Pain Perception
Pain is an unpleasant but essential experience that involves both physical and psychological components. It alerts us to injury, motivates us to protect ourselves, and helps prevent further damage. Although we naturally try to avoid pain, it plays a critical role in survival and adaptation.
Generally speaking, pain can be considered to be neuropathic or inflammatory in nature.
neuropathic and inflammatory pain
Pain generally falls into two main categories: inflammatory and neuropathic.
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Inflammatory pain signals tissue damage or irritation—such as a cut, sprain, or infection. It’s a normal part of the body’s healing response.
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Neuropathic pain results from damage or dysfunction within the nervous system itself (either the peripheral nerves or central pathways). In this case, the brain receives exaggerated or incorrect pain signals, even when no external injury is present.
When Pain Is Absent: Congenital Insensitivity to Pain
A very small number of people are born with a rare genetic disorder called congenital insensitivity to pain (CIP), or congenital analgesia. Individuals with CIP can feel touch, pressure, and temperature, but they do not experience pain.
As a result, they often sustain serious injuries without realizing it—such as burns, fractures, or self-inflicted wounds. Many children with CIP have mouth and tongue injuries from biting themselves without pain feedback.
Because pain helps prevent further injury, people with CIP often develop chronic damage, infections, and joint problems, leading to shorter life expectancy (U.S. National Library of Medicine, 2013).
Life Without Pain?
Imagine a life free of pain. How would it be—calm, fearless, serene? Would you feel invulnerable, invincible? Getting rid of pain is a popular quest—a quick search for “pain-free life” on Google returns well over 4 million hits—including links to various bestselling self-help guides promising a pain-free life in only 7 steps, 6 weeks, or 3 minutes. Pain management is a billion-dollar market, and involves much more than just pharmaceuticals. Surely a life with no pain would be a better one?
Well, consider one of the “lucky few”: 12-year-old “Thomas” has never felt deep pain. Not even when a fracture made him walk around with one leg shorter than the other, so that the bones of his healthy leg were slowly crushed to destruction underneath the knee joint. For Thomas and other members of a large Swedish family, life without pain is a harsh reality because of a mutated gene that affects the growth of the nerves conducting deep pain. Most of those affected suffer from joint damage and frequent fractures to bones in their feet and hands; some end up in wheelchairs even before they reach puberty (Minde et al., 2004). It turns out pain—generally—serves us well.
How Pain Signals Travel
When you step on a pin, your body activates nociceptors, specialized sensory receptors for pain. Action potentials in the receptor cells travel as nerve impulses with different speeds. You’ll notice two sensations:
- A sharp, immediate pain, which tells you to move your foot.
- A dull, aching pain, which lingers to remind you to protect the injury.
These signals travel along different nerve fibers at different speeds and are processed in separate brain regions. Researchers often distinguish between:
- Sensory–discriminatory pain – where pain is located and how intense it is.
- Affective–motivational pain – how unpleasant it feels and how motivated you are to escape it (Auvray et al., 2010; Price, 2000).
The Brain’s Role in Modulating Pain
Pain perception is not fixed—it is influenced by emotion, motivation, and context.
In April 2003, the climber Aron Ralston found himself at the floor of Blue John Canyon in Utah, forced to make an appalling choice: face a slow but certain death—or amputate his right arm. Five days earlier he fell down the canyon—since then he had been stuck with his right arm trapped between an 800-lb boulder and the steep sandstone wall. Weak from lack of food and water and close to giving up, it occurred to him like an epiphany that if he broke the two bones in his forearm he could manage to cut off the rest with his pocket knife. The thought of freeing himself and surviving made him so excited he spent the next 40 minutes completely engrossed in the task. The pain was unimportant. Only cutting through the main nerve made him stop for a minute—the flood of pain, he describes, was like thrusting his entire arm “into a cauldron of magma.” Finally free, he rappelled down a cliff and walked another 7 miles until he was rescued by some hikers (Ralston, 2010).
How is it possible to do something so excruciatingly painful to yourself, as Aron Ralston did, and still manage to walk, talk, and think rationally afterward? The answer lies within the brain, where signals from the body are interpreted. When we perceive somatosensory and nociceptive signals from the body, the experience is highly subjective and malleable by motivation, attention, emotion, and context.
