# Spectral and Nonlinear Analysis of Electrodermal Activity in Adolescent Anorexia Nervous
## 1 Introduction  
Anorexia nervosa (AN) is one of the most frequent eating disorders with the highest morbidity and mortality among intake malfunctions [1; 2]. Patients with AN are characterised by low weight as a result of the restriction of food intake, fear of weight gain, and distortion of self-image, including fear of being or becoming fat [3; 4]. Rates of lifetime prevalence of AN range from 1.4% to 3.3% inwomen [5]. AN mainly presents in mid- to late adolescence, although its occurrence in childhood and early adolescence is increasing [6]. In general, the autonomic nervous system (ANS) plays a crucial role in the maintenance of the homeostasis, flexibility and adaptability of the human body. It is assumed that disturbances of the ANS may participate in underlying mechanisms of cardiovascular disorders associated with AN [7]. For example, several studies revealed lower heart rate associated with higher heart rate variability indicating cardiovascular dominance, and sympathetic vascular underactivity indexed by lower blood pressure variability [7; 8; 9; 10; 11]. However, studies regarding other biosignals in AN are scarce. Electrodermal activity (EDA) represents a non-invasive index of sympathetic cholinergic activity. Specifically, changes in sympathetic activity lead to altered activity of ecrine sweat glands, which is related to skin's ability to conduct electricity and thus to EDA variations [12; 13]. Moreover, the ecrine sweat glands are involved in thermoregulation of organisms, and cover the majority of the organism, but are the densest on palms and soles. Indeed, these localizations are mainly used in psychophysiological research, because it is assumed that they are associated with emotion-evoked sweating [13]. From the physiological aspect, the skin is relatively non-conductive, due to its most upper layer of derma--the _stratum corneum_. A variable amount of sweat is released from the ducts of the ecrine sweat glands, which open onto the skin surface. The degree of sympathetic activation affects the amount of filled sweat ducts, and the amount of produced sweat; in turn, this activation influences the degree of skin conduction. Variations of the amount of sweat in the ducts relates to the conductivity of the skin and its measurable changes in EDA [14]. Additionally, [PERSON] et al. (2016) revealed that variation in the dynamics of the sympathetic activity evaluated by EDA represent one of the important prognostic and diagnostic tools in diseases associated with autonomic dysfunction [15]. With respect to methodology, [PERSON] et al. (2012) evaluated changes of EDA by standard time and frequency indices and nonlinear parameters, such as deterministic chaos, recurrence plot, and detrended fluctuation analysis in the healthy group during exposure of sets of images from the International Affective Picture System (IAPS) [16]. This study revealed that nonlinear indices of EDA contribute to better extraction of EDA features, such as characteristics of sympathetic activity without significant differences in two types of used measuring devices (special textile gloves vs. bipolar electrodes) [16]. Our previous study evaluated the difference between standard and nonlinear parameters of EDA (approximate and symbolic information entropy) after cognitive stressors (recovery phase) in healthy young adults. Specifically, whereas skin conductance amplitude remained increased in the recovery phase compared to the baseline period, nonlinear indices decreased under the baseline value. It is assumed that this could be caused by complex mechanisms consisting of feedback loops forming a chaotic system on the central level [17]. Recently, evaluation of EDA in AN has been attracting more attention from scientists. Several studies found a lower amplitude of skin conductance response (SCR) in adult females suffering from AN versus age-matched healthy control (HC) in resting conditions [18; 19; 20]. Similarly, [PERSON] et al.