A system for calculating an indicator associated to a brain activity of a subject, the system including an acquisition module configured to acquire at least an epoch of electroencephalographic signal of a subject from a plurality of electrodes and a data processing module configured to carry out the steps of: calculating an average vector (VA) using as input of an autoencoder neural network (aNN) an electroencephalographic signals (ES) of a subject acquired from a plurality of electrodes; detecting (DET) the presence of at least a predefined pattern in the consecutive average values of the average vector (VA); and generating an indicator of brain activity (Idx) of the subject when detecting the predefined pattern.