Postdoctoral Fellow, University of Connecticut
Can we detect cognitive stress using electrodermal activity under the water?
Divers face many stressful situations. If we can detect when they are suffering stress, we could improve their performance by guiding them to control it, or even save their life! In conditions of pressure and temperature associated with immersion in water, humans are more susceptible to severe stress, challenging the human physiological control systems. Reliable tools for the assessment of the stress underwater are needed. Electrodermal activity (EDA) is considered a promising alternative for the assessment of the level of stress in humans. EDA is a measure of the changes in conductance at the skin surface related to sweat production. In normal humidity conditions, EDA changes in response to stress in three main ways: the skin conductance level (SCL) is increased, the occurrence of non-specific skin conductance responses (NS.SCRs) increases, and the normalized spectral power in the band from (EDASympn) 0.045 to 0.25 Hz is elevated. When skin is immersed in water, the humidity blocks the sweat glands, changing the dynamics of EDA. For this reason, we wanted to explore the dynamics of EDA for subjects immersed in water, as response to cognitive stress.
Abstract: Time-domain features of electrodermal activity (EDA), the measurable changes in conductance at the skin surface, are typically used to assess overall activation of the sympathetic system. These time domain features, the skin conductance level (SCL) and the nonspecific skin conductance responses (NS.SCRs), are consistently elevated with sympathetic nervous arousal, but highly variable between subjects. A novel frequency-domain approach to quantify sympathetic function using the power spectral density (PSD) of EDA is proposed. This analysis was used to examine if some of the induced stimuli invoke the sympathetic nervous system’s dynamics which can be discernible as a large spectral peak, conjectured to be present in the low frequency band. The resulting indices were compared to the power of low-frequency components of heart rate variability (HRVLF) time series, as well as to time-domain features of EDA. Twelve healthy subjects were subjected to orthostatic, physical and cognitive stress, to test these techniques. We found that the increase in the spectral powers of the EDA was largely confined to 0.045–0.15 Hz, which is in the prescribed band for HRVLF. These low frequency components are known to be, in part, influenced by the sympathetic nervous dynamics. However, we found an additional 5–10% of the spectral power in the frequency range of 0.15–0.25 Hz with all three stimuli. Thus, dynamics of the normalized sympathetic component of the EDA, termed EDASympn, are represented in the frequency band 0.045–0.25 Hz; only a small amount of spectral power is present in frequencies higher than 0.25 Hz. Our results showed that the time-domain indices (the SCL and NS.SCRs), and EDASympn, exhibited significant increases under orthostatic, physical, and cognitive stress. However, EDASympn was more responsive than the SCL and NS.SCRs to the cold pressor stimulus, while the latter two were more sensitive to the postural and Stroop tests. Additionally, EDASympn exhibited an acceptable degree of consistency and a lower coefficient of variation compared to the time-domain features. Therefore, PSD analysis of EDA is a promising technique for sympathetic function assessment.
Pub.: 08 Apr '16, Pinned: 06 Oct '17
Abstract: Time-domain indices of electrodermal activity (EDA) have been used as a marker of sympathetic tone. However, they often show high variation between subjects and low consistency, which has precluded their general use as a marker of sympathetic tone. We surmise that this lack of consistency in certain sympathetic tone inducing conditions with previous time-invariant analysis of EDA may lie in its inability to characterize time-varying dynamics of the sympathetic tone. To overcome the disadvantages of time-domain and time-invariant power spectral indices of EDA, we developed a highly sensitive index of sympathetic tone, based on time-frequency analysis of EDA signals. Its efficacy was tested using experiments designed to elicit sympathetic dynamics. Twelve subjects underwent four tests known to elicit sympathetic tone arousal: cold pressor, tilt table, postural stimulation, and Stroop task. We hypothesize that a more sensitive measure of sympathetic control can be developed using time-varying spectral analysis. A recently-developed technique for time-frequency analysis was used to obtain spectral amplitudes associated with EDA. We found that the time-varying spectral frequency band 0.08-0.24 Hz was most responsive to stimulation. Spectral frequencies from 0.24 Hz and beyond were determined to be not related to the sympathetic dynamics because they comprised less than 5% of the total power. The mean value of time-varying spectral amplitudes in the frequency band 0.08-0.24 Hz were used as the index of sympathetic tone, termed TVSymp. TVSymp was found to exhibit low coefficient of variation (0.54), and higher consistency and sensitivity when compared to time-domain and time-invariant spectral indices.
Pub.: 22 Jul '16, Pinned: 06 Oct '17
Abstract: We analyzed multiple measures of the autonomic nervous system (ANS) based on electrodermal activity (EDA) and heart rate variability (HRV) for young healthy subjects undergoing 24-h sleep deprivation. In this study, we have utilized the error awareness test (EAT) every 2 h (13 runs total), to evaluate the deterioration of performance. EAT consists of trials where the subject is presented words representing colors. Subjects are instructed to press a button ("Go" trials) or withhold the response if the word presented and the color of the word mismatch ("Stroop No-Go" trial), or the screen is repeated ("Repeat No-Go" trials). We measured subjects' (N = 10) reaction time to the "Go" trials, and accuracy to the "Stroop No-Go" and "Repeat No-Go" trials. Simultaneously, changes in EDA and HRV indices were evaluated. Furthermore, the relationship between reactiveness and vigilance measures and indices of sympathetic control based on HRV were analyzed. We found the performance improved to a stable level from 6 through 16 h of deprivation, with a subsequently sustained impairment after 18 h. Indices of higher frequencies of EDA related more to vigilance measures, whereas lower frequencies index (skin conductance leve, SCL) measured the reactiveness of the subject. We conclude that indices of EDA, including those of the higher frequencies, termed TVSymp, EDASymp, and NSSCRs, provide information to better understand the effect of sleep deprivation on subjects' autonomic response and performance.
Pub.: 06 Jul '17, Pinned: 06 Oct '17
Abstract: Novel carbon/salt adhesive (CSA) electrodes have been found suitable for collecting electrodermal activity (EDA) signals. Ag/AgCl electrodes are considered the standard for collecting EDA signals, because it highly avoids electrodes' polarization. Ag is an expensive commodity. Furthermore, a hydrogel layer is needed for the Ag/AgCl electrodes to collect EDA signals. Adding hydrogel to the electrodes is a cumbersome process. Aforementioned circumstances highlight the need for a more accessible media to collect these signals, allowing EDA use to spread. Dry electrodes made with a mixture of carbon, salt and adhesive has shown to be suitable for collecting bioelectric signals. We have implemented a constant DC-source EDA circuit, with the intention of testing how these electrodes perform for collecting EDA signals. Recruited subjects (N=4) underwent a test including electric shocks, watching a disturbing video and performing the Stroop task. Time and frequency domain correlation were computed. For the obtained skin conductance responses (SCRs), amplitude, onset-to-peak time, and onset difference between the CSA and Ag/AgCl electrodes' acquired SCRs were computed. We found no significant differences on SCRs amplitude and onset-to-peak time between CSA and Ag/AgCl. Furthermore, the difference in onset time for simultaneous SCRs obtained using both media was not different to zero. We conclude that CSA electrodes are a suitable surrogate of Ag/AgCl electrodes for collecting EDA signals on healthy subjects using the implemented DC circuit.
Pub.: 24 Feb '17, Pinned: 06 Oct '17