But the new study uses a different form of stimulation, known as transcranial randomnoise stimulation (TRNS), which applies randomly fluctuating currents (within certain parameters) to the head.
Six months later, when participants were contacted again and asked to solve calculations, researchers found that the group which had received five days of transcranial randomnoise stimulation showed "superior long-term performance, compared to sham controls".
But he said transcranial randomnoise stimulation could help more people because it had the potential to improve the ability to add, subtract or multiply a string of numbers in your head - not just new number learning.
If there were no correlation between prosperity and distance from the equator, we would expect the green line to be centered at zero with random oscillations due to noise in the data.