*3.1. Participant Characteristics*

A total of 31 patients diagnosed with FMS were allocated to the probiotic or placebo group (Figure 1). Sociodemographic variables are shown in Table 1, which describe the sample that participated in the study. No statistically significant differences in any of the variables between either group (*p* > 0.05) were observed.

**Table 1.** Sociodemographic characteristics of study population.


**Figure 1.** Flow diagram of the progress through the phases of the pilot parallel randomized trial of two groups.

> *3.2. Performance on Cognitive Task*

3.2.1. Memory Task

> Digit Task

For each participant, the memory span (or capacity) score was calculated based on the longest sequence that was correctly remembered, forward and reverse (see Table 2), adding the corresponding score to all sequences answered correctly (two points were awarded when the two attempts of the sequence were reproduced correctly and one point when only one of them was remembered). These data were analyzed using an analysis of variance with one inter-subject manipulated factor, group (placebo, probiotic), and two within-subject manipulated factors, order (forward and reverse) and treatment (pre-, post-). No effect or interaction was statistically significant (*p* > 0.05).

**Table 2.** Span of memory expressed by mean and standard error.


Corsi Task

Span (or capacity) memory was calculated based on the longest sequence that each participant recalled correctly, directly and inversely, in at least one of the two sequences (Table 3). The average median RTs of the sequences correctly reproduced in forward and reverse order was also calculated (Table 4). These data were analyzed using analysis of variance with one factor manipulated between subjects, group (placebo, probiotic), and two factors manipulated within subjects, order (forward and reverse) and treatment (pre-, post-). No effect or interaction was statistically significant (*p* > 0.05).


**Table 3.** Span of memory expressed by mean and standard error.

**Table 4.** The average median of Reaction Times (RTs) expressed by mean and standard error.


## 3.2.2. Attention Task

Go/No-Go Task

In this task, we analyzed the error rate for the Go conditions, or errors of omission trials, the percentage of errors in the No-Go conditions, or errors of commission, and the average of the medians of the RTs obtained in the Go trials (Table 5) by participants in both groups. These data were analyzed using analysis of variance with one factor manipulated between subjects, group (placebo, probiotic), and one factor manipulated within subjects, treatment (pre-, post-). The ANOVA of errors of omission showed a marginal effect of the interaction group x treatment (F1, 24 = 3.62; *p* = 0.069). Furthermore, a marginal effect of group (F1, 24 = 3.56; *p* = 0.071) was observed post-treatment in the Go condition (Figure 2). No other effect or interaction was statistically significant (*p* > 0.05).

**Table 5.** The average median of RTs expressed by mean and standard error.


**Figure 2.** Percentage of omission errors (Go Errors) and of commission (No Go Errors) committed by participants in both groups depending on the treatment. Error bars represent the standard error of the mean. \* marginal effect of the interaction group x treatment.
