1. A method for assessing physiological signal reactivity of a subject in a single-trial analysis, the method comprising: obtaining a single continuous physiological signal from the subject; supplying a stimulus to the subject while the physiological signal is being obtained; constructing, based only on a time series of the physiological signal obtained from the subject, a signal model for the physiological signal using a processor; applying a single time reference to the time series corresponding to the time the stimulus is supplied; obtaining further signal data from the time series of the physiological signal, the further signal data being obtained from the subject subsequent to the time reference; testing whether the signal model is a valid signal model for the physiological signal, wherein the testing includes employing the signal model constructed from the physiological signal obtained at least partially prior to the single time reference and the further signal data obtained subsequent to the single time reference; and indicating, based on the testing, whether reactivity is present in the physiological signal.