The Laughing Brain

February 12, 2019

Written by: Sarah Reitz

 

When was the last time you laughed? Maybe it was after hearing a joke your friend told, reading a funny tweet, or watching one of the hundreds of Youtube videos of babies laughing. Laughter is formally defined as the physiological response to humor, and is present from the time we are infants. It is something that all humans have in common, regardless of age, sex, race, or language spoken. In fact, laughter even appears to cross species boundaries, with forms of laughter observed in many types of non-human primates, dolphins, and even rats (1)! Artificial intelligence (AI) researchers believe that laughter is such a critical part of being human that they are currently attempting to design robots and other AI that can laugh at the appropriate times, as well as make us laugh. But what purpose does our laughter serve? And do we know what happens in the brain to produce laughter?

 

Laughter and the social brain

Laughter is thought to be a social signal that is crucial for bonding and adapting to different social situations. It is a universal language: something that everyone can understand and bond over, even if we cannot communicate in any other way. Think about all of the times you’ve laughed in the last week or two. Chances are you were with another person! We rarely laugh by ourselves; when you get a text from a friend and respond with “lol”, you both know that you probably weren’t really laughing out loud. But if your friend made the same comment while you were physically together, chances are you actually would burst out laughing. In this way, laughter provides a sign of acceptance to our friends or anyone we are interacting with.

Laughter can also serve as an important social cue that helps us interpret our many social interactions. In fact, laughter is believed to predate human speech, with the thought that it may have helped our early ancestors to communicate their intentions when interacting with each other before languages were developed. As our societies grow increasingly complex, so does our laughter. Humans have different types of laughter that we use depending on the social situation we are in. The laugh you use when you hear your dad’s awful jokes is distinct from the laugh you use when recalling a funny situation with your best friend, and this is because the social intention behind the laughs is different. In one, you are trying to make your dad happy even though his joke isn’t really that funny, and in the other you are remembering a hilarious bonding moment shared between you and your best friend.

In fact, laughter is such an important social cue that neuroscientists are now studying what happens in our brain when we hear different kinds of laughter. A pair of studies found that the medial prefrontal cortex (mPFC) — a brain region involved in processing information about social situations and interactions, among other things — was significantly more active in people who heard forced laughter compared to people who heard genuine laughs (2). Additionally, the more active the mPFC was when listening to recordings of laughter, the better the participant was at determining whether the laughs were forced or genuine. These findings could be interpreted as the brain trying to decipher the intent of the laughter, perhaps to prevent you from being fooled. The brain seems to be able to detect genuine laughter right away, accepts it as genuine, and does not spend more time trying to analyze the meaning behind it. But when a more ambiguous laugh is heard, the brain devotes more energy trying to understand what this person is trying to communicate with this particular laughter. Another interesting finding was that the auditory cortex, which processes sound information, was more active when listening to laughs rated as happier and more authentic. If laughter is a source of positive social bonding, it makes sense that our brains would find true laughter pleasant and want to listen to more of it!

 

How does the brain produce laughter?

When we think about laughter, we can divide it into two main components. The first component is the emotional aspect, known as mirth, which occurs when you find something funny or joyful. The second component is the motor component, which is responsible for actually producing the physical smile and laughter. In other words, laughter likely consists of a “laugh detector” and a “laugh producer”. Though we still don’t fully understand how the brain detects and produces laughter, research over the past few decades suggests that more than one brain region is involved.

Much of what we know about the neuroscience behind laughter comes from studies conducted on epileptic patients. These patients have electrodes inserted into many regions of their brain to determine the origin of their seizures. In addition to being helpful for the patients, these electrodes also give neuroscientists a unique opportunity to observe what happens when different areas of the the human brain are activated. These studies support the idea of distinct brain regions controlling the emotional and motor components of laughter.

Current work suggests that one brain region controlling the motor aspect of laughter is the frontal operculum, a region of the frontal lobe (Figure 1). Brain imaging studies show that this area is active when we observe others smiling, and when we are imitating others’ smiles (3). When this area was activated in 4 separate patients, each patient smiled and laughed (4). Interestingly, these patients could not say why they were smiling or laughing, and did not appear to be experiencing the mirth that normally accompanies laughter.

To produce mirth along with laughter, it appears that regions in the frontal and temporal lobes must be activated. Three of the main regions that are involved are the superior frontal gyrus, the basal temporal lobe, and the pregenual anterior cingulate cortex. One study showed that activation of a small (2cm x 2cm) area of the superior frontal gyrus (Figure 2) consistently produced laughter and mirth in an 18 year old epileptic patient (5). Incredibly, the stronger this region was activated, the longer and harder the patient laughed! After each activation and bout of laughter, the researchers would ask the patient what she had found so funny, and each time she would attribute it whichever person or object was in the room at that moment.

Although clearly these regions appear to be involved in producing laughter and a sense of merriment, we still do not completely understand what it is about these regions that allows them to produce these behaviors. However, scientists believe that the brain areas responsible for producing laughter must be fairly small, since stimulation of the regions discussed above does not always result in laughter and mirth, perhaps because the wrong portion of that area is being activated.

As neuroscience advances, we will continue to learn more about the neural mechanisms behind laughter and further our understanding of such an arguably important part of being human. Perhaps as we further understand exactly how our brains produce laughter, we will also be able to understand why laughter is so contagious, or how laughter actually boosts our physical health. In the meantime, anytime you want laugh just do a quick Youtube search for “babies laughing” (or click here) and enjoy!

 

Image References:

Cover image by NicoBorie via Pixabay, https://pixabay.com/en/smile-man-worker-vertical-laughter-1591798/

Figure 1 from Gray’s Anatomy of the Human Body, via Wikimedia Commons, public domain. https://commons.wikimedia.org/wiki/File:Operculum.png

Figure 2 from Anatomography, via Wikimedia Commons, CCA-Share Alike 2.1 Japan. https://commons.wikimedia.org/wiki/File:Superior_frontal_gyrus_-_animation.gif

 

References:

1. Panksepp & Burgdorf (2000) 50kH chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables. Behav Brain Research 115(1):25-38
2. McGettigan, C et al. (2013) Individual differences in laughter perception reveal roles for mentalizing and sensorimotor systems in the evaluation of emotional authenticity. Cereb Cortex 25(1):246-57
3. Jabbi, M & Keysers, C (2008) Inferior frontal gyrus activity triggers anterior insula response to emotional facial expressions. Emotion, 8(6): 775-780.
4. Caruana et al. (2016) Smile and laughter elicited by electrical stimulation of the frontal operculum. Neuropsychologia 89:364-370
5. Fried et al. (1998) Electric current stimulates laughter. Nature, 391:650