疼痛與癢覺之大腦神經表徵及其交互作用

Abstract

痛與癢在人體中為不同的兩種防範身體受到外來潛在傷害的體感覺。這兩種感 覺,包含了不同的認知表徵以及能引起不同的動作反應。痛與癢之間,一般被廣 為接受的交互作用為疼痛能抑制癢覺。目前對於痛與癢兩者在於神經表徵上的不 同以及負責兩者之間交互作用的神經機制的了解仍相當有限。因此,本研究的目 的在於利用單一刺激媒介來引起痛與癢,以區分痛與癢不同的神經表徵,並研究 兩者間的交互作用。我們透過電刺激來引起健康受試者的痛、癢、痛與癢、不痛 不癢的感覺,並同步紀錄受試者大腦中的血氧濃度相依對比(Blood oxygen-level dependent, BOLD)訊號。受試者被要求在每次的刺激之後,針對其所感受到之 刺激強度(有多痛和有多癢)以及主觀的動作意向(多想收手和多想抓手)進行 評分。初步的行為結果顯示,癢覺相較於痛覺具有比較高比例的動作組成,並且 在伴隨疼痛出現的時候,癢的感覺與想抓手的衝動會被抑制。從神經性影像的結 果看來,相較於痛覺,癢覺在與運動相關的腦區,諸如前運動輔助區(pre- supplementary motor area)、前運動區(premotor area)和殼核(putamen),以 及大腦的高級聯絡區域下頂葉(inferior parietal lobule)、楔前葉(precuneus)和 背外側前額葉(dorsolateral prefrontal cortex)具有較強的反應。更重要的是,我們發現痛覺調控癢覺的機制來自框額皮質(orbitofrontal cortex)、視丘 (thalamus)以及小腦(cerebellum)的訊號。總結來說,我們的研究提供了證據 去支持在大腦中痛與癢具有不同的感覺與運動之神經表徵,並且會透過不同的腦 區去進行交互作用。

Pain and itch are different nociceptive sensations, which warn our body about potential damage. Both sensations encompass different cognitive attributes and elicit different motor responses, and the prevailing belief of their interaction is that pain inhibits itch. To date, little is known about the difference in the neural representation between pain and itch as well as the neural mechanism underlying their interaction. This may be attributed to the fact that most researchers evoked pain and itch by different modalities. As such, the aim of the current study is to isolate the difference in neural substrates for pain and itch and investigate their interaction by using a single modality to provoke pain and itch sensations. We applied electrical stimulation to induce pain, itch, pain and itch, and non-pain and non-itch sensations in healthy volunteers and simultaneously collected the blood oxygen-level dependent (BOLD) signals of their brain. After each stimulation, participants were required to report perceived stimulus intensity (how painful and how itchy) and subjective motor intention (the urge to withdraw for pain and to scratch for itch). Behavioral data revealed that itch sensation exhibited a higher proportion of motor component compared to pain, and the presence of pain suppressed both itch sensation and the urge to scratch. At the neural level, compared to pain, itch showed stronger activations in motor-related regions, including the pre-supplementary motor area, premotor area and putamen, and in highly associative brain regions, including the inferior parietal lobule, precuneus, and dorsolateral prefrontal cortex. Importantly, we observed that activation in the orbitofrontal cortex, thalamus and cerebellum underlay the inhibitory modulation of pain on itch. In conclusion, our findings suggest that different nociceptive sensations encompass dissimilar sensorimotor components, and distinct brain regions underlie the interaction between pain and itch in the human brain.