L. Zambreanu

Somatotopic organisation of the human insula to painful heat studied with high resolution functional imaging.

Pain perception is a multidimensional phenomenon, derived from sensory, affective, cognitive-evaluative and homeostatic information. Neuroimaging studies of pain perception have investigated the role of primary somatosensory cortex (SI); however, they have typically failed to demonstrate the expected somatotopy. An alternative network for the sensory component of pain has been proposed, involving a temperature and pain-specific nucleus of the thalamus (VMpo) and its projections to dorsal posterior insula (dpIns). According to this hypothesis, projections to the insula should be arranged somatotopically. In order to test for the presence of somatotopy in the operculo-insular brain region, we delivered moderately painful thermal stimuli to the right face, hand and foot in 14 healthy subjects and recorded brain responses using high resolution functional magnetic resonance imaging at 3 T. For each subject, the thermode temperature was adjusted to produce pain ratings of 5 to 6 out of 10, which corresponded to average temperatures for the face, hand and foot of 49.6, 48.5 and 48.5 degrees C, respectively. Examination of mixed effects group activation maps suggested a pain-related somatotopy in the contralateral posterior insula and putamen. Construction of frequency maps revealed that face activation within the posterior insula was anterior to both hand and foot, whilst foot activation was located medially in the circular sulcus. Single subject analysis demonstrated that only coordinates for dpIns activation were significantly dependent on stimulus location (Hotellings Trace, P = 0.012). Coordinates for face (paired t test, P = 0.004) and hand (P < 0.001) activity were more lateral than those for foot, whilst face activation was anterior to the foot (P = 0.037). Based on single subject analyses, the average standard space (MNI) coordinates for face, hand and foot activity were (-40,-16,11), (-40,-19,14) and (-35,-21,11) respectively.

A role for the brainstem in central sensitisation in humans. Evidence from functional magnetic resonance imaging

&NA; Animal studies have established a role for the brainstem reticular formation, in particular the rostral ventromedial medulla (RVM), in the development and maintenance of central sensitisation and its clinical manifestation, secondary hyperalgesia. Similar evidence in humans is lacking, as neuroimaging studies have mainly focused on cortical changes. To fully characterise the supraspinal contributions to central sensitisation in humans, we used whole‐brain functional magnetic resonance imaging at 3 T, to record brain responses to punctate mechanical stimulation in an area of secondary hyperalgesia. We used the heat/capsaicin sensitisation model to induce secondary hyperalgesia on the right lower leg in 12 healthy volunteers. A paired t‐test was used to compare activation maps obtained during punctate stimulation of the secondary hyperalgesia area and those recorded during control punctate stimulation (same body site, untreated skin, separate session). The following areas showed significantly increased activation (Z>2.3, corrected P<0.01) during hyperalgesia: contralateral brainstem, cerebellum, bilateral thalamus, contralateral primary and secondary somatosensory cortices, bilateral posterior insula, anterior and posterior cingulate cortices, right middle frontal gyrus and right parietal association cortex. Brainstem activation was localised to two distinct areas of the midbrain reticular formation, in regions consistent with the location of nucleus cuneiformis (NCF) and rostral superior colliculi/periaqueductal gray (SC/PAG). The PAG and the NCF are the major sources of input to the RVM, and therefore in an ideal position to modulate its output. These results suggest that structures in the mesencephalic reticular formation, possibly the NCF and PAG, are involved in central sensitisation in humans.

Operculoinsular cortex encodes pain intensity at the earliest stages of cortical processing as indicated by amplitude of laser-evoked potentials in humans

Converging evidence from different functional imaging studies indicates that the intensity of activation of different nociceptive areas (including the operculoinsular cortex, the primary somatosensory cortex, and the anterior cingulate gyrus) correlates with perceived pain intensity in the human brain. Brief radiant laser pulses excite selectively Adelta and C nociceptors in the superficial skin layers, provide a purely nociceptive input, and evoke brain potentials (laser-evoked potentials, LEPs) that are commonly used to assess nociceptive pathways in physiological and clinical studies. Adelta-related LEPs are constituted of different components. The earliest is a lateralised, small negative component (N1) which could be generated by the operculoinsular cortex. The major negative component (N2) seems to be mainly the result of activation in the bilateral operculoinsular cortices and contralateral primary somatosensory cortex, and it is followed by a positive component (P2) probably generated by the cingulate gyrus. Currently, early and late LEP components are considered to be differentially sensitive to the subjective variability of pain perception: the late N2-P2 complex strongly correlates with perceived pain, whereas the early N1 component is thought to be a pre-perceptual sensory response. To obtain physiological information on the roles of the pain-related brain areas in healthy humans, we examined the relationship between perceived pain intensity and latency and amplitude of the early (N1) and late (N2, P2) LEP components. We found that the amplitude of the N1 component correlated significantly with the subjective pain ratings, both within and between subjects. Furthermore, we showed that the N2 and P2 late LEP components are differentially sensitive to the perceived sensation, and demonstrated that the N2 component mainly explains the previously described correlation between perceived pain and the amplitude of the N2-P2 vertex complex of LEPs. Our findings confirm the notion that pain intensity processing is distributed over several brain areas, and suggest that the intensity coding of a noxious stimulus occurs already at the earliest stage of perception processing, in the operculoinsular region and, possibly, the primary somatosensory area.

