Desynchronization of the EEG in relation to mental activity can be demonstrated through various means. The acceleration spectrum entropy (ASE) is a method that quantifies the desynchronization of the EEG. It is a measure of the randomness or irregularity of a time series and ranges from 0 (straight line) to 100 (discrete random signal). The ASE was calculated for 20 EEGs under four conditions: "eyes closed," "eyes open," "eyes closed and mental arithmetic" and "eyes open and mental arithmetic." There was a significant increase between the conditions "eyes closed" and "eyes open" and a significant increase between the conditions "eyes closed" and "eyes closed and mental arithmetic." During eye opening the largest increase was seen in the occipital areas. Mental arithmetic showed an asymmetrical increase, namely in the left temporo-occipital region and in the right centro-temporal region. It is suggested that the ASE can be used to study cortical activation due to mental activity.