System and methods for objective evaluation of hearing using auditory steady-state responses

• US 20010049480A1
• Filed: 05/18/2001
• Published: 12/06/2001
• Est. Priority Date: 05/19/2000
• Status: Active Grant

• Current Assignee: Baycrest Centre For Geriatric Care (Baycrest)
• Original Assignee: Baycrest Centre For Geriatric Care (Baycrest)
• Inventors: John, Michael Sasha, Picton, Terence W.
• Granted Patent: US 6,602,202 B2
• Time in Patent Office: Days
• US Class Current: 600/559
• CPC Class Codes: A61B 5/0051 by applying vibrations
• CPC Class Codes: A61B 5/121 evaluating hearing capacity
• CPC Class Codes: A61B 5/38 Acoustic or auditory stimuli
• CPC Class Codes: A61B 5/7257 using Fourier transforms
• CPC Class Codes: H04R 25/70 Adaptation of deaf aid to hearing loss, e.g. initial electronic fitting

Abstract

This invention relates to an apparatus and method for assessing a subject'"'"'s hearing by recording steady-state auditory evoked responses. The apparatus generates a steady-state auditory evoked potential stimulus, presents the stimulus to the subject, senses potentials while simultaneously presenting the stimulus and determines whether the sensed potentials contain responses to the stimulus. The stimulus may include an optimum vector combined amplitude modulation and frequency modulation signal adjusted to evoke responses with increased amplitudes, an independent amplitude modulation and frequency modulation signal and a signal whose envelope is modulated by an exponential modulation signal. The apparatus is also adapted to reduce noise in the sensed potentials by employing sample weighted averaging. The apparatus is also adapted to detect responses in the sensed potentials via the Phase weighted T-test or Phase zone technique. The apparatus may furtherperform a number of objective audiological tests including latency tests, AM/FM discrimination tests, rate sensitivity tests, aided hearing tests, depth sensitivity tests, supra-threshold tests and auditory threshold tests. The apparatus is further adapted to perform multi-modality testing in which more than one sensory modality of the subject is tested simultaneously.

Claims

1. A method of testing the hearing of a subject, said method comprising the steps of:

(a) selecting at least one test signal;

(b) modulating at least one of the amplitude and frequency of said at least one test signal by an exponential modulation signal to produce at least one modulated test signal;

(c) transducing said at least one modulated test signal to create an acoustic stimulus and presenting said acoustic stimulus to said subject;

(d) sensing potentials from said subject while substantially simultaneously presenting said acoustic stimulus to said subject; and

,

(e) analyzing said potentials to determine whether said potentials comprise data indicative of the presence of at least one steady-state response to said acoustic stimulus.

2. The method of

claim 1

, wherein said exponential modulation signal comprises a sinusoidal signal having an exponent which is greater than 1 and less than or approximately equal to 4.

3. The method of

claim 1

, wherein said exponential modulation signal is defined by the formula:

y[n]=2*m_a*[(((1+sin(2*π

*f_am*t*n))/2)^eam−

0.5)+1]

in which;

y[n] is a time series of data points representing said exponential modulation signal;

m_a is the AM modulation depth;

f_am is the AM modulation frequency;

t is the time spacing between said data points;

n is a given data point; and

,

eam is the exponent of said exponential modulation signal.

4. The method of

claim 1

, wherein steps (b) and (e) of said method are effected by a software program.

5. A method of testing the hearing of a subject, wherein said method comprises the steps of:

(a) creating an optimum-vector mixed modulation test signal comprising at least one signal having an amplitude modulated component with a first phase and a frequency modulated component with a second phase wherein said second phase is adjusted relative to said first phase to evoke an increased response from said subject;

(b) transducing said test signal to create an acoustic stimulus and presenting said acoustic stimulus to said subject;

(c) sensing potentials from said subject while substantially simultaneously presenting said acoustic stimulus to said subject; and

,

(d) analyzing said potentials to determine whether said potentials comprise data indicative of the presence of at least one steady-state response to said acoustic stimulus.