Motivation–Decision Model and Descending Modulation of Pain
motivation-decision model
According to the motivation–decision model, the brain automatically and continuously evaluates the pros and cons of any situation—weighing impending threats and available rewards (Fields, 2004, 2006).
Anything more important for survival than avoiding the pain activates the brain’s descending pain modulatory system—a top-down system involving several parts of the brain and brainstem, which inhibits nociceptive signaling so that the more important actions can be attended to.
In Ralston’s extreme case, his actions were likely based on such an unconscious decision process—taking into account his homeostatic state (his hunger, thirst, the inflammation and decay of his crushed hand slowly affecting the rest of his body), the sensory input available (the silence around him indicating his solitude), and his knowledge about the threats facing him (death, or excruciating pain that won’t kill him) versus the potential rewards (survival, seeing his family again). Ralston’s story illustrates the evolutionary advantage to being able to shut off pain: the descending pain modulatory system allows us to go through with potentially life-saving actions.
However, when one has reached safety or obtained the reward, healing is more important. The very same descending system can then “crank up” nociception from the body to promote healing and motivate us to avoid potentially painful actions.
To facilitate or inhibit nociceptive signals from the body, the descending pain modulatory system uses a set of ON- or OFF-cells in the brainstem, which regulates how much of the nociceptive signal reaches the brain. The descending system is dependent on opioid signaling, and analgesics like morphine relieve pain via this circuit (Petrovic, Kalso, Petersson, & Ingvar, 2002).
Analgesic Power of Reward
Thinking about the good things, like his loved ones and the life ahead of him, was probably pivotal to Aron’s survival. The promise of a reward can be enough to relieve pain. Because expecting pain relief is a form of reward this can contribute to the placebo effect—where pain relief is due to the brain’s own opioid system (Eippert et al., 2009; Eippert et al.; Levine et al.). Eating tasty food, listening to good music, or feeling pleasant touch on your skin also decreases pain in both animals and humans, presumably through the same mechanism in the brain (Leknes & Tracey, 2008).
Pain for Chocolate
In a now classic experiment, Dum and Herz (1984) either fed rats normal rat food or let them feast on highly rewarding chocolate-covered candy (rats love sweets) while standing on a metal plate until they learned exactly what to expect when placed there. When the plate was heated up to a noxious/painful level, the rats that expected candy endured the temperature for twice as long as the rats expecting normal chow. Moreover, this effect was completely abolished when the rats’ opioid (endorphin) system was blocked with a drug, indicating that the analgesic effect of reward anticipation was caused by endorphin release.
For Aron the climber, both the stress from knowing that death was impending and the anticipation of the reward it would be to survive probably flooded his brain with endorphins, contributing to the wave of excitement and euphoria he experienced while he carried out the amputation “like a five-year-old unleashed on his Christmas presents” (Ralston, 2010). This altered his experience of the pain from the extreme tissue damage he was causing and enabled him to focus on freeing himself. Our brain, it turns out, can modulate the perception of how unpleasant pain is, while still retaining the ability to experience the intensity of the sensation (Rainville, Duncan, Price, Carrier, & Bushnell, 1997; Rainville, Feine, Bushnell, & Duncan, 1992).
Social rewards, like holding the hand of your significant other, have pain-reducing effects. Even looking at a picture of them can have similar effects—in fact, seeing a picture of a person we feel close to not only reduces subjective pain ratings, but also the activity in pain-related brain areas (Eisenberger et al., 2011). The most common things to do when wanting to help someone through a painful experience—being present and holding the person’s hand—thus seems to have a measurably positive effect.
Power of the Mind
The context of pain and touch has a great impact on how we interpret it.
The relative meaning of pain was illustrated by an experiment where the same moderate heat was rated as either painful, or as pleasant when it provided relief from a more intense pain (Leknes et al., 2013). The interpretation of touch also varies, as knowing who or what is touching us affects our response. Additionally, we can experience pain and pleasure vicariously, as the same brain areas that process these sensations when we experience them ourselves are also active when we observe someone else experiencing them (Singer et al., 2004).