Photograph by John Clark
Imagine for a moment a weather service that uses lots of lookouts, each of whom takes one measurement. Then all of them call in their results to a central phone exchange at once. One specialist reports whether it’s hot, another whether it’s cold, another whether it’s windy, and so on. To save time and money, each lookout indicates the severity of the condition being monitored by the number of times he lets the phone ring before hanging up. For example, one ring signals a light breeze and five rings a tornado. At the central exchange, a forecaster determines the overall conditions by monitoring the collective ringing of all the phones. That, in essence, is your brain analyzing your skin. One set of sensors reports cold, another warmth, another pain, and so forth. As you are about to discover, separate skin sensors sometimes get their lines crossed.
Find two glasses and three identical knives with metal handles. Fill one glass with cold water and ice cubes, and place one of the knives, handle first, into it. Next, fill the other glass with tap water that’s very warm but not painfully hot. Then dunk the remaining two knife handles into the heated water for 60 seconds. Ask a friend to extract the knives, fit the cold handle snugly between the two warm ones, and then quickly touch the three handles to the inside of your wrist as you close your eyes. You’ll experience a burning sensation that is more intense than what would be evoked by placing the two warm handles on the same spot.
This sensory confusion, which neurologists call the thermal-grill illusion, may occur because cold objects touching the skin simultaneously stimulate both fast-conducting (A-delta type) nerve fibers that signal cold and slower (C type) fibers that signal pain. Fast and slow sensory nerves connect to a single place in the spinal cord, and that pain information is then passed on to the brain. Signals from the faster, cold-transmitting fibers inhibit those spinal relays from the slower, pain-transmitting fibers. The net result is that a cold stimulus elicits cold but not pain sensations, even though some pain fibers are initially turned on. However, two warm stimuli placed right next to a cold object can dilute the amount of cold information flowing from a patch of skin. That decreases both the excitation of fast cold-signaling channels and the inhibition of pain signals, causing the illusory perception of pain.
The dilution of a cold stimulus by a warm one implies that the different signals are blended over a broad patch of skin—a phenomenon known as spatial summation. One way to test this is to see whether the pain illusion disappears when wide-area summing of the two sensations does not happen.
Repeat Experiment 1, but this time ask your accomplice to place the handles on your lips. You should perceive a three-part warm-cold-warm sensation (with no pain) as opposed to a single, hot-pain sensation. That is because your lips—unlike your wrists—do not lump together tactile inputs from broad swaths of skin and so can discern much finer details.
Neurologists study temperature-pain illusions because about 3 million Americans who have nerve damage from diabetes or trauma experience a disorder called neuropathic pain. For these individuals, even a light touch or a cool breeze can produce extremely painful, burning sensations. The most likely explanation is that the mechanism that allows fast sensory pathways to inhibit slow pain pathways has been disabled. Emerging therapies that untangle these circuits offer hope that people who suffer from neuropathic pain can someday count on more accurate sensory forecasts.