Human emotions may be studied by using a number of techniques. Unlike other species, the report of emotional behaviour according to the subject or an observer of the subject through a questionnaire may be used. Also, behavioural measures such as a rated observation of categorised facial expressions may be used. Alternatively, measures of psychophysiological changes in arousal, such as heart rate or electrical brain activity (using EEG), allow the monitoring of arousal levels during the presentation of an emotionally arousing stimulus. However, all of these measures are to some extent uncertain. Behavioural and psychophysiological measures are ambiguous since they may be associated with more than one state of arousal. Also, subjective reports are sometimes unreliable, especially in the clinical arena when patients may wish for more or less sympathy. Also, some observers may not be impartial. These problems of measurement contribute to the challenges in researching the nature and mechanisms underlying emotional behaviour.