2.2.2. Questionnaires

The following questionnaires (Figure 2) were used to collect psychological variables: Beck Depression Inventory—II (BDI—II) to determine the existence and severity of depression symptoms [29,30];

State—Trait Anxiety Inventory (STAI) to evaluate anxiety as a temporary state (state anxiety) or as a personal characteristic (trait anxiety) [31,32];

Emotion Regulation Questionnaire (ERQ) to separately assess cognitive reappraisal or regulation prior to an emotional experience and expressive suppression after an emotional experience [33,34];

Positive and Negative Affect Schedule (PANAS) to evaluate the subject's emotional recognition, as a positive or negative affect, either as a trait or a state (trait or state positive affect, trait or state negative affect) [35,36].

**Figure 2.** Methodological scheme.

#### *2.3. Methods*

Antidromic stimulation of the right median nerve was performed by applying an electrical stimulus to the anterior flexure of the wrist and collecting the sensory response in the second finger of the hand. The position of the recording, reference, ground, and stimulation electrodes remained unchanged throughout the procedure in order to achieve a reliable and reproducible recording [12,16,37–39].

The characteristics of the study and the subjective parameters to be collected were explained to all subjects. The following subjective responses reported by the subject were evaluated: the sensory perception threshold (SPT) or intensity at which the subject first recognized the applied stimulus and the pain perception threshold (PPT) or intensity at which the subject begins to recognize the stimulus as painful. The following objective responses observed during electrophysiology tests were evaluated: initial sensory response (ISR) or intensity at which the start of a detectable SNAP is observed and supramaximal response (SMR) or intensity at which the SNAP reaches its peak amplitude (Figure 2).

The intensity differences between thresholds (IDT) were determined in the following parameters: SPT-PPT, ISR-SMR, and SMR-PPT (Figure 2).

The electroneurographic parameters sensory nerve conduction velocity (SNCV) and SNAP amplitude were measured in m/s and μV, respectively, to determine normality according to the reference values (Figure 2).

The study was performed with the subject at rest, lying on an examination table (in position at least 15 min before the examination). The room temperature was maintained at 23–25 ◦C, and the skin temperature, measured with a temperature probe held by the patient, was between 30 and 32 ◦C. The electrical stimulus consisted of a quadrangular pulse lasting 0.1 ms, which was increased in increments of 1 mA until the perception threshold was reached and the initial SNAP obtained, after which it was increased in 2 mA increments until the remaining thresholds and responses had been obtained. The forced choice method was used to correctly measure the SPT and PPT, increasing and decreasing the intensity on at least 3 occasions to pinpoint the exact intensity generated by the sensation evaluated [40,41]. To determine ISR and SMR, 5 μV gains per division were used. ISR was defined as the minimum intensity needed to obtain a detectable SNAP without amplification. SMR was defined as the intensity needed to obtain an SNAP whose amplitude did not increase after applying a stimulus that was 20% more intense than the threshold. The stimuli were applied at intervals of between 25 and 55 s to avoid the expectation phenomenon [16].

#### *2.4. Statistical Analysis*

Data from the experimental and control groups were analyzed descriptively. The quantitative, psychological, and electrophysiological variables were expressed as mean and standard deviation (SD). The Kolmogorov–Smirnov normality test was used to test the normality of the distribution of quantitative data, and the Levene test was used to verify the equality of variances of all levels of each factor. Following this, the psychological and electrophysiological variables from both groups (CTTH and HC) were compared using the independent samples *t*-test in the case of normally distributed quantitative variables and the Mann–Whitney U test in the case of non-normally distributed variables.

Pearson's linear correlation coefficient was used to test for linear correlations between electrophysiological variables in the experimental and control groups separately. The linear correlation coefficients between psychological and electrophysiological variables were also calculated in each group.

Logistic regression analysis was performed to determine the capacity of the most sensitive electrophysiological variables to classify the data. The default variable input method was used. The effect was controlled by age and gender, taking age and sex as independent variables and SPT, ISR, and SMR as dependent variables and analyzing the relationship between each variable using the Pearson chi-squared goodness-of-fit test.

The diagnostic accuracy of electrophysiological variables that differed significantly between the HC and CTTH groups was evaluated with Receiver Operating Characteristic (ROC) curves, using cut-off points with the highest specificity and sensitivity.

Statistical analysis was performed on IBM SPPS Statistics v.27 (IBM, Armonk, NY, USA). A 95% confidence interval was used in all tests, and significance was set at *p* < 0.05.

#### **3. Results**
