A system and method for assessing the function of parts of the nervous system of a subject by measuring the pupillary responses to at least one stimulus ensemble comprising a plurality of individual stimuli the method comprising: presenting a sequence of selected individual stimuli from the at least one stimulus ensemble to the nervous system of a subject thereby evoking pupillary responses in at least one pupil of the subject, selected individual stimuli being concurrently presented in the sequence, wherein the individual stimuli are each individually balanced such that the pupillary responses evoked by individual stimuli in the ensemble are balanced according to the strength of the neural responses evoked by the individual stimuli detecting responses of the pupil or pupils evoked by the stimuli using a sensor and processing the detected responses to relate them to the function of the subjects neural responses to some or all of the individual stimuli of the ensemble.
