1. Baddeley A. Working memory: the interface between memory and cognition. J Cogn Neurosci. 1992;4(3):281–8.

2. Reuter-Lorenz PA, Park DC. Human neuroscience and the aging mind: a new look at old problems. J Gerontol Ser B Psychol Sci Soc Sci. 2010;65(4):405–15.

3. Wang M, Gamo NJ, Yang Y, Jin LE, Wang XJ, Laubach M, et al. Neuronal basis of age-related working memory decline. Nature. 2011;476(7359):210–3.

4. Zacks RT. Working memory, comprehension, and aging: a review and a new view. Psychol Learn Motiv. 1989;22(22):193–225.

5. Silver H, Feldman P, Bilker W, Gur RC. Working memory deficit as a core neuropsychological dysfunction in schizophrenia. Am J Psychiatr. 2003;160(10):1809–16.

6. Karatekin C, Asarnow RF. Working memory in childhood-onset schizophrenia and attention-deficit/hyperactivity disorder. Psychiatry Res. 1998;80(2):165–76.

7. Kensinger EA, Shearer DK, Locascio JJ, Growdon JH, Corkin S. Working memory in mild Alzheimer's disease and early Parkinson’s disease. Neuropsychology. 2003;17(2):230.

8. Barch DM. Pharmacological manipulation of human working memory. Psychopharmacology. 2004;174(1):126–35.

9. Farah MJ, Illes J, Cook-Deegan R, Gardner H, Kandel E, King P, et al. Neurocognitive enhancement: what can we do and what should we do? Nat Rev Neurosci. 2004;5(5):421–5.

10. Morrison AB, Chein JM. Does working memory training work? The promise and challenges of enhancing cognition by training working memory. Psychon Bull Rev. 2011;18(1):46–60.

11. Bestmann S, de Berker AO, Bonaiuto J. Understanding the behavioural consequences of noninvasive brain stimulation. Trends Cogn Sci. 2015;19(1):13–20.

12.• Dayan E, Censor N, Buch ER, Sandrini M, Cohen LG. Noninvasive brain stimulation: from physiology to network dynamics and back. Nat Neurosci. 2013;16(7):838–44. Authors provide a review of research outlining our current understanding of the mechanisms that underlie transcranial electrical stimulation and their effects on behavior.

13. Bindman LJ, Lippold OCJ, Redfearn JWT. The action of brief polarizing currents on the cerebral cortex of the rat (1) during current flow and (2) in the production of long‐lasting after‐effects. J Physiol. 1964;172(3):369–82.

14. Albert DJ. The effects of polarizing currents on the consolidation of learning. Neuropsychologia. 1966;4(1):65–77.

15. Utz KS, Dimova V, Oppenländer K, Kerkhoff G. Electrified minds: transcranial direct current stimulation (tDCS) and galvanic vestibular stimulation (GVS) as methods of non-invasive brain stimulation in neuropsychology—a review of current data and future implications. Neuropsychologia. 2010;48(10):2789–810.

16. Nitsche MA, Paulus W. Sustained excitability elevations induced by transcranial DC motor cortex stimulation in humans. Neurology. 2001;57(10):1899–901.

17. Park SH, Seo JH, Kim YH, Ko MH. Long-term effects of transcranial direct current stimulation combined with computer-assisted cognitive training in healthy older adults. Neuroreport. 2014;25(2):122–6.

18. Jones KT, Stephens JA, Alam M, Bikson M, Berryhill ME. Longitudinal neurostimulation in older adults improves working memory. PLoS ONE. 2015;10(4):e0121904. doi:10.1371/journal.pone.0121904.

19. Nitsche MA, Cohen LG, Wassermann EM, Priori A, Lang N, Antal A, et al. Transcranial direct current stimulation: state of the art 2008. Brain Stimul. 2008;1(3):206–23.

20. Nitsche MA, Paulus W. Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. J Physiol. 2000;527(3):633–9.

21. Jacobson L, Koslowsky M, Lavidor M. tDCS polarity effects in motor and cognitive domains: a meta-analytical review. Exp Brain Res. 2012;216(1):1–10.