6. The method of

claim 5

, wherein said method further comprises creating a database of normative optimal phase difference data correlated to subject characteristics and stimulus characteristics.

7. The method of

claim 5

, wherein the method further comprising using said database of normative phase difference data to adjust said second phase relative to said first phase.

8. A method of testing the hearing of a subject, wherein said method comprises the steps of:

(a) creating a test signal comprising at least one independent amplitude modulated and frequency modulated signal having an amplitude modulated component and a frequency modulated component, wherein said amplitude modulated component comprises a first modulation frequency and a first carrier frequency and said frequency modulated component comprises a second modulation frequency and a second carrier frequency wherein said first modulation frequency is substantially different from said second modulation frequency and said first carrier frequency is substantially similar to said second carrier frequency;

(b) transducing said test signal to create an acoustic stimulus and presenting said acoustic stimulus to said subject;

(c) sensing potentials from said subject while substantially simultaneously presenting said acoustic stimulus to said subject; and

,

(d) analyzing said potentials to determine whether said potentials comprise data indicative of a steady-state response to each amplitude modulated component and a steady-state response to each frequency modulated component.

9. An apparatus for testing the hearing of a subject, wherein said apparatus comprises:

(a) a signal creator adapted to create a test signal comprising at least one combined amplitude modulated and frequency modulated signal having an amplitude modulated component with a first phase and a frequency modulated component with a second phase wherein said signal creator comprises means for adjusting said second phase relative to said first phase;

(b) a transducer electrically coupled to said processor and adapted to transduce said test signal to create an acoustic stimulus and present said acoustic stimulus to said subject;

(c) a sensor adapted to sense potentials from said subject while said acoustic stimulus is substantially simultaneously presented to said subject; and

,

(d) a processor electrically coupled to said sensor and adapted to receive said potentials and analyze said potentials to determine if said potentials comprise data indicative of at least one response to said acoustic stimulus.

10. The apparatus of

claim 9

, wherein said apparatus further comprises a database of normative phase differences data correlated to subject characteristics and stimulus characteristics.

11. The apparatus of

claim 9

, wherein said processor is further adapted to create a second test signal comprising at least one independent amplitude modulated and frequency modulated signal.

12. A method of analyzing potentials to determine whether said potentials comprise data indicative of the presence of at least one steady-state response to a steady-state evoked potential stimulus, wherein said method comprises the steps of:

(a) presenting an evoked potential stimulus to a subject;

(b) sensing potentials from said subject while substantially simultaneously presenting said stimulus to said subject to obtain a plurality of data points;

(c) transforming said plurality of data points into a second plurality of data points;

(d) biasing said second plurality of data points with an expected phase value to obtain a plurality of biased data points; and

,

(e) applying a statistical test to said plurality of biased data points to detect said response.

13. The method of

claim 12

, wherein effecting steps (c) and (d) comprises the steps of:

(f) forming a plurality of sweeps from said plurality of data points;

(g) averaging said plurality of sweeps to obtain a plurality of averaged data points;

(h) calculating a plurality of Fourier components for said plurality of averaged data points;

(i) calculating the amplitude (a_i) and phase (θ

_I) for said plurality of Fourier components;

(j) biasing said amplitudes (a_i) to obtain biased data points (p_i) according to the formula;

p_i=a_i*cos(θ

_I−

θ

_e)

wherein θ

_e is said expected phase value.

14. The method of

claim 12

, wherein effecting step (e) comprises the steps of:

(k) calculating upper confidence limits using a one tailed Student t-test on biased amplitudes which represent noise in the vicinity of Fourier components where said response should occur; and

,

(l) comparing biased amplitudes of Fourier components where said response should occur to said upper confidence limits to determine if said biased amplitudes are larger than said upper confidence limit.

