Assessment of one of the sensory nervous systems of a human subject using patterns of null and non-null stimuli. Parts of the visual system for example, are presented with two simultaneous sequences of stimuli. Each sequence is varied over time between a null stimulus and one or more less frequent non-null stimuli. The variation of each sequence is also controlled so that neighbouring parts of the sensory system are less likely to receive simultaneous non-null stimuli. The stimuli are therefore sparse both in time and in some other aspect, typically a spatial dimension. One or more responses of the subject are measured and weight functions are determined for assessment of the sensory system.