the invention disclosed herein detects , processes and analyzes the eeg response of a subject to certain sound stimuli . a click sound stimulus is repetitively applied to the subject &# 39 ; s ear through a transducer . the click stimuli may be applied to one ear at a time ( monoaurally ), or to both ears simultaneously ( binaurally ). in a preferred embodiment , monoaural stimuli are applied at 37 hz . the eeg response is detected from surface electrodes . testing may be performed to ensure that the electrodes have been properly placed , and that nothing is impeding the electrodes &# 39 ; ability to detect the eeg response signal . ( see u . s . patent application entitled “ hearing evaluation device with patient connection evaluation capabilities ”). in a preferred embodiment , the electrodes are placed on the subject in the following manner : a positive electrode is placed on the forehead , a negative electrode is placed on the nape of the neck , and a ground electrode is placed on the mastoid or shoulder ( fig3 ). the eeg signal detected from these electrodes is filtered so as to exclude signals that are not applicable to the abr . the amplitude of the eeg response is digitized , and is assigned a binary value . this binary value represents the amplitude polarity of the waveform , that is , whether the eeg amplitude is positive or negative , at the measured time . the stimuli and responses are grouped into “ sweeps ” and “ blocks .” a sweep is a response waveform to a single click stimulus . a block is a series of sweeps , and in a preferred embodiment , represents 500 accepted click stimulus responses . we refer to “ accepted ” click stimulus responses , because some sweeps may be rejected due to problems with the testing conditions . upon completion of a block of accepted sweeps , signal averaging is used to compute the composite waveform that results from this block . in addition , signal averaging is also used to compute the average composite waveform from all blocks combined . this average composite waveform is then compared with an internal template , to determine if the null hypothesis ( h 0 ) can be rejected . the null hypothesis is the hypothesis that the baby is hearing - impaired , and will be rejected if the probability of an abr being present is above a certain pre - set statistical threshold . in the preferred embodiment , the null hypothesis is rejected , and the evaluation ceases , when sufficient data have been collected to conclude , with 99 . 96 % statistical confidence , that an abr waveform is present . a “ pass ” or other similar message may then be generated . if the average composite waveform is insufficient to reject the null hypothesis , then the evaluation continues until the total number of sweeps exceeds a preset threshold . in addition , for each block , the composite waveform is compared with the same internal template . the resultant comparison value is evaluated against the expected comparison value , based on the test conditions in the block . the differences between the calculated and expected comparison values ( based on normative data ) are measured for all blocks . the average value of these differences is determined and compared with a threshold , representing a confidence level that the hearing test will likely produce a “ refer ” result at the end of the test . the chief challenge in using abr to evaluate hearing loss is the difficulty in detecting the abr response ( if any ) from the noise within which it is buried . this noise is typically gaussian - distributed , with a mean amplitude of zero , and with changing variance . as stated above , the present invention detects the presence of an abr by repetitively applying click stimuli in blocks b of n b = 500 sweeps . each click stimulus is comprised of a brief acoustic pulse primarily composed of frequencies in the 500 - 4000 hz range . the repetition rate for the clicks is 37 hz . the polarities of the click stimuli are sequentially alternated between condensation ( positive square pulse ) and rarefaction ( negative square pulse ) stimuli . since the noise has a mean of zero and no component is synchronous with the stimulus repetition rates , it is likely to sum toward zero with increasing sweeps , leaving the abr . under the preferred embodiment of the present invention , the amplitude sequence of each click stimulus response is converted into a sequence of polarities ( positive or negative ) which , in turn , is summed with the other response polarity sequences in block b , to form the array x b . for instance , an amplitude sample in the click stimulus response would be given a “ 1 ” if this amplitude were positive ( no matter how high ), and a “ 0 ” if this amplitude were negative ( no matter how low ). and , if no abr were present the expected proportion of polarities , which is the same as the polarities of the abr waveform would be 0 . 