Cognitive Effects of Theta Burst Stimulation in Mild Cognitive Impairment and Alzheimer's Disease

Transcranial magnetic stimulation (TMS) is a noninvasive brain stimulation technique that is increasingly used for a growing number of research and clinical applications.Typically, this transient magnetic field is focally applied with a figure-of-eight coil that is carefully placed on the surface of the scalp over a targeted stimulation site. Patterned repetitive TMS (rTMS), such as theta burst stimulation (TBS) can produce long-lasting effects on neural activity and behavior beyond the stimulation period (Chou et al., 2015a; Fitzgerald et al., 2006). In general, high frequency (> 5 Hz) rTMS and its newer version, intermittent theta burst stimulation (iTBS), facilitate cortical excitability, whereas low frequency (about 1 Hz) rTMS and continuous theta burst stimulation contribute to opposite effects (Pascual-Leone et al., 2000; Huang et al., 2005; Wassermann and Zimmermann, 2012).Careful manipulation of the parameters comprising these patterned rTMS pulse trains can induce neuroplastic changes that resemble either long-term potentiation (LTP) or depression (Chen et al., 1997; Pascual-Leone et al., 1994). Early studies targeting the motor cortex helped elucidate which rTMS parameters promote particular responses and their neurophysiological underpinnings (Klomjai et al., 2015).

In recent years, rTMS has been closely investigated to evaluate its potential to modulate cognitive functions in Alzheimer'sdisease (AD) and mild cognitive impairment (MCI). As compared to conventional excitatory rTMS protocols, iTBS leads to comparable effects with similar number of pulses but considerable shorter duration and lower intensity of stimulation (Bakker et al., 2015; Rossi, Hallett, Rossini, Pascual-Leone, & Safety, 2009). Recent literature also suggest that TBS has lower rates of reported adverse event (AE) compared to rTMS (Najib & Horvath, 2014). Therefore, iTBS is assumed to modulate cognitive function in people with cognitive impairments.