15. The method of

claim 12

, wherein said expected phase value is obtained from a database of normative expected phase values correlated to subject characteristics and stimulus characteristics.

16. The method of

claim 12

, wherein said expected phase value is obtained from previous testing on said subject using stimuli having a higher intensity.

17. The method of

claim 12

, wherein said stimulus contains other components for which responses have been detected and said expected phase value is obtained from extrapolation of said phase values for said responses which have already been detected.

18. A method of detecting a response to an evoked potential stimulus, wherein said method comprises the steps of:

(a) presenting an evoked potential stimulus to a subject;

(b) sensing potentials from said subject while substantially simultaneously presenting said stimulus to said subject to obtain a plurality of data points; and

,

(c) calculating phase values for said plurality of data points, wherein, a response is detected if an adequate number of said calculated phase values fall within a predetermined phase value range.

19. The method of

claim 18

, wherein said effecting step (c) further comprises the steps of:

(d) separating said plurality of data points into epochs;

(e) calculating a Fourier component for the frequency at which said response should occur for each epoch; and

,

(f) calculating the phase of each Fourier component calculated in step (e).

20. The method of

claim 19

, wherein said method further comprises the steps of:

(g) calculating a target phase range;

(h) calculating the number of said Fourier components that have a phase that is within the target phase range (N); and

,

(i) using binomial analysis with N to determine whether said plurality of data points contain a response.

21. The method of

claim 20

, wherein said target phase range is calculated based on a database of normative expected phases correlated to subject characteristics and stimulus characteristics.

22. An apparatus for testing the hearing of a subject, wherein said apparatus comprises:

(a) a signal creator adapted to create a test signal;

(b) a transducer electrically coupled to said signal creator and adapted to transduce said test signal to create an acoustic stimulus and present said acoustic stimulus to said subject;

(c) a sensor adapted to sense potentials from said subject while said acoustic stimulus is substantially simultaneously presented to said subject; and

,

(d) a processor electrically coupled to said sensor and adapted to receive said potentials and analyze said potentials to determine if said potentials comprise data indicative of at least one response to said acoustic stimulus;

wherein said analysis involves biasing said potentials based on an expected phase value.

23. The apparatus of

claim 22

, wherein said apparatus further comprises a database of expected phase value data correlated to subject characteristics and stimulus characteristics.

24. A method of noise reduction for a plurality of data points which are obtained during steady-state evoked potential testing, wherein said plurality of data points comprise at least one signal and noise and wherein said method comprises the steps of:

(a) obtaining said plurality of data points;

(b) separating said plurality of data points into a plurality of epochs; and

,

(c) applying an adaptive noise reduction method to each epoch.

25. The method of

claim 24

, wherein said adaptive noise reduction method comprises effecting sample weighted averaging wherein weights are calculated based on the variance of noise, wherein the frequency of the noise is in close proximity to the frequency of the signal.

26. The method of

claim 25

, wherein said sample weighted averaging is effected according to the steps of:

(d) forming a plurality of sweeps by concatenating the epochs together;

(e) filtering each sweep to obtain a plurality of filtered sweeps;

(f) aligning each sweep to form a first matrix in which the sweeps are the rows of the matrix and the epochs within the plurality of sweeps are the columns of the matrix and aligning each filtered sweep in a similar fashion to form a filtered matrix which is used to calculate weights;

(g) calculating the variance of each epoch in the filtered matrix to obtain a noise variance estimate for each epoch in the filtered matrix;

(h) normalizing the noise variance estimate for each epoch in the filtered matrix by dividing the noise variance estimate for each epoch in the filtered matrix by the sum of all noise variance estimates for the epochs along the column of the filtered matrix which contains the epoch to obtain a normalized noise variance estimate for each epoch;

(i) inverting each normalized noise variance estimate to obtain a weight for each epoch and multiplying each corresponding epoch in the first matrix by its respective weight to obtain a plurality of weighted epochs; and

,

(j) summing all of the weighted epochs in the first matrix along the columns of the first matrix to obtain a signal estimate.

