Med Biol Eng Comput DOI 10.1007/s11517-015-1251-5
ORIGINAL ARTICLE
A new low‑noise signal acquisition protocol and electrode placement for electrocochleography (ECOG) recordings Chathura Kumaragamage · Brian Lithgow · Zahra Moussavi
Received: 9 May 2014 / Accepted: 6 February 2015 © International Federation for Medical and Biological Engineering 2015
Abstract Electrocochleography (ECOG) is a low-amplitude electrophysiological measurement technique primarily used as an assistive tool for the diagnosis of Ménière’s disease. Of the two types of ECOG, transtympanic (TT) and extratympanic (ET), ET-ECOG has gained popularity due to its noninvasive nature; however, it suffers from increased susceptibility to various types of noise, due to the low-signal amplitude (~1 µV scale) associated with the method. Therefore, reliably obtaining ECOG recordings involves an environment that minimally interferes with the recording, a low-noise signal recorder, and a carefully executed recording protocol. We propose a new method that involves a modified ear electrode and electrode placement protocol that offers a solution to reducing noise in ET-ECOG. Noise suppression is achieved by minimizing background biological noise, and thermal noise from electrode impedances, which were identified to be the main contributors to signal degradation in ET-ECOG. Results show that the proposed method yields a >2.6 dB improvement in SNR in comparison with the conventional method (p 10 ms after the ECOG response of interest, there is minimal interference between the evoked PAM and ECOG responses.
saturation. PLI typically occurs [1, 27, 38] from a common mode voltage induced on the patient due to parasitic coupling, which gets converted to differential mode by mismatched differential electrode impedances. Most ECOG recording protocols described in the literature [5, 6, 10, 19] have one notable feature in common: The reference electrode is a different type compared to the active ear electrode resulting in mismatched interface impedances [25]. As a result, susceptibility to PLI is inherent in traditional ECOG (compared to electrocardiography, or electroencephalography where identical electrode types are typically used). Hence, electrical shielding methods and the use of a Faraday cage are commonly reported in ECOG literature [2, 10].
2.1.3 Power line interference Power line interference (PLI) typically appears as peaks of 50/60 Hz harmonics in frequency domain and can severely affect high gain (>89 dB) measurements leading to amplifier
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Med Biol Eng Comput
Fig. 2 a A component of inner ear activity may arise due to responses generated by the tensor tympani, stapedius, and the postauricular [3, 30] muscles (PAM) correlated with the sound stimulus.
These muscles are triggered by loud sounds [3] as a protective mechanism. b ECOG response which is the zoomed in version of the circled waveform in (a)
Fig. 3 a Sources of biological activity present in the ear canal. b Currently used (OEM) ECOG electrode placement setup, where an ear electrode is placed proximal to the TM, and an OEM reference electrode is placed on the contralateral ear lobe. c The proposed electrode placement setup involves a modified electrode as the active ear electrode positioned proximal to the TM (similar to the OEM ear electrode), and a similar modified electrode used as the reference, fixated on the base of the ipsilateral ear canal (incisura). Finally, an ear plug with an opening to deliver the auditory stimulus is inserted into
the ear canal which also assists in fixating the electrodes in place. d–e The modified electrode was fabricated by wrapping an Ag/AgCl wire with cotton wool that appears similar to a cotton swab. f Next, the cotton wrapped electrode is soaked in a viscous mixture of saline and electrode gel to establish a low-impedance contact interface between the electrode wire and conductive gel. Since the whole cotton wool tip is soaked in saline, a larger surface area is established for both interfaces, resulting in lower impedance compared to the currently used OEM electrode
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Med Biol Eng Comput
2.2 Experimental methods
2.2.2 Investigation of low‑noise electrodes and their placement protocol
2.2.1 ECOG recording protocol In this work, a popular ET-ECOG electrode protocol [11] is used as the referring standard wherein a disposable TMECochGtrode (Bio-Logic) (or OEM) ear electrode coated with conductive gel is placed proximal to the TM, called the ‘active electrode.’ The contralateral earlobe is connected with an OEM electrode (Biopac–ELS254S) coated with gel, called the ‘reference’ for the active electrode (Fig. 3b illustrates the electrode placement of the differential pair, where the reference electrode is shown on the ipsilateral earlobe for simplicity). An OEM electrode (Biopac–ELS258S) also coated with gel is placed on the forehead; this is considered as the common/ground electrode. All electrodes, except the ear electrode, are fixated by surgical tape, and the ear electrode is fixated using an ear plug (see Fig. 3b). All recordings were obtained with a low-noise √ (
