Incomplete evidence that increasing current intensity of tDCS boosts outcomes

Abstract

The presence or absence of simple dose response does not impact how efficacious a given tDCS dose is for a given indication. Understanding dose-response in human applications of tDCS is needed for protocol optimization including individualized dose to reduce outcome variability, which requires intelligent design of dose-response studies.

Methodology for tDCS integration with fMRI

“…Local EF intensities do not have a trivial relationship with resulting neuromodulation in neuronal network activity level and ultimately complex behaviors (Esmaeilpour et al, ). Therefore, combining imaging data and current flow predictions requires assumptions on (a) the scaling of “neuromodulation” with regional EF (which itself requires decision on if to use maximum e‐field, average, or some other aggregate measure) and (b) the measure of brain “activation” based on imaging.…”

“…The relationship between applied current over skin and induced electric field in brain is linear and determined by the physics of tDCS (i.e., electric field intensity in the brain will increase linearly with applied current; more applied current, more electric field in brain [Bikson et al, ; Esmaeilpour et al, ]). However, the challenging question is whether increasing current intensity necessarily increases neural and behavioral responses?…”

“…Anatomical and biological factors such as head size (Bikson, Rahman, & Datta, 2012), skull thickness and individual gyrification of cortex (Opitz, Paulus, Will, Antunes, & Thielscher, 2015), structural and functional connections (Rosso et al, 2014) neurotransmitter levels (Krause & Cohen Kadosh, 2014), age (Minhas, Bikson, Woods, Rosen, & Kessler, 2012) and genetics (Thair, Holloway, Newport, & Smith, 2017) are among the parameters that can cause variability in response to a given dose across participants. Notably, models and measures of current flow show that for the same applied current, substantial differences in induced electric field in brain can occur across individuals (Esmaeilpour et al, 2018;Truong et al, 2014). it may be important to consider the differences between diffuse stimulation when conventional two-pads as opposed to focal stimulation with HD-tDCS (Datta et al, 2009).…”

Current intensity‐ and polarity‐specific online and aftereffects of transcranial direct current stimulation: An fMRI study

“…For example, duration of tDCS after‐effects are generally influenced by the stimulation duration, current intensity, and the number of treatment sessions; however, the dose–response to manipulating these parameters (ex: by prolonging the stimulation duration (Monte‐Silva et al, ; Monte‐Silva, Kuo, Liebetanz, Paulus, & Nitsche, ), or increasing the current intensity (Batsikadze, Moliadze, Paulus, Kuo, & Nitsche, ; Jamil et al, )) do not necessarily lead to greater effects. This nonlinearity is an important concern, as it indicates a need for more nuanced experimental approaches where stimulation parameters are investigated through systematic titrations (Esmaeilpour et al, ). Moreover, given the extant nature of physiological findings on areas other than the M1, an open question for development of clinical protocols is whether the physiological findings of tDCS‐MEP studies conducted in the M1 can be extrapolated in order to understand effects of tDCS in other cortical areas.…”

“…For example, duration of tDCS after-effects are generally influenced by the stimulation duration, current intensity, and the number of treatment sessions; however, the dose-response to manipulating these parameters (ex: by prolonging the stimulation duration (Monte-Silva et al, 2013;Monte-Silva, Kuo, Liebetanz, Paulus, & Nitsche, 2010), or increasing the current intensity (Batsikadze, Moliadze, Paulus, Kuo, & Nitsche, 2013;Jamil et al, 2017)) do not necessarily lead to greater effects. This nonlinearity is an important concern, as it indicates a need for more nuanced experimental approaches where stimulation parameters are investigated through systematic titrations (Esmaeilpour et al, 2018).…”