Physiological changes of spectral frequency content during d2 test. Depiction of frequency changes at all electrode positions during the performance of the d2 test in comparison to the relaxed eyes-open state at the first visit (day A). Colour vision map with interpolated data is constructed as described under methods. Statistically significant changes between these two states are marked by an asterisk and depicted with respect to position in the map. Left side of figure corresponds to frontal brain region.