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Excerpt:

Introduction Glaucoma is an eye disease that damages the optic nerve, causing peripheral vision loss. Once damage occurs, conventional treatments (like lowering eye pressure) cannot restore the lost vision. Researchers have therefore explored whether non-invasive brain stimulation might help improve remaining vision. Two common methods are transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS), which apply weak electrical or magnetic pulses to the scalp to modulate the brain’s activity. Small studies have tested such techniques on glaucoma patients to see if visual processing (contrast sensitivity, field defects, etc.) can be enhanced. We review these pilot and controlled trials, noting where electrodes or coils were placed, the stimulation settings, the measured vision gains, and how long those gains lasted. We also discuss possible mechanisms (like boosting brain plasticity or reducing neural “noise”) and the importance of good sham-controlled study designs (since practice or placebo effects can mimic improvement). Brain Stimulation Techniques tDCS uses a mild constant electrical current applied through electrodes on the scalp. Depending on polarity, it can increase (anodal) or decrease (cathodal) cortical excitability. Typically, one electrode is placed over the target brain region (often the occipital visual cortex), and the other electrode (reference) is placed elsewhere (e.g. the cheek or forehead). Treatment sessions often last 10–20 minutes at 1–2 mA. TMS uses brief magnetic pulses through a coil to induce electrical currents in the underlying cortex. Both methods have been used for many brain disorders; for vision, they aim to “boost” residual visual function by recruiting plasticity in visual pathways. tDCS in Glaucoma In glaucoma studies, researchers have generally targeted the visual cortex (occipital lobe). A recent randomized trial had patients receive one session of anodal tDCS (a-tDCS) at 2 mA for 20 minutes. The anode was placed at Oz (midline occiput) and the cathode on the cheek. This single session modestly improved visual field detection accuracy (about 3–5% gain in high-resolution perimetry) compared to sham (). The multifocal visual-evoked potentials (mfVEP) also showed slightly higher signal-to-noise and faster responses after a-tDCS. These gains were statistically significant versus sham, but very small in magnitude, roughly on the order of test-retest variability (). In other words, vision improved on some tests, but only by a few percent, which may not be noticeable in daily life. Session parameters: Typical pilot studies used a single 20-minute session of 1–2 mA a-tDCS to the occiput (Oz). One study also tried alternative waveforms (alternating current tACS at 10 Hz, and random-noise tRNS) versus sham, but only a-tDCS showed any clear effect (). No study has used very high intensity or very long duration beyond 20–30 minutes. Vision outcomes: Measured outcomes have included visual field indices (e.g. detection accuracy or mean defect in perimetry) and sometimes contrast sensitivity or visual acuity. In the above trial, a-tDCS produced a small increase in detection accuracy on a high-res perimetry test (). No large change in standard automated perimetry (mean defect) was shown, nor in visual acuity. Contrast sensitivity was not always measured in glaucoma trials, though in other eye disorders tDCS can transiently boost contrast thresholds. Crucially, the Glaucoma RCT noted that the tiny impr