A system and method for assessing the function of parts of the nervous systemof a subject by measuring the pupillaryresponses to at least one stimulus ensemble comprising a plurality ofindividual stimuli the method comprising: presenting asequence of selected individual stimuli from the at least one stimulusensemble to the nervous system of a subject thereby evokingpupillary responses in at least one pupil of the subject, selected individualstimuli being concurrently presented in the sequence,wherein the individual stimuli are each individually balanced such that thepupillary responses evoked by individual stimuli in theensemble are balanced according to the strength of the neural responses evokedby the individual stimuli detecting responses ofthe pupil or pupils evoked by the stimuli using a sensor and processing thedetected responses to relate them to the function of thesubjects neural responses to some or all of the individual stimuli of theensemble.