The final strength of tDCS that we want to emphasize is that it is easily combined with other methods used to measure neural activity. This is because the effects of tDCS last for several hours, allowing sufficient signal averaging of the changed brain. We will briefly describe a couple of examples in which tDCS was combined with functional MRI and recordings of participants’ EEG and the averaged ERPs.
The downside of the long-lasting effects of tDCS is that this type of stimulation results in essentially static changes in the brain. These are slowly evolving effects, and a skeptic might argue that tDCS provides essentially no temporal resolution. However, when it comes to combining tDCS with fMRI, this can be made to be an advantage. For example, with fMRI it is possible to measure far field effects across the entire brain. When this has been done, researchers have shown that tDCS stimulation can result in changes across a large brain-wide network. Chib, Yun, Takahashi and Shimojo (2013) (link) provide a nice example of this combination of methods. They showed that stimulation of prefrontal cortex (i.e., anode at Fp1 and cathode at F3) resulted in signal change in the ventral medial cortex of participants viewing face stimuli. That is, this experiment showed that stimulation of relatively remote brain areas can result in far field activations that are not in the current path. This is a natural combination of methods because fMRI excels in measuring whole brain activity to understand the potentially board networks influenced by the tDCS that is long lasting enough to perform the necessary scans.
We might expect tDCS to have poor temporal resolution due to the sluggish nature of this causal manipulation. However, there is evidence from EEG and ERP studies that tDCS can have surprisingly specific effects at certain points in time during the flow of information processing (Reinhart and Woodman, 2015b (link)). For example, several recent studies showed that tDCS at different locations on the head changed one specific ERP component (lasting approximately 100 ms) while leaving temporally adjacent components unchanged. One such study showed that tDCS stimulation applied to parietal cortex changed the N1 component elicited by visual stimuli, but not P1 or N2pc measured just tens of milliseconds on either side of the N1 (Reinhart and Woodman, 2015e (link)). Another study showed that medial-frontal stimulation changed the error-related negativity (ERN) measured in the first 150 ms following a response, but no other components during the entire flow of information processing in a visual discrimination task (Reinhart and Woodman, 2014 (link)). This means that the temporal precision of tDCS may be better than we think, changing activity during just one 100–150 ms period and not any other periods of processing during a trial lasing 1 second or more. Thus, combining the slow after effects of tDCS with a high temporal resolution technique like electrophysiology can demonstrate that tDCS can have effects with high temporal specificity.
The downside of the long-lasting effects of tDCS is that this type of stimulation results in essentially static changes in the brain. These are slowly evolving effects, and a skeptic might argue that tDCS provides essentially no temporal resolution. However, when it comes to combining tDCS with fMRI, this can be made to be an advantage. For example, with fMRI it is possible to measure far field effects across the entire brain. When this has been done, researchers have shown that tDCS stimulation can result in changes across a large brain-wide network. Chib, Yun, Takahashi and Shimojo (2013) (link) provide a nice example of this combination of methods. They showed that stimulation of prefrontal cortex (i.e., anode at Fp1 and cathode at F3) resulted in signal change in the ventral medial cortex of participants viewing face stimuli. That is, this experiment showed that stimulation of relatively remote brain areas can result in far field activations that are not in the current path. This is a natural combination of methods because fMRI excels in measuring whole brain activity to understand the potentially board networks influenced by the tDCS that is long lasting enough to perform the necessary scans.
We might expect tDCS to have poor temporal resolution due to the sluggish nature of this causal manipulation. However, there is evidence from EEG and ERP studies that tDCS can have surprisingly specific effects at certain points in time during the flow of information processing (Reinhart and Woodman, 2015b (link)). For example, several recent studies showed that tDCS at different locations on the head changed one specific ERP component (lasting approximately 100 ms) while leaving temporally adjacent components unchanged. One such study showed that tDCS stimulation applied to parietal cortex changed the N1 component elicited by visual stimuli, but not P1 or N2pc measured just tens of milliseconds on either side of the N1 (Reinhart and Woodman, 2015e (link)). Another study showed that medial-frontal stimulation changed the error-related negativity (ERN) measured in the first 150 ms following a response, but no other components during the entire flow of information processing in a visual discrimination task (Reinhart and Woodman, 2014 (link)). This means that the temporal precision of tDCS may be better than we think, changing activity during just one 100–150 ms period and not any other periods of processing during a trial lasing 1 second or more. Thus, combining the slow after effects of tDCS with a high temporal resolution technique like electrophysiology can demonstrate that tDCS can have effects with high temporal specificity.
