Slow Cortical Potentials [SCP’s] are very slow electrical shifts in brain activity. They change periodically from electrically negative to positive and play a significant role in the regulation of attention. A negative shift in the SCP signal represents activation of the cortex, increasing the firing probabilities of the underlying cortical areas. A positive shift represents an inhibition of the cortex and a decrease in firing probabilities. A well-researched negative SCP is the Contingent Negative Variation (CNV), which may reflect expectancy, motivation, intention to act or attention.
SCP’s are direct current (DC) shifts in the EEG and last from 300 msec to several seconds. The SCP itself does not have an absolute DC level, it is always a relative change from the starting point in the DC potential of the EEG. SCP’s are measured with an EEG electrode on the skin of the scalp, using a true DC-amplifier. As the SCP signal is very vulnerable to artifacts, the SCP signal needs to be corrected for (especially vertical) eye movements. NeXus uses a uniquely designed SCP sensor with two active EEG electrodes combined in one electrode for measuring SCP signals. This combined signal is referenced in a two-channel setup against both A1 and A2. As such, horizontal eye movements are balanced out online and subtracted from the EEG signal.