Functional magnetic resonance imaging was used to assess the cortical areas active during the observation of mouth actions performed by humans and by individuals belonging to other species (monkey and dog). Two types of actions were presented: biting and oral communicative actions (speech reading, lip-smacking, barking). As a control, static images of the same actions were shown. Observation of biting, regardless of the species of the individual performing the action, determined two activation foci (one rostral and one caudal) in the inferior parietal lobule and an activation of the pars opercularis of the inferior frontal gyrus and the adjacent ventral premotor cortex. The left rostral parietal focus (possibly BA 40) and the left premotor focus were very similar in all three conditions, while the right side foci were stronger during the observation of actions made by conspecifics. The observation of speech reading activated the left pars opercularis of the inferior frontal gyrus, the observation of lip-smacking activated a small focus in the pars opercularis bilaterally, and the observation of barking did not produce any activation in the frontal lobe. Observation of all types of mouth actions induced activation of extrastriate occipital areas. These results suggest that actions made by other individuals may be recognized through different mechanisms. Actions belonging to the motor repertoire of the observer (e.g., biting and speech reading) are mapped on the observer's motor system. Actions that do not belong to this repertoire (e.g., barking) are essentially recognized based on their visual properties. We propose that when the motor representation of the observed action is activated, the observer gains knowledge of the observed action in a “personal” perspective, while this perspective is lacking when there is no motor activation.

Objective: Tactile spatial acuity is routinely tested in neurology to assess the state of the dorsal column system. In contrast, spatial acuity for pain is not assessed, having never been systematically characterized. More than a century after the initial description of tactile acuity across the body, we provide the first systematic whole‐body mapping of spatial acuity for pain.Methods: We evaluated the 2‐point discrimination thresholds for both nociceptive‐selective and tactile stimuli across several skin regions. Thresholds were estimated using pairs of simultaneous stimuli, and also using successive stimuli.Results and interpretation: These two approaches produced convergent results. The fingertip was the area of highest spatial acuity, for both pain and touch. On the glabrous skin of the hand, the gradient of spatial acuity for pain followed that observed for touch. On the hairy skin of the upper limb, spatial acuity for pain and touch followed opposite proximal–distal gradients, consistent with the known innervation density of this body territory. Finally, by testing spatial acuity for pain in a rare participant completely lacking Aβ fibers, we demonstrate that spatial acuity for pain does not rely on a functioning system of tactile primary afferents. This study represents the first systematic characterization of spatial acuity for pain across multiple regions of the body surface. Ann Neurol 2014;75:917–924

Looking at still images of body parts in situations that are likely to cause pain has been shown to be associated with activation in some brain areas involved in pain processing. Because pain involves both sensory components and negative affect, it is of interest to explore whether the visually evoked representations of pain and of other negative emotions overlap. By means of event-related functional magnetic resonance imaging, here we compare the brain areas recruited, in female volunteers, by the observation of painful, disgusting, or neutral stimuli delivered to one hand or foot. Several cortical foci were activated by the observation of both painful and disgusting video clips, including portions of the medial prefrontal cortex, anterior, mid-, and posterior cingulate cortex, left posterior insula, and right parietal operculum. Signal changes in perigenual cingulate and left anterior insula were linearly related to the perceived unpleasantness, when the individual differences in susceptibility to aversive stimuli were taken into account. Painful scenes selectively induced activation of left parietal foci, including the parietal operculum, the postcentral gyrus, and adjacent portions of the posterior parietal cortex. In contrast, brain foci specific for disgusting scenes were found in the posterior cingulate cortex. These data show both similarities and differences between the brain patterns of activity related to the observation of noxious or disgusting stimuli. Namely, the parietal cortex appears to be particularly involved in the recognition of noxious environmental stimuli, suggesting that areas involved in sensory aspects of pain are specifically triggered by observing noxious events.

Despite growing interest in the placebo effect, the neural correlates of conditioned analgesia are still incompletely understood. We investigated herein on brain activity during the conditioning and post-conditioning phases of a placebo experimental paradigm, using event-related fMRI in 31 healthy volunteers. Brief laser heat stimuli delivered to one foot (either right or left) were preceded by different visual cues, signalling either painful stimuli alone, or painful stimuli accompanied by a (sham) analgesic procedure. Cues signalling the analgesic procedure were followed by stimuli of lower intensity in the conditioning session, whereas in the test session both cues were followed by painful stimuli of the same intensity. During the first conditioning trials, progressive signal increases over time were found during anticipation of analgesia compared to anticipation of pain, in a medial prefrontal focus centered on medial area BA8, and in bilateral lateral prefrontal foci. These frontal foci were adjacent to, and partially overlapped, those active during anticipation of analgesia in the test session, whose signal changes were related to the magnitude of the placebo behavioral response, and those active during placebo analgesia. Specifically, a large focus in the right prefrontal cortex showed activity related to analgesia, irrespective of the expected side of stimulation. Analgesia was also related to decreased activity, detectable immediately following noxious stimulation, in parietal, insular and cingulate pain-related clusters. Our findings of dynamic changes in prefrontal areas during placebo conditioning, and of direct placebo effects on cortical nociceptive processing, add new insights into the neural bases of conditioned placebo analgesia.

Most imaging studies on the human pain system have concentrated so far on the spatial distribution of pain-related activity. In the present study, we investigated similarities and differences between the spatial and temporal patterns of brain activity related to touch vs. pain perception. To this end, we adopted an event-related functional magnetic resonance imaging (fMRI) paradigm allowing us to separately assess the activity related to stimulus anticipation, perception, and coding. The fMRI signal increases following brief mechanical noxious or non-noxious stimulation of the hand dorsum were largely overlapping in the contralateral and ipsilateral hemispheres, including portions of the parietal, insular, frontal and cingulate cortices. Higher activity following noxious stimulation was found in the contralateral mid-anterior insular cortex, in the anterior mid-cingulate cortex (aMCC) and in the adjacent dorso-medial frontal cortex. Significant decreases in fMRI signals following both tactile and painful stimuli were found in perigenual cingulate (pACC)/medial prefrontal cortex (MPF) and in the posterior cingulate/precuneus/paracentral lobule; more intense decreases were found in the pACC/MPF following painful stimuli. fMRI signal increases in the contralateral insula and in aMCC, but not in the parietal cortex, were more prolonged following painful than tactile stimuli. Moreover, a second peak of signal increases (albeit of lower intensity) was found in anterior insula and aMCC during pain intensity rating. These results show specific spatio-temporal patterns of cortical activity related to processing noxious vs. non-noxious mechanical stimuli.