Laughter: it's not always fun and games.

For many of us, laughter is a perfectly normal part of human existence: it helps us build relationships, generates good brain chemicals, and releases pent-up tension. It can even improve your cardiovascular function.

But not all laughter is healthy. For some, it's no laughing matter at all.

When Tigist Feyisa noticed her two-year-old son breaking into peals of uncontrollable giggles that left him "confused and disoriented," she grew concerned. Then she learned that Matthew's pathological laughter episodes -- gelastic seizures, in scientific parlance -- were actually the result of a tumor in his brain. "It's heartbreaking," Feyisa said. "I mean, heartbreaking to find out your child has a tumor."

Thankfully, such lesions are usually benign, and Ephrem appears to be functioning normally after being operated on earlier this month. Still, the growths are linked to all sorts of developmental and behavioral problems, even if they themselves aren't inherently life-threatening. Uncontrollable laughter is just the beginning -- patients have experienced symptoms ranging from early-onset puberty to full-blown intractable epilepsy. To date, what we know about gelastic seizures mostly revolves around how the disease progresses in patients over time. Why the tumors develop is still largely a puzzle.

If research on what scientists call hypothalamic hamartoma is rather limited, so are the treatments. Doctors have tried everything from anticonvulsant medications to radiation therapy. Removing the tumor surgically seems to work, but it's an invasive and risky procedure, especially when many of the patients tend to be younger children (the tumors appear to be congenital).

Under normal conditions, according to brain scans conducted by William and Mary researcher Peter Derks, laughter works like this: first, the left side of your cerebral cortex interprets the content and structure of a piece of humor. The right side of the cortex performs the analysis enabling you to get the joke. An electrical wave then pulses through your cerebral cortex about four-tenths of a second later; you laugh if the wave takes a negative charge.

For whatever reason, hypothalamic hamartoma subverts this process so that patients suffer from spontaneous explosions of laughter. What's scary is that the chuckles may or may not come along with the mirthful feeling the rest of us experience when we see or hear something funny. Instead, it's a purely physical impulse that seems almost entirely isolated from a patient's surroundings. That can be both disorienting and frightening.