Table 3.
Summary of all studies reviewed, listing respective cognitive assessments including the timepoints of the assessment, exact measures, and main outcomes.
| References | Cognitive assessment (to evaluate tDCS effects on cognition) | ||
|---|---|---|---|
| Timepoints | Measures* | Outcomes | |
| Im et al. (2019) | Baseline and after 6 months of treatment | MMSE, CDR, neurological test battery (digit span test, BNT, RCFT with immediate and delayed recall and recognition, clock drawing test, SVLT with immediate and delayed recall and recognition, contrasting program, Go-no go test, COWAT, Stroop word and color reading) | • MMSE and BNT scores significantly improved after active tDCS compared to sham • Active tDCS resulted in consistent performance (at lower score levels) in contrasting program and Stroop word reading while scores decreased for sham |
| Sandrini et al. (2016) | Learning performance, recall after 48 h and recall after 1 month | Learning and recall of a list of 20 words | • Significant effect for recall after 48 h: Active tDCS group recalled significantly more words compared to sham • No significant differences after 1 month |
| Meinzer et al. (2015) | During stimulation (and fMRI) | Semantic word retrieval task | • tDCS significantly improved semantic word-retrieval performance in the patients to the level of controls |
| Yun et al. (2016) | Baseline and after 3-weeks of treatment | MMQ (MMQ-C, MMQ-A, MMQ-S) | • MMQ-C significantly increased after active tDCS compared to sham • Results for MMQ-A were similar to MMQ-C results but not significant between the active and sham groups • No significant difference for MMQ-S between active tDCS and sham |
| Fiori et al. (2017) | During word retrieval: 1–10 presentations for each picture-word pair (T1-T10) | Training, verification, and word retrieval of 20 pseudoword-picture associations (bisyllabic pseudowords) | • Bihemispheric: higher number of correct responses in the old group during T10 compared to T1 compared to unihemispheric and sham condition • No differences between the three conditions in the young age group • During T10 the young group was significantly more accurate than the old group for unihemispheric and sham; no significant difference in the bihemispehric condition • Same results for vocal reaction times |
| Westerberg et al. (2015) | Before a 90-min nap and 30 min after | Two declarative memory tests (word-pair recall, fast recognition test), 1 non-declarative test (object-priming test) | • Recall improvement from pre-nap to post-nap was significantly larger for active so-tDCS compared to sham • No significant fast recognition or object priming performance difference between active and sham so-tDCS after the nap (both increased significantly) |
| Cotelli et al. (2014) | Baseline (T0), after 2 weeks of treatment (T1), after 3 months (T2), after 6 months (T3) | Face-name association task (FNAT), neuropsychological tests (picture naming task, BADA, Rivermead behavioral memory test, Rey auditory verbal learning test, Rey-Osterrieth test, complex figure copy, TMT A and B) | • FNAT: active or sham tDCS + memory training group showed significantly improved performances compared to active tDCS + motor training group at T1 and similar for T2, at T3 sham + memory training was still significant compared to the other two groups • No differences in neuropsychological tests (except an improvement for both tDCS and sham + memory training at T3 in the TMT A score) |
| Bystad et al. (2016) | Before and after stimulation | Primary: immediate and delayed recall and recognition of CVLT-II Secondary: MMSE, clock drawing test, TMT A and B |
• CVLT-II: no significant differences between active and sham tDCS but a tendency toward higher improvement in CVLT-II recall after active tDCS • No significant differences for secondary outcome measures |
| Lu et al. (2019) | Baseline (T0), after 4 weeks of treatment (T1), 4 weeks after post-intervention (T2), 8 weeks after post-intervention (T3) | Primary: WM test (RT), ADAS-Cog Secondary: CVFT, TMT, Chinese neuropsychiatric inventory (CNIP) |
