Pain: sensory processing in the human brain

I had a specific interest in spatial coding of noxious input, which underlies our ability to estimate where it hurts.

When I started becoming interested in pain, during my PhD, I was surprised to discover how little it was known about spatial coding of noxious input. I felt we were lacking the ABC: such as basic information about cortical topographical organization and spatial acuity. So there I started. My primary model of human pain has been laser pain: radiant heat generated by high-energy infrared lasers (for example, Nd:YAP lasers) selectively activate skin nociceptors (which mediate the sense of pain), without co-activating skin mechanoreceptors (which mediate the sense of touch). Depending on the energy used, laser stimulation of skin nociceptors can safely elicit sensations of both first pain (pinprick, sharp pain) and second pain (dull warmth or burning heat).

First we asked: are there fine-grained maps of noxious input in the human primary somatosensory cortex? We only had evidence of coarse maps, discriminating hand and foot, but there was no evidence of a detailed map of noxious input to the hand. By collaborating with Marty Sereno and Patrick Haggard, we combined phase-mapping fMRI methods with laser stimulation of skin nociceptors and found, in each of the subjects we studied, a fine-grained map of nociceptive input to the digits in the central sulcus of the human brain. These pain maps were co-aligned to maps of tactile input to the same digits.

Average maps of pain (A-delta) and touch (A-beta) in the human central sulcus

 

We also described for the first time the distribution of spatial acuity across the whole body surface, and its relation to innervation density. By studying patients with a complete loss of the sense of touch (due to a rare neuropathy), we found that spatial acuity for pain does not require a functioning tactile system.

Mapping of spatial acuity for pain and touch

 

In addition to spatial coding, I have investigated how vision and touch influence the perception of pain. We have shown that looking at our body while experiencing pain can reduce pain thresholds, through changes in functional connectivity between visual and somatosensory brain regions. Furthermore, experiencing touch together with pain can also reduce pain, by inhibiting nociceptive processing at subcortical level.