22. Russo R, Wallace D, Fitzgerald PB, Cooper NR. Perception of comfort during active and sham transcranial direct current stimulation: a double blind study. Brain Stimul. 2013;6(6):946–51.

23. Palm U, Reisinger E, Keeser D, Kuo MF, Pogarell O, Leicht G, et al. Evaluation of sham transcranial direct current stimulation for randomized, placebo-controlled clinical trials. Brain Stimul. 2013;6(4):690–5.

24. Krishnan C, Santos L, Peterson MD, Ehinger M. Safety of noninvasive brain stimulation in children and adolescents. Brain Stimul. 2015;8(1):76–87.

25. Brunoni AR, Amadera J, Berbel B, Volz MS, Rizzerio BG, Fregni F. A systematic review on reporting and assessment of adverse effects associated with transcranial direct current stimulation. Int J Neuropsychopharmacol. 2011;14(8):1133–45.

26. Andrews SC, Hoy KE, Enticott PG, Daskalakis ZJ, Fitzgerald PB. Improving working memory: the effect of combining cognitive activity and anodal transcranial direct current stimulation to the left dorsolateral prefrontal cortex. Brain Stimul. 2011;4(2):84–9.

27. Berryhill ME, Wencil EB, Coslett HB, Olson IR. A selective working memory impairment after transcranial direct current stimulation to the right parietal lobe. Neurosci Lett. 2010;479(3):312–6.

28. Boehringer A, Macher K, Dukart J, Villringer A, Pleger B. Cerebellar transcranial direct current stimulation modulates verbal working memory. Brain Stimul. 2012;6(4):649–53.

29.• Carvalho S, Boggio PS, Gonçalves OF, Vigário AR, Faria M, Silva S, et al. Transcranial direct current stimulation based metaplasticity protocols in working memory. Brain Stimul. 2015;8(2):289–94. This study systematically examined the effects of tDCS on working memory using varying stimulation protocols. Findings from this research suggest that tDCS-related polarity effects are influenced by previous levels of activity.

30. De Putter LM, Vanderhasselt MA, Baeken C, De Raedt R, Koster EH. Combining tDCS and working memory training to down regulate state rumination: a single-session double blind sham-controlled trial. Cogn Ther Res. 2015;39(6):754–65.

31. Fregni F, Boggio PS, Nitsche M, Bermpohl F, Antal A, Feredoes E, et al. Anodal transcranial direct current stimulation of prefrontal cortex enhances working memory. Exp Brain Res. 2005;166(1):23–30.

32. Giglia G, Brighina F, Rizzo S, Puma A, Indovino S, Maccora S, et al. Anodal transcranial direct current stimulation of the right dorsolateral prefrontal cortex enhances memory-guided responses in a visuospatial working memory task. Funct Neurol. 2014;29(3):189.

33. Gladwin TE, den Uyl TE, Fregni FF, Wiers RW. Enhancement of selective attention by tDCS: interaction with interference in a Sternberg task. Neurosci Lett. 2012;512(1):33–7.

34. Hoy KE, Emonson MR, Arnold SL, Thomson RH, Daskalakis ZJ, Fitzgerald PB. Testing the limits: investigating the effect of tDCS dose on working memory enhancement in healthy controls. Neuropsychologia. 2013;51(9):1777–84.

35. Jeon SY, Han SJ. Improvement of the working memory and naming by transcranial direct current stimulation. Ann Rehabil Med. 2012;36(5):585–95.

36. Keeser D, Padberg F, Reisinger E, Pogarell O, Kirsch V, Palm U, et al. Prefrontal direct current stimulation modulates resting EEG and event-related potentials in healthy subjects: a standardized low resolution tomography (sLORETA) study. Neuroimage. 2011;55(2):644–57.

37.• Kim JH, Kim DW, Chang WH, Kim YH, Kim K, Im CH. Inconsistent outcomes of transcranial direct current stimulation may originate from anatomical differences among individuals: electric field simulation using individual MRI data. Neurosci Lett. 2014;564:6–10. Authors used individual structural MRI and current flow modeling to account for differences in tDCS efficacy across groups.

38. Marshall L, Mölle M, Siebner HR, Born J. Bifrontal transcranial direct current stimulation slows reaction time in a working memory task. BMC Neurosci. 2005;6(1):23.