ORIGINAL ARTICLE
A new low‑noise signal acquisition protocol and electrode placement for electrocochleography (ECOG) recordings Chathura Kumaragamage · Brian Lithgow · Zahra Moussavi
Received: 9 May 2014 / Accepted: 6 February 2015 © International Federation for Medical and Biological Engineering 2015
Abstract Electrocochleography (ECOG) is a low-amplitude electrophysiological measurement technique primarily used as an assistive tool for the diagnosis of Ménière’s disease. Of the two types of ECOG, transtympanic (TT) and extratympanic (ET), ET-ECOG has gained popularity due to its noninvasive nature; however, it suffers from increased susceptibility to various types of noise, due to the low-signal amplitude (~1 µV scale) associated with the method. Therefore, reliably obtaining ECOG recordings involves an environment that minimally interferes with the recording, a low-noise signal recorder, and a carefully executed recording protocol. We propose a new method that involves a modified ear electrode and electrode placement protocol that offers a solution to reducing noise in ET-ECOG. Noise suppression is achieved by minimizing background biological noise, and thermal noise from electrode impedances, which were identified to be the main contributors to signal degradation in ET-ECOG. Results show that the proposed method yields a >2.6 dB improvement in SNR in comparison with the conventional method (p 10 ms after the ECOG response of interest, there is minimal interference between the evoked PAM and ECOG responses.
saturation. PLI typically occurs [1, 27, 38] from a common mode voltage induced on the patient due to parasitic coupling, which gets converted to differential mode by mismatched differential electrode impedances. Most ECOG recording protocols described in the literature [5, 6, 10, 19] have one notable feature in common: The reference electrode is a different type compared to the active ear electrode resulting in mismatched interface impedances [25]. As a result, susceptibility to PLI is inherent in traditional ECOG (compared to electrocardiography, or electroencephalography where identical electrode types are typically used). Hence, electrical shielding methods and the use of a Faraday cage are commonly reported in ECOG literature [2, 10].
2.1.3 Power line interference Power line interference (PLI) typically appears as peaks of 50/60 Hz harmonics in frequency domain and can severely affect high gain (>89 dB) measurements leading to amplifier
13
Med Biol Eng Comput
Fig. 2 a A component of inner ear activity may arise due to responses generated by the tensor tympani, stapedius, and the postauricular [3, 30] muscles (PAM) correlated with the sound stimulus.
These muscles are triggered by loud sounds [3] as a protective mechanism. b ECOG response which is the zoomed in version of the circled waveform in (a)
Fig. 3 a Sources of biological activity present in the ear canal. b Currently used (OEM) ECOG electrode placement setup, where an ear electrode is placed proximal to the TM, and an OEM reference electrode is placed on the contralateral ear lobe. c The proposed electrode placement setup involves a modified electrode as the active ear electrode positioned proximal to the TM (similar to the OEM ear electrode), and a similar modified electrode used as the reference, fixated on the base of the ipsilateral ear canal (incisura). Finally, an ear plug with an opening to deliver the auditory stimulus is inserted into
the ear canal which also assists in fixating the electrodes in place. d–e The modified electrode was fabricated by wrapping an Ag/AgCl wire with cotton wool that appears similar to a cotton swab. f Next, the cotton wrapped electrode is soaked in a viscous mixture of saline and electrode gel to establish a low-impedance contact interface between the electrode wire and conductive gel. Since the whole cotton wool tip is soaked in saline, a larger surface area is established for both interfaces, resulting in lower impedance compared to the currently used OEM electrode
13
Med Biol Eng Comput
2.2 Experimental methods
2.2.2 Investigation of low‑noise electrodes and their placement protocol
2.2.1 ECOG recording protocol In this work, a popular ET-ECOG electrode protocol [11] is used as the referring standard wherein a disposable TMECochGtrode (Bio-Logic) (or OEM) ear electrode coated with conductive gel is placed proximal to the TM, called the ‘active electrode.’ The contralateral earlobe is connected with an OEM electrode (Biopac–ELS254S) coated with gel, called the ‘reference’ for the active electrode (Fig. 3b illustrates the electrode placement of the differential pair, where the reference electrode is shown on the ipsilateral earlobe for simplicity). An OEM electrode (Biopac–ELS258S) also coated with gel is placed on the forehead; this is considered as the common/ground electrode. All electrodes, except the ear electrode, are fixated by surgical tape, and the ear electrode is fixated using an ear plug (see Fig. 3b). All recordings were obtained with a low-noise √ (
