Neural Control of Gait & Navigation in ADRD

The investigators will conduct this experiment with individuals that have subjective cognitive decline. At pre- and post-test, they will receive structural MRI and resting state fMRI so that we can quantify brain vestibular-motor and vestibular-hippocampal network segregation. These will serve as the primary outcome metrics. Participants will perform a walking + spatial navigation task. Their performance in the first 15 minutes will count as their pre-test behavioral score (gait speed, gait variability, navigation path integration task error). Participants will then receive either sham or active bilateral vestibular cortical tDCS. They will be randomly assigned into the active (n=30) or sham tDCS group (n=30) without their awareness. The active group will receive 20 minutes of 2mA tDCS bilaterally to the vestibular cortex (right parietal operculum 2 (OP2), MNI coordinates 50,-32,18; left OP2, MNI coordinates 38,-20,16); these regions have been identified through meta-analyses as being responsive to various types of vestibular stimulation. Electrodes will be placed using a Brainsight (Rogue Research, Montreal, QC, Canada) neuro-navigation system in which we will fit each participant's head to a standardized template space (MNI 152 template) from their MRIs (neuro-navigated stimulation). The anode electrode will be placed over the left vestibular cortex, whereas the cathode will be placed over the right. Stimulation at 2 mA has previously been shown to result in net excitation in both regions stimulated, as opposed to the traditional approach that increases excitability under the anode and inhibits excitability under the cathode. The sham tDCS group will receive stimulation for only 30 seconds during an active ramp up, and then immediate ramp down. This is to ensure that neither group is actively aware if they are receiving tDCS stimulation. The experimenter will also be blinded to whether a participant is receiving active or sham stimulation; they will enter a code into the stimulation box, making the protocol double blind. Participants will perform walking + spatial navigation on three days for 30 minutes per day (tDCS will be on for the first 20 minutes), under either active or sham stimulation. This protocol will be applied over three sessions, each separated by at least 24 hours and all three completed within 10 days. The investigators will determine whether bilateral tDCS increases vestibular-motor and vestibular-hippocampal network segregation. They will examine behavioral task improvements as secondary outcomes. Using an ANOVA model, a total sample size of 60 (n=30 per group) results in 80% power to detect an effect size as small as Cohen f= 0.37 at significance level 0.05. Using a two-sided two-sample t-test with the same sample size, power and significance level (if the investigators were to analyze each variable separately), the minimum detectable effect size is 0.75 and for a nonparametric Wilcoxon-Mann-Whitney test, it becomes 0.76 under the same settings.