Enhancing Cognitive Control in Mild Cognitive Impairment Via Non-invasive Brain Stimulation
Phase: NA
Conditions: Mild Cognitive Impairment
Sponsor: University of California, San Francisco

Study Protocol & Statistical Analysis Plan 
 
Title: Enhancing Cognitive Control in Mild Cognitive Impairment Via Non-
invasive Brain Stimulation 
 
NCT#: NCT04647032 
 
Last Updated: 11/2/2021 
 
 

Study Procedures  
This study was preregistered at ClinicalTrials.gov with identifier NCT04647032. In this 
study, participants first met with researchers at the University of California, San 
Francisco and provided informed consent. Participants received two iPad tablets, one for 
tACS days and one for outcome measures days. Participants were instructed on the use 
of each device (with a brief demonstration in front of a mirror with the tACS patches), the 
timeline of events, and provided with detailed written instructions. Once home and with 
active internet, participants first completed the outcome measure tasks without any 
neurostimulation (baseline). On the following day, participants self-applied the 
neurostimulation device and then completed 20-minutes of adaptive CCT. Participants 
completed the paired tACS+CCT task in the same manner for the following four days, 
totaling five consecutive days. On the following (6th) day, participants completed the 
same outcome measures as baseline without any concurrent tACS (1-week 
assessment). The next day, one week from the initial session, participants completed the 
adaptive CCT task with paired tACS again. Participants then had two additional weekly 
sessions of tACS+CCT. Finally, on the day after the last (8th) tACS session, participants 
completed a final tACS-free outcome measures session (1-month assessment).  
 
Participants 
In this double-blind parallel-group study, we enrolled participants aged 60-80 years until 
we reached 30 total participants. Participants were randomly assigned following simple 
randomization procedures (computerized random numbers) to 1 of 2 treatment groups. 
The randomization was completed by a researcher who did not have face to face contact 
with the aMCI patients and the patients completed the training at home without 
researchers present. The patients and experimenter assessing adherence were blinded 
to the random group assignment. We anticipated a 10% attrition rate, in line with our 
previous tACS studies [1-3]. To be included in this study, participants had to be English 
speaking, have at least 12 years of education, have normal or corrected to normal vision 
(without glaucoma, macular degeneration, amblyopia, or strabismus) and hearing, be 
able to complete cognitive tasks and study procedures, and be able to tolerate tACS. To 
be categorized as md-aMCI, participants scored between 17 and 28 on the Montreal 
Cognitive Assessment (MoCA), had an age-matched Z score of at least -1 on immediate 
memory or delayed memory (as measured by the California Verbal Learning Test) and 
at least -1 Z score on verbal and semantic fluency (D words, animals), processing speed 
(digit symbol and number trails tasks), or task switching (number letter trails task). 
Finally, participants needed a self-reported memory complaint. Exclusion criteria 
consisted of a family history of epilepsy or seizures, serious head trauma, pregnant, any 
implanted electrical devices, IQ below 80, substance abuse, or on contraindicated 
medications that may alter brain states (anti-depressant, anti-anxiety, psychotropic 
medications). Of the 30 participants, 1 dropped out for personal reasons and two 
participants did not adhere the CCT schedule during the initial week. This resulted in 27 
participants who were randomly assigned to receive theta tACS (N: 14) or control tACS 
(N:13). All participants signed informed consent documents approved by the University 
of San Francisco, California Institutional Review Board (IRB). The study took place 
entirely within the greater San Francisco Bay region, within individual participant’s 
homes, as participants needed to visit UCSF on the initial day to consent, receive 
instructions, and equipment. Participants received $20 per hour for participation and a 
$50 bonus for completion of the study. 
 
