1. INTRODUCTION
Nonspecific electrodermal
responses (NS.EDRs) by definition, are those responses, which are occurred
without any externally applied stimuli or motor activity. They are evoked due
to the occurrence of some kind of internal or subliminal stimulus (Bari, 2019, Boucsein, 2012a). According
to Boucsein et al., (Boucsein, 2012b) the amplitude for
NS.EDRs can be defined as 0.01 μS for computerized scoring and 0.05
μS for hand-scoring NS.EDRs records. In contrast, electrodermal responses that
are related to externally applied stimuli are called specific electrodermal
responses (Boucsein, 2012b).
The number of occurrences of NS.EDRs is termed frequency
of NS.EDRs. The frequency of NS.EDRs is appeared to be stable personality trait
(Schell et al., 1988). In
addition, it is suggested to be a reliable indicator of tonic activation.
Moreover, the frequency of NS.EDRs is a significant psychophysiological
variable that reflects individual differences, termed electrodermal lability (Vossel and Zimmer, 1990). Labiles are
individuals who exhibit high NS.EDRs frequency, whereas stabiles are those
individuals who show low NS.EDRs frequency (Crider and Lunn, 1971). This trait
view of NS.EDRs frequency is supported by some evidences. Such evidences include
counts of NS.EDRs, which have been shown to have moderate to high retest reliabilities
over intervals of up to one year.
There is also a genetic contribution to the variance in
NS.EDRs frequency. Finally, people who exhibit high NS.EDRs frequency have been
found to differ from those who exhibit low NS.EDRs frequency in behavioral
measures of attention and vigilance, as well as EEG (Boucsein, 2012a). Recently, it has been
shown that, NS.EDRs are mainly used to investigate individual differences like
personality,(Crider, 2008, Norris et al., 2007),
health vulnerability,(El‐Sheikh and Arsiwalla, 2011, El-Sheikh et al., 2007), and antisocial or aggressive behavior (Gatzke-Kopp et al., 2002).
Two quantitative parameters are important for analyzing
NS.EDRs, which are frequency and amplitude. Both parameters are investigated
for specific responses during the exposition of emotional or other specific
stimuli, for example (Bari et al., 2018, Lipp et al., 1998, Turpin and Siddle, 1979,
Goshvarpour et al., 2014). However, investigation of variations
in frequency and amplitude of NS.EDRs before and after specific stimuli
(responses) remain unclear. In addition, there are a limited number of studies
on NS.EDRs. For instance, an increase in the frequency of NS.EDRs during mental
load compared to rest was observed (Dawson et al., 2007, Nikula, 1991).
In another study, (Bari, 2019) both quantitative parameters of
NS.EDRs were investigated only during two resting periods (before and after a
stimulus), where a significant difference was obtained between parameters
recorded at two resting periods. In the current study, the experiment has been expanded
to cover more resting periods and after five different types of stimuli,
instead of only one stimulus. The target was to know the effects of each type
of stimulus on NS.EDRs, particularly on the frequency and amplitude of NS.EDRs,
which occurred at resting conditions after the specific responses recovered to
the initial reading before the specific response. These continuous
measurements of NS.EDRs parameters might be of physiological interest and be employed
to examine the electrodermal processes in detail.
The main goal of this investigation was to obtain a better
understanding of the NS.EDRs. The specific aim was to find out and analyze variations
in the frequency and amplitude of the three parameters of NS.EDRs, which were
NS.SCRs, NS.SPRs, and NS.SSRs at various resting periods (pre and post various
external stimuli) simultaneously at the same skin site. To do that a
non-invasive bioimpedance technique was utilized. The technique is dependent on
recording the three EDA parameters, skin conductance (SC), skin potential (SP),
and skin susceptance (SS) simultaneously at the same skin site.
2. RELATED WORKS
The studies conducted on NS.EDRs are rare. Only a few
studies in the literature focused on the analysis of NS.EDRs. For example, Boucsein
et al. (Boucsein,
2012b)
noted that NS.EDRs are short-lasting changes in EDA, which might occur in the
absence of obvious external stimuli. According to a study by Bernstein (
1979),
the frequency of NS.EDRs exhibited by a subject which can vary depending on the
conditions or setting in which the frequency is measured. The interplay between
the subject and the conditions or sessions can make changes in NS.EDRs
frequency and this frequency may reflect a trait-like characteristic of the
subject. Nikula (
1991)
investigated the association between cognitions and NS.EDRs. The author showed
that NS.EDRs may reflect arousal in the service of increased cognitive
capacity. Franz et al. (
2003)
recorded NS.SCRs and used to compare subjects with high alexithymic to
normal or low alexithymics during stimuli presentation. Authors reported that,
subjects with high alexithymic showed a decreased number of NS.SCRs under all
load conditions in contrast to low alexithymics. According to Dawson et al. (
2007), frequency of NS.EDRs increases during mental load compared to rest
period. NS.SCRs were quantified in a study conducted by Rachow et al. (
2011)
to investigate differences between normal and patients with acute
schizophrenia.
