Background: Functional electrical stimulation (FES) improves unilateral spatial neglect (USN). However, only few studies have reported on the relationship between FES and cognitive function. Therefore, this study aimed to investigate the effects of FES on attentional function in healthy participants using the Posner task, which is generally used to assess patients with USN.

Methods: In this interventional study, we conducted experiments on healthy participants to obtain fundamental knowledge before investigating effective ways of adapting FES to patients with USN as our final purpose. A total of 28 right-handed healthy young participants were assigned to the online-effect group or after-effect group and underwent FES simultaneously with or immediately before the Posner task, respectively. FES was applied to the extensor muscles of the left forearm to induce involuntary movement.

Results: A significant difference was observed in the effects of FES in the Posner task between the online-effect group or after-effect group in the left-valid conditions (online-effect, 26.7 ms delay; after-effect, -0.4 ms [reduction]; p = 0.01). No interaction was noted between the groups and the type of target of the Posner task presentation condition. The effects of FES differed between the online- and after-effect condition groups.

Conclusion: Performing the attention task after FES may enhance attention compared with performing the task with FES simultaneously.