and most preferably about 85 Hz 4 kHz: about 65-200 + Hz, preferably 85-110 Hz, and most preferably about 97 Hz Hence, 60-165 Hz for CF < 1.5 kHz 65-200 + Hz for CF > 1.5 kHz (b) Normal sleeping adults 250 Hz: about 70-130 Hz, preferably about 80-115 Hz, and most preferably about 85-95 Hz 500 Hz: about 70-180 Hz, preferably about 80-115 Hz, and most preferably about 85-95 Hz 1 kHz: about 70-200 Hz, preferably about 80-115 Hz, and most preferably about 95 Hz 2 kHz: about 75-200 + Hz, preferably about 85-195 Hz, and most preferably about 105-160 Hz 4 kHz: about 75-200 + Hz, preferably about 85-200 + Hz, and most preferably about 120-190 Hz Hence, 70-180 Hz for CF < 1 kHz 75-200 + Hz for CF > 1 kHz ______________________________________

It is expected that as infants mature, their responses will become more like those of adults. Accordingly, there will be a shift of optimum MF ranges.

Thus other modulating frequencies will be determined experimentally for other types of patients.

The use of modulation frequencies in excess of 60 Hz to evoke the responses allows the most efficient detection of a response in the type of patient being tested and at the carrier frequency being used. The system may be designed to choose the optimum modulation frequency automatically, based on the type of subject and the carrier frequency used. The audiometer embodying the present invention has the advantage over prior art audiometers in that it may make use of the widest possible range of modulation types (limited only by the requirement of reasonable frequency specificity), in that it makes use of the modulation frequencies that allow most efficient detection of a response during sleep (namely those in excess of 60 Hz) and that it employs a frequency specific stimulus. It also detects a response In real-time enabling the transfer to a new stimulus automatically.

The brain potentials are preferably recorded by means of electrodes on the vertex or forehand and on the mastoids of the patient, in the preferred embodiment, the patient is presented with a band limited tone burst or a tone that is simultaneously amplitude and frequency modulated or an amplitude modulated tone. The EEG signal is Fourier analysed to extract the components at the modulation frequency and its second harmonic, as these have been found to be the predominant components of the response. The use of low-pass filters following the multiplication of the EEG signal by the modulation frequency waveform and its second harmonic provides a time "window" which samples the EEG waveform for an interval of, typically, 64 periods of the modulation waveform. The filters are sampled twice every such interval (that is, typically, once every 32 modulation periods) resulting in a set of samples, each of which contains measurements of amplitude and phase of the EEG components present in very narrow frequency bands centered on the modulation frequency and its second harmonic. The phase measurements are made relative to the modulation frequency envelope.

The sets of samples are analysed to provide mean amplitudes, mean phases and probabilities that the distributions of the angles of the samples could have occurred by chance (i.e. in the absence of a phase-locked response). The said probabilities enable the system to decide in real-time whether a response is present. As the system is able to vary both the loudness and the carrier frequency of the auditory signal presented to the patient, it allows objective testing of hearing, which may be performed automatically.