5 . however , if an abr were present , the proportion would likely be higher . the proportion of polarities in an evoked response matching the abr waveform is related to the amount of signal noise . after each block of sweeps , the summed polarity sequence x b for block b is summed with the other summed polarity sequences into an array x . also , the total number of sweeps n is calculated as the sum of the number of sweeps in each block :  { x = ∑ b = 1 b  x b n = ∑ b = 1 b  n b b = 1 , 2 , …  , b  the summed polarity sequence x is then compared with a template waveform , which has been compiled with the use of normative data ( fig1 ). it will be necessary to use different template abr waveforms for different subject populations and for different hardware configurations . the template waveform is comprised of m weighted points , strategically placed to match the typical abr waveform . at each of the m points , a weight is assigned , reflecting the importance and polarity of the given measurement point in ascertaining the presence of an abr , as derived from normative data . ( see fig2 ). the importance of each measurement point is a function of its amplitude . for any given point m , the sum of the polarities would be x m . the sum of the weights equals zero . the present invention uses a test statistic z to aid in determining if an abr is present . this test statistic is defined as : z = ∑ m m = 1  w m  ( x m - μ x ) npq   ∑ m = 1 m  w m 2 where n is the number of sweeps , p is the probability of positive polarity , and q is the complementary probability . the test statistic z scores the random binary array x by multiplying its elements x m at each template point m with the corresponding weight w m , and summing these results into a single , normalized number . now , in the absence of an abr , the peak of the distribution of z would remain at zero , while in the presence of an abr , the test statistic would grow with increasing number of sweeps n . subjects exhibit variability in the latency of the abr waveform , so that different subjects , each of whom can hear , may exhibit abr waveforms at different times after the click stimulus . in order to compensate for this variability , the test statistic z may be recalculated at various times . the highest z from each of these time - shifted samples , z max , can be saved and used to determine the presence of the abr . in a preferred embodiment of the present invention , a “ pass ” is indicated when z max reaches a value that is 4 standard deviations from zero . additionally , it has been found that the peak - to - peak amplitude of the abr in normal - hearing babies varies from baby to baby . as explained below , the present invention accounts for this variability in abr amplitude , by making conservative assumptions about the abr amplitude of the subject , who is being tested . the present invention also discloses a method and apparatus for predicting a failure to pass . this is achieved by the statistical methods described below . for each block , the statistic z max and the noise variance σ 2 b , are calculated . using normative data , and based on the assumption of a small ( 10 th percentile ) abr waveform , an expected or mean μz maxb is derived . in deriving this expected μz maxb , the present invention takes into the account the eeg signal variance . it has been found that the eeg signal variance affects z max ( see fig4 ), and therefore it is important to take this relationship into account in determining the expected z max , based upon the smallest abr . in particular , tests with normative data indicate that with decreasing eeg signal variance the mean of the test statistic z max grows at increased rates , but the standard deviation of z max remains approximately constant ( σz max = 0 . 65 ). the predicted refer evaluation relies on an analysis of the difference between the expected and actual z max . for any given block , z max can be written as : z max b = μ z maxb + y b σ z maxb b = 1 , 2 , . . . , b where μ zmaxb and σ zmaxb represent the mean and standard deviation of z maxb , and y b represent a distance factor from the mean . it has been found that the standard deviation of z maxb is approximately constant , and therefore σ zmax approximately equals σ zmaxb . therefore , the preceding equation can be transformed into : y b = z max b  - μ z max b σ z max   b = 1 , 2 , …  , b the probability density function of y b is a standard normal distribution , representing a normalized distance measure of the outcomes of z maxb from their respective means . using this information , the present invention implements a threshold for the triggering of a predicted refer , as follows : μ y b = 1 b  ∑ b = 1 b  y b & lt ; y threshold   b = 1 , 2 , …  , b in a preferred embodiment , the present invention uses a μ yb threshold of − 1 . 