Abstract:
A method and system for modulating a steady state stimulus involving first generating a steady state stimulus signal and then modulating the steady state stimulus signal by at least a portion of a sinusoidal signal where the stimulus is set to zero during another portion of the sinusoidal signal. The steady state stimulus may also be offset by the addition of a constant or may be passed through a bandpass filter to reduce or eliminate frequency components that are outside the desired range of stimulation.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to a method and system for modulating a steady state stimulus and more particularly to a method and system for testing sensory responses of a subject using a modulated stimulus, such as a stimulus for Auditory Steady State Response (ASSR), for auditory testing.  
       BACKGROUND  
       [0002]     Living animals generate electrical potentials which, when collected, detected and analysed, can be used for a variety of purposes. For example, synchronous neural activity in a live animal or human brain produces electrical potentials that can be detected at the surface of the scalp with conductive electrodes. These detected potentials can then be used in a wide variety of clinical applications, particularly diagnostic applications.  
         [0003]     Stimulation of sensory systems, including but not limited to the auditory and visual systems, with amplitude-modulated (AM) and/or frequency-modulated (FM) tones evoke electrical responses from the brain that correspond to the frequency of modulation. These responses, sustained for the duration of stimulation are known as steady-state responses or steady-state evoked potentials (SSEP). Particularly, Auditory Steady State Responses (ASSR) have been shown to have clinical value in objectively estimating the threshold of hearing at specific frequencies.  
         [0004]     An ASSR is an electric sinusoidal signal, which is believed to originate in the brainstem, typically elicited by a modulated sinusoidal stimulus. For example, the stimulus may be a carrier tone of audible frequency range, the amplitude or frequency of which is modulated with a low modulation frequency, typically between 40 and 100 Hz. The ASSR signal evoked then has exactly the same frequency of such modulation. Unfortunately, the ASSR also has a very low amplitude, which causes difficulty in reliably extracting the ASSR signal from noise.  
         [0005]     The general principle of ASSR measurement is described as follows. A modulated pure tone is presented to the ear. The carrier frequencies are usually conventional audiometric tones, from 125 to 8000 Hz. The levels of the frequencies are at or higher than 20 dB SPL. The modulation frequencies are typically 40 Hz or within the 70 to 100 Hz range (usually 80 Hz if they are in the 70 to 100 Hz range), with a 0.95 modulation index.  
         [0006]     At the time of stimulation, a sinusoidal electric signal (the ASSR), which has a frequency equal to the modulation frequency of the stimulus, appears on the surface of the skull. The ASSR can be recorded from the surface of the skull with three electrodes, typically on the vertex, on the temporal bone, and on the lobule. This electric signal, whose magnitude is typically from 40 to 400 nV, is then amplified with a typical gain of approximately 10,000 V/V. It is then passed through a band pass filter, with a typical lower frequency cutoff at 10 to 30 Hz and a higher frequency cutoff at 100 to 300 Hz. The filtered signal is then converted into its digital form and processed.  
         [0007]      FIG. 1  depicts a pure-tone ASSR carrier modulated by a pure sinusoidal modulation in the case where Amplitude Modulation is used. The frequency spectrum of the resulting stimulus waveform contains only the carrier frequency (f c ) and the two side-band frequencies (f c +f m  and f c −f m ). Since the modulation frequency f m  is typically much smaller than f c , only those sensorineural pathways that are sensitive to frequencies very close to the carrier are stimulated. This frequency specificity in hearing threshold estimates is clinically valuable for optimizing intervention in patients with hearing loss. Although ASSR signals have the advantage that they are frequency-specific, as noted above, ASSR signals are difficult to detect in the presence of electromagnetic interference due to the ASSR signal&#39;s low amplitude relative to other electrical signals recorded from the scalp, such as electroencephalography (EEG), electrocardiography (ECG), electrooculography (EOG), and electromyography (EMG).  
         [0008]     An alternative method for estimating frequency-specific hearing thresholds is to use a transient stimulus that consists of a short tone burst. The auditory brainstem response (ABR) to the tone burst, a response recognizable to trained clinicians, can be detected if the transient sound is above the patient&#39;s threshold for hearing. The disadvantage of this method is that a rapid stimulus onset is required to evoke a response that is easier to recognize, but a rapid stimulus onset also results in a broad stimulus spectrum that is not limited to the specific frequency of the tone, i.e. the tone-burst test is not as frequency-specific as the steady-state response is.  
