<b>Mechanisms of intracerebral pain and itch perception </b><b>in humans using neuroimaging methods </b>

Abstract

Recently, both electrophysiological studies such as magnetoencephalography (MEG) and hemodynamic studies such as functional magnetic resonance imaging (fMRI) are intensively being used to elucidate underlying mechanisms of human pain and itch perception. MEG following A-delta (first pain) and C fiber stimulation (second pain) were similar except for a longer latency for the latter. At first, primary somatosensory cortex (SI) contralateral to the stimulation is activated and then secondary somatosensory cortex (SII), insula, amygdala and anterior cingulate cortex (ACC) in the bilateral hemispheres are activated sequentially. As for findings using fMRI, the stimulation of both C and A-delta fibers activated the bilateral thalamus, bilateral SII, right (ipsilateral) middle insula, and bilateral Brodmann's area (BA)24 ⁄32, with the majority of activity found in the posterior portion of the ACC. However, magnitude of activity in the BA 32 ⁄8 ⁄6, including ACC and pre-supplementary motor area (pre-SMA), and the bilateral anterior insula was significantly stronger following the stimulation of C nociceptors than A-delta nociceptors. The cerebral mechanism of itch has been investigated using positron emission tomography (PET) and fMRI. These studies have identified brain regions associated with itch. However, no study has investigated the temporal aspect of the cerebral mechanism of itch because of no itch stimulus useful for electroencephalography (EEG) and MEG recordings. Thus, we developed a new itch stimulus (i. e., electrical itch stimulus). We confirmed that the electrical itch stimulus evoked itch sensation and that itch stimulus-related brain response can be observed using EEG and the stimulus. In addition, the conduction velocity related to the electrical itch stimulus estimated by EEG data was about 1 m/s, indicating that itch sensation evoked by the electrical itch stimulus would be associated with C-fibers. We also performed magnetoencephalography (MEG) recording using the electrical itch stimulus and observed parts of the temporal aspect of itch stimulus-related brain processing. For example, it was suggested that neural information related to itch was transmitted from the contralateral secondary somatosensory cortex /insula (SII /insula) to the ipsilateral SII/insula. In addition, we also observed the activation of the precuneus. Activity of his region was also observed by fMRI study, speculating that the precuneus may be selective for itch. These findings indicate that EEG and MEG recordings using the electrical itch stimulus would be useful to investigate the temporal aspect of the cerebral mechanism of itch.