27. The method of

claim 26

, wherein filtering comprises using a bandpass filter having a passband region which is local to the frequency region where the at least one signal resides such that the calculated weights are based on the frequency region local to the frequency region where the at least one signal resides.

28. A method of objectively testing the hearing of a subject, wherein said method comprises the steps of:

(a) selecting an auditory test;

(b) creating an appropriate test signal comprising at least one component for said auditory test;

(c) transducing said test signal to create a stimulus and presenting said stimulus to said subject;

(d) sensing potentials from said subject while substantially simultaneously presenting said stimulus to said subject; and

,

(e) analyzing said potentials to detect at least one response.

29. The method of

claim 28

, wherein said auditory test is a latency test which comprises calculating a latency value for a detected response and comparing said latency value to a database of normative latency values to obtain an indication of the normal/abnormal status of the auditory system of said subject.

30. The method of

claim 29

, wherein said latency test comprises the steps of:

(f) calculating an onset phase for each detected response in said sensed potentials;

(g) calculating a phase delay (P) for each detected response by subtracting each onset phase from 360 degrees.

(h) calculating a latency value (L) for each detected response according to the following formula;

L=1000*(P+N*360)/(360*f_m)

where N is the number of cycles that have occurred in the stimulus before each detected response to a respective component of said stimulus occurs and f_m is the frequency of said detected response; and

,

(i) comparing each latency value to a database of normative latency values to obtain an indication of the normal/abnormal status of the auditory system of said subject.

31. The method of

claim 30

, wherein N is approximately 1 and fm is in the range of approximately 75 to 100 Hz or in the range of approximately 150 to 190 Hz.

32. The method of

claim 29

, wherein said latency test further comprises the steps of:

(j) calculating a first latency value for a first detected response and calculating a second latency value for a second detected response;

(k) finding the difference between said first latency value and said second latency value to obtain a differential latency value; and

,

(l) comparing said differential latency value to a database of normative differential latency values to obtain an indication of the status of the auditory system of said subject.

33. The method of

claim 28

, wherein said auditory test is an aided hearing test which comprises the following steps:

(m) fitting said subject with a hearing aid;

(n) performing steps (b) to (e);

(o) adjusting the hearing aid if an inadequate number of responses are detected in step (e); and

,

(p) performing steps (n) and (o) until an adequate number of responses have been detected in said potentials.

34. The method of

claim 33

, wherein said method further comprises effecting step (o) by adjusting the gain of said hearing aid for a frequency region which is substantially similar to the frequency region of a component in said test signal for which a response was not detected.

35. The method of

claim 28

, wherein said auditory test is an AM/FM discrimination test and said test signal comprises at least one independent amplitude modulation and frequency modulation signal, wherein, said AM/FM discrimination test comprises the steps of:

(q) performing steps (b) to (e);

(r) calculating a response ratio according to the number of detected responses divided by the total number of responses which could have occurred in response to said stimulus; and

,

(s) estimating a word recognition score correlated to said response ratio.

36. The method of

claim 35

, wherein said test signal further comprises noise masking.

37. The method of

claim 28

, wherein said auditory test is an AM/FM discrimination test and said test signal comprises an amplitude modulated component and a frequency modulated component, wherein, said AM/FM discrimination test comprises the steps of:

(t) performing steps (b) to (e);

(u) calculating a first amplitude of a response to said frequency modulated component;

(v) calculating a second amplitude of a response to said amplitude modulated component;

(x) calculating an amplitude ratio by dividing said first amplitude by said second amplitude; and

,

(y) comparing said amplitude ratio to a database of normative amplitude ratios to obtain an indication of the status of the auditory system of said subject.

38. The method of

claim 37

, wherein said test signal comprises a plurality of pairs of amplitude modulated components and frequency modulated components and the method further comprises carrying out steps (u) to (y) for each of said pairs.