39. Martin DM, Liu R, Alonzo A, Green M, Loo CK. Use of transcranial direct current stimulation (tDCS) to enhance cognitive training: effect of timing of stimulation. Exp Brain Res. 2014;232(10):3345–51.

40. Meiron O, Lavidor M. Unilateral prefrontal direct current stimulation effects are modulated by working memory load and gender. Brain Stimul. 2013;6(3):440–7.

41. Mulquiney PG, Hoy KE, Daskalakis ZJ, Fitzgerald PB. Improving working memory: exploring the effect of transcranial random noise stimulation and transcranial direct current stimulation on the dorsolateral prefrontal cortex. Clin Neurophysiol. 2011;122(12):2384–9.

42. Mylius V, Jung M, Menzler K, Haag A, Khader PH, Oertel WH, et al. Effects of transcranial direct current stimulation on pain perception and working memory. Eur J Pain. 2012;16(7):974–82.

43. Nikolin S, Loo CK, Bai S, Dokos S, Martin DM. Focalised stimulation using high definition transcranial direct current stimulation (HD-tDCS) to investigate declarative verbal learning and memory functioning. NeuroImage. 2015;117:11–9.

44. Ohn SH, Park CI, Yoo WK, Ko MH, Choi KP, Kim GM, et al. Time-dependent effect of transcranial direct current stimulation on the enhancement of working memory. Neuroreport. 2008;19(1):43–7.

45. Sandrini M, Fertonani A, Cohen LG, Miniussi C. Double dissociation of working memory load effects induced by bilateral parietal modulation. Neuropsychologia. 2012;50(3):396–402.

46. Sellers KK, Mellin JM, Lustenberger CM, Boyle MR, Lee WH, Peterchev AV, et al. Transcranial direct current stimulation (tDCS) of frontal cortex decreases performance on the WAIS-IV intelligence test. Behav Brain Res. 2015;290:32–44.

47. Teo F, Hoy KE, Daskalakis ZJ, Fitzgerald PB. Investigating the role of current strength in tDCS modulation of working memory performance in healthy controls. Front Psychiatry. 2011;2:45.

48. Weigand A, Richtermeier A, Feeser M, Guo JS, Briesemeister BB, Grimm S, et al. State-dependent effects of prefrontal repetitive transcranial magnetic stimulation on emotional working memory. Brain Stimul. 2013;6(6):905–12.

49. Wu YJ, Tseng P, Chang CF, Pai MC, Hsu KS, Lin CC, et al. Modulating the interference effect on spatial working memory by applying transcranial direct current stimulation over the right dorsolateral prefrontal cortex. Brain Cogn. 2014;91:87–94.

50. Xu J, Healy SM, Truong DQ, Datta A, Bikson M, Potenza MN. A feasibility study of bilateral anodal stimulation of the prefrontal cortex using high-definition electrodes in healthy participants. Yale J Biol Med. 2015;88(3):219.

51. Zaehle T, Sandmann P, Thorne JD, Jäncke L, Herrmann CS. Transcranial direct current stimulation of the prefrontal cortex modulates working memory performance: combined behavioural and electrophysiological evidence. BMC neuroscience. 2011;12:2.

52. Lally N, Nord CL, Walsh V, Roiser JP. Does excitatory fronto-extracerebral tDCS lead to improved working memory performance? F1000Research. 2013;2:219.

53. Martin DM, Liu R, Alonzo A, Green M, Player MJ, Sachdev P, et al. Can transcranial direct current stimulation enhance outcomes from cognitive training? A randomized controlled trial in healthy participants. Int J Neuropsychopharmacol. 2013;16(9):1927–36.

54. Motohashi N, Yamaguchi M, Fujii T, Kitahara Y. Mood and cognitive function following repeated transcranial direct current stimulation in healthy volunteers: a preliminary report. Neurosci Res. 2013;77(1):64–9.

55. Richmond LL, Wolk D, Chein J, Olson IR. Transcranial direct current stimulation enhances verbal working memory training performance over time and near transfer outcomes. J Cogn Neurosci. 2014;26(11):2443–54.

