Cortical Representation Of Facial Tactile And Painful Heat Stimulation In Humans: Evidence From FMRI

The face component of the classical somatosensory homunculus has long been thought to be oriented right-side up along the central sulcus of the human brain, the rest is upside down. A variety of neuroimaging tools have supported that the somatotopic representation of the trunk and limb is upside down, it seems to be sure. However, direct recording from the somatosenseory cortex in monkeys, as well as clinical and imaging findings in man, have shown that the face representation within the primary somatosensory cortex (SI) is much more complex than the one depicted in the classical somatosensory homunculus. The findings are still controversial.Sensation is one of the most important orofacial functions. The functional neuroanatomy of face perception including tactile and pain remains largely unexplored. The previous studies of facial tactile and pain carried out in animals were mostly on peripheral and brainstem mechanisms, but studies on cortical mechanisms were scarce. Functional magnetic resonance imaging (fMRI) is a noninvasive neuroimaging technique which has high spatial and temporal resolutions. It has been extensively used in the exploration of cerebral activities in auditory, visual, sensory, motor and cognitive procedures. The emerging of fMRI allows investigators to insight the functional activities related to brain. In this study we employed high spatial resolution neuroimaging method fMRI to noninvasively clarify the different face representation of maxillary trigeminal division (V2) and mandibular trigeninal division (V3) in the SI cortex, and primarily evaluate cortical response to facial tactile and heat stimulated pain.This study consists of the following two parts: Part 1. Cortical representation of facial tactile: evidence from fMRIObjective: To distinguish the different face representation of the V2 and V3in the SI cortex, and evaluate brain activation response to facial tactile. Methods: 8 right handed healthy volunteers (4 women, 4 men, mean 30.4 years) participated in the study. Cotton-tipped swab stimuli was applied separately to right side of face: maxillary trigeminal division and mandibular trigeminal division, and block designed BOLD functional MRI scan covering the whole brain was carried out. The fMRI data were analyzed by SPM99 software to generate the activation map. Results: Increased BOLD signals during V2 tactile stimulation were found in contralateral SI (BA3), insula/SII, superior temporal gyrus, ipsilateral postcentral gyrus (BA40), bilateral prefrontal cortex (PFC), inferior temporal gyrus, transverse temporal gyrus. V3 stimulation activated the contralateral SI (BA3), premotor cortex(BA6), insula/SII, precuneus, parahippocampal gyrus, superior temporal gyrus, ipsilateral prefrontal cortex, inferior parietal lobule, putamen, bilateral middle temporal gyrus. Conclusion: 1. The face representation of the V2 was superior to V3 in the SI cortex. 2. Tactile stimuli activated contralateral primary and secondary somatosensory cortex, and ipsilateral sensory cortex. 3. Activations were found not only in sensory cortex during tactile stimulation but also in some other parts, including frontal lobe, temporal lobe and occipital lobe. These regions may be involved in the facial tactile processing. Part 2. Cortical representation of heat induced facial pain: evidence from fMRIObjective: To evaluate the neural cortical response to facial skin heat stimulated pain, and provide foundation for further investigation of the complex role of the central nervous system in the chronic orofacial pain conditions. Methods: 6 right handed healthy volunteers (2 women, 4 men, mean 31.1 years) participated in the study. Heat pain stimuli was applied to right side of right side of face: maxillary trigeminal division, and block designed BOLD functional MRI scan covering the whole brain was carried out. The fMRI data were analyzed by SPM99 software to generate the activation map. Results: Increased BOLD signals during heat stimulation were found in bilateral anterior cingulated cortex (ACC),insula/SII , contralateral postcentral gyrus, inferior frontal gyrus, cerebellum, ipsilateral superior temporal gyrus, inferior parietal lobule. The contralateral activation in ACC and insula/SII were stronger than that in ipsilateral side. Conclusion: 1. Brain activation of facial pain agrees with previous neuroimaging findings in somatotopic pain perception, including insula/SII, ACC, PFC, parietal lobe, cerebellum. 2. The facial pain evoked a stronger activation of contralateral hemisphere than ipsilateral hemisphere. 3. Compared with facial tactile, the facial pain evoked the activation of brain regions related to pain affective.