**3. Results**

#### *3.1. Literature Search and Study Characteristics*

After a comprehensive search, 68 relevant articles were identified in the databases. After eliminating duplicates, a total of 36 articles were selected for this review. The PRISMA flow diagram (for details, see Section 2.1. Search strategy) shows the exclusion of studies at each screening stage (Figure 1). Finally, 15 full-text articles were included; they were checked for suitability according to the predefined inclusion criteria, and then subjected to data extraction (Table 1) and quality assessment.

Regarding the characteristics of the selected studies, the year of publication rangedfrom 1989 to 2021. Most of the studies included a control group of healthy participants [2,7,9,25,26,28,73–78], although two were uncontrolled clinical trials [1,28]. The location of the studies varied widely: 11 were conducted in Europe (Spain [25,26], United Kingdom [69,76,78], Sweden [9], Germany [7], Austria [2], Netherlands [28], and Belgium [1]), 1 in the United States [77], and 4 in Canada [27,73–75]. Further details of the characteristics of the selected studies can be found in Table 1.

In terms of the study designs, an experimental design was used in the majority of cases [1,2,25–28,69,74,75,77,78], although two studies used a factorial design [9,76], one used a case-control design [7], and one used a pilot randomized controlled trial design [73].

Regarding the results of the studies (Table 1) related to performance on the emotional Stroop task, longer reaction times and delayed responses to negative emotional words (associated with pain) were observed in patients with chronic pain, especially those with FMS [7,9,25,26,74–78]. Most studies revealed an emotional interference effect in FMS patients [25,79,80]. Greater processing of negative and/or positive words was also observed in patients with FMS [79,81]. Moreover, greater responses were observed in regions related to pain and emotional regulation (the somatosensory region, and the cingulate and prefrontal cortices, among other regions) compared to healthy controls [27,69,73]. Specifically, in the presence of negative stimuli with emotional content, patients with chronic pain showed greater activation in the aforementioned brain regions, indicating greater processing of pain and negative emotions in these patients [27,69,73]. Previous evidence indicates that patients with CLBP and FMS have an attentional bias towards negative words with emotional content [1,2,9,74].