         [0009]     In order to achieve better frequency specificity without compromising response level, Gorga et al. (Gorga M. P. et al. (1992), Journal of the American Academy of Audiology, 3, 159-165, “Auditory brainstem responses elicited by 1000 Hz tone bursts in patients with sensorineural hearing loss”, which is hereby incorporated herein by reference) explored the use of a variety of tone-burst stimulus onset and offset waveforms. This included the use of a waveform providing a short tone-burst described as a “Blackman Envelope”. Because waveforms such as the “Blackman envelope” have broader frequency spectrum in their main frequency lobe but smaller side lobes compared to linearly gated tone bursts, it was presumed by Gorga that these waveforms would produce the effects of a rapid onset stimulus while maintaining more frequency specificity. Purdy et al (Purdy et al. (2002), Ear &amp; Hearing 2002;23;358-368, “ABR Thresholds to tonebursts gated with Balckman and Linear Windows in Adults with High Frequency Sensorineural Hearing Loss”, which is hereby incorporated herein by reference) showed, however, that there is no significant difference between linear and Blackman windows in actual threshold determination.  
         [0010]     John et al. (John, M. S. et al (2002), Ear and Hearing 2002; 23; 106-117, “Auditory Steady-State Responses to Exponential Modulation Envelopes”, which is hereby incorporated herein by reference) explored boosting the Auditory Steady-State Response by using a stimulus consisting of a pure tone modulated by a cosine-squared function.  FIG. 2  illustrates cosine-squared exponential modulation for the case of amplitude modulation (“AM”). John et al. studied modulation functions using other exponents as well, with similar results. This set of modulation functions is known generally as “exponential modulation”. The cosine-squared AM technique has the property that it adds a single harmonic to the fundamental modulation frequency. The resulting waveform has frequency components at fc, fc +/− fm, and fc +/− 2 fm, but at no other frequency. John et al. reports that the ASSR signal was enhanced significantly using this technique without compromising frequency specificity. John et al. theorized that the improvement in ASSR performance was possibly due to: the cosine squared function causing the stimulus to be close to zero between peaks allowing for the neurons involved in the response to complete their refractory period before being re-stimulated; and the maximum slope of the modulation signal being greater for the cosine-squared function than that for pure sinusoidal modulation.  
         [0011]     While exponential modulation appears to result in good frequency specificity for the stimulus and improves the ASSR response level, this technique requires additional computational complexity in calculating the modulation function. For example, if a lookup table is used to calculate the sine function, the cosine squared function would have to be calculated from this or a separate lookup table would be required. Furthermore, the exponentiation of the cosine function requires more floating-point precision than the calculation of a simple cosine function. The increased computational cost of exponential modulation generally results in additional costs, requires larger equipment, and the like, when used for ASSR related tests. The additional costs and larger equipment may not be as effective in a clinical environment where it can be important to have a steady-state stimulus/response system, such as an ASSR system, that is, for example, battery-powered, inexpensive, small, lightweight and possibly handheld.  
         [0012]     As such, there is a need for an improved stimulus/response system and method and system of modulating a steady state stimulus that overcomes at least some of the disadvantages noted above.  
       SUMMARY  
       [0013]     The present invention overcomes at least some of the disadvantages of previous approaches by providing a method and system for generating a steady-state stimulus, for example an ASSR stimulus, using a partially rectified cosine as a modulation signal, thus reducing computational complexity, and reducing the size and cost of systems for testing ASSR.  
         [0014]     According to one exemplary embodiment, there is provided a method of modulating a steady state stimulus, the method including: generating a steady state stimulus signal; and modulating the steady state stimulus signal by at least a portion of a sinusoidal signal where the stimulus is set to zero during another portion of the sinusoidal signal. In this way, the stimulus provided to a subject is zero for a portion between peaks and it is believed that this allows the neurons involved to complete their refractory period before being re-stimulated.  
         [0015]     It will be understood by one of skill in the art that the modulation may be amplitude modulation or frequency modulation.  
         [0016]     In a particular case, the portion of the sinusoidal signal may be the positive portion of the sinusoidal signal.  
         [0017]     In another particular case, the sinusoidal signal may be offset by the addition of a constant.  
         [0018]     In yet another particular case, the sinusoidal signal may be multiplied by a constant greater than 1 to increase to rate of stimulus onset.  
         [0019]     In a case where there is an offset and a multiplier, the sinusoidal signal, the offset and the multiplier may be chosen so that the peak, off time and onset rate of the modulation emulate that for cosine squared modulation.  
         [0020]     In another particular case, the sensory stimulus may be passed through a bandpass filter to eliminate or reduce frequency components of the stimulus that are outside the desired range of stimulation.  
         [0021]     According to another exemplary embodiment, there is provided a system for modulating a steady state stimulus, the system including: a steady state stimulus signal generator; a sine wave signal generator; a rectifier for causing the sine wave signal to be zero for all values of the waveform below a predetermined threshold; and a multiplier for combining the steady state stimulus signal and sine wave signal.  
         [0022]     In a particular case, the system further includes an offset generator to offset the sine wave signal by the addition of a constant.  
         [0023]     In another particular case, the multiplier mutiplies the sine wave signal by a constant greater than 1 to increase the rate of stimulus onset.  
