3D重建脑表面中央前回辨识及初级皮层运动功能分布研究

目的研究利用三维重建脑表面成像辨识中央前回的方法及其可靠性。方法难治性癫痫经术前评估需要进行颅内电极植入的病例12例,术前行3T磁共振结构像扫描及运动功能磁共振测试,颅内电极植入后行头颅CT扫描。利用磁共振数据,用Brainvoige软件进行脑表面三维重建,并与硬膜下电极CT融合。根据中央前回的形状走行特点,在重建的脑表面上标记中央前回,进而辨识标记术区中央前回。用皮层电刺激和功能磁共振验证所标记中央前回的可靠性。结果 12例均进行了脑表面三维重建,并标记出了中央前回,硬膜下电极CT电极点与三维脑表面融合。统计中央前回上电极点101个,电刺激运动响应73个,响应率72%;对照组中央前回前1cm（统计中央前沟前2个电极范围）,130个电极,17个电极有运动响应,响应率13%,p值小于0.05;功能磁共振手的运动100%中央前回有激活,中央前回前无明显激活。电刺激及功能核磁均验证了本研究方法确定中央前回准确可靠。结论脑表面成像辨识中央前回准确可行,可以利用各初级运动皮层分布的距离进行详细的初级运动皮层的绘制定位。     

Cortical activation changes associated with motor recovery in patients with precentral knob infarct

We investigated the cortical activation changes associated with motor recovery in six hemiparetic patients with precentral knob infarct. fMRI at 1.5 T with finger movements at a fixed rate was performed twice in each patient, 1 and 6 months after stroke onset. From the images obtained, the LI (laterality index) for the primary sensorimotor cortex (SM1) was calculated to measure the degree of the cortical activity concentration in the contralateral hemisphere. Our results showed that a greater improvement in motor function scores was significantly correlated with a greater increment in LI induced by affected finger movements (p < 0.05). Motor recovery after precentral knob infarct was found to be positively related with the concentration of SM1 activity in the ipsilesional hemisphere. This finding may imply motor recovery through cortical reorganization after precentral knob infarct in the human brain.     

Motor cortex activation is preserved in patients with chronic hemiplegic stroke

Many central nervous system conditions that cause weakness, including many strokes, injure corticospinal tract but leave motor cortex intact. Little is known about the functional properties of surviving cortical regions in this setting, in part because many studies have used probes reliant on the corticospinal tract. We hypothesized that many features of motor cortex function would be preserved when assessed independent of the stroke-affected corticospinal tract. Functional MRI was used to study 11 patients with chronic hemiplegia after unilateral stroke that spared regions of motor cortex. Activation in stroke-affected hemisphere was evaluated using 3 probes independent of affected corticospinal tract: passive finger movement, a hand-related visuomotor stimulus, and tapping by the nonstroke index finger. The site and magnitude of cortical activation were similar when comparing the stroke hemisphere to findings in 19 control subjects. Patients activated each of 8 cortical regions with similar frequency as compared to controls, generally with a smaller activation volume. In some cases, clinical measures correlated with the size or the site of stroke hemisphere activation. The results suggest that, despite stroke producing contralateral hemiplegia, surviving regions of motor cortex actively participate in the same proprioceptive, visuomotor, and bilateral movement control processes seen in control subjects.     

Apomorphine reduces BOLD signal in fMRI during voluntary movement in Parkinsonian patients

Our group investigated modulatory effects of apomorphine on cerebral activation patterns during finger tapping movements in seven Parkinson's disease (PD) patients off medication. Cerebral activation was measured according to an established fMRI protocol. Apomorphine application disclosed a reduction of cerebral activation patterns to the contralateral precentral gyrus affecting both clinically affected and unaffected sides; tapping with the unaffected hand additionally revealed activation in the contralateral postcentral gyrus. These findings contradict those of similar functional imaging studies performed in PD to date, which variously found augmentation of cerebral activation patterns in Parkinsonian patients after dopaminergic stimulation. One conceivable explanation for our singular results would be preferred binding of apomorphine to presynaptic dopaminergic receptors, leading to inhibition of endogenous dopamine release and resultant diminished dopaminergic stimulation, reflected in diminished cerebral activation patterns. These findings warrant future consideration and further investigation of possible central inhibitory effects of dopaminergic therapy in functional imaging studies in PD.     

