Evoked response audiometer for testing sleeping subjects

• US 5,023,783 A
• Filed: 06/08/1990
• Issued: 06/11/1991
• Est. Priority Date: 11/11/1987
• Status: Expired due to Term

• Current Assignee: University of Melbourne
• Original Assignee: University of Melbourne
• Inventors: Cohen, Lawrence T., Rickards, Field W.
• Primary Examiner(s): Jablon, Clark J.
• Application Number:
  US07/534,356
• Time in Patent Office: 368 Days
• Field of Search: 128/746, 364/413.02, 364/413.05
• US Class Current: 600/559
• CPC Class Codes: A61B 5/12 Audiometering
• CPC Class Codes: A61B 5/38 Acoustic or auditory stimuli
• CPC Class Codes: A61B 5/7225 Details of analog processing, e.g. isolation amplifier, gain or sensitivity adjustment, filtering, baseline or drift compensation A61B5/30, A61B5/316 take precedence; transformers for specific applications H01F38/00; amplifiers H03F
• CPC Class Codes: A61B 5/7257 using Fourier transforms
• CPC Class Codes: Y10S 128/92 Computer assisted medical diagnostics

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.

Claims

1. An evoked response audiometer comprising means of supplying to a sleeping patient an auditory signal consisting of a carrier frequency which is periodically modulated 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 of at least 70 Hz, the frequency of modulation being varied in a generally increasing manner for auditory signals of higher frequencies.

2. The audiometer of claim 1, wherein said frequency of modulation is about 70-115 Hz for auditory signals having frequencies less than or equal to 1.5 kHz, and said frequency of modulation is about 75-200 Hz (or more) for auditory signals having frequencies in excess of 1.5 kHz.

3. A method of testing the hearing of a sleeping patient comprising the steps of:

supplying to said sleeping patient an auditory signal consisting of a carrier frequency which is periodically modulated at frequencies in excess of 70 Hz such that the stimulus is at least substantially frequency specific, said auditory signal being presented for sufficiently extended period of time to enable phase-locked steady-state potentials to be evoked in the brain,

sampling the brain potential signals evoked by said signal, and

analysing said brain potentials to determine whether phase-locking of said brain potentials to the modulated auditory signal has occurred, the frequency of modulation being varied in a generally increasing manner for auditory signals of higher frequencies.

4. An evoked response audiometer comprising means of supplying to a sleeping patient an auditory signal consisting of a carrier frequency which is periodically modulated 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 analyzing said brain potentials to determine whether phase-locking of said brain potentials to the modulated auditory signals has occurred, said auditory signal means being controlled so that said auditory signals are periodically modulated at frequencies selected according to the following table:

space="preserve" listing-type="tabular">______________________________________ Auditory Signal Modulation Frequency ______________________________________ (a) Normal Sleeping Neonates 500 Hz;

at least 70 Hz and substantially within the range of 70-140 Hz, 1.5 kHz;

at least 75 Hz and substantially within the range of 75-165 Hz, 4 kHz;

at least 75 to at least 200 Hz, (b) Normal Sleeping Adults 250 Hz;

at least 70 Hz and substantially within the range of 75-130 Hz, 500 Hz;

at least 75 Hz and substantially within the range of 75-180 Hz, 1 kHz;

at least 75 Hz and substantially within the range of 75-200 Hz, 2 kHz;

at least 75 to at least 200 Hz, 4 kHz;

at least 75 to at least 200 Hz. ______________________________________

5. The audiometer of claim 4, wherein, for maturing infants, the modulation frequency for each auditory signal is gradually increased with age towards the modulation frequency specified for adults.

6. The audiometer of claim 4, wherein said auditory signal supplying means is controlled by a computer programmed to select the most appropriate modulation frequency for each stimulus frequency depending on the nature of the patient and their state of arousal.

7. The audiometer of claim 4 wherein said modulation frequencies are selected from:

space="preserve" listing-type="tabular">______________________________________ Auditory Signal Modulation Frequency ______________________________________ (a) Normal Sleeping Neonates 500 Hz;

substantially within the range of 70-95 Hz, 1.5 kHz;

substantially within the range of 75-115 Hz, 4 kHz;

at least 85 to at least 200 Hz, (b) Normal Sleeping Adults 250 Hz;

substantially within the range of 80-115 Hz, 500 Hz;

substantially within the range of 80-115 Hz, 1 kHz;

substantially within the range of 80-115 Hz, 2 kHz;

substantially within the range of 85-195 Hz, 4 kHz;

at least 85 to at least 200 Hz. ______________________________________

8. The audiometer of claim 4 wherein said modulation frequencies are:

space="preserve" listing-type="tabular">______________________________________ Auditory Signal Modulation Frequency ______________________________________ (a) Normal Sleeping Neonates 500 Hz;

about 72 Hz, 1.5 kHz;

about 85 Hz, 4 kHz;

about 97 to at least 200 Hz, (b) Normal Sleeping Adults 250 Hz;

about 85-95 Hz, 500 Hz;

about 85-95 Hz, 1 kHz;

about 95 Hz, 2 kHz;

about 105-160 Hz, 4 kHz;

about 120 to at least 190 Hz. the range of 80-115 Hz, 2 kHz;

substantially within the range of 85-195 Hz, 4 kHz;

