Phantom arms, legs and now cell phone vibrations -- you can feel them, you can sense them, but they aren't really there.
Chalked up largely to a natural anomaly in the wiring of the brain, such experiences blur the boundaries between reality and imagination in those who experience them.
It is hard, for example, to understand how patients who have had an arm or leg amputated can experience acute pain in a limb that is no longer there. But for these people, the pain is uncomfortably real.
"In the past, it was thought that people with phantom limbs were crazy, but today we know that people aren't crazy," says Dr. Jack Tsao, an assistant professor of neurology at the Uniform Services University in Maryland. "There is a physiological basis to this sensation."
And phantom sensations don't just affect amputees. Though it is a complaint of an entirely different magnitude, many cell phone and BlackBerry users report feeling vibrations when their phones are, in fact, silent.
Although such sensations are nothing like pain from a phantom limb, doctors say the two phenomena may be somewhat related.
"If you use your cell phone a lot, it becomes part of you," says Dr. William Barr, the chief of neuropsychology at the New York University School of Medicine. "You become habituated to it.
"It's like wearing a tight sock all day," he explains. "When you take it off, you still feel it there on your foot. If your cell phone is not there, you still feel like it is."
It's in Your Mind
Although the reasons for these false perceptions are not definitely known, researchers agree that, in both cases, a certain part of the brain plays a key role.
For amputees who experience phantom limbs, the part of the brain affected is called the somatosensory cortex. This region contains nerves that process information related to touch.
For example, if someone touches your hand, nerves in a particular area of the somatosensory cortex are activated, allowing you to feel that your hand is being touched. All of your body parts are mapped out to a certain area of the cortex.
Jon Kaas, a professor of psychiatry at Vanderbilt University, explains that after losing a limb, the neurons that control the movement and the sensation of that limb are still present in the brain.
"After losing a limb, the brain is reorganized," he says. "When neurons are deprived of their normal source of stimulus, they send out new connections and start to respond to new signals."
Kaas goes on to explain how patients with an amputated arm can still feel a sensation in their arm when they are touched on the face. The reason for this is that the nerves that originally controlled the hand have formed new connections so that they now respond to a touch on the face.
"There is an orderly organization of the body in the cortex of the brain," Kaas says. "If you lose input from your hand, the section of neurons that control the hand are initially inactive, but then recover and start to respond to other connections.
"These brain parts still have the capacity to signal to the limb even though the limb is not present, which gives the sensation of a phantom limb."
Like the phantom limb phenomenon, mysterious cell phone vibrations can also be explained by changing nerve connections in the brain.
"Cell phones enter into the neuromatrix of the body -- they become appendages," says Barr.
So when you leave your cell phone at home, the brain interprets it as it would a phantom limb -- it's not present, but you feel as though it is.
"It's an interesting technological statement about society that our machines are becoming part of us," says Barr.
Press the Button, Get the Message
Kaas says another principle, known as operant conditioning, may also be at play in phantom phone vibrations.
In studies of operant conditioning, researchers have found that rats that are rewarded after pressing a lever will learn to press the lever more frequently. The pressing becomes habitual.
In the case of cell phones, people are rewarded when they pick up their calls and read their incoming text messages, which causes them to pick up their cell phones more and more frequently.
"People are rewarded when they are able to detect low amplitude vibrations so they get better and better at responding," says Kaas. "It is very rewarding to get the message, so people are able to train their system to detect that signal."
As people repeat this behavior over and over again, connections between nerves in their brain become stronger and new ones are formed, which helps to make the behavior automatic.
And sometimes, as is the case with vibrating cell phones, the behavior becomes too automatic.
"People have gotten so good at detecting vibrations that they start responding to false positives -- they think something is there when it is not," says Kaas.
Tsao agrees. "Most people keep BlackBerrys on their hips or in a shirt pocket, and so the body is used to picking up sensations in this area," he says. "Even when the BlackBerry isn't present, the body gives the signal that something is going off."
Kicking the 'CrackBerry' Addiction
And just as the brain changes to create phantom sensations, it can also change back to get rid of them. Over time, the phantom limb syndrome goes away as other parts of the brain take over the part that controls the limb.
"Sometimes other parts of the brain will move into the real estate occupied by the amputated limb," says Barr. "Over time, other parts of the brain start to encroach on the part of the brain that represents the phantom limb."
Similarly, experts say that those haunted with BlackBerry vibrations should simply stop using them.
"The problem will stop if people stop carrying BlackBerrys," says Kaas. "It's not a permanent condition. If people stop carrying their BlackBerrys, the connections between neurons will degrade, and people will be able to retain their neurons to do other things."
However, while phantom sensations arise from similar brain functions, phantom limbs and phantom phone vibrations are in no way similar in how they affect the lives of those experiencing these sensations. Losing a limb and losing a cell phone are not at all comparable, and many experts emphasize that the pain experienced by some amputee victims can be seriously disabling.