TRANSCRANIAL ELECTRICAL STIMULATION MODULATES POWER AND FIELD-FIELD COHERENCY IN MACAQUE VISUAL CORTEX

Transcranial direct or alternating current stimulation (tDCS or tACS) are used for the treatment of several cognitive disorders, many of which are due to imbalances in excitatory-inhibitory (E-I) interactions, but how stimulation affects the underlying neural network remains an open question. In the primary visual cortex (V1), E-I interactions due to presentation of large gratings induce slow (20 Hz-35 Hz) and fast gamma (40 Hz-70 Hz) oscillations, which weaken with ageing and neurodegeneration. However, the effect of long duration tDCS/tACS on stimulus-induced gamma is unknown.
We applied 3 mA cathodal tDCS and ±1.5 mA or ±2.5 mA 20 Hz tACS targeting V1 for 20 minutes, while full-screen gratings were shown to two monkeys before, during and after stimulation. We analyzed the power and field-field coherency (FFC), a measure of phase consistency, to assess changes in power and connectivity due to tDCS/tACS.
We found an increase in fast gamma power (40 Hz-70 Hz) and FFC (60 Hz–100 Hz) with tDCS post stimulation which persisted for about 1.5 hours. This observation aligns with a previous finding that during tDCS, inter-areal coherency is enhanced in the gamma range. Conversely, tACS at 20 Hz reduced slow gamma (20 Hz-35 Hz) power and FFC (40 Hz–60 Hz), although the effect was weaker and less consistent across monkeys.
Long-lasting changes in power and coherency within a brain area might modulate information processing, learning, and adaptation. Our findings illuminate the potential to modulate synchrony via transcranial electrical stimulation that outlasts the stimulation period.