at least 85 to at least 200 Hz. ______________________________________

9. A method of testing the hearing of a sleeping patient comprising the steps of supplying to said sleeping patient an auditory signal consisting of a carrier frequency which is periodically modulated 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, sampling the brain potentials to determine whether phase-locking of said brain potentials to the modulated auditory signal has occurred, said auditory signals being periodically modulated at frequencies selected according to the following table:

space="preserve" listing-type="tabular">______________________________________ Auditory Signal Modulation Frequency ______________________________________ (a) Normal Sleeping Neonates 500 Hz;

at least 70 Hz and substantially within the range of 70-140 Hz, 1.5 kHz;

at least 75 Hz and substantially within the range of 75-165 Hz, 4 kHz;

at least 75 to at least 200 Hz, (b) Normal Sleeping Adults 250 Hz;

at least 70 Hz and substantially within the range of 75-130 Hz, 500 Hz;

at least 75 Hz and substantially within the range of 75-180 Hz, 1 kHz;

at least 75 Hz and substantially within the range of 75-200 Hz, 2 kHz;

at least 75 to at least 200 Hz, 4 kHz;

at least 75 to at least 200 Hz. ______________________________________

10. The method of claim 9 wherein said frequency of modulation is selected according to the following table:

space="preserve" listing-type="tabular">______________________________________ Auditory Signal Modulation Frequency ______________________________________ (a) Normal Sleeping Neonates 500 Hz;

substantially within the range of 70-95 Hz, 1.5 kHz;

substantially within the range of 75-115 Hz, 4 kHz;

at least 85 to at least 200 Hz, (b) Normal Sleeping Adults 250 Hz;

substantially within the range of 80-115 Hz, 500 Hz;

substantially within the range of 80-115 Hz, 1 kHz;

substantially within the range of 80-115 Hz, 2 kHz;

substantially within the range of 85-195 Hz, 4 kHz;

at least 85 to at least 200 Hz. ______________________________________

11. The method of claim 9 wherein said frequency of modulation is selected according to the following table:

space="preserve" listing-type="tabular">______________________________________ Auditory Signal Modulation Frequency ______________________________________ (a) Normal Sleeping Neonates 500 Hz;

about 72 Hz, 1.5 kHz;

about 85 Hz, 4 kHz;

about 97 to at least 200 Hz, (b) Normal Sleeping Adults 200 Hz;

about 85-95 Hz, 500 Hz;

about 85-95 Hz, 1 kHz;

about 95 Hz, 2 kHz;

about 105-160 Hz, 4 kHz;

about 120 to at least 190 Hz. ______________________________________

12. An evoked response audiometer comprising means of supplying to a sleeping patient an auditory signal consisting of a carrier frequency which is periodically modulated 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 analyzing 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 of at least 70 Hz, the frequency of modulation being varied in a generally increasing manner for auditory signals of higher frequencies;

said sampling means comprising means for multiplying said brain potential signals by said modulation frequency waveform and its quadrature component and by the waveform of the second harmonic of the modulation frequency and its quadrature component to produce product waveforms;

means for low-pass filtering said product waveforms to provide a time window which samples the brain potential for a predetermined interval to provide a set of Fourier analysis samples containing amplitude and phase data in narrow bands centered on the modulation frequency and its second harmonic.

13. The audiometer of claim 12, wherein said analysing means further comprises means for analysing said Fourier analysis samples to extract mean values of the amplitudes and phase angles of said signals, means for extracting from said mean values of said phase angles the probabilities (P%) that the distributions of said phase angles could have occurred by chance, whereby the existence of said phase locking is determined.

14. The audiometer of claim 13, wherein said Fourier analysis is performed by filters operating according to an approximation of the Hanning function [1-cos(2π

t/t_H)]/2 where t_H is the width of the same time window.

15. The audiometer of claim 14, wherein said sampling means further comprises:

a computer,

wherein said time window has a width of approximately 64 cycles and the computer samples the signals every 32 cycles to provide sample overlap.

16. The audiometer of claim 15, wherein said probability P% is extracted according to the functions:

##EQU3## where Q is a constant ≦

1.0, being approximately 0.625,

N is the number of said samples,

θ

_i are said phase angles, and

Probability P%=100 exp. (-0.5027F).

17. A method of testing the hearing of a sleeping patient comprising the steps of:

supplying to said sleeping patient an auditory signal consisting of a carrier frequency which is periodically modulated at frequencies in excess of 70 Hz 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;

sampling the brain potential signals evoked by said signal;

analysing said brain potentials to determine whether phase-locking of said brain potentials to the modulated auditory signal has occurred, the frequency of modulation being varied in a generally increasing manner for auditory signals of higher frequencies;

multiplying said brain potential signals by said modulation frequency waveform and its quadrature component and by the second harmonic of the modulation frequency and its quadrature component to produce product waveforms;

low-pass filtering said product waveforms to provide a time window which samples the brain potential for a predetermined interval to provide sets of samples containing amplitude and phase data in narrow bands centered on the modulation frequency and its second harmonic, said low-pass filtering providing Fourier analysis of said product waveforms to produce mean values of the amplitude and phaseangles of said signals; and

extracting from said mean values of said phase values the probabilities P% that the distributions of said phase angles, could have occurred by chance, whereby the existence of said phase locking is determined.

18. The method of claim 17, wherein said probability P% is extracted according to the functions:

##EQU4## where Q is a constant ≦

1.0, being approximately 0.625,

N is the number of said samples,

θ

i are said phase angles for said samples, and

Probability P%=100 exp. (-0.5027F).