全喉切除发声重建过程中语言功能脑区的神经重组特点：fMRI随访研究

全喉切除语音恢复过程中,语言皮质区将产生新的神经连接。观察了18例全喉切除食道发声的患者,在患者发音差、中等、良时,利用功能磁共振方法观察发现,发音中等时患者左侧额中回、左侧中央前回、左侧中央后回、左侧辅助运动区和右侧梭状回区域激活体素和强度变化率明显高于发音差和良时。推测这些脑区在语音恢复过程中发挥了重要作用,能指导患者更好地学习发音。然而,在语音恢复后期这些地区的激活体素和强度变化下降至正常对照水平,说明一旦掌握了发音技巧,学习和指导效果将会减弱。     

Neural mechanism and heritability of complex motor sequence and audiovisual integration: A healthy twin study

Complex motor sequencing and sensory integration are two key items in scales assessing neurological soft signs. However, the underlying neural mechanism and heritability of these two functions is not known. Using a healthy twin design, we adopted two functional brain imaging tasks focusing on fist‐edge‐palm (FEP) complex motor sequence and audiovisual integration (AVI). Fifty‐six monozygotic twins and 56 dizygotic twins were recruited in this study. The pre‐ and postcentral, temporal and parietal gyri, the supplementary motor area, and the cerebellum were activated during the FEP motor sequence, whereas the precentral, temporal, and fusiform gyri, the thalamus, and the caudate were activated during AVI. Activation in the supplementary motor area during FEP motor sequence and activation in the precentral gyrus and the thalamic nuclei during AVI exhibited significant heritability estimates, ranging from 0.5 to 0.62. These results suggest that activation in cortical motor areas, the thalamus and the cerebellum associated with complex motor sequencing and audiovisual integration function may be heritable.     

Spatiotemporal mapping of cortical activity accompanying voluntary movements using an event-related beamforming approach

We describe a novel spatial filtering approach to the localization of cortical activity accompanying voluntary movements. The synthetic aperture magnetometry (SAM) minimum-variance beamformer algorithm was used to compute spatial filters three-dimensionally over the entire brain from single trial neuromagnetic recordings of subjects performing self-paced index finger movements. Images of instantaneous source power ("event-related SAM") computed at selected latencies revealed activation of multiple cortical motor areas prior to and following left and right index finger movements in individual subjects, even in the presence of low-frequency noise (e.g., eye movements). A slow premovement motor field (MF) reaching maximal amplitude approximately 50 ms prior to movement onset was localized to the hand area of contralateral precentral gyrus, followed by activity in the contralateral postcentral gyrus at 40 ms, corresponding to the first movement-evoked field (MEFI). A novel finding was a second activation of the precentral gyrus at a latency of approximately 150 ms, corresponding to the second movement-evoked field (MEFII). Group averaging of spatially normalized images indicated additional premovement activity in the ipsilateral precentral gyrus and the left inferior parietal cortex for both left and right finger movements. Weaker activations were also observed in bilateral premotor areas and the supplementary motor area. These results show that event-related beamforming provides a robust method for studying complex patterns of time-locked cortical activity accompanying voluntary movements, and offers a new approach for the localization of multiple cortical sources derived from neuromagnetic recordings in single subject and group data.     

Intrahemispheric cortical connexions and visual guidance of hand and finger movements in the rhusus monkey.

An attempt has been made to elucidate in the rhesus monkey the role of intrahemispheric cortico-cortical connexions in visual guidance of relatively independent hand and finger movements which are governed mainly from the precentral motor cortex. These movements were tested by requiring the animals to retrieve with their fingers small food pellets from a special test board in which the pellets were easily visible but were more difficult to palpate. Unilateral occipital lobectomy combined with a commissurotomy impaired the performance of the contralateral hand. The same was true for the parietal leucotomy of Myers et al. (1962) which transects the bulk of the intrahemispheric occipitofrontal cortical fibres. Tests of the visual discrimination of the leucotomized hemispheric showed that the motor impairment after this leucotomy did not represent a visual defect. In control animals no impairment was found after ablation of the cortex on the surface of the postcentral gyrus and the superior parietal lobule. A mild impairment occurred, however, when the lesion either involved also the inferior parietal lobule or was accompanied by a white matter infarct deep under the postcentral gyrus. The findings make it likely that the intrahemispheric cortical fibres to the frontal lobe play a role in visual guidance of relatively independent hand and finger movements. This conclusion is also supported by some preliminary findings after frontal lobe lesions, but further experiments are necessary to establish it firmly.     

