Cross-modal Neuromodulation to Improve Balance and Cognition in Older Adults With Glaucoma: A Randomized Trial of Cerebellar Transcranial Electrical Stimulation

The Narrow Path Walking Test (NPWT) is a modified version of the Time Up and Go Test (TUGT) that assesses dynamic balance function during gait in a more challenging environment. In the TUGT, subjects stand up from a 46 cm-tall chair, walk 3 m at their normal walking speed, bypass an obstacle, return, and sit down. NPWT requires participants to perform the same procedure as the TUGT. In NPWT, a 40 x 300 cm green mat with 24% Michelson contrast against the walking path to create a challenging environment, dividing the path into two 20 cm-wide narrow pathways to test mediolateral dynamic balance. The NPWT time will be measured. To add difficulty, the dual task of the serial subtraction will be added in the walking procedure.To achieve stable performance, each test will be repeated 3 times, and the results will be averaged to ensure consistency. The recording parameters for NPWT will include trial time, mistakes, cognitive performance, and dual-task cost.

The Mini-BESTest will be utilized to assess participants' balance through a series of 14 tasks that encompass both static and dynamic balance challenges, including activities such as sit-to-stand, walking with head turns, standing on one leg, and Time up and Go with a dual task. Each task is scored on a scale from 0 to 2, with higher scores reflecting superior balance abilities. To provide a comprehensive evaluation, we will include variations in task difficulty, enabling us to analyze how different levels of complexity influence balance performance. The total score from the Mini-BESTest will be examined to gauge participants' balance proficiency and to identify any improvements resulting from the intervention. All tasks will be administered in a standardized sequence, ensuring a systematic and reliable approach to balance assessment for all participants.

The SOT provides a quantitative assessment of an individual's ability to utilize specific sensory cues to maintain stability during quiet stance. The protocol consists of six standardized conditions designed to systematically disrupt or isolate sensory information: (1) eyes open, firm surface; (2) eyes closed, firm surface; (3) sway-referenced vision, firm surface; (4) eyes open, sway-referenced surface; (5) eyes closed, sway-referenced surface; and (6) sway-referenced vision and surface. During the assessment, participants will be instructed to stand still with their feet shoulder-width apart for 20 seconds per trial. The primary outcome measures include the Composite Equilibrium Score, representing a weighted average of postural stability across all conditions, and Sensory Analysis Ratios, which are calculated to pinpoint specific impairments in the visual, somatosensory, and vestibular systems.

The UFOV test will include three conditions:

• Stimulus Identification: A central target will be displayed on the screen for 60 frames, before being masked by a 'white noise' screen. The display duration will be adjusted using a 3-up/1-down adaptive staircase procedure to determine the threshold, with testing concluding after eight reversals or seventy-two total trials.
• Divided Attention: Both a central and a peripheral target will be displayed simultaneously, the latter appearing randomly at one of eight meridian locations (0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°, 360°) at 20° from the center. The presentation duration will be adjusted using the algorithm as in test 1 to reach a threshold.
• Selective Attention: This condition will include a central target, a peripheral target, and additional peripheral distractors. The distractors will subtend the same visual angle as the peripheral target. Apart from the introduction of distractors, the procedure is the same as for test 2.

The total Reaction Time Test will be divided into two parts. The first part will involve a simple reaction assessment, utilizing Python software to create the stimulus.

Participants will be instructed to respond to a stimulus shown in the center of the screen. The second part of the test will involve a choice reaction test, in which four squares will be arranged horizontally on the screen. The stimulus will appear randomly in one of the squares, and the participants will be guided to react to the position as quickly as possible. For the first part, the corresponding time to the stimulus will be recorded. For the second part, the reaction time to the stimulus and the task accuracy will be recorded. Each part will be repeated 3 times to get the average score.

To evaluate whether tACS or tDCS can modulate visual cognitive function by modulating ocular blood flow, further providing insight into the effects of these methods on retinal and optic nerve function, the LSFG RetFlow will be used to evaluate the change in the optic blood flow before and after the stimulation