Oscillations in cortical networks are likely to support the basic neurophysiology behind cognitive and movement-related processes (Buzsaki & Draguhn, 2004; Varela et al., 2001). Transcranial electric stimulation (tES) has successfully produced changes in neurotransmitter concentrations (Stagg et al., 2009) and modulates functional connectivity as measured by functional magnetic resonance imaging (Polania et al., 2012). The combination of tES with techniques like magnetencephalography (MEG) enables oscillatory responses in specific frequency bands to be investigated during the application of transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS) with high temporal resolution. Such combinations will improve understanding of the neurophysiological mechanism mediating the effects of tES (Dayan et al., 2013). Moreover, the effect of tES on the underlying cortex is likely to be greatest in proximity to or directly underneath the stimulating electrode. Unlike EEG, MEG is capable of making such measurements during tES.
tDCS-MEG & tACS-MEG
The first issue arising from applying tES with MEG is that tDCS produces artefacts in MEG measurements. However, changes in gamma-band and alpha power evoked by visual stimulation can successfully be measured during the tDCS (Hoogenboom et al., 2006; Marshall et al., 2016). Subsequent investigations have successfully revealed that the oscillatory response to visual stimuli and voluntary movements can be modulated by tDCS. For example, Hanley et al. (2015) found a significant reduction in alpha power following anodal stimulation relative to sham. Moreover, when participants complete a task, polarity-dependent effects of tDCS have been found on slow cortical magnetic fields (0 - 4 Hz) evoked by finger tapping, which were also reflected in a change in reaction time (Garcia-Cossio et al., 2016). Such investigations reveal the feasibility and potential utility of concurrent tDCS-MEG experiments.
tACS-MEG, on the other hand, presented a slightly different challenge. The challenge is that the frequency-specific effect of tACS on the oscillatory response overlaps with the frequency of the tACS artefact measured by MEG (Witkowski et al., 2016), which is a particular issue when brain oscillations are expected to align in phase with electric current applied to the scalp (e.g. Thut et al., 2011; Zaehle et al., 2010). A developing approach to deal with this potential issue is to recruit a specific type of tES waveform, which applies amplitude modulation to a sinewave. An amplitude modulated tACS waveform is illustrated in figure 1. Amplitude modulation involves two frequencies: a carrier frequency and an envelope frequency. The carrier frequency is the stimulation frequency of tACS whereas the envelope frequency enables the amplitude of tACS to a change as a function of time.
Amplitude-modulated tACS (AM-tACS) was developed to reduce the the impact of tACS frequency-related artefacts, which often overlap with the frequency of interest to MEG measurements. Here, the frequency of electric current administered by the stimulator - the carrier frequency - would be much higher than the frequency of interest during MEG (Witkowski et al., 2016). The carrier frequency, on the the hand, would be a lower frequency. Artefacts of tACS occur at the frequency of stimulation, meaning that the artefact will be present at the carrier frequency, but not the envelope frequency of AM-tACS (Witkowski et al., 2016). There is evidence that such an approach does not interfere with the beta rebound associated with voluntary movement (Witkowski et al., 2016), suggesting that conventional MEG phenomena can be successfully retrieved during the application of amplitude-modulated tACS (AM-tACS). Such an approach would be limited if effects were not produced using amplitude modulated waveforms were not associated with a measurable effect. However, Minami & Amano (2017) showed that AM-tACS can shift the peak alpha frequency (PAF), which was reflected in a change in the perception for perceived jitter. This suggests that AM-tACS is useful for not only avoiding artefacts during MEG but also for producing effects that are functionally relevant.
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