Habituation of Brain Responses to Sensory Stimuli in Fibromyalgia

The goal of this observational study is to assess event-related potentials and habituation in patients with fibromyalgia. The main research questions are:

* Is there a measurable difference in sensory responses between fibromyalgia patients and healthy control?
* Could these measures provide evidence supporting claims of hypersensitivity?

Participants will:

- receive randomized sensory stimuli (auditory, visual, somatosensory, audiovisual, auditory-somatosensory, visual-somatosensory, and auditory-visual-somatosensory) in blocks of 20 trials.

Fibromyalgia is a disorder characterized primarily by widespread musculoskeletal pain. It is often accompanied by symptoms such as fatigue, insomnia, cognitive difficulties, etc. In addition to chronic pain, patients frequently report hypersensitivity to sensory stimuli, both nociceptive and non-nociceptive. Studies using psychophysical and electrophysiological tests have found altered sensory processing in fibromyalgia patients (Lim et al., 2016; McDermid et al., 1996; Staud et al., 2021).

Research focused on event-related potentials (ERPs) have found differences in amplitude in the responses to auditory, nociceptive, and non-nociceptive somatosensory stimuli in fibromyalgia patients (Alanoğlu et al., 2005; De Tommaso et al., 2011; Lorenz, 1998; Montoya et al., 2006). One key aspect evaluated through ERPs is habituation, defined as a decrease in response due to repeated stimulation (Thompson \& Spencer, 1966). In healthy individuals, ERPs amplitude decreases with repeated sensory input due to progressive neuronal response reduction. In fibromyalgia patients, reduced habituation has been observed for laser-induced ERPs (De Tommaso et al., 2011) and somatosensory ERPs (Montoya et al., 2006), though no differences were found for auditory ERPs.

ERPs not only provide information about sensory modalities but also contain non-specific components related to stimulus expectation, motor preparation, and attentional orientation (Gondan \& Röder, 2006). Previous studies have attempted to isolate this non-specific component, as part of habituation may stem from a decrease in its amplitude rather than from a direct sensory response reduction (Young et al., 2022). Using an analytical model, researchers successfully separated this component in experiments with individually or simultaneously presented stimuli, yielding promising results.

In the present study, participants (both fibromyalgia patients and healthy controls) will attend one experimental session in which stimulation in different sensory modalities will take place along with electroencephalographic (EEG) recordings. For each stimulus type, 2 blocks of 20 trials will be performed.The order of the stimulus will be selected randomly, but they can be divided into three categories:

* Unimodal stimulus

  * Auditory (A): a 1000 Hz tone delivered at a comfortable volume through a speaker
  * Visual (V): a checkerboard pattern, inverted every 25 ms, presented on a computer screen.
  * Somatosensory (S): a train of 3 electrical stimuli that elicits a pricking sensation.
* Bimodal stimulus

  * Auditory-Visual (AV)
  * Auditivo-Somatosensory (AS)
  * Visual-Somatosensory (VS)
* Trimodal Stimulus

  * Auditory-Visual-Somatosensory (AVS) The aim of the study is to evaluate neurophysiological parameters related to habituation in multimodal sensory stimulation environments to characterize ERPs in fibromyalgia patients.

Eligibility Criteria:
Inclusion Criteria:

Fibromyalgia patients:

* Primary diagnosis of fibromyalgia according to the criteria used by the healthcare professional in charge of the patient.
* Other comorbidities are accepted as long as the pain caused by them is less severe than the pain caused by fibromyalgia.
* Willingness and ability to fully understand the content and scope of the experiment and to comply with the experiment instructions.
* Normal or corrected-to-normal vision.
* Normal hearing

Healthy controls:

* No history of neurological diseases, chronic pain or musculoskeletal disorders.
* Willingness and ability to fully understand the content and scope of the experiment and to comply with the experiment instructions.
* Normal or corrected-to-normal vision.
* Normal hearing

Exclusion Criteria:

Fibromyalgia patients:

* Pregnancy
* History of addictive behavior, defined as alcohol, cannabis, opioids or other drugs abuse.
* Presence of fever, tuberculosis, malignant tumors, infectious processes, acute inflammatory processes.
* Lack of cooperation

Healthy controls:

* Pregnancy
* History of chronic pain or musculoskeletal or articular disorders
* History of addictive behavior, defined as alcohol, cannabis, opioids or other drugs abuse.
* Presence of fever, tuberculosis, malignant tumors, infectious processes, acute inflammatory processes.
* Lack of cooperation