Frequency‐dependent t ACS modulation of BOLD signal during rhythmic visual stimulation

Chai, Yuhui, Jingwei Sheng, Peter A Bandettini, and Jia‐Hong Gao. 2018. 'Frequency‐dependent t ACS modulation of BOLD signal during rhythmic visual stimulation', Human brain mapping, 39: 2111-20.
 
Summary:        Transcranial alternating current stimulation (tACS) is able to modulate neural activity. However, the spread of tACS effects in the brain is unknown. Using concurrent tACS and fMRI Chai et al. delivered 8 Hz stimulation to visual cortex during presentation of a flickering checkerboard gradient at 8 Hz. They found that stimulus-modulated visual cortical activation was increased by stimulation matched to the rate of the flickering checkerboard. These effects were strongest with 8 Hz tACS, weaker at its harmonic 4 Hz, and no present with 16 Hz tACS. TACS may be most effective at boosting ongoing neural activity when aligned to that activity or at its harmonic frequency.
 
Pros:

• Concurrent tACS-fMRI is novel and provides insight into the spatial specificity of tACS
• Simple and elegant task design provides interpretable findings

 
Open questions:

• Follow-up experiment that found that these effects were only present for the 8Hz flickering visual stimulus, and matched 16Hz tACS resulted in decreased neural activity for a 16Hz flickering stimulus. Could this suggest a special role for alpha oscillations (8-12Hz) in visual cortex?
• Could the effects be stronger by considering the individual alpha frequency at which visual cortex resonates?
• Does stimulation alter perception or subjective experience?

 
Contributed by: Justin Riddle, PhD