• ADAS-Cog: significant improvement for all groups at T1, but no difference between groups, tendency of falling back to baseline at T2 and T3 for all groups • WM test: significant improvement for all groups until T3, tDCS+WMT showed highest WM capacity at T1 compared to other groups • CVFT: tDCS-WMT showed a greater improvement in delayed recall compared to single-modality interventions; at T3 only the tDCS+WMT group showed significant enhancement on delayed recall performance over baseline • tDCS-WMT group showed better performance of logical memory at 12th week |
| Meinzer et al. (2013) | During stimulation (and fMRI) | Overt semantic word generation task | • During sham younger adults produced significantly less errors than elderly • Older subjects produced significantly less errors during active tDCS in comparison to sham |
| • Response times (RTs) were comparable between young and old subjects during sham; no difference in RTs for elderly during active tDCS compared to sham | |||
| Khedr et al. (2014) | Baseline (T0), after 10 days of treatment (T1), after 1 month (T2), after 2 months (T3) | MMSE, WAIS-III (verbal comprehension, arithmetic and digit span, perceptual organization, processing speed) | • MMSE: significant improvement in anodal and cathodal tDCS compared to sham (increase of nearly 2 points at T1 and further increase of 2 points at T2 and T3); anodal tDCS group showed better improvement in orientation, registration, attention, and naming object compared to cathodal tDCS • WAIS-III: only cathodal and not anodal tDCS showed improved performance IQ compared to sham |
| Prehn et al. (2017) | Immediate recall, delayed recall after 6 h, 1 day later and 1 week later | Object-location learning task (LOCATO), primary outcome: immediate recall, secondary outcome: delayed recall | • Significant effect of SSRI but not of stimulation on immediate recall scores • Young and old group profited most from atDCS+SSRI • No significant effects on delayed recall |
| Manenti et al. (2017) | Baseline (after learning), free recall and recognition 48 h and 30 days after learning (and tDCS) | Learning, recall, and recognition of a list of 20 words | • Significant difference on hit-false alarms score between atDCS and sham at day 30, anodal tDCS significantly improved memory recognition on day 30 • atDCS and sham group showed similar free recall performance at day 30 |
| Ferrucci et al. (2008) | Baseline and 30 min after stimulation | Word recognition task (WRT), visual attention task | • atDCS improved WRT accuracy, while ctDCS significantly worsened it, sham left it unchanged; same results for DI (derived by subtracting false positive from true positive responses) • No significant differences in RTs in the visual attention task for atDCS or ctDCS compared to sham |
| Boggio et al. (2012) | Baseline (T0), at the end of treatment day 5 (T1), 1 week later (T2), 4 weeks later (T3) | MMSE, ADAS-Cog, visual recognition task (VRT), visual attention task (VAT) | • No significant effects for MMSE, ADAS-Cog, and VAT scores between active and sham tDCS • VRT: significant main effect for tDCS performance changes from baseline: 8.99% after anodal and 2.62% after sham tDCS (for T1, T2 and T3) |
| Cruz Gonzalez et al. (2018) | Screening, baseline (after CS training), after sham+CS, after tDCS+CS, post assessment (after CS) | Cognitive stimulation (planning ability and divided attention, processing speed and selective attention, short-term memory, calculation and WM), CDR, MoCA | • Enhanced cognitive performance in processing speed, selective attention, WM activities, completion time in planning ability and divided attention tasks for active tDCS compared to sham • Variable CS outcomes but subjects did not show significantly better outcomes in sham intervention compared to baseline CS |
The only two studies that did not reveal significant effects are highlighted in gray. Cognitive measures:
*
MMSE, Minimal Mental State Examination; CDR, Clinical Dementia Rating (Morris, 1993); BNT, Boston Naming Test; RCFT, Rey Complex Figure Test; SVLT, Seoul Verbal Learning Test; COWAT, Controlled Oral Word Association Test; MMQ, Multifactorial Memory Questionnaire; BADA, Battery for the Analysis of the Aphasic Deficit; TMT, Trail Making Test; CVLT-II, California verbal learning test second edition; WM, Working Memory; ADAS-Cog, Alzheimer's Disease assessment scale-cognition subscale; CVFT, Category Verbal Fluency Test; WAIS-III, Wechsler Adult Intelligence Subscales; MoCA, Montreal Cognitive Assessment; CS, Cognitive Stimulation.