1. A method for detecting relevant stimuli using pupillary response as a correlate to cognitive response, comprising: providing a classifier, said classifier trained to detect patterns of spatio-temporal pupil features and generate an output indicative of the occurrence or absence of a relevant stimulus that evokes both a cognitive response and a pupillary response in a subject; using a sensor to measure the pupillary response of a subject subjected to stimuli in an environment, wherein the occurrence of a relevant stimulus is initially unknown to the classifier; deriving data samples d(n) of the pupil diameter from the pupillary response; segmenting the data samples d(n) into a sequence of time-shifted windows, each window including a response period and a baseline period; extracting a plurality of spatio-temporal pupil features from the data samples d(n) in said response and baseline periods in each said window; and for each said window, presenting the extracted pupil features to the classifier to generate an output indicative of the occurrence or absence of a relevant stimulus.