Neuroanatomical Hypotheses

There has been very little direct empirical investigation of neuroanatomical correlates of normal laughter. A hypothetical neural circuit was first theorized in 1924 by Kinnier Wilson. According to Wilson's model, laughter is produced by a medullary effector center that links the seventh nerve nucleus in the pons with the 10th motor nucleus in the medulla and with phrenic nuclei in the upper cervical cord. This center is modulated by the cerebral cortex and limbic structures by means of an integrative center in the mesial thalamus, hypothalamus, and subthalamus.

Activity of the laughter center is thought to be determined by a voluntary pathway (corticobulbar fibers) and fibers extending from the orbital surface of the frontal lobes through the bulbar nuclei. Input to these fibers comes from an involuntary pathway (the basal ganglia), which appears to be inhibited by the voluntary one. No single cortical area supplies the origins for the voluntary and involuntary fibers; the input derives from diffuse cortical regions including the frontal, premotor, motor, parietal, temporal, and hippocampal regions.

The notion of hypothalamic integration of cortical and limbic control on the brain stem laughter center remains to be substantiated, although it is consistent with clinical observations.

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