Patent Abstract:
the invention relates to a method to design and generate frequency - specific electrical or acoustical stimuli for the recording of auditory steady - state responses , assr , from human individuals , where the stimuli are generated as a combination of a series of three or more spectral components , each having a specified frequency , amplitude and phase and where the frequency difference between the successive pure tones in the series preferably is constant = f s . the invention further relates to a device for detection of assr and to a software program for use in such device .

Detailed Description:
as an example , such a stimulus could be generated by five pure tones with the frequencies corresponding to 800 , 900 , 1000 , 1100 and 1200 hz . in this example , the tones with the lowest and highest frequency have half the amplitude than that of the other three tones and all tones ( sinusoidal waveforms ) have the same starting phase . the corresponding five pure tones are shown in fig1 . in this example the frequency difference between the pure tones is constant (= 100 hz ) and the central spectral component has a frequency (= 1000 hz ) that is an integer of the frequency difference (= 10 * 100 hz ). the five pure tones are plotted on top of each other in fig2 , and the figure demonstrates that with a repetition frequency that corresponds to the frequency difference ( viz . 100 hz ) all five pure tones become in - phase ( viz . every 10 ms ). this repetition frequency is some times referred to as the ‘ beat ’- frequency . the final stimulus — which is the sum ( or the mean ) of the five pure tones — is plotted in fig3 , and has some similarities to a carrier frequency that is modulated by a complex window function . with a classical amplitude modulation approach the periodicity of the final stimulus waveform is defined by the characteristics of the modulation signal . contrary to this , the periodicity of the final stimulus waveform that is designed via the spectral approach ( or the summation approach ) would be given — as a first order approximation — by the spacing between the individual frequency components . for example , if the stimulus consists of n spectral components ( n = three or more , uneven numbers ) with a spacing of f . [ hz ], then the frequency area that is covered by the stimulus would correspond to ( n − 1 )* f s [ hz ] placed around the central spectral component , and the temporal periodicity would correspond to t s = 1 / f s [ s ]. it can further be demonstrated that the temporal envelope of the final stimulus corresponds to a window function with a complexity , which depends on the exact parameter values ( the number , frequencies , amplitudes and phases ). such a window function could for instance be described as a special blackman - harris window ( harris , 1978 ). with a stimulus designed through the spectral domain approach , each individual spectral components of the assr - stimulus could be given a time alignment ( or phase adjustment ) that compensates for the corresponding cochlear delay . this would result in a stimulus , which would produce larger response amplitudes than those obtained with a corresponding stimulus without such compensation . preliminary test results have in fact been able to demonstrate this . as an example , such a stimulus could be generated by seven pure tones with the frequencies corresponding to 200 , 300 , 400 , 500 , 600 , 700 and 800 hz . in this example , the tones with the lowest and highest frequency have half the amplitude than that of the other five tones and each pure tone is given a time - lag which compensates for its cochlear delay . this delay can be estimated from a cochlear model — for instance as the one shown in fig4 that is based on de boer ( 1980 ) and greenwood ( 1990 ). the time - lagged pure tones are shown in fig5 , and are plotted on top of each other in fig6 . the final stimulus — which is the sum ( or the mean ) of the five pure tones — is plotted in fig7 . due to the introduced time - lag the resulting ‘ time - pulse ’ now appears with a left - right asymmetry . by the spectral design approach ( or summation approach ) some response - artifact problems can be avoided especially for low frequency stimuli . this is obtained by allowing the individual pure tones to be located at frequencies that not are integer values of the constant frequency difference between the different spectral components . following the example in fig1 , such a stimulus could for instance be generated by six pure tones with equal amplitude and phase and with the frequencies corresponding to 250 , 350 , 450 , 550 , 650 and 750 hz . the tones with the lowest and highest frequency have half the amplitude than that of the other four tones and all tones ( sinusoidal waveforms ) have the same starting phase as shown in fig8 . as before , the frequency difference between the pure tones is constant (= 100 hz ) but now the spectral components have frequencies that do not correspond to integer multiples of the frequency difference . the six pure tones are plotted on top of each other in fig9 and the figure demonstrates — as before — that with a repetition frequency that corresponds to the frequency difference ( viz . 100 hz ˜ 10 ms ) all six pure tones appear in - phase . the final stimulus is plotted in fig1 , and because the pure tones in this example are located half - way between integer values of the frequency difference , the temporal waveform demonstrates polarity - inversion from period to period . this corresponds to the polarity change of brief stimuli that was mentioned previously . if the frequencies of the pure tones were not exactly located half - way between the integer values of the frequency difference , the temporal waveform of the resulting stimulus would have the same envelope but a rolling phase from period to period . in any case , the spectrum of the final stimulus will only have components at the used pure tone frequencies and therefore , will not share any frequency bins with the evoked response . dau , t ., wegner , o ., melleret , v . and kollmeier , b . ‘ auditory brainstem responses with optimized chirp signals compensating basilar - membrane dispersion ’. j . acoust . soc . am . 107 , 1530 - 40 . 2000 . cebulla , m ., stürzebecher , e . and elberling , c . ‘ objective detection of auditory steady - state response : comparison of one - sample and q - sample tests ’. presented at : “ the international conference on newborn hearing screening , diagnosis and intervention ”. cernobbio , italy . 2004 . de boer , e . ‘ auditory physics . physical principles in hearing theory i ’. phys . rep . 62 , 87 - 174 . 1980 . dolphin , w . f . ‘ the envelope following response to multiple tone pair stimuli . hear . res . 110 , 1 - 14 . 1997 . don , m ., masuda , a ., nelson , r . and brackmann , d . ‘ successful detection of small acoustic tumors using the stacked derived - band auditory brain stem response amplitude . am . j . otol . 18 , 608 - 21 . 1997 . eggermont , j . j . ‘ electrocochleography ’. in : keidel & amp ; 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