transcranial Direct Current Stimulation, or tDCS, and transcranial Current Stimulation, or tACS, are close cousins when it comes to brain stimulation technology. Both tDCS and tACS are non-invasive (read “wearable”) devices that pass a little current to the brain to boost brain function and help treat brain diseases. Both are low-current, meaning the total voltage applied is about a 9V battery and the total current is a few mA. Both are considered well “tolerated” (read: does not hurt a lot) when a good device and technique is used. Though tACS is more likely to make you see imaginary white flashes, non-harmful “light shows” that scientists call phosphenes. Both require you to place electrodes on the head, which are then connected to a device. In fact, some devices can be switched between tDCS and tACS like this Soterix 1×1 tES device. Both tDCS and tACS can be upgraded to High-Definition: so HD-tDCS and HD-tACS respectively.
The key difference between tACS and tDCS is the waveform. tDCS uses a DC waveform. Think of the battery, the current coming out is just steady the whole time. In contract, tACS makes a sinusoidal waveform. Think of a wave that is constantly going up and down. Actually, with tACS, the direction of current flow switches as the wave swings one way and the other. Remember in tDCS people talk about anode and cathode (explained here) which is only possible because the current is always flowing one way. In tACS, people do not talk about anode and cathode because the direction of current flow is always switching back and forth. So when you set up tDCS you need to be very careful where you put the anode and cathode (see our montage guide). With tACS, there is no difference. With tACS, the wave can go up and down slowly (low frequency) or up and down fast (high-frequency). So with tACS, you need to specify the wave rate, as in 10 Hz tACS or 40 Hz tACS. Hz means how many times up and down per second. With DC there is no frequency to report (or you could say the frequency is zero)
So whats better, tDCS or tACS? There is no simple answer. It firstly depends on what you plan to use them for. Depending on the application you may find there is more evidence (papers) for tDCS or for tACS. Which does not mean the one with less published evidence does not work. It can also depend on the WAY you want to influence the brain. tDCS changes excitability (read: max capacity) and plasticity (read: learning). tACS is thought to lock in on the brain’s ongoing oscillations and boost them.