Here's one of the eerier conditions out there. Pathological laughter and crying can cause patients to go into fits of laughter, or tears, out of the blue. The people who have it generally feel calm, but can't physically restrain themselves in any way. Why?
Exterior and Interior Emotions
Pathological laughter and crying (PLC) has earned itself many names. It's sometimes referred to as pseudobulbar affect, sometimes as emotional lability, and sometimes as emotional incontinence. Of its many names, the last one is probably the most inappropriate. PLC has very little to do with emotions. It's the fact that most of us assume that when a person is crying they must be grief-stricken, or when they're laughing they have to be full of joy, that causes misdiagnoses.
People with PLC are sometimes diagnosed with bipolar disorder or depression. Both of those conditions are disorders of mood and emotion. PLC is almost mechanical. Beyond dismay that they're unable to stop, people who go on these laughing or crying jags don't feel deep emotions during their outbursts. If they feel anything, the apparent emotion doesn't linger after the fit has ended. It's almost like the body is having a aesthetically-guided spasm. The eyes and mouth work mechanically, and coordinately, in a way that is meant to convey an intense emotion, but the emotion isn't there.
The Search for a Cause
All patients with PLC have damaged brains. Occasionally, the condition can surface after strokes, or in the early stages of Alzheimer's or Parkinson's disease. It can also start after the brain takes a physical hit, but the condition most often associated with PLC is amyotrophic lateral sclerosis, which is a deterioration of the spinal nerves, and the cerebellum. In the past, the fact that PLC is caused by degenerative diseases has made finding the cause of the outbursts extremely difficult. The only way to examine the problem directly was through an autopsy, and by the time the patient died, they had lesions all over their brains.
Over time scientists discovered more sophisticated tools than a sharpened knife. Areas of the brain can be kicked into action (or inaction) through use of electrostimulation. Surprisingly, it was fairly simple to activate emotions, and the physical expression of those emotions. Finding which parts of the brain to stimulate to activate the expression of emotion without the emotion itself as harder. One patient, while undergoing a study for Parkinson's disease, could be moved to uncontrollable tears without sadness by stimulation of the left subthalamic nucleus, a region just above the cerebellum. (Interestingly, a change in frequency in the same spot provoked no tears, but did cause a feeling of intense anxiety.) Stimulation of the right anterior cingulate cortex, an area towards the front of the brain, caused patients to burst into "mirthless laughter," which sounds really creepy, both for the subject and the researchers.
The Surprise In Our Brain
In the end, there are several areas of the brain that can cause joyless laughter and sorrowless tears, but they tend to cluster around the base of the brain and top of the spine. This may be why amyotrophic lateral sclerosis, which causes deterioration begining in that area, is so often associated with PLC. The question is, what is happening in the brain to bring on the outbursts?
There are two theories. One simply holds that the lesions destroy emotional control. The crying and laughing "centers" of the brain are "disinhibited," and just happen to be disinhibited without bringing the actual emotions along for the ride.
There is a more recent, and intriguing, theory. We don't generally decide to burst into tears. We don't even decide to laugh. Still, the process is initiated by some conscious recognition. If we don't recognize a situation, we can't be either amused or grieved by it. Recognition of certain contexts brings on a physical reaction, as well as a mental one. Some researchers believe that, when certain areas of the brain are damaged, we lose the ability to recognize the appropriate contexts for those physical reactions. The part of the brain in charge of the physical response to emotional situations loses the ability to distinguish between the right response to being told we're going to be executed and the right response to being told what's for lunch. Physically we respond the same way, while emotionally don't feel the same distress. This would mean that our brain has a trigger for physical responses to emotions, and a separate trigger for mental responses to emotions - and they can see the same situation very different ways.