NeuroImage
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Motor cortex stimulation (MCS) is relatively recent neurosurgical technique for pain control, the use of which is growing steadily since its description in the last decade. While clinical series show that at least 50% of patients with chronic, pharmacoresistant neuropathic pain may benefit from this technique, the mechanisms of action of MCS remain elusive. In this review, we synthesise a number of studies that, combining electrophysiology and functional imaging, have permitted to proceed from phenomenology to models that may account for part of such mechanisms. ⋯ Current hypotheses suggest that MCS may act through at least two mechanisms: activation of perigenual cingulate and orbitofrontal areas may modulate the emotional appraisal of pain, rather than its intensity, while top down activation of brainstem PAG may lead to descending inhibition toward the spinal cord. Recent evidence also points to a possible secretion of endogenous opioids triggered by chronic MCS. This, along with the delayed and long-lasting activation of several brain structures, is consistent with the clinical effects of MCS, which may also last for hours or days after MCS discontinuation.
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Many studies have focused on defining the network of brain structures involved in normal physiological pain. The different dimensions of pain perception (i.e., sensory discriminative, affective/emotional, cognitive/evaluative) have been shown to depend on different areas of the brain. In contrast, much less is known about the neural basis of pathological chronic pain. ⋯ Both PET and fMRI have been used to investigate the basis of allodynia. The results obtained have been very variable, probably reflecting the heterogeneity of patients in terms of etiology, lesion topography, symptoms and stimulation procedures. Overall, these studies indicated that acute physiological pain and neuropathic pain have distinct although overlapping brain activation pattern, but that there is no unique "pain matrix" or "allodynia network".
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Using direct cortical stimulation to map language function during awake craniotomy is a well-described and useful technique. However, the optimum neuropsychological tasks to use have not been detailed. We used both functional MRI (fMRI) and direct cortical stimulation to compare the sensitivity of two behavioral paradigms, number counting and object naming, in the demonstration of eloquent cortical language areas. ⋯ In both patients and controls, fMRI activation maps demonstrated greater left lateralization for object naming as compared to number counting in both frontal and temporal language areas. Number counting resulted in a more bihemispheric distribution of activations than object naming. Both cortical stimulation testing and fMRI suggest that automated speech tasks such as number counting may not fully engage putative language networks and therefore are not optimal for language localization for surgical planning.
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To investigate brain mechanisms whereby electrical stimulation of the motor cortex (MCS) may induce pain relief in patients with neuropathic pain, cerebral blood flow (CBF) changes were studied using H2O PET in 19 consecutive patients treated with MCS for refractory neuropathic pain. Patients were studied in three conditions, (a) before MCS (Baseline, stimulator stopped 4 weeks before), (b) during a 35-min period of MCS and (c) during a 75-min period after MCS had been discontinued (OFF). Compared to Baseline, turning on the stimulator was associated with CBF increase in the contralateral (anterior) midcingulate cortex (aMCC, BA24 and 32) and in the dorso-lateral prefrontal (BA10) cortices. ⋯ Functional connectivity analysis showed significant correlation between pgACC and PAG, basal ganglia, and lower pons activities, supporting the activation of descending ACC-to-PAG connections. MCS may act in part through descending (top-down) inhibitory controls that involve prefrontal, orbitofrontal and ACC as well as basal ganglia, thalamus and brainstem. These hemodynamic changes are lengthened and might therefore underlie the long-lasting clinical effects that largely outlast the actual stimulation periods.
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Sensitivity to spoken language is an integral part of infants' formative development, yet relatively little is known about the neural mechanisms that underlie the emerging ability to perceive and process speech. This is in large part because there are a limited number of non-invasive techniques available to measure brain functioning in human infants. Near-infrared spectroscopy (NIRS), an optical imaging technique that estimates changes in neuronal activity by measuring changes in total hemoglobin concentration and oxygenation, may be a viable procedure for assessing the relation between speech processing and brain function in human infants. ⋯ Here we studied cortical activity in infants aged 6-9 months, as measured by NIRS, during exposure to linguistic stimuli paired with visual stimuli and compared this to the activity observed in the same regions during exposure to visual stimuli alone. Results dissociate infants' hemodynamic responses to multimodal and unimodal stimuli, demonstrating the utility of NIRS for studying perceptual development in infants. In particular, these findings support the use of NIRS to study the neurobiology of language development in older infants, a task that is difficult to accomplish without the use of attention-getting visual stimuli.