The first-of-its-kind imaging study showed that women activate the parts of the brain involved in language processing and working memory more than men when viewing funny cartoons. Women were also more likely to activate with greater intensity the part of the brain that generates rewarding feelings in response to new experiences.
"The results help explain previous findings suggesting women and men differ in how humor is used and appreciated," said Allan Reiss, MD, the Howard C. Robbins Professor of Psychiatry and Behavioral Sciences and director of the Center for Interdisciplinary Brain Sciences Research. He added that the results, which appear in the Nov. 7 online issue of the Proceedings of the National Academy of Sciences, could lead to a better understanding of medical conditions such as depression and cataplexy.
Researchers know that a number of brain structures, including the prefrontal cortex, which is involved in language processing and memory, are involved in humor appreciation. In 2003, Reiss and colleagues showed for the first time that the brain's mesolimbic reward center, which is responsible for the rewarding feelings that follow such events as monetary gain or cocaine use, is also activated by humor.
Past studies have shown gender differences in the use and appreciation of humor and the meaning and function of laughter, but no previous research has examined sex-specific differences in the brain's response to humor.
During this study, 20 healthy adults (10 men and 10 women) viewed 70 black-and-white cartoons and then rated the cartoons on a one-to-10 "funniness scale." During the screenings, the researchers used functional magnetic resonance imaging to monitor participants' brain function. They then analyzed blood oxygenation-level-dependent signal activation, a measure of neural activity, in various parts of the brain.
After analyzing the data, the researchers found that men and women share much of the same humor-response system; both use to a similar degree the part of the brain responsible for semantic knowledge and juxtaposition and the part involved in language processing. But they also found that some brain regions were activated more in women. These included the left prefrontal cortex, suggesting a greater emphasis on language and executive processing in women, and the nucleus accumbens, or NAcc, which is part of the mesolimbic reward center.
Reiss said he was taken off guard by the NAcc finding. After puzzling over it, he and his colleagues theorized that because the women in this study used more analytical machinery when deciphering humorous material, it signaled that they weren't necessarily expecting the cartoons to be as rewarding as did the men. When a woman's brain encountered the punch line, her reward center lit up. According to Reiss, the activation of this center not only signals the presence of something pleasant, but that the pleasure was unexpected.
"Women appeared to have less expectation of a reward, which in this case was the punch line of the cartoon," said Reiss. "So when they got to the joke's punch line, they were more pleased about it."
The researchers also found that the funnier the cartoon, the more the reward center was activated in women. That was not the case in men who seemed to "expect" the cartoons to be funny from the start. If subsequent studies show that women's reward center and other regions of the brain are more sensitive to emotional stimuli, including negative stimuli, that could help explain why depression strikes twice as many women as men, potentially leading to new therapies, Reiss said.
The results of the study also have potential implications for individuals who suffer from cataplexy, in which a sudden loss of motor control is precipitated by strong emotions, most notably humor.
In other findings, men and women showed no significant difference in the number of stimuli they rated as funny, nor how funny they found the humorous stimuli. Response time for both funny and unfunny cartoons was also similar, although women were quicker at identifying material they considered unfunny.
In a related study that also appears in Proceedings of the National Academy of Sciences, Reiss and his colleagues, including Dean Mobbs, now a PhD student at University College London, found that personality traits, such as extroversion and introversion, affect how humor is processed. "The combined results of these two studies suggest that humor taps into several neural systems associated with gender or personality and helps to explain individual differences in humor appreciation," said Reiss.
The gender study was funded by the National Institutes of Health and a Howard Hughes Summer Fellowship from the Department of Biological Sciences at Stanford. Undergraduate student Eiman Azim, now a graduate student at Harvard University, was a key collaborator and the first author.
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