39. The method of

claim 28

, wherein said auditory test is a depth sensitivity test and said test signal comprises one or more amplitude modulated components having a modulation depth or one or more frequency modulated components having a modulation depth, wherein, said depth sensitivity test comprises the steps of:

(z) carrying out steps (b) to (e);

(aa) obtaining an estimate of response amplitude for each detected response; and

,

(bb) comparing each estimated response amplitude with a database of normative response amplitudes to obtain an indication of the status of the auditory system of said subject.

40. The method of

claim 39

, wherein said depth sensitivity test further comprises the steps of:

(cc) detecting responses to test signals comprising a first modulated component having a first modulation depth and a second modulated component having a second modulation depth, wherein said modulation depths are different;

(dd) analyzing said detected responses to obtain an estimate of a first response amplitude for a detected response to said first modulated component and a second response amplitude for a detected response to said second modulated component;

(ee) calculating a response amplitude ratio from said first response amplitude and said second response amplitude; and

,

(ff) comparing said response amplitude ratio with a database of normative response amplitude ratios to obtain an indication of the status of the auditory system of said subject.

41. The method of

claim 28

, wherein said auditory test is a rate sensitivity test which comprises the following steps:

(gg) creating a set of modulation frequencies comprising at least two modulation frequencies;

(hh) creating a set of test signals wherein each test signal comprises at least one amplitude modulated component having a unique modulation frequency chosen from said set of modulation frequencies;

(ii) carrying out steps (c) and (d) for each test signal in said set of test signals;

(jj) analyzing said set of potentials to detect a response for each amplitude modulated component in said set of test signals and estimating a response amplitude for each detected response;

(kk) calculating a rate sensitivity value based on said estimated response amplitudes; and

,

(ll) comparing said rate sensitivity value to a database of normative rate sensitivity values to obtain an indication of the status of the auditory system of said subject.

42. The method of

claim 41

, wherein said rate sensitivity value is the slope of the line resulting from a plot of estimated response amplitude for each amplitude modulated component versus modulation frequency for each amplitude modulated component.

43. The method of

claim 28

, wherein said auditory test is a supra-threshold test comprising an intensity limen test and said test signal comprises an amplitude modulated component having a modulation depth of approximately 100%, wherein said intensity limen test comprises the steps of:

(mm) performing steady state evoked potential testing while minimizing the modulation depth of the test signal upon each detected response to determine a minimum modulation depth at which a response is detected; and

,

(nn) comparing said minimum modulation depth with a database of normative minimum modulation depths to obtain an indication of the status of the auditory system of said subject.

44. The method of

claim 43

, wherein said supra-threshold test further comprises a frequency limen test and said test signal comprises an amplitude modulated component having a frequency modulation depth, wherein, said frequency limen test comprises the steps of:

(oo) determining a minimum modulation depth at which a response is detected; and

,

(pp) comparing said minimum modulation depth with a database of normative minimum modulation depths to obtain an indication of the status of the auditory system of said subject.

45. The method of

claim 28

, wherein said auditory test is an auditory threshold test and said test signal comprises two or more combined amplitude modulation and frequency modulation signals having carrier frequencies which are separated by more than one-half octave, wherein, each combined amplitude modulation and frequency modulation signal has a frequency modulated component and an amplitude modulated component and the phase of the frequency modulated component is adjusted relative to the phase of the amplitude modulated component, wherein said auditory threshold test comprises the step of:

(qq) carrying out steps (c) to (e) to determine a minimal stimulus intensity for which a response is detected for each combined amplitude modulation and frequency modulation signal.

46. The method of

claim 45

, wherein the envelope of each combined amplitude modulation and frequency modulation signal is modulated by an exponential modulation signal.

47. The method of

claim 45

, wherein said auditory threshold test is conducted for a maximum time limit which is adjusted depending on the amount of noise in said potentials.

