Excitatory Conditions

The most common excitatory condition that is associated with pathological laughter occurs in epileptics. Laughter as part of an epileptic seizure was documented by the neurologist Trousseau in 1873 and was described in Dostoevsky's novel The Idiot. In 1957, the term ''gelastic epilepsy'' was coined to refer to epileptic fits in which laughter is the only or the most common symptom. More than 160 cases of gelastic epilepsy have been reported in the literature. Gelastic epilepsy often occurs in patients with hypothalamic hamarto-mas and precocious puberty, in patients with complex partial seizures and temporal lobe origin seizures, and in children with infantile spasms. The seizures usually begin in infancy or childhood and are associated with cognitive decline in later years. In patients with these seizures, involuntary, mechanical giggling typically occurs as the initial ictal behavior before an alteration in consciousness. The duration of the laughter in patients with hamartomas is usually less than 30 seconds and the seizures occur several times a day.

In a recent review of published reports of seizures involving laughter, Biraben and colleagues proposed the following neural hypotheses for the genesis of laughter:

1. Laughter arising as a reactional behavior (i.e., in response to a pleasant feeling or mirth: Only a few such cases have been observed, all involving seizures with a temporal focus. In one such case, involving stimulation of the left temporobasal region (fusiform and para-hippocampal gyri), the individual experienced a change in the semantic connotation of stimuli (things became funny); in another case with stimulation in the same region, the modification experienced was perceptual (things changed in a funny way). Biraben et al. suggest that the laughter in these cases might be a physiological response to a modified cognitive process.

2. Laughter arising as a forced action or an automatism: Seizures with a frontal focus have characteristically produced laughter described as forced and unmotivated. Arroyo and colleagues describe one patient with a cavernoma of the anterior cingulate gyrus whose laughter appeared to be an irrepressible motor behavior; resection of the lesion eliminated the laughing behavior. They suggest that the premotor mesial system acts as an interface between the limbic loop, which includes the anterior cingulate gyrus, and the motor loop, which includes the supplementary motor area. Biraben et al. propose that cases of gelastic seizure originating in the anterior cingulum involve a critical functional disconnection between the motor loop and the limbic loop within the mesial premotor system. The laughter arising in these cases reflects a behavioral output from a motor program separated from all motivation.

Laughter in one other epileptic patient reviewed, a nongelastic case, occurred after cortical electrical stimulation of the lateral border of the rostral part of the supplementary motor area. The laughter was natural, rather than forced, and was accompanied by a general feeling of amusement toward the environment at large. Biraben et al. suggest that such laughter may reflect an imbalance between the mesial and lateral premotor systems.

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