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."
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.