Although laughter and humour have been constituents of humanity for thousands if not millions of years, their systematic study has begun only recently. Investigations into their neurological correlates remain fragmentary and the following review is a first attempt to collate and evaluate these studies, most of which have been published over the last two decades. By employing the classical methods of neurology, brain regions associated with symptomatic (pathological) laughter have been determined and catalogued under other diagnostic signs and symptoms of such conditions as epilepsy, strokes and circumspect brain lesions. These observations have been complemented by newer studies using modern non-invasive imaging methods. To summarize the results of many studies, the expression of laughter seems to depend on two partially independent neuronal pathways. The first of these, an 'involuntary' or 'emotionally driven' system, involves the amygdala, thalamic/hypo- and subthalamic areas and the dorsal/tegmental brainstem. The second, 'voluntary' system originates in the premotor/frontal opercular areas and leads through the motor cortex and pyramidal tract to the ventral brainstem. These systems and the laughter response appear to be coordinated by a laughter-coordinating centre in the dorsal upper pons. Analyses of the cerebral correlates of humour have been impeded by a lack of consensus among psychologists on exactly what humour is, and of what essential components it consists. Within the past two decades, however, sufficient agreement has been reached that theory-based hypotheses could be formulated and tested with various non-invasive methods. For the perception of humour (and depending on the type of humour involved, its mode of transmission, etc.) the right frontal cortex, the medial ventral prefrontal cortex, the right and left posterior (middle and inferior) temporal regions and possibly the cerebellum seem to be involved to varying degrees. An attempt has been made to be as thorough as possible in documenting the foundations upon which these burgeoning areas of research have been based up to the present time.