56. Berryhill ME, Jones KT. tDCS selectively improves working memory in older adults with more education. Neurosci Lett. 2012;521(2):148–51.

57. Nilsson J, Lebedev AV, Lövdén M. No significant effect of prefrontal tDCS on working memory performance in older adults. Front Aging Neurosci. 2015;7:230.

58. Seo MH, Park SH, Seo JH, Kim YH, Ko MH. Improvement of the working memory by transcranial direct current stimulation in healthy older adults. J Korean Acad Rehabil Med. 2011;35(2):201–6.

59. Boggio PS, Khoury LP, Martins DC, Martins OE, De Macedo EC, Fregni F. Temporal cortex direct current stimulation enhances performance on a visual recognition memory task in Alzheimer disease. J Neurol Neurosurg Psychiatry. 2009;80(4):444–7.

60. Moreno ML, Vanderhasselt MA, Carvalho AF, Moffa AH, Lotufo PA, Benseñor IM, et al. Effects of acute transcranial direct current stimulation in hot and cold working memory tasks in healthy and depressed subjects. Neurosci Lett. 2015;591:126–31.

61. Oliveira JF, Zanao TA, Valiengo L, Lotufo PA, Benseñor IM, Fregni F, et al. Acute working memory improvement after tDCS in antidepressant-free patients with major depressive disorder. Neurosci Lett. 2013;537:60–4.

62. Ferrucci R, Bortolomasi M, Vergari M, Tadini L, Salvoro B, Giacopuzzi M, et al. Transcranial direct current stimulation in severe, drug-resistant major depression. J Affect Disord. 2009;118(1):215–9.

63. Fregni F, Boggio PS, Nitsche MA, Rigonatti SP, Pascual‐Leone A. Cognitive effects of repeated sessions of transcranial direct current stimulation in patients with depression. Depress Anxiety. 2006;23(8):482–4.

64. Loo CK, Sachdev P, Martin D, Pigot M, Alonzo A, Malhi GS, et al. A double-blind, sham-controlled trial of transcranial direct current stimulation for the treatment of depression. Int J Neuropsychopharmacol. 2010;13(1):61–9.

65. Loo CK, Alonzo A, Martin D, Mitchell PB, Galvez V, Sachdev P. Transcranial direct current stimulation for depression: 3-week, randomised, sham-controlled trial. Br J Psychiatry. 2012;200(1):52–9.

66. Salehinejad MA, Rostami R, Ghanavati E. Transcranial direct current stimulation of dorsolateral prefrontal cortex in major depression: improving visual working memory, reducing depressive symptoms. NeuroRegulation. 2015;2(1):37.

67. Boggio PS, Ferrucci R, Rigonatti SP, Covre P, Nitsche M, Pascual-Leone A, et al. Effects of transcranial direct current stimulation on working memory in patients with Parkinson’s disease. J Neurol Sci. 2006;249(1):31–8.

68. Hoy KE, Arnold SL, Emonson MR, Daskalakis ZJ, Fitzgerald PB. An investigation into the effects of tDCS dose on cognitive performance over time in patients with schizophrenia. Schizophr Res. 2014;155(1):96–100.

69. Smith RC, Boules S, Mattiuz S, Youssef M, Tobe RH, Sershen H, et al. Effects of transcranial direct current stimulation (tDCS) on cognition, symptoms, and smoking in schizophrenia: a randomized controlled study. Schizophr Res. 2015;168(1):260–6.

70. Jo JM, Kim YH, Ko MH, Ohn SH, Joen B, Lee KH. Enhancing the working memory of stroke patients using tDCS. Am J Phys Med Rehabil. 2009;88(5):404–9.

71. Ulam F, Shelton C, Richards L, Davis L, Hunter B, Fregni F, et al. Cumulative effects of transcranial direct current stimulation on EEG oscillations and attention/working memory during subacute neurorehabilitation of traumatic brain injury. Clin Neurophysiol. 2015;126(3):486–96.

72. Liu A, Bryant A, Jefferson A, Friedman D, Minhas P, Barnard S, et al. Exploring the efficacy of a 5-day course of transcranial direct current stimulation (TDCS) on depression and memory function in patients with well-controlled temporal lobe epilepsy. Epilepsy Behav. 2016;55:11–20.

