Characterization of Ag/AgCl Dry Electrodes for Wearable Electrophysiological Sensing

With the rising need for on-body biometric sensing, the development of wearable electrophysiological sensors has been faster than ever. Surface electrodes placed on the skin need to be robust in order to measure biopotentials from the body reliably and comfortable for extended wearability. The electrical stability of nonpolarizable silver/silver chloride (Ag/AgCl) and its low-cost, commercial production have made these electrodes ubiquitous health sensors in the clinical environment, where wet gels and long wires are accommodated by patient immobility. However, smaller, dry electrodes with wireless acquisition are essential for truly wearable, continuous health sensing. Currently, techniques for the robust fabrication of custom Ag/AgCl electrodes are lacking. Here, we present three methods for the fabrication of Ag/AgCl electrodes: oxidizing Ag in a chlorine solution, electroplating Ag, and curing Ag/AgCl ink. Each of these methods is then used to create three different electrode shapes for wearable application. Bench-top and on-body evaluation of the electrode techniques was achieved by electrochemical impedance spectroscopy (EIS), calculation of variance in electrocardiogram (ECG) measurements, and analysis of auditory steady-state response (ASSR) measurement. Microstructures produced on the electrode by each fabrication technique were also investigated with scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The custom Ag/AgCl electrodes were found to be efficient in comparison with standard, commercial Ag/AgCl wet electrodes across all three of our presented techniques, with Ag/AgCl ink shown to be the better out of the three in bench-top and biometric recordings.

Neural auditory contrast enhancement in humans

The perception of sensory events can be enhanced or suppressed by the surrounding spatial and temporal context in ways that facilitate the detection of novel objects and contribute to the perceptual constancy of those objects under variable conditions. In the auditory system, the phenomenon known as auditory enhancement reflects a general principle of contrast enhancement, in which a target sound embedded within a background sound becomes perceptually more salient if the background is presented first by itself. This effect is highly robust, producing an effective enhancement of the target of up to 25 dB (more than two orders of magnitude in intensity), depending on the task. Despite the importance of the effect, neural correlates of auditory contrast enhancement have yet to be identified in humans. Here, we used the auditory steady-state response to probe the neural representation of a target sound under conditions of enhancement. The probe was simultaneously modulated in amplitude with two modulation frequencies to distinguish cortical from subcortical responses. We found robust correlates for neural enhancement in the auditory cortical, but not subcortical, responses. Our findings provide empirical support for a previously unverified theory of auditory enhancement based on neural adaptation of inhibition and point to approaches for improving sensory prostheses for hearing loss, such as hearing aids and cochlear implants.

Neurophysiological Evaluation of Right-Ear Advantage During Dichotic Listening

Right-ear advantage refers to the observation that when two different speech stimuli are simultaneously presented to both ears, listeners report stimuli more correctly from the right ear than the left. It is assumed to result from prominent projection along the auditory pathways to the contralateral hemisphere and the dominance of the left auditory cortex for the perception of speech elements. Our study aimed to investigate the role of attention in the right-ear advantage. We recorded magnetoencephalography data while participants listened to pairs of Japanese two-syllable words (namely, “/ta/ /ko/” or “/i/ /ka/”). The amplitudes of the stimuli were modulated at 35 Hz in one ear and 45 Hz in the other. Such frequency-tagging allowed the selective quantification of left and right auditory cortex responses to left and right ear stimuli. Behavioral tests confirmed the right-ear advantage, with higher accuracy for stimuli presented to the right ear than to the left. The amplitude of the auditory steady-state response was larger when attending to the stimuli compared to passive listening. We detected a correlation between the attention-related increase in the amplitude of the auditory steady-state response and the laterality index of behavioral accuracy. The right-ear advantage in the free-response dichotic listening was also found in neural activities in the left auditory cortex, suggesting that it was related to the allocation of attention to both ears.

Selective Attention Measurement of Experienced Simultaneous Interpreters Using EEG Phase-Locked Response

We quantified the electroencephalogram signals associated with the selective attention processing of experienced simultaneous interpreters and calculated the phase-locked responses evoked by a 40-Hz auditory steady-state response (40-Hz ASSR) and the values of robust inter-trial coherence (ITC) for environmental changes. Since we assumed that an interpreter's attention ability improves with an increase in the number of years of experience of simultaneous interpretation, we divided the participants into two groups based on their simultaneous interpretation experience: experts with more than 15 years of experience (E group; n = 7) and beginners with <1 year (B group; n = 15). We also compared two conditions: simultaneous interpretation (SI) and shadowing (SH). We found a significant interaction in the ITC between years of SI experience (E and B groups) and tasks (SI and SH). This result demonstrates that the number of years of SI experience influences selective attention during interpretation.

Assessing Agreement between Frequency-Specific Chirp Auditory Steady-State Response and Pure Tone Audiometry in Adults by Intraclass Correlation Coefficient

<b><i>Introduction:</i></b> Chirp auditory steady-state response (ASSR) can be used to assess frequency-specific hearing thresholds. However, its reliability has not been confirmed yet. The purpose of this proposed study is to analyze the agreement of thresholds measured by chirp-ASSR and pure tone audiometry (PTA) to investigate the value of chirp-ASSR in hearing threshold evaluation. <b><i>Methods:</i></b> Participants with normal hearing (age: 18–66, 108 ears) and patients with sensorineural hearing loss (age: 22–82, 75 ears) were tested using PTA and chirp-ASSR at 0.5, 1, 2, and 4 kHz, respectively. Intraclass correlation coefficient (ICC) and Bland-Altman plot were introduced to analyze the agreement between the 2 methods. <b><i>Results:</i></b> One-hundred eight participants underwent both chirp-ASSR and PTA to estimate their thresholds. The ICCs yielded by these 2 methods are 0.757, 0.893, 0.883, and 0.921 (<i>p</i> &#x3c; 0.001) at 0.5, 1, 2, and 4 kHz carrier frequency, respectively. However, there is a significant difference between the 2 methods at 2 kHz: the mean value of the ASSR thresholds is 5.27 dB HL higher than the value of PTA thresholds. Additionally, the 95% limits of agreement range from −27.48 to 26.66 dB HL at 0.5 kHz, from −18.19 to 17.87 dB HL at 1 kHz, from −12.01 to 22.55 dB HL at 2 kHz, and from −21.29 to 19.17 dB HL at 4 kHz, which are large enough to affect clinical decision-making. <b><i>Conclusion:</i></b> In this study, we have confirmed good to excellent correlation between chirp-ASSR and PTA in threshold estimation for adults with and without hearing loss. The degree of correlations is higher for participants with hearing loss and for measurements at high frequencies. However, significant systematic difference and large limits of agreement between the 2 methods have been found. These findings show that chirp-ASSR can be treated as a supplementary method to PTA when evaluating the hearing level, but the 2 methods are not interchangeable due to their systematic difference and large limits of agreement.