Impact of apomorphine on BOLD signal during movement in normals

Our group investigated modulatory effects of apomorphine on cerebral activation patterns during finger tapping movements in six healthy right-handed volunteers using an established fMRI protocol. Apomorphine application disclosed a reduction of cerebral activation to the contralateral precentral and postcentral gyrus and ipsilateral cerebellum, with a prominent net reduction of BOLD signal in cerebellar areas. These findings contradict those of similar studies performed on dopaminergic function and Parkinson's disease (PD), which predominantly found augmentation of cerebral activation patterns in normal volunteers and PD patients after dopaminergic stimulation. One conceivable explanation for our singular results would be preferred binding of apomorphine to presynaptic dopaminergic receptors, leading to inhibition of endogenous dopamine release and resultant diminished dopaminergic stimulation, reflected in diminished cerebral activation patterns. These findings warrant future consideration and further investigation of possible central inhibitory effects of dopaminergic therapy in functional imaging studies of the dopaminergic system in general and PD in particular.     

Hypofunctioning of sensory gating mechanisms in patients with obsessive-compulsive disorder.

BACKGROUND: In obsessive-compulsive disorder (OCD) patients, functional abnormalities in basal ganglia/precentral circuitries cause cortical hyperexcitability and lack of inhibitory control. These loops can be partly explored by median-nerve somatosensory evoked potentials (SEPs), which functionally reflect the brain responsiveness to somatosensory stimuli. In healthy humans, SEPs' amplitude during voluntary finger movements is lower than during muscular relaxation (i.e., sensory gating). Cortical hyperexcitability in OCD could be eventually responsible for a reduction of sensory gating. This might have pathophysiologic implications for motor compulsions. METHODS: Median-nerve SEPs were recorded in 11 OCD patients and 9 healthy volunteers during muscle relaxation ("Relax") or finger movements of the stimulated hand ("Move"). Latencies and amplitudes of pre- and postcentral SEP components were compared between groups during "Relax" and "Move" conditions. RESULTS: In OCD patients, the responsiveness to sensory stimuli was enhanced for precentral SEPs. Sensory gating ("Relax" vs. "Move") in control subjects involved both pre- and postcentral SEPs, the former being reduced in amplitude by approximately 60%. In OCD patients, sensory gating was spatially restricted to precentral SEP components and was significantly reduced compared with control subjects (approximately 30%). CONCLUSIONS: Enhanced precentral SEPs and hypofunctioning of centrifugal sensory gating in OCD might reflect the inability to modulate sensory information due to a "tonic" high level of cortical excitability of motor and related areas, likely resulting from basal ganglia dysfunction. This might offer new insights into the pathophysiology of OCD.     

A functional MRI study of three motor tasks in the evaluation of stroke recovery

Functional brain imaging studies have provided insights into the processes related to motor recovery after stroke. The comparative value of different motor activation tasks for probing these processes has received limited study. We hypothesized that different hand motor tasks would activate the brain differently in controls, and that this would affect control-patient comparisons. Functional magnetic resonance imaging (MRI) was used to evaluate nine control subjects and seven patients with good recovery after a left hemisphere hemiparetic stroke. The volume of activated brain in bilateral sensorimotor cortex and four other motor regions was compared during each of three tasks performed by the right hand: index-finger tapping, four-finger tapping, and squeezing. In control subjects, activation in left sensorimotor cortex was found to be significantly larger during squeezing as compared with index-finger tapping. When comparing control subjects with stroke patients, patients showed a larger volume of activation in right sensorimotor cortex during index-finger tapping but not with four-finger tapping or squeezing. In addition, patients also showed a trend toward larger activation volume than controls within left supplementary motor area during index-finger tapping but not during the other tasks. Motion artifact was more common with squeezing than with the tapping tasks. The choice of hand motor tasks used during brain mapping can influence findings in control subjects as well as the differences identified between controls and stroke patients. The results may be useful for future studies of motor recovery after stroke.     