73. Datta A, Bansal V, Diaz J, Patel J, Reato D, Bikson M. Gyri-precise head model of transcranial direct current stimulation: improved spatial focality using a ring electrode versus conventional rectangular pad. Brain Stimul. 2009;2(4):201–7.

74. Stagg CJ, Nitsche MA. Physiological basis of transcranial direct current stimulation. Neuroscientist. 2011;17(1):37–53.

75. Nitsche, M. A., Kuo, M. F., Paulus, W., & Antal, A. (2015). Transcranial direct current stimulation: protocols and physiological mechanisms of action. In Textbook of Neuromodulation (pp. 101-111). Springer New York.

76.•• Summers JJ, Kang N, Cauraugh JH. Does transcranial direct current stimulation enhance cognitive and motor functions in the ageing brain? A systematic review and meta-analysis. Ageing Res Rev. 2016;25:42–54. To date, this is the most comprehensive review and meta-analysis of the effects of tDCS on older adults.

77. Witte AV, Kürten J, Jansen S, Schirmacher A, Brand E, Sommer J, et al. Interaction of BDNF and COMT polymorphisms on paired-associative stimulation-induced cortical plasticity. J Neurosci. 2012;32(13):4553–61.

78. Peña-Gómez C, Sala-Lonch R, Junqué C, Clemente IC, Vidal D, Bargalló N, et al. Modulation of large-scale brain networks by transcranial direct current stimulation evidenced by resting-state functional MRI. Brain Stimul. 2012;5(3):252–63.

79. Polanía R, Paulus W, Nitsche MA. Modulating cortico‐striatal and thalamo‐cortical functional connectivity with transcranial direct current stimulation. Hum Brain Mapp. 2012;33(10):2499–508.

80. Edwards D, Cortes M, Datta A, Minhas P, Wassermann EM, Bikson M. Physiological and modeling evidence for focal transcranial electrical brain stimulation in humans: a basis for high-definition tDCS. Neuroimage. 2013;74:266–75.

81. Zito GA, Senti T, Cazzoli D, Müri RM, Mosimann UP, Nyffeler T, et al. Cathodal HD-tDCS on the right V5 improves motion perception in humans. Front Behav Neurosci. 2015;9:257.

82. Herrmann CS, Rach S, Neuling T, Strüber D. Transcranial alternating current stimulation: a review of the underlying mechanisms and modulation of cognitive processes. Front Hum Neurosci. 2013;7:279.

83. Hoy KE, Bailey N, Arnold S, Windsor K, John J, Daskalakis ZJ, et al. The effect of γ-tACS on working memory performance in healthy controls. Brain Cogn. 2015;101:51–6.

84. Kuo MF, Nitsche MA. Exploring prefrontal cortex functions in healthy humans by transcranial electrical stimulation. Neurosci Bull. 2015;31(2):198–206.

85. Paulus W. Transcranial electrical stimulation (tES - tDCS; tRNS, tACS) methods. Neuropsychol Rehabil. 2011;21:602–17. doi:10.1080/09602011.2011.557292.

86. Chaieb L, Antal A, Paulus W. Transcranial random noise stimulation-induced plasticity is NMDA-receptor independent but sodium-channel blocker and benzodiazepines sensitive. Front Neurosci. 2015;9:125.

87. Herpich F, Melnick M, Huxlin K, Tadin D, Agosta S, Battelli L. Transcranial random noise stimulation enhances visual learning in healthy adults. J Vis. 2015;15(12):40.

88. Vanneste S, Fregni F, De Ridder D. Head-to-head comparison of transcranial random noise stimulation, transcranial AC stimulation, and transcranial DC stimulation for tinnitus. Front Psychiatry. 2013;4:158.

89. Jonides J, Lewis RL, Nee DE, Lustig CA, Berman MG, Moore KS. The mind and brain of short-term memory. Annu Rev Psychol. 2008;59:193.

90. Wechsler D. WAIS-III: administration and scoring manual: Wechsler Adult Intelligence Scale. San Antonio, TX: Psychological Corporation; 1997.

