Abstract: The invention relates to a method and apparatus for assessing the reactivity observable in a certain physiological signal, especially the EEG signal, of a comatose subject. In order to obtain an objective and a reliable measure of the reactivity automatically and without the presence of a trained EEG specialist, a valid signal model is constructed for an EEG signal obtained from the subject. A time reference corresponding to a stimulus is applied and further signal data is obtained from the time series, the further signal data being subsequent to the time reference. By employing the further signal data, the method tests whether the signal model remains to be a valid signal model for the EEG signal also after the stimulus, and indicates, based on the test, whether reactivity is present in the physiological signal.