 
 

Neuromodulation 
Participants were randomized to receive theta tACS (6 Hz) or control stimulation (1 Hz) 
using a self-applied humm tACS device (humm, California, USA). Humm tACS devices 
consist of a single patch with two electrodes covered in adhesive gel that fixes directly 
below the hairline and extends from AF3 to AF4 (10-20 EEG system). During each CCT 
session, tACS was applied for 15 minutes at 1.5 mA (baseline-to-peak; 3 mA peak-to-
peak). Stimulation also included 30 seconds of ramping up from 0 mA to full intensity 
and 30 seconds of ramping down to 0 mA at the end of the 15 minutes, for a total of 16 
minutes. Participants first cleansed their forehead with an isopropanol wipe where the 
humm device was to be applied, then pressed a button at the center of the patch to 
begin the stimulation. After an automated impedance check, stimulation began and then 
automatically ended after the prescribed 16-minute duration. Participants were provided 
with four humm patches as each patch was used twice across the eight stimulation 
sessions. The theta and control humm patches were identical in appearance and could 
only be identified by the serial number listed on the device, ensuring participants were 
blind to the existence of different conditions. Following the end of each tACS session, 
participants filled out a survey of side effects by rating the following 11 measures on a 
scale from 0 (not noticeable) to 10 (not tolerable): headache, neck pain, scalp pain, 
tingling, itching, burning sensation, increased alertness, increased sleepiness, trouble 
concentrating, acute mood change, phosphenes. Participants who rated any measure to 
be 7 or more would have been removed from the study; however, no participants 
reported ratings that would have warranted such removal. In addition to the surveys, 
tolerability was initially assessed in-lab during the instructional period. 
 
Cognitive Control Training  
On each of the eight CCT sessions participants began the self-applied tACS and 
immediately began the multitasking CCT task, AKL-T01 (Akili Interactive Labs, Inc). 
AKL-T01 is a proprietary system based on patented technology underlying the 
NeuroRacer paradigm that challenges cognitive control by requiring multitasking 
performance (for more details see: [4]). Briefly, participants guide a character down a 
path by tilting the iPad similar to a steering wheel (visuomotor/sensorimotor task). At the 
same time, participants were engaged in a perceptual discrimination task, where they 
tapped on the screen in response to target items (e.g., green fish) and ignored all 
distractors (e.g., blue fish). Importantly, AKL-T01 employs algorithms that continuously 
adapts to individual performance in real time with feedback provided. Correctly 
identifying consecutive targets lowered the response time to be counted as correct, 
visually depicted as faster moving targets. Consecutive obstacles (gates) avoided in the 
visuomotor task increased the speed of the vehicle and misses slowed down the speed. 
Participants completed five ‘missions’ per day, which lasted approximately 20-minutes in 
total (i.e., 4 minutes longer than tACS). Participants received ‘stars’ as credit for multiple 
correct responses in a row and advanced to the next level every five stars. Participants 
were instructed to administer the tACS and play AKL-T01 at the same time of day during 
all eight CCT sessions. Participants were provided with detailed step-by-step instructions 
with pictures that walked them through setting up the humm devices, how to use the 
tablets, and the order of operations each day. Participants were also provided a calendar 
that listed their activities on each day over the course of four weeks. Researchers were 
able to remotely monitor adherence to the CCT sessions as performance data was 
uploaded online during each session.  
 
 
 

