Laughter and Happiness
It is not known why laughter is so frequent in AS. More recently, advances in neuroimaging have assisted researchers in uncovering the cortical and subcortical regions of the brain that are associated with laughter in normal individuals. Specifically, the results of these studies indicate that the humor-processing pathway includes parts of the frontal lobe brain area, important for cognitive processing; the supplementary motor area, important for movement; and the nucleus accumbens, associated with pleasure. The supplementary motor area is most involved in the motor aspects of humor (laughing and smiling). Recent neuroimaging studies show that increased activation in the supplementary motor area is correlated with laughter [1, 2], as is activation in the dorsal anterior cingulate area. Both of these regions receive rich dopamine input from the ventral striatum. A recent functional imaging study also showed that the cortical structures involved with humor and laughter include: the temporo-occipital junction, inferior frontal gyrus/temporal pole, and the supplementary motor area/dorsal anterior cingulate, all in the left hemisphere [3]. The same study also implicated that several subcortical structures, including the amygdala, ventral striatum/nucleus accumbens, ventral tegmental area, thalamus, and hypothalamus are also involved in humor and laughter. Taken together, the results suggest that the left hemisphere plays a distinct role in the humor processing pathway, and subcortical, dopaminergic structures play an important role in humor and laughter.
Studies of the brain in AS, using MRI or CT scans, have not shown any defect suggesting a site for a laughter-inducing abnormality. Recent neuroimaging studies in deletion positive patients with AS do reveal some abnormalities within the humor processing pathway. Specifically, individuals with AS exhibit reduced volume in the nucleus accumbens as well as the globus pallidus within the left hemisphere, and these regions are associated with both the reward mechanisms associated with humor/laughter as well as the motor aspects of laughter. The results of diffusion tensor imaging studies also reveal abnormalities (reduced fiber density and coherence) within white matter pathways in the limbic system in patients with AS, as well as prefrontal regions, and differences in the fibers projecting to and from the internal capsule that may account for difficulties with the regulation of laughter. The results of magnetization transfer imaging studies reveal differences in the thalamus (fibers projecting to/from the thalamus) that could also account for some of these difficulties[4]. Although there is a type of seizure associated with laughter, termed gelastic epilepsy, this is not what occurs in AS. The laughter in AS seems mostly to be an expressive motor event; most reactions to stimuli, physical or mental, are accompanied by laughter or laughter-like facial grimacing. Although AS children experience a variety of emotions, apparent happiness predominates.
The first evidence of this distinctive behavior may be the onset of early or persistent social smiling at the age of 1-3 months. Giggling, chortling and constant smiling soon develop and appear to represent normal reflexive laughter but cooing and babbling are delayed or reduced. Later, several types of facial or behavioral expressions characterize the infant's personality. A few have pronounced laughing that is truly paroxysmal or contagious and “bursts of laughter” occurred in 70% in one study [5]. More often, happy grimacing and a happy disposition are the predominant behaviors. In rare cases, the apparent happy disposition is fleeting as irritability and hyperactivity are the prevailing personality traits; crying, shrieking, screaming or short guttural sounds may then be the predominant behaviors.
1. Iwase M, Ouchi Y, Okada H, et al. Neural substrates of human facial expression of pleasant emotion induced by comic films: a PET Study. Neuroimage, 2002. 17(2): p. 758-68.
2. Osaka N, Osaka M, Kondo H, et al. An emotion-based facial expression word activates laughter module in the human brain: a functional magnetic resonance imaging study. Neurosci Lett, 2003. 340(2): p. 127-30.
3. Mobbs D, Greicius MD, Abdel-Azim E, et al. Humor modulates the mesolimbic reward centers. Neuron, 2003. 40(5): p. 1041-8.
4. Peters SU, Bacino CA, Chu Z, et al. Inside the brain in Angelman Syndrome: Phenotypic characterization using advanced neuroimaging techniques (452). The Am Soc Hum Genet Mtg (www.ashg.org/2008meeting/abstracts/fulltext), 2008.
5. Buntinx IM, Hennekam RC, Brouwer OF, et al. Clinical profile of Angelman syndrome at different ages. Am J Med Genet, 1995. 56(2): p. 176-83.