A monitoring device for monitoring an individual under investigation comprises a recording device for recording a neuronal signal from the individual, and a calculating device for calculating a parameter representing a quantitative measure of an anaesthesia or coma condition of the individual and being derived from the neuronal signal, wherein the calculating device is adapted for calculating a complexity parameter representing the quantitative measure of the anaesthesia or coma condition, and the calculating device includes a buffer circuit for storing at least one time series of the neuronal signal and an analysis circuit for subjecting the time series to a recurrence quantification analysis (RQA) providing the complexity parameter. Furthermore, a method includes estimating a parameter representing a quantitative measure of an anaesthesia or coma condition of an individual.