Identifying Brain Activity Specifically Related to the Maintenance and Perceptual Consequence of Central Sensitization in Humans

Central sensitization (CS) refers to an increase in the excitability of spinal dorsal horn neurons that results from, and far outlasts the initiating nociceptive input. Here, functional magnetic resonance imaging was used to examine whether supraspinal activity might contribute to the maintenance of CS in humans. A crossover parametric design was used to distinguish and control for brain activity that is related to the consequence of increased pain experienced during CS. When the intensity of pain during CS and normal states were matched, only activity within the brainstem, including the mesencephalic pontine reticular formation, and the anterior thalami remained increased during CS. Further analyses revealed that activity in the isolated brainstem area correlated positively with the force of noxious stimulation only during CS, whereas activity in the isolated thalamic area was not modulated parametrically in either CS or normal states. Additionally, the mean activity in the isolated brainstem area was increased only during CS, whereas the mean activity in the isolated thalamic area was increased in both states, albeit less so in the normal state. Together, these findings suggest a specific role of the brainstem for the maintenance of CS in humans. Regarding brain areas related to the consequence of increased pain perception during CS, we found that only cortical activity, mainly in the primary somatosensory area, was significantly correlated with intensity of pain that was attributable to both the force of noxious stimulation used and state in which noxious stimulation was applied.

Multiple Somatotopic Representations of Heat and Mechanical Pain in the Operculo-Insular Cortex: A High-Resolution fMRI Study

Whereas studies of somatotopic representation of touch have been useful to distinguish multiple somatosensory areas within primary (SI) and secondary (SII) somatosensory cortex regions, no such analysis exists for the representation of pain across nociceptive modalities. Here we investigated somatotopy in the operculo-insular cortex with noxious heat and pinprick stimuli in 11 healthy subjects using high-resolution (2 × 2 × 4 mm) 3T functional magnetic resonance imaging (fMRI). Heat stimuli (delivered using a laser) and pinprick stimuli (delivered using a punctate probe) were directed to the dorsum of the right hand and foot in a balanced design. Locations of the peak fMRI responses were compared between stimulation sites (hand vs. foot) and modalities (heat vs. pinprick) within four bilateral regions of interest: anterior and posterior insula and frontal and parietal operculum. Importantly, all analyses were performed on individual, non-normalized fMRI images. For heat stimuli, we found hand-foot somatotopy in the contralateral anterior and posterior insula [hand, 9 ± 10 (SD) mm anterior to foot, P < 0.05] and in the contralateral parietal operculum (SII; hand, 7 ± 10 mm lateral to foot, P < 0.05). For pinprick stimuli, we also found somatotopy in the contralateral posterior insula (hand, 9 ± 10 mm anterior to foot, P < 0.05). Furthermore, the response to heat stimulation of the hand was 11 ± 12 mm anterior to the response to pinprick stimulation of the hand in the contralateral (left) anterior insula (P < 0.05). These results indicate the existence of multiple somatotopic representations for pain within the operculo-insular region in humans, possibly reflecting its importance as a sensory-integration site that directs emotional responses and behavior appropriately depending on the body site being injured.

Simultaneous recording of laser-evoked brain potentials and continuous, high-field functional magnetic resonance imaging in humans

Simultaneous recording of event-related electroencephalographic (EEG) and functional magnetic resonance imaging (fMRI) responses has the potential to provide information on how the human brain reacts to an external stimulus with unique spatial and temporal resolution. However, in most studies combining the two techniques, the acquisition of functional MR images has been interleaved with the recording of evoked potentials. In this study we investigated the feasibility of recording pain-related evoked potentials during continuous and simultaneous collection of blood oxygen level-dependent (BOLD) functional MR images at 3 T. Brain potentials were elicited by selective stimulation of cutaneous Adelta and C nociceptors using brief radiant laser pulses (laser-evoked potentials, LEPs). MR-induced artifacts on EEG data were removed using a novel algorithm. Latencies, amplitudes, and scalp distribution of LEPs recorded during fMRI were not significantly different from those recorded in a control session outside of the MR scanner using the same equipment and experimental design. Stability tests confirmed that MR-image quality was not impaired by the evoked potential recording, beyond signal loss related to magnetic susceptibility differences local to the electrodes. fMRI results were consistent with our previous studies of brain activity in response to nociceptive stimulation. These results demonstrate the feasibility of recording reliable pain-related LEPs and fMRI responses simultaneously. Because LEPs collected during fMRI and those collected in a control session show remarkable similarity, for many experimental designs the integration of LEP and fMRI data collected in separate, single-modality acquisitions may be appropriate. Truly simultaneous recording of LEPs and fMRI is still desirable in specific experimental conditions, such as single-trial, learning, and pharmacological studies.