         [0024]     In yet another particular case, the system further includes a bandpass filter to filter frequency components of the stimulus signal that are outside the desired range of stimulation.  
         [0025]     According to yet another exemplary embodiment, there is provided a system for testing sensory systems of a subject, the system including: a test signal generator for providing at least one test signal; a modulator for modulating at least one of the at least one test signals by a portion of a sinusoidal signal where the stimulus is set to zero during the other portion of the sinusoidal signal; a transducer for transducing the at least one modulated test signal to create a sensory stimulus, and presenting the stimulus to the subject; a detector for detecting potentials from the subject while substantially simultaneously presenting the stimulus to the subject; and a processor for analyzing the potentials to determine whether the potentials are indicative of the presence of at least one steady-state response to the stimulus.  
         [0026]     In a particular case, the portion of the sinusoidal signal is the positive portion of the sinusoidal signal.  
         [0027]     In another particular case, the system further includes an offset generator for offsetting the sinusoidal signal by the addition of a constant.  
         [0028]     In yet another particular case, the system further includes a multiplier for multiplying the sinusoidal signal by a constant greater than 1 to increase to rate of stimulus onset.  
         [0029]     In still yet another particular case, the system further includes a bandpass filter to act on the sensory stimulus to eliminate or reduce frequency components of the stimulus that are outside the desired range of stimulation.  
         [0030]     It will be further understood that the modulation described above may be generated by hardware components acting on an analog or digital signal and/or by application of a software program to a digital signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]     These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein:  
         [0032]      FIG. 1  shows an example of conventional amplitude modulation of a sinusoidal ASSR stimulus;  
         [0033]      FIG. 2  shows an example of exponential modulation of a cosine ASSR stimulus;  
         [0034]      FIG. 3  shows a side view of an examplary embodiment of stimulus response system;  
         [0035]      FIG. 4  shows a top view of the stimulus response system of  FIG. 3 ;  
         [0036]      FIG. 5  shows an example of the partially rectified cosine wave superimposed on a cosine squared modulated waveform; and  
         [0037]      FIG. 6  shows the power spectrum FFT for partially rectified modulation versus that for cosine squared modulation. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0038]      FIG. 3  shows a side view of an examplary stimulus response system  10 . Stimulus response system  10  includes a reference electrode module  12 . In this exemplary embodiment, reference electrode module  12  includes an electrode  12   a  and an amplifier component  12   b  that have been adapted to fit together as a module. Such a reference electrode module is described in greater detail in U.S. patent application Ser. No. 10/690,630, filed Oct. 23, 2003, by Sokolov et al., which is hereby incorporated herein by reference.  
         [0039]     Reference electrode module  12  is electrically coupled to two or more signal electrodes  14 . In  FIG. 3 , two signal electrodes  14  (also referred to herein as simply “electrodes  14 ”) are illustrated—a first electrode  14   a  is illustrated in the foreground while a second electrode  14   b  (shown in dotted line) is in the background. Electrodes  14   a ,  14   b  are electrically coupled to reference electrode module  12  by lead wires  20   a ,  20   b , respectively.  
         [0040]     In this arrangement, reference electrode module  12  also acts as the reference electrical ground for electrodes  14   a  and  14   b . Electrical potentials detected by electrodes  14  will be passed into amplifier component  12   b  for signal amplification. The use of short lead wires  20  (usually less than 15-20 cm in length on an adult human&#39;s head and even shorter on an infant&#39;s or small animal&#39;s head) results in far less noise being inductively or capacitively coupled to the lead wires that carries the signal from electrodes  14  to the amplifier component  12   b  than conventional electrode-lead and wire-amplifier arrangements. Additionally, since lead wires  20  are preferably taut, motion artifacts that induce noise in the lead wires  20  as they move through static (i.e., time invariant) electromagnetic fields are reduced. The motion artifact noise is reduced compared to known arrangements because lead wires  20   a  and  20   b  are likely to move through very similar paths and remain fixed relative to each other through these time-invariant electromagnetic fields. Consequently, there is likely to be only very small differential potentials resulting from these differential motion artifacts that will be detected by amplifier component  12   b.    
         [0041]     Reference electrode module  12  and electrodes  14  are affixed or mounted to subject  22  through known adhesives or other fixation methods or mechanisms. Additionally, a conductive substance such as electrode gel, for example, may be used to enhance or ensure electrical conduction between the skin of subject  22  and electrodes  12 ,  14 .  
         [0042]     Reference electrode module  12  is also electrically coupled to signal processing device  18  by way of connector  16 . More particularly, amplifier component  12   b  is adapted to be electrically connected to lead wires  20   a ,  20   b  and connector  16 . Connector  16  is preferably a shielded wire allowing amplified electrical potential signals to be transmitted from reference electrode module  12  to signal-processing device  18 .  