Cortical gamma‐oscillations modulated by auditory–motor tasks‐intracranial recording in patients with epilepsy

Human activities often involve hand‐motor responses following external auditory–verbal commands. It has been believed that hand movements are predominantly driven by the contralateral primary sensorimotor cortex, whereas auditory–verbal information is processed in both superior temporal gyri. It remains unknown whether cortical activation in the superior temporal gyrus during an auditory–motor task is affected by laterality of hand‐motor responses. Here, event‐related γ‐oscillations were intracranially recorded as quantitative measures of cortical activation; we determined how cortical structures were activated by auditory‐cued movement using each hand in 15 patients with focal epilepsy. Auditory–verbal stimuli elicited augmentation of γ‐oscillations in a posterior portion of the superior temporal gyrus, whereas hand‐motor responses elicited γ‐augmentation in the pre‐ and postcentral gyri. The magnitudes of such γ‐augmentation in the superior temporal, precentral, and postcentral gyri were significantly larger when the hand contralateral to the recorded hemisphere was required to be used for motor responses, compared with when the ipsilateral hand was. The superior temporal gyrus in each hemisphere might play a greater pivotal role when the contralateral hand needs to be used for motor responses, compared with when the ipsilateral hand does. Hum Brain Mapp, 2010. © 2010 Wiley‐Liss, Inc.     

A functional magnetic resonance imaging study of paced finger tapping in children

Fourteen typically developing children from 7.9-11.3 years in age were studied with functional magnetic resonance imaging to identify the cerebral loci involved in performance of paced finger tapping by children. Each child performed two bimanual alternating paced finger-tapping tasks. In the first, paced finger tapping was conducted to external 3-Hz pacing provided by a metronome. In the second, the metronome was turned off and finger tapping continued while each child tried to maintain the 3-Hz rhythm by self pacing. Individual and group data were analyzed with statistical parametric mapping techniques that resulted in activation maps for the two tasks. Metronome tapping produced activation of the posterior regions of both superior temporal gyri, both primary sensorimotor cortices, anterodorsomedial cerebellum and supplementary motor area. Self-tapping resulted in recruitment of pre-supplementary motor area and cerebellum in addition to bilateral supplementary motor area and primary sensorimotor cortical activation. Bimanual alternating paced finger tapping performed by children activates a neural network involving primary motor cortex, supplementary motor area, and cerebellum. Posterior superior temporal gyrus may be important for encoding auditory information, and presupplementary motor area and midline cerebellum play an important role in self-paced finger tapping.     

年轻人与老年人简单乘法运算之功能磁共振成像研究

目的应用fMRI技术探讨中国青年和老年人群在简单运算任务下脑激活模式及其与行为学之间的关系。方法分别对青年组(19例)和老年组(20例)健康志愿者进行对照任务和简单运算任务下的fMRI检查。结果两组受试者受教育程度(P=0.125)、智力水平(P=0.921),以及完成对照任务(P=0.142)和简单乘法运算任务(P=0.880)之正确率差异无统计学意义,但老年组受试者完成对照任务(P=0.000)和简单乘法运算任务(P=0.005)反应时间明显延长。青年组受试者在任务刺激下可激活右侧缘上回并向顶内沟和颞中上回后部延伸,中央前回和运动前区、前额叶,左侧缘上回并向颞上回后部和角回延伸,顶内沟区域、颞中下回,内侧后扣带回、楔前叶、辅助运动区、海马沟、海马旁回及前额叶内侧;老年组受试者则分别激活右侧缘上回和顶下区域并向颞中上回后部延伸,中央前回和运动前区、前额叶,左侧缘上回和角回并向顶下延伸,中央前回和运动前区、岛叶及前额叶,内侧后扣带回和中央旁小叶、前扣带回及前额叶内侧;两组受试者共激活脑区包括顶下区域、楔前叶、中央前后回和额顶叶网络,以及颞叶、海马旁回、钩回、屏状核和后扣带回等皮质下结构。结论数学事实提取相关网络的主要成分受年龄影响较小,老年人群的任务激活脑区主要向任务相关顶区集中。     

Functional MRI activation of primary and secondary motor areas in healthy subjects☆

