Anodal HD-tDCS on the dominant anterior temporal lobe and dorsolateral prefrontal cortex: clinical results in patients with mild cognitive impairment

Abstract

Objectives: Mild cognitive impairment (MCI) is a neurocognitive disorder in which the cognitive and mental abilities of humans are declined. Transcranial direct-current stimulation (tDCS) is an emerging noninvasive brain stimulation technique aimed at neuromodulation. In this study, we investigate whether high-definition anodal tDCS stimulation (anodal HD-tDCS) in MCI patients in two different brain regions will be effective in improving cognitive function.

Methods: This study was done as a randomized, double-blind clinical trial. Sixty MCI patients (clinically diagnosed by expert neurologists) were randomly divided into three groups. Two groups received 2-mA anodal HD-tDCS for 20 min for 2 weeks (5 consecutive days in each week, 10 days in total). In the first group (twenty patients), the left dorsolateral prefrontal cortex (left DLPFC) was targeted. In the second group (twenty patients), the target zone was the dominant anterior temporal lobe (DATL). The third group (twenty patients) formed the Sham group. The Montreal Cognitive Assessment (MoCA) and Quality of Life in Alzheimer's Disease (QoLAD) were considered as the outcome measures.

Results: MCI patients obtained the highest MoCA mean scores in both left DLPFC and DATL groups versus the study baseline 2 weeks after the intervention. In addition, the MoCA mean scores of MCI patients were greater in both intervention groups compared to the Sham group up to 3 months post-stimulation (p-value ≤ 0.05). However, as we moved away from the first stimulation day, a decreasing trend in the MoCA mean scores was observed. Moreover, in the left DLPFC and DATL groups, higher QoLAD mean scores were observed 3-month post-stimulation, highlighting the effectiveness of anodal HD-tDCS in improving the quality of life in MCI patients.

Conclusion: In this research, it was shown that applying anodal HD-tDCS at left DLPFC and DATL brain regains for two successive weeks improves cognitive function in MCI patients (by obtaining higher values of MoCA scores) up to 3 months after the intervention compared to the Sham group. This illustrates the positive effects of HD-tDCS, as a non-pharmacological intervention, for improving cognitive function and quality of life in MCI patients.

Significance: Two weeks after anodal HD-tDCS of the DLPFC and DATL brain regions, the MCI patients achieved the highest MoCA mean scores compared to the Sham group across all measurement intervals.

Keywords: Dominant anterior temporal lobe; HD-tDCS; Left dorsolateral prefrontal cortex; Mild cognitive impairment; MoCA.

Fig. 1

Schematic illustration of the experimental procedure. Sixty participants signed consent. There were three participants excluded before completing the baseline. Sixty participants were randomized and then divided into three groups. 2-mA anodal HD-tDCS for 20 min was applied for 2 weeks (5 consecutive days in each week) on the left DLPFC (20 patients, first group) and on the DATL (20 patients, second group). The third group (20 patients) received the Sham stimulation. All 60 patients completed a 2-week treatment and 1-month and 3-month follow-up. The CONSORT/2010 statement was utilized as a model to construct this figure

Fig. 2

Two different 4 × 1 anodal HD-tDCS interventions. The position of electrodes is based on the international 10–10 system. Indicated locations were used for stimulation (blue: cathode; red: anode). For “Group_1,” the target zone was located on the left dorsolateral prefrontal cortex, and the electrode montage was as follows: cathode placed on the Fp2 location and the other four anode electrodes placed at F1, F3, FC1, and FC3 locations. For “Group_2,” the target zone was in the dominant anterior temporal lobe, and the electrode montage was as follows: cathode positioned on the FP2 and the other four anode electrodes on F7, FT7, T9, and FT9. The Neuroelectrics Instrument Controller (NIC2) software was used to create this figure

Fig. 3

The study timeline explains three stages of assessments including pre-intervention (baseline), intervention (treatment), and follow-up (post-intervention). Pre-intervention period for 1 month, 2-week anodal HD-tDCS treatment period (W1, W2), and a 3-month follow-up period from the first day of stimulation. One month before treatment, MRI, Peterson’s criteria, and DBI tests were also done. Montreal Cognitive Assessment (MoCA) test was taken for each patient 1 month before treatment (M_-1), the second week (W2), and the first (M1) and the third month (M3). Quality of Life in Alzheimer’s Disease (QoLAD) at baseline and the third month was measured

Fig. 4

The experimental protocol of stimulation for intervention and Sham groups. A Intervention group. Patients received a direct current for 20 min. B Sham group. Patients received direct current for only 60 s, while electrodes remained for 20 min. During the first 15 s, the current flow increased slowly, and during the last 15 s, the current flow decreased slowly

Fig. 5

The MoCA mean scores between different time intervals: baseline, 2 weeks later, 1 month later, and 3 months later. Two weeks later, patients achieved highest scores in the MoCA test in the left DLPFC and DATL groups compared to the other time intervals. “*” shows the statistically significant cases (p-value ≤ 0.05). The data are presented using mean ± SD (standard deviation)

Fig. 6

Comparison of MoCA mean sub-scale scores during different time intervals according to the groups. In general, the maximum score was observed after 2 weeks of intervention