91. Sternberg S. Memory-scanning: mental processes revealed by reaction-time experiments. Am Sci. 1969;57(4):421–57.

92.•• Horvath JC, Forte JD, Carter O. Quantitative review finds no evidence of cognitive effects in healthy populations from single-session transcranial direct current stimulation (tDCS). Brain Stimul. 2015;8(3):535–50. An important meta-analysis of single-session tDCS on healthy young adults’ cognitive performance that is the subject of some debate.

93.•• Hill AT, Fitzgerald PB, Hoy KE. Effects of anodal transcranial direct current stimulation on working memory: a systematic review and meta-analysis of findings from healthy and neuropsychiatric populations. Brain Stimul. 2015;9(2):197–208. A recent review and meta-analysis that examines the effects of tDCS on WM, specifically, in both healthy and neuropsychiatric samples.

94.• Brunoni AR, Vanderhasselt MA. Working memory improvement with non-invasive brain stimulation of the dorsolateral prefrontal cortex: a systematic review and meta-analysis. Brain Cogn. 2014;86:1–9. A meta-analysis focusing specifically on non-invasive brain stimulation of the DLPFC on working memory improvements.

95. Price AR, Hamilton RH. A re-evaluation of the cognitive effects from single-session transcranial direct current stimulation. Brain Stimul. 2015;8(3):663–5.

96.• Flöel A. tDCS-enhanced motor and cognitive function in neurological diseases. Neuroimage. 2014;85:934–47. Authors provide one of the more wide-ranging reviews of tDCS effects in neuropsychiatric populations.

97. Burke SN, Barnes CA. Neural plasticity in the ageing brain. Nat Rev Neurosci. 2006;7(1):30–40.

98.• Teixeira-Santos AC, Nafee T, Sampaio A, Leite J, Carvalho S. Effects of transcranial direct current stimulation on working memory in healthy older adults: a systematic review. Principles and Practice of Clinical Research. 2015;1(3):73–81. Complementing Summers et al., this review examines the effects of tDCS in older adults only on measures of working memory.

99.• Tremblay S, Lepage JF, Latulipe-Loiselle A, Fregni F, Pascual-Leone A, Théoret H. The uncertain outcome of prefrontal tDCS. Brain Stimul. 2014;7(6):773–83. This comprehensive review examines the effects of tDCS to DLPFC on multiple cognitive measures.

100. Moliadze V, Atalay D, Antal A, Paulus W. Close to threshold transcranial electrical stimulation preferentially activates inhibitory networks before switching to excitation with higher intensities. Brain Stimul. 2012;5(4):505–11.

101. Lustig, C., & Sarter, M. (2016). Attention and the cholinergic system: relevance to schizophrenia. Current Topics in Behavioral Neurosciences.

102. Reuter-Lorenz PA, Cappell KA. Neurocognitive aging and the compensation hypothesis. Curr Dir Psychol Sci. 2008;17(3):177–82. doi:10.1111/j.1467-8721.2008.00570.x.

103. López-Alonso V, Cheeran B, Río-Rodríguez D, Fernández-del-Olmo M. Inter-individual variability in response to non-invasive brain stimulation paradigms. Brain Stimul. 2014;7(3):372–80.

104. Wiethoff S, Hamada M, Rothwell JC. Variability in response to transcranial direct current stimulation of the motor cortex. Brain Stimul. 2014;7(3):468–75.

105. Brunoni AR, Kemp AH, Shiozawa P, Cordeiro Q, Valiengo LCL, Goulart AC, et al. Impact of 5-HTTLPR and BDNF polymorphisms on response to sertraline versus transcranial direct current stimulation: implications for the serotonergic system. Eur Neuropsychopharmacol. 2013;23(11):1530–40.

106. Plewnia C, Zwissler B, Längst I, Maurer B, Giel K, Krüger R. Effects of transcranial direct current stimulation (tDCS) on executive functions: influence of COMT Val/Met polymorphism. Cortex. 2013;49(7):1801–7.

107. Krause B, Kadosh RC. Not all brains are created equal: the relevance of individual differences in responsiveness to transcranial electrical stimulation. Front Syst Neurosci. 2014;8:25.