BACKGROUND： Functional MRI （fMRI） demonstrates the localization of hand representation in the motor cortex, thereby providing feasible noninvasive mapping of functional activities in the human brain.
OBJECTIVE： To observe cortical activation within different cortical motor regions during repetitive hand movements in healthy subjects through the use of fMRI.
DESIGN： An observational study, with each subject acting as his own control.
SETTING： Department of Radiology, the First Affiliated Hospital of Nanchang University.
PARTICIPANTS： Seven healthy volunteers, 4 males and 3 females, aged 19 to 38 years, participated in the study. All subjects were right-handed, with no neurological or psychological disorders. Informed written consent was obtained from all subjects, and the study was approved by the Institutional Review Board of the First Affiliated Hospital of Nanchang University.
METHODS： The study was performed at the Department of Radiology between June-August 2005. A 1.5 Tesla Siemens MRI scanner （Symphony, Germany） was used to acquire T1-weighted structural images, which were oriented parallel to the line running through the anterior and the posterior commissures. Subjects were instructed on a task and were allowed to practice briefly prior to the imaging procedure. The motor activation task consisted of the right hand performing a clenching movement. The T1-W images were acquired from six alternating epochs of rest and activation from all seven healthy subjects. Data were collected with echoplanar imaging of brain oxygen level dependent （BOLD） sequence. Each series comprised six cycles of task performance （30 seconds）, alternating with rest （30 seconds） periods, and 3-second time intervals. The differences between active and baseline fMRI imaging were calculated using the student t-test. Differential maps were overlaid on the high resolution TI-W structural image for neuroanatomical correlation of activation areas.
MAIN OUTCOME MEASURES： The omega-shaped hand knob     

Motor cortex localization using functional MRI and transcranial magnetic stimulation.

OBJECTIVE: Congenital brain lesions producing focal seizures may be accompanied by reorganization of the areas responsible for motor and sensory functions within the brain due to a phenomenon that has been termed "neuronal plasticity." This can be studied using functional MRI (fMRI) and transcranial magnetic stimulation (TMS). Using either method, the motor cortex can be localized noninvasively, but to date there have been few studies correlating the level of agreement between the two techniques. METHODS: We used fMRI and TMS to localize the motor cortex in a young woman with intractable focal seizures, congenital left arm weakness, and a dysplastic right hemisphere on MRI. RESULTS: There was excellent agreement in the localization of motor representation for each hand. Both were predominantly located in the left hemisphere. fMRI also showed an area of posterior activation in the right hemisphere, but there was no evidence of descending corticospinal projections from this site using TMS, direct cortical stimulation, and Wada testing. CONCLUSIONS: Functional MRI (fMRI) and transcranial magnetic stimulation (TMS) were successfully used to localize cortical motor function before epilepsy surgery. Each technique demonstrated migration of motor function for the left hand to the left motor cortex. After resection of the dysplastic right precentral gyrus there was no permanent increase in weakness or disability. The two techniques are complementary; fMRI indicates all cortical areas activated by the motor task, whereas TMS identifies only those areas giving rise to corticospinal projections.     

Cheiro-oral syndrome due to a cortical infarction in the precentral gyrus: a case report]

We report a 65-year-old woman with sudden onset of paresthesia on the left side of the lip and left thumb. Neurological examinations did not demonstrate any disturbance of higher brain function or motor function except for subjective thermohyperesthesia of the left thumb. Brain MRI demonstrated a small high intensity lesion on T2-weighted images (T2 WI) and diffusion-weighted images (DWI) in the right anterior precentral gyrus. She was diagnosed with cerebral infarction presenting with pure cheiro-oral syndrome (COS). Her neurological symptoms were completely abolished 4 or 5 days later. The thalamus is the region responsible for COS in the majority of cases while reports that a cortical infarction causes COS are rare, because it is difficult to show a small cortical lesion on CT or conventional T2WI. DWI is superior to T2 WI in discriminating between acute and chronic ischemic lesions as well as in detecting small cortical lesions adjacent to the cerebrospinal fluid. In this patient, the lesion was vaguely hyperintense on T2 WI and difficult to differentiate from an artifact caused by cerebrospinal fluid. The precentral gyrus infarction detected on DWI was thought to cause COS in this patient. Not only lesions of the postcentral gyrus as a primary sensory cortex but also those of the precentral gyrus or opercula causing COS were described in previous reports, indicating that the hand and mouth sensory areas may be widely distributed. Cortical mapping studied by electrical stimulation through subdural grid electrodes also supports this finding. Further examination of the relation between neurological symptoms and localization on MR images such as DWI is needed to clarify the distribution of the sensory cortex.     

