The transcranial direct current stimulation (tDCS) protocol will involve the application of a continuous current for 30 minutes, with an intensity ranging from 1 to 1.5 mA, targeting the left dorsolateral prefrontal cortex (DLPFC) during each session. Two saline-soaked sponge electrodes, each measuring 35 cm², will be employed to ensure adequate skin contact. In accordance with the international 10-20 EEG electrode placement system, the anodal electrode will be positioned at the F3 location, specifically targeting the left DLPFC. The reference electrode will be placed above the left eye to facilitate efficient current flow through the prefrontal region of the brain. To enhance both safety and efficacy, a 30-second ramp-up and ramp-down period will be incorporated at the commencement and conclusion of the stimulation session. This methodology is intended to minimize potential adverse effects and ensure an optimal stimulation experience for participants.

The device will be configured to produce an upward gradient for 30 seconds, identical to that used in the experimental group, followed by a downward gradient for another 30 seconds. Consequently, the control group will feel a similar tingling on their scalp as the experimental group. This stimulation will occur for a total of 60 seconds, which is insufficient to induce changes in cortical excitability. Studies have demonstrated that this approach effectively ensures patient blinding.

Catastrophizing is understood as a set of negative emotional and cognitive processes that, by themselves, lead to an increase in pain symptoms and fatigue. It also involves a perceived helplessness in coping with pain and an inability to disengage from pain-related thoughts, whether in anticipation or recollection of the painful experience. Catastrophizing comprises three components: rumination, which refers to persistent worry and the inability to suppress thoughts related to pain; magnification, the tendency to exaggerate the unpleasantness of pain and amplify negative thoughts about pain relief; and helplessness, which arises from the perceived inability to cope with painful situations.

To measure the level of catastrophizing in the present study, the Spanish validated version of the Pain Catastrophizing Scale (PCS) was used. This version demonstrates an internal consistency of 0.79 and a test-retest reliability of 0.84. The PCS is a self-administered questionnaire consisting of 1

The SF-12 measures Health-Related Quality of Life (HRQoL) and comprises several dimensions: (a) physical functioning, (b) physical role, (c) emotional role, (d) social functioning, (e) bodily pain, and (f) vitality. Additionally, it includes two overarching dimensions that encompass the previous ones: (a) general health and (b) mental health. It is a widely used instrument in the healthcare field and has demonstrated satisfactory psychometric properties, including internal consistency and test-retest reliability.

The VAS is considered the most representative scale and is regarded as the best option due to its simplicity and ease of use. The version employed in this study ranges from 0 to 10. This scale provides an effective tool for subjectively and discriminatively quantifying pain intensity. A score of 0 indicates no pain as reported by the individual, while 10 represents the worst imaginable pain. Due to these characteristics, the VAS has shown greater utility compared to descriptive or fixed-value scales. The VAS has demonstrated high reliability (Cronbach's α = 0.97) [95% CI = 0.96 to 0.98].

The DN4 is one of the most sensitive questionnaires for diagnosing neuropathic pain, demonstrating a high capacity to distinguish between neuropathic and nociceptive pain. The DN4 has been validated in patients with low back pain (LBP) due to disc herniation, spinal stenosis, degenerative disc disease, spinal pain, lumbar spine degeneration, spinal surgery, and lumbar scoliosis. A DN4 cutoff score of 4 yielded a sensitivity of 80% and a specificity of 92% for the diagnosis of neuropathic pain in individuals with LBP. Each patient was asked to describe their pain using the first seven descriptors of neuropathic pain in the DN4, in order to calculate the DN4 interview subscore (range: 0-7).

The Beck Depression Inventory - Second Edition (BDI-II) is a self-report instrument consisting of 21 items used to measure the severity of depression in adults and adolescents over the age of 13. This version of the inventory (BDI-II) was developed to assess symptoms corresponding to the criteria for diagnosing depressive disorders outlined in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV; 1994)

For this reason, a scale was designed to evaluate self-efficacy expectations regarding symptom management, physical functioning, and pain control. The Chronic Pain Self-Efficacy Questionnaire by Martín-Aragón et al. (1998), validated in Spanish, was used, as its validation results are comparable to those of the original scale by Anderson et al. (1995). This version demonstrates high internal consistency (α = 0.87). The questionnaire consists of 19 items, uses a Likert-type scale, and has a response range from 0 to 10, where 0 represents feeling completely incapable, 5 moderately capable, and 10 fully capable.