Outcome Measures  
Primary outcome measures consisted of 1) multitasking ability on the CCT task, 2) a 
sustained attention task and 3) a working memory task. On each of the three sessions 
where outcome measures were assessed (baseline, 1-week, 1-month), participants 
completed the outcome assessments on a separate color-coded iPad than the CCT 
task. To assess multitasking ability on the CCT task, participants completed Monitor 
(AKL-M01, Akili Interactive Labs, Inc), which is an assessment version of AKL-T01. 
Monitor consists of a set of game-like tasks lasting approximately 7 minutes in which 
participants were engaged in a dynamically adjusting perceptual discrimination task with 
and without concurrent visuospatial tracking. Similar to AKL-T01, participants completed 
the perceptual discrimination task by correctly tapping on the tablet’s screen when the 
correct target appears (i.e. only green items) without having to navigate an avatar. In 
addition, participants completed a visuomotor/sensory motor tracking task by tilting the 
iPad to steer an avatar vehicle through a racecourse with obstacles present in the road 
that must be avoided, without having to discriminate targets. Finally, participants 
completed the discrimination and tracking tasks concurrently to measure multitasking 
abilities. Both discrimination and tracking tasks use adaptive algorithms that 
continuously adjust task difficulty to converge on a consistent proportional correct value, 
which is saved as an individual threshold of discrimination and tracking performance. 
These threshold values were subsequently used in statistical analyses to assess 
multitasking performance. 
Participants also completed the Adaptive Cognitive Evaluation-Explorer (ACE-X). 
The tasks in ACE-X are standard tests that assess different aspects of cognitive control 
(attention, working memory), modified by incorporating adaptive algorithms, immersive 
graphics, video tutorials, motivating feedback, and a user-friendly interface. Each task 
within ACE-X was completed in approximately 5 minutes. Two of these tasks were 
sustained attention tasks used to assess attentional control, and were modeled after the 
Test of Variables of Attention task (TOVA). In both tasks, participants were instructed to 
respond as quickly as possible to a target stimulus that appeared at the top of the screen 
and ignore any stimuli that appeared at the bottom of the screen. Both stimuli were 
presented for 100 ms, had 2.1 seconds to respond, and trials occurred every 2 seconds. 
The two tasks differ in their proportion of target stimuli. The task with frequent stimuli (32 
of 40 trials) assesses inhibitory control abilities. The task with infrequent stimuli (16 of 80 
trials) assesses sustained attention abilities. There was no adaptivity of response 
window or feedback for these tasks to closely follow the design of the TOVA.  
To assess working memory, two ACE-X tasks were used (based on the Corsi 
block task), which measures visuospatial working memory capacity (WM). Briefly, a field 
of randomly distributed diamonds appeared on the screen and participants were to tap 
the order (or backwards order) of the diamonds that were illuminated for 1 second each. 
Participants begin with three targets and if they complete two consecutive trials 
successfully, they advance to the next level with one additional target. If a participant 
fails three trials in a row, the task ends. Participants can complete up to a total of nine 
targets. Participant must select all target gems in the correct order within the maximum 
response time (5 seconds + 1 second per target).  
In addition to our primary outcome measures assessing multitasking, sustained 
attention, and working memory, we also collected data from 4 other ACE-X tasks to be 
used in an exploratory analysis of other aspects of attentional control: 1) Boxed, a visual 
search interference processing based on the visual search paradigm [5]. 2) Color 
Tricker, based on the Stroop task measuring central visual inhibitory control. 3) Flanker, 
a peripheral visual inhibitory control task. 4) Compass, a spatial selective attention task 
based on the Posner cueing paradigm.  

Boxed is a forced-voice task with four stimuli conditions of 20 trials each: set size 
of 4 or 12 and feature or congruent stimuli. The stimuli are colored landolt squares (red 
and green) with side openings (left, right, bottom, or top) and participants are instructed 
to attend to the green with top or bottom openings (target) and ignore all other red and 
green squares. Participants respond to whether the target has an opening on the top or 
bottom. The feature 4 condition includes one green target and three red distractors. 
Feature 12 includes 11 red distractors and one green target. Conjunction 4 includes the 
green target, green distractor, and two red distractors. Conjunction 12 includes the green 
target, six red distractors, and five green distractors.  
Color tricker, based on the Stroop task, is designed to measure response 
inhibition performance. Participants viewed colored words that spell a color and are 
instructed to identify the color of the word (target) and ignore the color the word spells 
(distractor). The task has two conditions, congruent, matching color and word, and 
incongruent, mismatching color and word. In both conditions participants selected the 
color that matches the color of the word regardless of what is spelled. Each condition 
has 20 pseudorandomized trials with an equal number of targets of each color. A target 
color was never the word from the previous trial and a target color was never repeated 
two trials in a row.  
Flanker is designed to measure selective attention and interference resolution 
performance. Participants view an array of five arrows and are instructed to identify the 
direction of a central arrow (target) surrounded by four flanking arrows (distractors). In 
congruent trials (14 trials) the center arrow points the same direction as the four flanking 
arrows and in incongruent trials (14 trials) the target arrow points in the opposite 
direction as the four flanking arrows. Trial types are mixed pseudo randomly such that 
no more than three trials in a row are the same condition.  
Compass is based on the Posner cueing paradigm and is designed to measure 
spatial selective attention. Participants were instructed to look at the center of the screen 
where they saw an arrow pointing to the likely location of a target symbol. The task has 
three conditions, 40 valid trials (arrow points in correct direction), 10 invalid trials (arrow 
points in incorrect direction), and 10 neutral trials (arrow points in both directions). In 
each condition the participant tapped the side of the screen where the symbol appeared 
regardless of the direction the arrow was pointing. All trials are mixed pseudo randomly 
with no more than three of the same targets in a row (e.g. left) and no more than two 
invalid trials or 10 valid trails in a row. 
On all adaptive ACE-X tasks, the window to respond was adaptive based on 
accuracy from the previous trial. This allows for the same tasks to be used in a range of 
participants that vary in age and clinical condition. If the participant is correct and 
responds quickly enough, the response window decreases by 10 ms. If the participant 
responds incorrectly or too slow, the response window increases by 40 ms. The 
minimum response window floor is 150 ms. For all adaptive modules, the inter-trial 
interval was 800-1200 ms and feedback occurred for 200 ms post response.  
The secondary outcome measures consisted of the Instrumental Activities of 
Daily Living (IADL) survey, where participants rated their level of independence 
(independent, needs help, dependent, or cannot do) on 16 metrics (e.g. driving, 
managing finances, bathing) on a scale of 1 (cannot do) to 4 (independent). Mean 
ratings across all metrics were used for statistical analysis of IADL. 
 