3 , which provides for a 90 % confidence level . this threshold level is set so that only the subjects with the 10 % lowest z max values for the 10 th percentile abr will trigger a predicted refer . also , rather than automatically terminating the evaluation , in accordance with the present invention the operator may be notified that a predicted refer has been triggered . using the same principle , the operator may also be notified of the current likelihood that the subject would pass . the operator could then be given the choice of deciding whether to continue with the evaluation or not . in the preferred embodiment , the operator would continue with the test , so that if the subject does not ultimately pass the test , the operator can feel confident that the failure to pass was not caused by exogenous factors . also in a preferred embodiment , the predicted refer evaluation is only conducted after the completion of each block , starting at 12 blocks of sweeps . if the electrodes are improperly placed , the abr waveform may be inverted , and z max would grow in the opposite direction as the number of sweeps increases . this could falsely trigger a predicted refer , so predicted refer analysis is also conducted using an inverted polarity condition ( { circumflex over ( z )} inv = −{ circumflex over ( x )} ), where z maxb is replaced with the inverse of z maxb , and the threshold for a predicted refer is restated as : μ y b inv = 1 b  ∑ b = 1 b  y b inv & lt ; y threshold   b = 1 , 2 , …  , b with predicted refer analysis conducted for both z max and the inverse of z max , the operator can determine if a predicted refer is triggered even if the electrodes are switched . if electrode reversal is detected , the operator may be notified , or the inverted data can be analyzed as if it were not inverted . ( ee u . s . patent application entitled “ hearing evaluation device with patient connection evaluation capabilities ”). these techniques can be used to detect adverse evaluation conditions , such as device errors or noise conditions that erroneously cause the test statistic z max to grow excessively quickly . for example the test could be stopped and the user alerted if u y exceeded a preset threshold . this threshold could represent the largest u y expected for the ( i . e . the maximum amplitude abr ) based on normative data . additionally , failure to pass can also be predicted using standard regression techniques . extrapolating from the current and previous z max values , one can determine how many sweeps will likely be necessary before z max reaches the threshold value required to generate a “ pass .” if that number of sweeps exceeds the maximum , then a predicted refer can be generated . moreover , the present invention can also be used with different test statistics using other types of signal parameters . for instance , test statistics have been used which employ the ratio between the estimated variance of the averaged total signal and the estimated variance of the background noise . see c . elberling and m . don , quality estimation of averaged auditory brainstem responses , scand . audiol . 13 : 187 - 197 ( 1984 ). it is believed that the prediction techniques described above could be used in conjunction with this test statistic , as well as other test statistics using different signal parameters . the generation of the click stimulus , the detection of the eeg response signal , the processing and analysis of the eeg response signal including the predicted refer function , and the display of the results are performed by conventional electronic means , e . g ., digital microprocessor controlled devices . such devices include a transducer to generate the auditory stimulus , and conventional electrodes to detect the eeg response signal . to analyze the eeg response signal a processing unit , such a conventional microprocessor , and memory unit are needed . additionally , a display unit and optionally an input device , such as a mouse and / or a keyboard , provide operator interface . as shown in fig3 stimulus generator 10 generates the click stimulus , and eeg transducer 20 detects the eeg response to the stimulus . next , eeg signal conditioning 30 and signal processing 40 occur , readying the eeg response for analysis . statistical analysis for the presence of an abr 50 then occurs , and predictive statistical analysis for abr presence 60 simultaneously occurs , in accordance with the present invention . normative data 70 is used in latter type of analysis . finally , a control device with user interface 80 displays the results . one skilled in the art will appreciate that the present invention can be practiced by other than the preferred embodiments , which are presented for purposes of illustration and not of limitation . in addition , the specific parameter values identified are useful or representative parameter values , and other values or ranges of values for these parameters may be used without departing from the spirit and scope of the invention .
an apparatus and method for evaluation of hearing loss is disclosed . the apparatus and method use evoked auditory brainstem responses to determine if the subject is able to hear repeatedly administered click stimuli . in order to expedite evaluation , the present invention uses normative data to accurately predict when a subject is likely to fail the test , so that evaluation will not needlessly continue for a subject who is very unlikely to pass .