         [0043]     Signal-processing device  18  operates to generate and modulate a steady state stimulus signal and to receive and process signals received from reference electrode module  12  via connector  16 . Signal-processing device  18  may be, for example, a conventional signal-processing device that has been adapted to also receive amplified electrical potential signals rather than electrical potentials that have yet to be amplified. Signal-processing device  18  may include, for example, a visual display for displaying the generated and received signals, a signal recorder component for recording the signal received for later review and analysis, and various signal-processing circuits and software for processing any signals generated and received. Such signal processing may include circuitry and/or software for further reducing any noise contained in the received amplified signals.  
         [0044]     The signal-processing device  18  is also connected, via transducer connector  24 , to a transducer  26 . The transducer  26  is for applying the stimulus to the subject  22 . In this embodiment, the transducer  26  may be, in this embodiment, an external speaker, a headphone, or an ear inserted speaker, as are known in the art.  
         [0045]     Referencing  FIG. 4 , stimulus response system  10  is illustrated in a top view of subject  22 . As noted above, lead wires  20   a  and  20   b  electrically connect electrodes  14   a ,  14   b  to reference electrode module  12 .  
         [0046]     Referencing  FIGS. 3 and 4 , in operation of system  10 , an operator affixes reference electrode module  12  and electrodes  14  to a subject in the relevant areas of interest in a manner known to those of ordinary skill in the art. The operator also electrically connects, by way of a lead wire  20 , each electrode  14  to reference electrode module  12 . In the exemplary embodiment, electrode  14   a  is connected to reference electrode module  12  by way of lead wire  20   a  and electrode  14   b  is connected to reference electrode module  12  by way of lead wire  20   b . Lead wires  20  may be connected to electrodes  12 ,  14  prior or after fixation to the subject. An operator also electrically connects reference electrode module  12  to signal processing device  18  by way of connector  16 . The operator then positions the transducer  26  to appropriately deliver the stimulus signal to the subject  22 . In this case, by placing an ear insertion speaker.  
         [0047]     After the electrodes and transducer (speaker) are arranged appropriately, the stimulus for testing the sensory response of the subject may be applied to the subject.  
         [0048]     As described above, the stimulus may be a carrier tone of audible frequency range (“steady state stimulus signal”) that is modulated with a low modulation frequency, typically between 40 and 100 Hz. In an exemplary embodiment, a partially rectified cosine wave is applied as a modulation signal rather than the conventional sine function or an exponential cosine function. signals. Alternatively, the technique may be implemented using software on a digital microcomputer or microcontroller to calculate the function or lookup the function.  
         [0049]     The exemplary embodiment also provides a waveform that is less frequency specific than the cosine squared waveform.  FIG. 6  shows the power spectrum FFT for partially rectified modulation versus that for cosine squared modulation. From  FIG. 6 , it is apparent that, for a 1 kHz carrier, the spectrum of the waveform modulated by a partially rectified cosine contains more power in the range of 500-1500 Hz than the spectrum for the waveform modulated by a cosine squared function. The signal power contained outside this range, however, is small compared to the components close to the center frequency and are not likely to affect threshold detection. In one embodiment of this invention, the stimulus may also be band pass filtered to eliminate or reduce these extra frequency components.  
         [0050]     It will be understood that various elements of the embodiments of the invention can be effected by a software program or effected by hardware components or by a combination thereof.  
         [0051]     The above embodiments describe a system in which the reference electrode includes an amplifier. It will be understood by one of skill in the art that a more conventional system in which amplification is performed at the signal processing device could also be used.  
         [0052]     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art  
         [0053]     The partially rectified cosine wave has the form: 
        f m =cos(2*pi*f*t)*m/n+b, for values of t such that −cos(2*pi*f*t)*m/n&lt;b and     f m =0 for all other values of t.     Here, b is a bias or offset that determines what portion of the modulation signal is nonzero and m/n is a fraction, usually greater than 1, that determines the slope of the onset of modulation.        
 
         [0057]      FIG. 5  shows an example of the resulting modulated AM waveform (for a value of b=0.5 and m/n=48/33) superimposed on a cosine squared modulated waveform. It is apparent from  FIG. 5  that the partially rectified waveform has the advantages of: being zero for a significant portion between peaks allowing for the neurons involved in the response to complete their refractory period before being re-stimulated; and having a maximum slope of the modulation signal that is greater than that for pure sinusoidal modulation.  
         [0058]     A further advantage of the present technique is that it requires no additional computational power compared to the calculation of conventional sinusoidal modulation. It will be understood to one of skill in the art that the present technique may be implemented using analog components for the generation of a modulating cosine function, adding the offset “b”, rectifying the signal so that the resulting waveform is zero for all negative values of the waveform and multiplying the carrier and modulated without departing from the spirit and scope of the invention as outlined in the claims appended hereto.