1. A device for generating a representation of a subject's attention level comprising: means for measuring brain signals from the subject; means for extracting temporal features from the brain signals; means for classifying the extracted temporal features using a classifier to give a score x 1 ; means for extracting spectral-spatial features from the brain signals; means for selecting spectral-spatial features containing discriminative information between concentration and non-concentration states from the set of extracted spectral-spatial features; means for classifying the selected spectral-spatial features using a classifier to give a score x 2 ; means for combining the scores x 1 and x 2 to give a single score; and means for presenting said single score to the subject.