An apparatus for assessing the brain state of a patient comprises at least one sensor to monitor at least one physiological variable, which provides at least one output signal. In addition, the apparatus for assessing the brain state of a patient comprises a computer processor for receiving the at least one output signal, wherein the computer processor filters the at least one output signal and applies pattern recognition techniques in evaluating the brain state of the subject. The computer processor comprises a first filtering module, which removes transient events not indicative of a brain state, and a second filtering sub-module, which groups together multiple modalities on the basis of classification techniques and incorporates additional information regarding mental states.