The Myelin Content of the Human Precentral Hand Knob Reflects Interindividual Differences in Manual Motor Control at the Physiological and Behavioral Level

The primary motor cortex hand area (M1_HAND) and adjacent dorsal premotor cortex (PMd) form the so-called motor hand knob in the precentral gyrus. M1_HAND and PMd are critical for dexterous hand use and are densely interconnected via corticocortical axons, lacking a sharp demarcating border. In 24 young right-handed volunteers, we performed multimodal mapping to delineate the relationship between structure and function in the right motor hand knob. Quantitative structural magnetic resonance imaging (MRI) at 3 tesla yielded regional R1 maps as a proxy of cortical myelin content. Participants also underwent functional MRI (fMRI). We mapped task-related activation and temporal precision, while they performed a visuomotor synchronization task requiring visually cued abduction movements with the left index or little finger. We also performed sulcus-aligned transcranial magnetic stimulation of the motor hand knob to localize the optimal site (hotspot) for evoking a motor evoked potential (MEP) in two intrinsic hand muscles. Individual motor hotspot locations varied along the rostrocaudal axis. The more rostral the motor hotspot location in the precentral crown, the longer were corticomotor MEP latencies. “Hotspot rostrality” was associated with the regional myelin content in the precentral hand knob. Cortical myelin content also correlated positively with task-related activation of the precentral crown and temporal precision during the visuomotor synchronization task. Together, our results suggest a link among cortical myelination, the spatial cortical representation, and temporal precision of finger movements. We hypothesize that the myelination of cortical axons facilitates neuronal integration in PMd and M1_HAND and, hereby, promotes the precise timing of movements.SIGNIFICANCE STATEMENT Here we used magnetic resonance imaging and transcranial magnetic stimulation of the precentral motor hand knob to test for a link among cortical myelin content, functional corticomotor representations, and manual motor control. A higher myelin content of the precentral motor hand knob was associated with more rostral corticomotor presentations, with stronger task-related activation and a higher precision of movement timing during a visuomotor synchronization task. We propose that a high precentral myelin content enables fast and precise neuronal integration in M1 (primary motor cortex) and dorsal premotor cortex, resulting in higher temporal precision during dexterous hand use. Our results identify the degree of myelination as an important structural feature of the neocortex that is tightly linked to the function and behavior supported by the cortical area.     

Effects of polarity of bipolar sensorimotor direct cortical stimulation on intraoperative motor evoked potentials

ObjectiveThe present study investigated the effects of the stimulus polarity and location of motor evoked potential (MEP) to establish a stimulation protocol.MethodsNineteen patients who intraoperatively underwent MEP in bipolar direct cortical stimulation were enrolled in the present study. Somatosensory evoked potentials (SEP) of the contralateral median nerve stimulation were recorded to determine stimulation sites. MEP was performed under two settings in all patients: 1. Anodal bipolar stimulation: an anode on the precentral gyrus and a cathode on the postcentral gyrus, 2. Cathodal bipolar stimulation: a cathode on the precentral gyrus and an anode on the postcentral gyrus. MEP amplitudes and the coefficient of variation (CV) at a stimulation intensity of 25 mA and the thresholds of induced MEP were compared between the two settings.ResultsAn electrical stimulation at 25 mA induced a significantly higher amplitude in cathodal bipolar stimulation than in anodal bipolar stimulation. Cathodal bipolar stimulation also showed significantly lower thresholds than anodal stimulation. CV did not significantly differ between the two groups.ConclusionsThese results indicate that cathodal bipolar stimulation is superior to anodal bipolar stimulation for intraoperative MEP monitoring.SignificanceMEP in cathodal bipolar cortical stimulation may be used in a safe and useful evaluation method of motor fiber damage that combines sensitivity and specificity.     

Motor cortex excitability after thalamic infarction.

Transcranial magnetic stimulation was used to map hand muscle representations in the motor cortex of a patient in whom infarction of the sensory thalamus deprived the sensorimotor cortex of sensory input. The threshold for activation of the motor cortex on the affected side was higher and the cortical representational maps of individual muscles were less well defined than those on the normal side. It is concluded that electrophysiological changes in cortical organisation can be demonstrated following withdrawal of, or imbalance in sensory afferent activity to the cerebral cortex in humans.