Table 1.
tDCS studies on language functions in healthy individuals
| Studies on healthy subjects | Subjects | Age mean±SD years | Education years | Polarity | Electrode size (cm) | Stimulated areas | Reference electrode | Control areas | Intensity/duration | Task | Online/offline | Effects | Follow-up |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Frontal tDCS | |||||||||||||
| Iyer et al39 | 103 (47 men) | 37.5±12.9 | ≥12 | A/C/S | 5×5 | Left dorsolateral prefrontal cortex | Contralateral supraorbital area | No | 1 and 2 mA/20 min | Verbal fluency (phonemic cue) | Offline | Anodal tDCS (2 mA) improves verbal fluency | No follow-up |
| Fertonani et al41 | 12 (4 men) | 24.1±3.7 | DNR | A/C/S | 5×7 | Left dorsolateral prefrontal cortex | Right shoulder | No | 2 mA/8 and 10 min | Picture naming | Offline | Anodal tDCS reduces latency of response | No follow-up |
| 12 controls (6 men) | 21.8±1 | ||||||||||||
| De Vries et al42 | 44 (21 men) 6 excluded | 22.6±2.1 | >15 | A/S | 5×7 | Left inferior frontal gyrus | Contralateral supraorbital area | Right inferior frontal gyrus | 1 mA/20 min | Artificial grammar learning and grammatical decision | Online | Left anodal tDCS improves the overall performance in the task of grammatical decision | No follow-up |
| 10 controls (5 men) | 23.7±2.4 | ||||||||||||
| Liuzzi et al43 | 30 (12 men) | 24.97±0.56 | >12 | A/C/S | 5×5 | Left motor cortex | Contralateral supraorbital area | Left dorsolateral prefrontal cortex | 1 mA/20 min | Action/objects word learning paradigm | Offline | Cathodal tDCS on left motor cortex reduces success rates in action words vocabulary | 7, 14, 28 days after tDCS |
| 27 controls (A) (12 men) 6 controls (B) (3 men) |
24.96±0.43 24.50±0.50 |
||||||||||||
| Cattaneo et al44 | 10 (4 men) | 23.6±3.2 | >12 | A/S | 5×7 | Left inferior frontal gyrus | Contralateral supraorbital area | Right inferior frontal gyrus | 2 mA/20 min | Verbal fluency (phonemic and semantic cue) | Offline | Left tDCS improves verbal fluency | No follow-up |
| 8 controls (3 men) | 23.8±123.5 | ||||||||||||
| Holland et al45 | 10 (3 men) | 69±DNR | DNR | A/S | 5×7 | Left inferior frontal cortex | Contralateral frontopolar cortex | No | 2 mA/20 min | Picture naming | Online and during fMRI study | Anodal tDCS has significant behavioural and regionally specific neural facilitation effect | No follow-up |
| Wirth et al46 | 20 (10 men) | 23.5±3.7 | >12 | A/S | 5×7 | Left dorsolateral prefrontal cortex | Right shoulder | No | 1.5 mA/30 min | Semantic blocking paradigm and picture naming | Online/offline (EEG) | Anodal tDCS is capable of enhancing neural processes during and after application | No follow-up |
| Temporal tDCS | |||||||||||||
| Sparing et al47 | 15 (10 men) | 26.9±3.7 | DNR | A/C/S | 5×7 | Left posterior perisylvian area | Vertex | Right posterior perisylvian area | 2 mA/7 min | Picture naming | Offline/online | Left anodal tDCS reduces latency of response | 5/10 min after the end of tDCS |
| Floel et al48 | 19 (10 men) | 25.36±2.7 | DNR | A/C/S | 5×7 | Left posterior perisylvian area | Contralateral supraorbital area | No | 1 mA/20 min | Verbal learning | Online | Anodal tDCS facilitates learning speed and accuracy | No follow-up |
| Fiori et al49 | 10 (7 men) | 55±7.9 | >12 | A/S | 5×7 | Left posterior perisylvian area | Contralateral fronto-polar cortex | Right occipitoparietal area | 1 mA/20 min | Associative verbal learning | Online | tDCS on left posterior perisylvian area reduces naming response latency | No follow-up |
| Ross et al51 | 15 (4 men) | 25.6±DNR | DNR | A/S | 5×7 | Left anterior temporal lobe | Contralateral cheekbone | Right anterior temporal lobe | 1.5 mA/15 min | People and landmark naming | Online | Right tDCS increases naming performance for famous people | No follow-up |
A, anodal tDCS; C, cathodal tDCS; DNR, data not reported; mA, milliampere; offline, the subject executes the task before and after stimulation; online, the subject executes the task during stimulation; S, sham tDCS; SD, standard deviation; tDCS, transcranial direct current stimulation.