48. The method of

claim 45

, wherein said auditory threshold test comprises upwardly adjusting the intensities of components in said stimulus which tend to evoke responses having smaller amplitudes, so that all components in said stimulus evoke responses having similar amplitudes.

49. The method of

claim 45

, wherein step (e) comprises the step of:

(rr) reducing noise in said potentials to obtain reduced noise potentials by employing sample weighted averaging.

50. The method of

claim 45

, wherein step (e) comprises the step of:

(ss) performing a phase weighted t-test on said potentials to determine whether said potentials comprise data indicative of at least one response to said stimulus.

51. The method of

claim 45

, wherein step (e) comprises the step of:

(tt) performing a modified Rayleigh test of circular uniformity (MRC) on said potentials to determine whether said potentials comprise data indicative of at least one response to said stimulus.

52. The method of

claim 45

, wherein adjusting said intensity of one of said components comprises using the Stimulus Flux method which comprises creating another waveform of appropriate phase and amplitude, which when added to said test signal, results in the desired adjustment of said intensity of one of said components.

53. An apparatus for objectively testing the hearing of a subject, wherein said apparatus comprises:

(a) a selector adapted for selecting an auditory test to perform on said subject;

(b) a signal creator electrically coupled to said selector and adapted to create an appropriate test signal comprising at least one component for said test;

(c) a transducer electrically coupled to said signal creator and adapted to transduce said test signal to create an acoustic stimulus and present said acoustic stimulus to said subject;

(d) a sensor adapted to sense potentials from said subject while said acoustic stimulus is substantially simultaneously presented to said subject;

(e) a processor electrically coupled to said sensor and adapted to receive said potentials and analyze said potentials to determine if said potentials comprise data indicative of at least one response to said acoustic stimulus; and

,

(f) a programmable hearing aid coupled to said processor, wherein, said programmable hearing aid comprises a plurality of programmable gain factors for different frequency regions and at least one programmable filter slope.

54. A method of testing at least two senses of a subject, wherein said method comprises the steps of:

(a) selecting a first steady-state test signal to test a first sensory modality;

(b) transducing said first steady-state test signal to create a first stimulus and presenting said first stimulus to said subject;

(c) selecting a second steady-state test signal to test a second sensory modality;

(d) transducing said second steady-state test signal to create a second stimulus and presenting said second stimulus to said subject;

(e) sensing potentials while substantially simultaneously presenting both stimuli to said subject; and

,

(f) analyzing said potentials to determine whether said potentials comprise data indicative of at least one steady-state response to said stimuli.

55. The method of

claim 54

, wherein said steady-state test signals comprise an auditory signal, a visual signal and a tactile signal.

56. The method of

claim 54

, wherein said method further comprises ensuring that all modulation frequencies in the stimuli are not integer multiples of one another.

57. An apparatus for testing at least two senses of a subject, wherein said apparatus comprises:

(a) a signal creator adapted to create a first steady-state test signal and a second steady-state test signal;

(b) a first transducer electrically coupled to said selector and adapted to transduce said first test signal to create a first stimulus and present said first stimulus to said subject;

(c) a second transducer electrically coupled to said selector and adapted to transduce said second test signal to create a second stimulus and present said second stimulus to said subject;

(d) a first sensor adapted to sense first potentials from said subject while said first stimulus is substantially simultaneously presented to said subject;

(e) a second sensor adapted to sense second potentials from said subject while said second stimulus is substantially simultaneously presented to said subject;

(f) a processor electrically coupled to said first sensor and adapted to receive said first potentials and analyze said first potentials to determine if said first potentials comprise data indicative of at least one response to said first stimulus; and

,

(g) the processor, electrically coupled to said second sensor and adapted to receive said second potentials and analyze said second potentials to determine if said second potentials comprise data indicative of at least one response to said second stimulus, wherein, each stimulus is presented substantially simultaneously.