However, their results did not reveal a significant difference between
patients and controls. Bari (
2019)
compared variations in NS.EDRs at two different resting phases, before and
after a specific stimulus. The author found significant changes in NS.EDRs that
were recorded in the resting period following the specific stimulus. Gertler et
al. (
2020)
studied the neural correlates of NS.SCRs occurred during resting state fMRI
scan. They showed that the NS.SCRs are regulated by a wide cortical network of
brain regions that engage in a complex, seemingly biphasic manner. In a
controlled laboratory study, NS.SCRs were examined by Van Der Mee et al. (
2021)
during various conditions including mental stressors or physical activities to
predict changes in the sympathetic nervous system. Researchers suggested that
NS.SCRs frequency is useful to predicate changes in sympathetic activity during
daily life. Kurinec et al. (
2022)
examined NS.SCRs to predicate total sleep deprivation (TSD). Authors found that
NS.SCRs were sensitive to TSD, with significant systematic inter-individual
differences.
The above cited studies are focused on
recording only one parameter (NS.SCRs) and one resting period. In the current study,
variations in frequency and amplitude of three EDA (NS.SCRs, NS.SSRs and NS.SPRs)
parameters at various resting periods before and after five different external
stimuli are investigated with the aim of better understanding the waveforms of
the NS.EDRs and electrodermal processes in general.
3.
MATERIALS AND METHOD
A new technique of recording the
three EDA parameters simultaneously at the same skin site was used (Bari
et al., 2018). The technique was dependent on the computerized
system, which consisted of a small (matchbox-sized) front-end electronic box
connected by using a National Instruments DAQ card- NI USB-6211 to a PC laptop
running custom-written software in LabVIEW, v. 14. Bari et al.(
2018)
who used a three electrode setup, with one measuring electrode (M), one
reference electrode (R), and a current-sink electrode (C) . The C and R
electrodes were used for unipolar AC SC and SS measurements, whereas M and R
electrodes were utilized for SP measurement. The employed electrodes were
KendallTM KittyCatTM1050NPSM, which is shown to be reliable for EDA recordings (Tronstad
et al., 2010).
2.1 Participants and Measurements
EDA measurements were done on 20 apparently
healthy participants (10 males and 10 females, 21–39 years old, average age 29
yrs). All participants were recruited from the University of Zakho and gave
written informed consent. During the data collection, the participants were
sitting in an armchair in a silent laboratory with a temperature 22±2 °C.
The electrodes were affixed five minutes before the EDA recordings started so
that they would have time to stabilize. Also, this was done for the
participants to relax in order to get steady EDA recordings when the measurement
started. Then measurement of EDA parameters started and NS.SCRs, NS.SPRs and
NS.SSRs were computed at resting periods before and after five (asking a mathematical question, exposing to unwarned sound,
looking at a scary image, having a deep breath, and touching or mild clap on
the test subject’s shoulder) different stimuli. It should be
noted that, the duration for each resting period was 60 seconds. Throughout the
whole recordings, participants were asked to relax, and to keep the testing
hand calm.
2.2 Data Analysis
The Statistical Package for Social Sciences (SPSS) was used for data analysis.
The differences in the NS. EDRs were evaluated statistically in terms of different
resting periods by utilizing one-way repeated analysis of variation (ANOVA)
followed by post hoc multiple pairwise comparisons using Sidak correction.
Ethical approval
The protocol of this study has been complied with all the relevant national
regulations, institutional policies and in accordance with the tenets of the
Helsinki Declaration (Carlson
et al., 2004).
4. RESULTS
3.1 Frequency of Nonspecific Electrodermal Responses
3.1.1
Frequency of NS. SCRs
Table 1 shows the frequency of NS. SCRs obtained for each
subject before and after five stimuli. It is clear that almost all participants
gave a series of NS.SCRs. In addition, there is a high difference among
participants with respect to the frequency of NS.SCRs. Moreover, Figure 1 indicates
the frequency count of NS.SCRs for all subjects per resting periods. The
highest frequency (141) is recorded at the first resting period (i.e. before
stimuli), whereas the lowest responding frequency (89) is recorded at the final
resting period (i.e. after the touch stimulus). Furthermore, it can be seen
from the figure that the frequency of NS.SCRs are decreased with time (by
comparing the first resting period with the rest of the periods).
5. DISCUSSION
The present study was designed to
examine changes in the frequency and amplitude of NS.EDRs during a fixed
sequence of resting periods, which were administered to all subjects. This study demonstrated that the frequency of NS.EDRs (NS.SCRs, NS.SPRs, and NS.SSRs) recorded
at the first resting period (i.e. before stimuli) were more than those recorded
at the periods after stimuli. Based on the above
results, the frequency counts gradually decreased to the lowest value at the
last resting period, which was after the final stimulus (touch). On the other
hand, there were fluctuations in the amplitude of NS.SCRs, NS.SPRs, and NS.SSRs
recorded before and after stimuli.
