Table 4.

Summary of outcomes.

Study	Study design	Time points measured	Outcome	RoB
Bak et al. (2016)	CT	Baseline, a week later, and 9 months follow-up	Significant improvement in attentional switching (subtest 3) after just 1 week of SL training compared to passive controls, and maintenance of scores if Gaelic was practiced more than 5 h per week at follow-up 9 months later. All Gaelic learners showed significant improvement, regardless of age group.	7/14 Fair
Berggren et al. (2020)	RCT	Baseline and 11 weeks later	No significant differences were found between the SL group and the relaxation group, even though the SL group did demonstrate success in learning Italian vocabulary.	8/14 Fair
Bubbico et al. (2019)	RCT	Baseline and 4 months later	The MMS scores of the SL group remained stable, but the control group's scores significantly decreased. No other significant differences in cognition were found, although the SL group's functional connectivity increased in the right inferior and superior frontal gyrus, as well as in the left superior parietal lobule.	7/14 Fair
Klimova et al. (2020)	RCT	Baseline and 12 weeks later	Increases in MoCA scores were revealed in some of the participants in the SL group, yet they did not meet significance. Yet, decreases in scores in the SL group were also revealed, as well as increases in scores of the control group.	6/14 Fair
Long et al. (2019)	No CG	Baseline, and a week later	The TEA tests were found to be significantly correlated to Gaelic level. After the course, the beginner group evidenced the most improvement in TEA scores. Comparisons were made for different age groups, and all groups improved significantly in attentional switching and inhibition subtests 2 and 3.	5/11 Fair
Pfenninger and Polz (2018)	No CG	Baseline and 4 weeks later	Significantly improved Stroop and A-K-T scores for monolinguals, and significantly less errors on the language proficiency C-test. Qualitative results showed a positive effect on social life, improved memory, and boosted well-being.	8/11 Fair
Ramos et al. (2017)	CT	Baseline and 8 months later	No significant increases in switching ability among the intervention group were found in their test designed for the study.	3/14 Poor
Ware et al. (2017)	No CG	Baseline and 4 months later	No significant differences in MoCa and UCLA between pre-and post-intervention.	6/11 Fair
Wong et al. (2019)	RCT	Baseline and 6 months later and 3-month follow-up	Significant improvement was found for the active intervention groups, with the English group significantly improving in the ADAS-Cog, the auditory reading span, and the backward digit span tests. Yet the ANT, the forward digit span test, and the Simon task scores did not reach significance.	8/14 Fair

For the quality assessment scores, it should be noted that the studies without control groups were scored over 11 points, while those with control groups were assessed with a 14-point scale. CT, Controlled trial; RCT, randomized controlled trial; CG, control group; SL, second language; MMS, Mini Mental State; TEA, Test of Everyday Attention; MoCA, Montreal Cognitive Assessment; ADAS-Cog, Alzheimer's Disease Assessment Scale -Cognitive Subscale; ANT, Attention Network Test; BMT, Babcock Memory Test; FAB, Frontal Assessment Battery; VFT, Verbal Fluency Test; A-K-T, Geriatric Concentration Test; UCLA, University of California Loneliness Assessment.