Testing the limits: Investigating the effect of tDCS dose on cognitive performance in healthy controls and patients with schizophrenia

• ¹ Monash University, Monash Alfred Psychiatry Research Centre, Australia
• ² University of Toronto, Centre for Addiction and Mental Health, Canada

Background: Transcranial Direct Current Stimulation (tDCS) is a non-invasive form of brain stimulation which has been shown to induce changes in brain activity and subsequent functioning. In particular, there is a rapidly growing evidence base showing that anodal tDCS applied to the left dorsolateral prefrontal cortex (DLPFC) is able to enhance aspects of cognitive functioning. This has led to excitement regarding the potential treatment of cognitive dysfunction in illness, such as schizophrenia, as well as the possibility of ‘electrodoping’ in healthy controls to greatly improve cognitive performance.

Objective: This excitement surrounding the potential of tDCS, for both healthy individuals and those with cognitive impairment, is contingent on its ability to meaningfully improve cognitive performance. If substantial effects
are possible, there should be a dose related effect with greater benefits apparent at higher doses. To date such a relationship has not been investigated in either healthy controls or patients with schizophrenia.

Methods: Here I present the results of two parallel studies investigating the effects of increasing the current (or ‘dose’) of tDCS on the degree of working memory (WM) improvement in 18 healthy controls and 18 patients with schizophrenia. Single sessions of 1mA, 2mA and sham anodal tDCS to the left DLPFC were undertaken over a period of two to three weeks. Participants underwent a WM task at three time points post stimulation (0, 20 and 40 mins).

Results: Our results showed that while active tDCS can enhance behavioural performance in healthy controls there was no dose response relationship. These findings are somewhat unexpected as tDCS dose-response relationships for cognitive enhancement have been seen in patient populations. Indeed we saw such a relationship in our schizophrenia group, with significant effects of 2mA only on working memory performance over time in patients with schizophrenia.

Conclusions: While tDCS was shown to enhance cognitive functioning in healthy controls there was no evidence of greater or longer enhancements at a higher tDCS dose. This is in contrast to what was seen in the schizophrenia group where cognition, and underlying neurophysiology, is impaired. These findings provide greater understanding of the mechanisms of tDCS induced cognitive enhancement and provide evidence of the possible limits of such enhancement for the healthy population.