Are pain and itchiness the same thing?

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If you'd asked that question a few years ago, neuroscientists might have scratched their heads (heh) and shrugged. The sensation of pain and what scientists call "itchy" are so closely linked in your brain that it's almost impossible to tell where one begins and the other ends. But a new study has changed all that.

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In a study published last week in PLoS One, delightfully named "Analysis for Distinctive Activation Patterns of Pain and Itchy in the Human Brain Cortex Measured Using Near Infrared Spectroscopy," a group of neuroscientists led by Chih-Hung Lee and Takashi Sugiyama used a sophisticated imaging technique to see what happens to blood flow in the brain when subjects feel pain, versus when they are itchy. Previous studies had used fMRI imaging, which revealed only that the two sensations seemed to blossom in the same region of the prefrontal cortex.


Write the researchers:

Distinct from pain, itch is an unpleasant perception that evokes the desire to scratch. Although there are dissimilarities between their processing, pain and itch are thought to be closely related in that weak activation of nociceptors mediates itch, while strong activation of the same receptors results in weak pain. Moreover, there is a broad overlap in neuromediators of pain and itch signal processing. Interestingly, scratch-induced pain can abolish itching, suggesting reciprocal control of pain and itch.


So we know that itchy and pain feel subjectively similar. And brain imaging shows why that is — some of the same receptors in your nervous system are involved in processing both sensations. But did that mean itchy and pain were basically variations on the same neurological phenomenon?

That's what Lee and colleagues wanted to know. So they got seven test subjects and tried a new imaging technique, near-infrared spectroscopy (NIRS), which allowed them to see how blood flow in the brain changed in real time:

We observed distinct activation patterns in the frontal cortex for acute pain and histamine-induced itch. The prefrontal cortex exhibited a pain-related and itch-related activation pattern of blood flow in each subject. Although it looked as though that activation pattern for pain and itching was different in each subject, further cross correlation analysis of NIRS signals between each channels showed an overall agreement with regard to prefrontal area involvement. As a result, pain-related and itch-related blood flow responses (delayed responses in prefrontal area) were found to be clearly different between pain (τ = +18.7 sec) and itch (τ = +0.63 sec) stimulation. This is the first pilot study to demonstrate the temporal and spatial separation of a pain-induced blood flow and an itch-induced blood flow in human cortex during information processing.


So pain and itchy may be in roughly the same region of the brain, but blood flows into those regions at a different rate depending on whether you're feeling itchy or feeling pain.

What's hopeful about this discovery is that it might allow researchers down the line to treat pain and itchy disorders with more precision. But it's also just a really interesting development in how we understand the way physical sensations are translated into brain activity.


Read the full study on PLoS One