The effects of noninvasive tDCS have been studied in both heal-thy individuals and those with neurological disorders. Based on stimulation polarity, tDCS can either increase or decrease the excitability of the underlying cortex. Anodal stimulation increases excitability with neuronal depolarization and cathodal stimulation decreases excitability with neuronal hyperpolarization [141516]. Synaptic activity controls the changes in intracortical inhibition or facilitation and can affect the results of tDCS [17]. Anodal tDCS of the left temporoparietal area (LTA) and dorsolateral prefrontal cortex (DLPFC) are potentially the most favorable polarity and stimulation sites for tinnitus relief [18192021]. tDCS of LTA results in more widespread diffused impact on cortical areas beyond the target region. However, tDCS of DLPFC results in a more localized impact on the target region itself [22].

Anodal tDCS of the LTA has led to transient suppression of tinnitus in 42% [23] and 35% [18] of participants. A comparatively long-lasting impact on tinnitus perception, lasting up to a few days, was observed in a recent double-blind, sham-controlled study conducted by Garin et al. [18] using tDCS for 20 minutes with 1 mA current intensity. Anodal tDCS produced more favorable effects compared with cathodal or sham tDCS. Vanneste and colleagues [19] explored whether tDCS of DLPFC would suppress tinnitus and used a slightly higher current intensity (1.5 mA) than other authors [1823]; they reported a 29.9% positive response rate with bifrontal tDCS using an anode on the right DLPFC and cathode on the left DLPFC. However, a review of studies conducted since 1998 using tDCS in humans under various clinical conditions showed that no studies used a current intensity greater than 2 mA [24]. The use of a current higher than 2 mA would require a preparatory investigation of safety issues, which was beyond the scope of this study.

tDCS can potentially be applied in clinical settings for patients with tinnitus, although more research is needed in this area.

Transcranial magnetic stimulation (TMS) is a non-invasive tool that can be used to modulate neural activity. TMS delivers a high intensity and short-lasting current pulse via an insulated stimulating coil. This produces a magnetic field perpendicular to the coil which penetrates the underlying scalp and brain at a reduced intensity ultimately affecting the activity of cortical neurons in the brain region beneath the coil (Fig. 1). TMS can be administered as a single pulse or as paired pulses.

The magnetic field reaches a maximum of 1.5-2 T (same size as a MRI scanner) in approximately 100 µs and then decays back to zero [25]. The magnetic coils have different shapes. Round coils are the most powerful. Figure-eight-shaped coils or double-cone coils are more focused with a maximal current delivered at the intersection of the 2 round components which can deliver the current to the targets more precisely (Fig. 2).

Reportedly, single magnetic pulses have no long-lasting effects on the brain, however multiple pulses of rTMS tend to show prolonged effects on the brain. Cortical excitability was repeatedly decreased with low-frequency (≤1 Hz) rTMS [26], while high-frequency (5-20 Hz) rTMS resulted in increased excitability [27].

These types of stimulations are used for neurophysiological exploratory purposes. rTMS administers multiple TMS pulses to a patient's head during a single period [28]. Repetitive magnetic fields produced by rTMS can decrease neural overactivity in cortical areas and alleviate tinnitus [29]. Specifically, rTMS of the auditory cortex is effective in the treatment of tinnitus [30]. Neuroplastic changes in the brain, as a reaction to sensory deafferentation, are considered a cause of tinnitus [31]. Networks of several cortical areas may cause tinnitus, but the exact area remains unknown. Progress in functional imaging has helped explain the pathophysiology and identified brain lesions related to tinnitus [32]. Several studies have reported that positron emission tomography (PET) can be used to identify the region of unbalanced cortical activity where rTMS could be applied [3334]; however, controversy remains regarding the usefulness of PET for rTMS targeting [35] and the function of PET and other neuroimaging modalities in determining the stimulation site is debatable. Rossi [36] reported that stimulation (5 days, 1-Hz rTMS) in the left temporoparietal area was effective and not related to tinnitus laterality. Moreover, only a few studies showed that stimulation of the left temporoparietal cortex produced better results than stimulation of the right cortex [3637].

Since rTMS showed positive outcomes in treatment for tinnitus, many groups have reported their own treatment protocols and results. Specifically, Eichhammer [30] reported significant improvement in 3 patients treated with brain stimulation. Subsequently, many other studies have reported the effectiveness of rTMS stimulation in the temporoparietal region [373839] ranging from 8-50%. Kim et al. safely applied rTMS to the temporoparietal cortex daily for 5 days with long-term benefits [40].

rTMS is widely used to treat various psychological diseases. Specifically, stimulation of the prefrontal area with rTMS was proven effective in patients with depression [41]. rTMS of the auditory cortex showed positive effects in the treatment of chronic tinnitus [30]. However, stimulation-related issues, including intensity, duration or predictors of response, are uncertain. Recently, rTMS of multiple brain cortices including the auditory cortex was perform-ed to treat tinnitus [42]. Kleinjung reported that combined rTMS of the temporal and frontal brain cortices was more effective than rTMS of the temporal area alone [43]. Park et al. reported that combined rTMS of the auditory and prefrontal cortices was more beneficial than rTMS of the auditory cortex alone to treat tinnitus in patients with depression [44]. However, low-frequency left temporal rTMS combined with low-frequency right DLPFC rTMS did not show increased benefit [45].

Previous studies reported adverse effects including sensorineural hearing loss after temporal rTMS or epileptic seizures after high-frequency and high-intensity rTMS [46]. However, to date, the results reported showed rTMS is a well-tolerated and safe technique [47].