Fig. 4

A subgroup analysis for the effects of different BCI mental tasks. The 12 included studies were categorized into motor imagery-based BCIs (eight studies), movement attempt-based BCIs (two studies), and action observation-based BCIs (two studies), depending on the nature of the mental tasks. The results indicted that both movement attempt-based (SMD = 0.69; 95% CI = 0.16–1.22; I² = 0%; P = 0.010; random-effects model) and action observation-based BCIs (SMD = 1.25; 95% CI = 0.0.05–2.45; I² = 72%; P = 0.040; random-effects model) tended to show superior clinical effects, compared to MI-based BCIs (SMD = 0.16; 95% CI = − 0.13 – 0.45; I² = 0%; P = 0.290; random-effects model) in regard to improving upper extremity function. However, the difference among subgroups was not significant (P = 0.070)