Based on the findings of this study, NS.SCRs, NS.SPRs, and
NS.SSRs were highly sensitive to observe variations in the frequency of NS.EDRs
and accordingly changes in sympathetic system and individuals’ differences
(labiles and stabiles). There was a great variation among the participants according
to the frequency of NS.SCRs, NS.SPRs, and NS.SSRs.
The high frequency of NS.SCRs, NS.SPRs, and NS.SSRs of some subjects may be
due to the high existence and intensity of personally significant thoughts. On
the other hand, the low frequency of NS.SCRs, NS.SPRs, and NS.SSRs in other
subjects may be due to a general disengagement of goal-oriented cognitions (Nikula, 1991).
Notably, as mentioned above, the frequency of NS.SCRs,
NS.SPRs, and NS.SSRs at the resting period before stimuli were more (see Tables
1,2, and 3, and Figures 1, 2, and 3) than the frequency detected after stimuli
for the three parameters. This difference may be due to the fact that, at the beginning
of the experiment the participants were under a higher level of stress than the
rest periods of the experiments. In addition, with time, the level of stress
decreased for all participants, which led to a noticeable reduction in the frequency
of NS.SCRs, NS.SPRs, and NS.SSRs at the last resting period (after touch
stimulus). According to Bernstein, ( 1973) , the status of an individual
in terms of NS.EDRs frequency is dependent on the occasion or conditions under
which the frequency index is recorded, then the interaction of individuals and
condition (e.g. stressful) or individuals and session should be extensive
contributors to variations in NS.EDRs frequency, and this frequency might
reflect a trait-like characteristic of subjects.
According to the study findings, there were differences among
the detected frequencies of the three parameters (NS.SCRs, NS.SPRs, and
NS.SSRs). By comparing the results shown in Figures 1, 2, and 3, it is evident
that the number of detected NS.SPRs was higher than both NS.SCRs and NS.SSRs. This
suggests that SP is more sensitive than both SC and SS for detecting NS.EDRs. In
addition, the frequency counts of NS.SSRs recorded from all the test subjects
were very small compared to both NS.SCRs and NS.SPRs, and even NS.SSRs were not
seen from some participants at all as revealed in Table 3. This suggests that
NS.SSRs are not sensitive to detect variations in NS.EDRs in particular small
responses, indicating that the role of SS is less crucial in the analysis of
NS.EDRs compared to the SC and SP (Bari, 2019).
Variations in amplitudes of NS.SCRs, NS.SPRs, and NS.SSRs
were also observed during different resting periods. In addition, amplitudes of
NS.SCRs, NS.SPRs, and NS.SSRs decreased during resting periods after all
stimuli (as seen in Figures 4, 5, and 6) in line with (Bari, 2019); except after sound stimuli,
which were higher before stimuli. The occurrence of higher amplitudes of
NS.SCRs, NS.SPRs, and NS.SSRs after sound stimulus could be due to the prior
level of sweat in the duct as a result of the high intensity of sound. Also, it
might be due to sympathetic overactivity throughout resting periods meaning a potential
delayed inhibitory process of sympathetic activity (Visnovcova et al., 2016). The
inhibitory and excitatory influences of sympathetic nervous system activity,
which affect the central regulation of EDA arise in different structures of the
central nervous system. Boucsein et al. ( 2012a) explained two relatively
independent pathways which affected the production of sweat and accordingly EDA:
Cortical level- central regulation of EDA associated with the affection of basal
ganglia and cortical centers as brain structures, (Sequeira and Roy, 1993) and
subcortical level-EDA regulation involves influences from the limbic system and
hypothalamus connected with thermoregulatory sweating. Moreover, the amygdala
mediates the excitatory influences of EDA, while the hippocampus affects the
inhibitory influence (Visnovcova et al., 2016).
The rise time or time parameter of NS.SCRs is also changed
in resting periods. Figure 7 shows that rise time of NS.SCRs decreased in the
resting periods following the five external stimuli. The shorter rise time is
associated with the weaker NS.SCRs, which is attributed to the same reasons
discussed above. These findings are in agreement with Bari (20219), who also showed
that the rise time of the NS.SCRs decreased in the resting time after an
external stimulus.
CONCLUSION
The three parameters of NS.EDRs (NS.SCRs, NS.SPRs, and
NS.SSRs) showed variations in the frequency and amplitude of NS.EDRs. NS.SPRs
were more sensitive to detect or respond to slight changes in EDA waveforms
compared to other two parameters. On average subjects showed a higher frequency
and lower amplitude of NS.EDRs at the first resting period (i.e. before
stimuli), than those recorded at resting times after the stimuli. This
suggests that prior level of sweat in the duct due to
external stimuli has impacts on the frequency and amplitude of NS.EDRs. NS.EDRs
due to their sensitivity may be used in clinical applications for monitoring
arousal, emotional behavior, psychophysiological variables, and goal-directed
thinking.
In future studies, a wider age range can be targeted
with a larger sample size to further investigate NS.EDRs.
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