Here’s a question that lies at the intersection of biology and philosophy: Is itch a unique form of touch that is qualitatively different from the other touch modalities, or is it merely a different pattern of stimulation that relies upon one or more of the touch senses we have already encountered in this book? By analogy, is the relationship between itch and other touch sensations like that between a saxophone and a piano? Each produces sound, but those sounds are qualitatively different. Or is it like the relationship between bebop jazz played on the piano and classical music of the Romantic period played on the piano? They, too, are clearly distinguishable because of their musical structure and context, but they come into being on the same sound-producing device. In the past, this type of question would have been left to philosophers. Today, biology can add to the discussion.
Some who believe that the itch is a pattern rather than a unique type of touch contend that it is merely a particular type of pain—one of a weak, dilute character. They point out, correctly, that itch and pain have certain similarities. Both can be triggered by a wide variety of stimuli: mechanical, chemical, and sometimes thermal. In particular, both pain and itch can be activated by chemical products of inflammation and can sometimes be relieved by anti-inflammatory drugs. Both are subject to strong modulation by cognitive and emotional factors, including attention, anxiety, and expectation. And both pain and itch signal the intrusion of things in the environment that should be avoided—they are, in other words, motivational senses that demand action. Pain leads to a reflexive withdrawal response; itch leads to a reflexing scratching response. Scratching in response to itch, like withdrawal from pain to prevent tissue damage, is thought to be protective. It can cause us to dislodge venomous arthropods, like spiders, wasps, or scorpions, or those than transmit disease-causing pathogens, like malarial mosquitoes or plague-bearing fleas.
If itch were merely a weak or intermittent form of pain, then one would imagine that increasing the integrity or frequency of an itchy stimulus could raise it to the threshold of feeling painful, or, conversely, that attenuating a painful stimulus could cause it to evoke an itch sensation. However, when studied in a lab with carefully controlled stimuli, this never happens. Weak pain is just weak pain, and intense itching is just intense itching. Another key distinction between itch and pain involves their location on the body. While pain can be felt widely, in the skin, muscles, joints, and viscera, itching is restricted to the outer layer of the skin and the mucous membranes that adjoin the skin, like those that line the mouth, throat, eyes, nose, labia minora, and anus. You can have pain in your guts, but not itchy guts.