Transcranial Direct Current Stimulation as a memory enhancer in healthy participants and patients with Alzheimer`s disease

The aim of this thesis was to investigate the effects of transcranial direct current stimulation (tDCS) as a memory enhancer in Alzheimer’s disease patients and healthy individuals. In addition, we wanted to study how verbal memory functions are related to hippocampus subfield volumes. This thesis consists of three reports, in which two of the reports (I and II) aimed to study the effects of tDCS, and the other report (III) focused on verbal memory and subfields of the hippocampus. In all three reports, the California Verbal Learning Test II (CVLT-II) was used to assess verbal memory functions. The CVLT-II is normed for age and sex and is a widely used memory test, in both experimental and clinical settings. In reports I and II, the effect of a stimulation method called “transcranial direct current stimulation (tDCS)” was investigated. This is a noninvasive method in which two or more electrodes are placed on the scalp. The electrode positioning depends on the area intended to be stimulated. A weak direct current is delivered through the scalp and aims to increase cortical excitability (i.e., aims to make the neurons more capable of responding to stimuli). The stimulation electrode (the anode) was placed over the temporal cortex, whereas the reference electrode (the cathode) was placed over the right frontal cortex. In report I, we used a randomized controlled trial design in which 26 patients with Alzheimer’s disease underwent six 30-minute sessions of tDCS stimulation during a two-week period. Half of them received active tDCS stimulation, while the other half received placebo tDCS. We found no significant differences between active and placebo tDCS, neither in the primary outcome nor in the secondary outcome measures. In report II, 40 healthy participants underwent six tDCS sessions for two consecutive days. Half of the participants received active tDCS, and the other half received placebo tDCS. No significant differences were found in verbal memory outcomes. However, in the young participants there was a significant difference between active and placebo tDCS in executive functions measured by the Trail Making Test, part B (TMT B). In report II, we investigated the relation between verbal memory and hippocampal subfield volumes in 47 right-handed healthy adults. T1-weighted MRI results were obtained using a 1,5 Tesla scanner. The results showed a significant correlation between left hippocampal subfields volumes and verbal memory. However, no significant correlations were found between right hippocampal volumes and verbal memory. The overall conclusions are as follows: 1) In patients with Alzheimer’s disease, six 30-minute sessions of active tDCS over a period of two weeks did not offer any significant improvements in memory functions, compared to the placebo tDCS results. However, the generalizability is limited due to the small sample size. 2) In healthy participants, six 30-minute sessions of active tDCS for two consecutive days did not offer significantly better memory outcomes, compared to the placebo tDCS results. 3) In healthy adults, there was a significant correlation between verbal memory performance and left hippocampal subfield volume