The invention relates to a method and apparatus for assessing the reactivity observable in a certain physiological signal, especially the EEG signal, of a subject. In order to obtain a compact measurement arrangement that enables reliable assessment of changes in the reactivity of the subject even during a long time period, a conventional monitoring device is provided with a stimulation module configured to supply stimuli to the subject through a measurement probe that the device uses to receive physiological signal data from the subject. The measurement probe may comprise, for example, an arm cuff through which the device monitors the blood pressure of the subject.