Analyses  
To measure protocol adherence, we conducted independent-samples t-tests between 
the tACS groups on the percentage of tasks completed as compared to what was 
expected (CCT: 8 AKL-T01 sessions, Outcomes: 3 Monitor sessions, 3 ACE-X sessions, 

3 IADL surveys). Side effects were averaged across the 8 CCT sessions per participant 
for each of the 11 categories. We then conducted independent-samples t-tests between 
the two tACS groups for each of the 11 categories.  
To assess divided attention (multitasking), we measured the difficulty threshold 
level at each time point for both the perceptual discrimination and visuomotor tracking 
multitasking modules. To assess sustained attentional control, we averaged RT across 
both portions of the ACE-X continuous performance task (frequent and infrequent 
targets). To assess working memory, we averaged the maximum span correctly 
achieved on the forwards and backwards span tasks. To assess inhibitory control on the 
Flanker and Color Tricker tasks, we measured the average rate correct score (RCS) 
together for all trial types, which was defined as the number of overall correct trials 
divided by mean response time. To assess attentional orienting on the Compass task, 
we analyzed RCS across all trial types together. To assess visuospatial search on the 
Boxed task, analyzed RCS across all trial types together. For all outcome measure 
comparisons, we conducted repeated-measures ANOVAs (rm-ANOVA) using JASP, 
with a within-subjects factor of time (baseline, 1-week, 1-month) and a between-subjects 
factor of tACS group (theta or control). A Greenhouse-Geisser correction was used 
when appropriate. Effect sizes in ANOVAs are reported as partial eta squared (η2p) and 
with Cohen’s d for independent samples t-tests. Post-hoc comparisons within rm-
ANOVAs were Bonferroni-corrected. 
 
 
References 
[1] 
Kueider AM, Parisi JM, Gross AL, Rebok GW. Computerized cognitive training 
with older adults: A systematic review. PLoS One 2012. 
https://doi.org/10.1371/journal.pone.0040588. 
[2] 
Reinhart RMG, Nguyen JA. Working memory revived in older adults by 
synchronizing rhythmic brain circuits. Nat Neurosci 2019;22:820–7. 
https://doi.org/10.1038/s41593-019-0371-x. 
[3] 
Wei J, Zhang Z, Yao Z, Ming D, Zhou P. Modulation of Sustained Attention by 
Theta-tACS over the Lateral and Medial Frontal Cortices. Neural Plast 2021;2021. 
https://doi.org/10.1155/2021/5573471. 
[4] 
Anguera JA, Boccanfuso J, Rintoul JL, Al-Hashimi O, Faraji F, Janowich J, et al. 
Video game training enhances cognitive control in older adults. Nature 
2013;501:97–101. 
[5] 
Treisman AM, Gelade G. A feature-integration theory of attention. Cogn Psychol 
1980.