340 SIMILAR NOCICEPTIVE AFFERENTS MEDIATE PSYCHOPHYSICAL AND ELECTROPHYSIOLOGICAL RESPONSES TO THERMAL STIMULATION

Type II A-fibre nociceptors (AMHs), the peripheral afferents mediating first pain to heat stimuli in primate hairy skin, have not been described in glabrous skin. The aim of this study was to assess the effect of hairy and glabrous skin stimulation on neural transmission of nociceptive inputs elicited by different kinds of thermal heating. We recorded and analysed automatically at a single-trial level the brain potentials evoked by heat stimuli (thermal radiation, LEPs, and thermal conduction, CHEPs) of hand dorsum and palm in healthy volunteers. Laser stimulation of hairy and glabrous skin at the same energy elicited remarkably similar psychophysical ratings and LEPs. This finding provides evidence that first pain to heat does exist in glabrous skin, and suggests that similar type-II like nociceptive afferents mediate first pain to heat stimulation of glabrous and hairy skin in humans. In contrast, CHEPs following glabrous skin stimulation had significantly longer latencies (N2-wave: +25%, P2-wave: +24%) and smaller amplitudes (N2-wave: −40%, P2-wave: −44%). Independently of the stimulated territory, CHEPs always had significantly longer latencies (hairy skin: N2-wave: +75%, P2-wave: +56%) and smaller amplitudes (hairy skin N2-wave: −42%, P2-wave: −19%) than LEPs. These findings are consistent with the thickness-dependent delay and attenuation of the temperature waveform at nociceptor level when conductive heating is applied, and suggest that the previously reported lack of first pain and microneurographical type II AMH responses following glabrous skin stimulation could have been the result of a search bias consequent to the use of long-wavelength radiant heating as stimulation procedure.

307 THE SUPRASPINAL REPRESENTATION OF CENTRAL SENSITIZATION IN HUMANS

Background. The increased brain activation observed in previous neuroimaging studies of experimental hyperalgesia reflects not only the process of central sensitization, but also the increase in pain perception. This study aims to isolate brain activity specifically related to central sensitization, by matching perceived pain intensity between normal and central sensitization states. Methods. A cohort of right handed, mixed gender healthy volunteers known to develop demonstrable hyperalgesia in response to an intra-epidermal injection of capsaicin was recruited. Punctate mechanical stimulation of varying force (64 mN, 128 mN, 256 mN and 512 mN) was applied to hyperalgesic and control areas (same site, untreated skin) in separate sessions (balanced order), and brain responses were recorded using functional magnetic resonance imaging. The visual analogue scale (VAS) was used to rate the intensity of sharp or prickling sensations. Results. In agreement with previous studies, the comparison between activation maps following punctate stimulation of control and hyperalgesic areas with identical force, revealed significant increases in activation of the bilateral thalamus, basal ganglia, insula, secondary somatosensory cortex, pre-frontal cortices as well as the anterior cingulate cortex during hyperalgesia. However, preliminary analysis of brain activity to punctate stimulation at forces resulting in matched VAS ratings in control and hyperalgesic areas revealed additional significant activation in the brainstem. Conclusion. To date, these results support previous findings highlighting the brainstem’s critical role in the generation and maintenance of hyperalgesia. Ongoing analyses will dissect further the neural correlates specific to the central sensitization state. doi:10.1016/j.ejpain.2007.03.237

Response to Dr Ochoa

Zambreanu et al. (2005) show that punctate stimulation of an area of secondary cutaneous hyperalgesia, induced experimentally by topical heat/capsaicin application in the leg of human volunteers, causes activation of multiple sites in cortex, thalamus, brainstem, and cerebellum, unilaterally or bilaterally, as per functional brain imaging. Prominent brainstem sites are the NCF and PAG in the mesencephalic reticular formation. Given that in experimental animals such sites have a purported role in development and maintenance of central sensitization and secondary hyperalgesia, the authors suggest that the same structures may be involved in central sensitization in human chronic pain. However, the authors do not conceive a mandatory alternative explanation, namely, that the diffuse changes recorded in the CNS are an exaggerated response to stimulation of hyperexcitable primary nociceptors sensitized by the heat/capsaicin challenge in the area of experimental secondary hyperalgesia (Serra et al., 2004). Comparison with results of stimulation in the area of experimental primary hyperalgesia would have been instructive, and perhaps not dissimilar. There is another concern: whereas neuronal activation in the mesencephalon was actually demonstrated, the concept that it reflects sensitization is an assumption (Ochoa, 2004), and who knows whether the active neurons may be facilitatory on-cells or inhibitory off-cells (Fields et al., 1983; Heinricher et al., 1987) driving protective descending modulation (Ren and Dubner, 2002).