Fiber connections between the cerebral cortex and the corpus callosum in Alzheimer's disease: a diffusion tensor imaging and voxel-based morphometry study

Regional cortical atrophy in Alzheimer's disease (AD) most likely reflects the loss of cortical neurons. Several diffusion tensor imaging studies reported reduced fractional anisotropy (FA) in the corpus callosum in AD. The aim of this study was to investigate the association between reduced FA in the corpus callosum and gray matter atrophy in AD. Thirteen patients with AD with a mean (+/-standard deviation) age of 68.3 years (+/-11.5) and mean Mini Mental State Examination (MMSE) score of 21.8 (+/-4.8) were recruited. There were 13 control subjects with a mean age of 66.7 years (+/-6.4) and MMSE of 29.1 (+/-0.7). We used voxel-based morphometry of gray matter maps and region of interest-based analysis of FA in the corpus callosum. FA values of the anterior corpus callosum in AD patients were significantly correlated with gray matter volume in the prefrontal cortex and left parietal lobes. FA values of the posterior corpus callosum were significantly correlated with gray matter volume in the bilateral frontal, temporal, right parietal, and occipital lobes. In control subjects, no correlations were detected. Our findings suggest that decline of FA in the corpus callosum may be related to neuronal degeneration in corresponding cortical areas.     

Functional and anatomical cortical underconnectivity in autism: evidence from an FMRI study of an executive function task and corpus callosum morphometry

The brain activation of a group of high-functioning autistic participants was measured using functional magnetic resonance imaging during the performance of a Tower of London task, in comparison with a control group matched with respect to intelligent quotient, age, and gender. The 2 groups generally activated the same cortical areas to similar degrees. However, there were 3 indications of underconnectivity in the group with autism. First, the degree of synchronization (i.e., the functional connectivity or the correlation of the time series of the activation) between the frontal and parietal areas of activation was lower for the autistic than the control participants. Second, relevant parts of the corpus callosum, through which many of the bilaterally activated cortical areas communicate, were smaller in cross-sectional area in the autistic participants. Third, within the autism group but not within the control group, the size of the genu of the corpus callosum was correlated with frontal-parietal functional connectivity. These findings suggest that the neural basis of altered cognition in autism entails a lower degree of integration of information across certain cortical areas resulting from reduced intracortical connectivity. The results add support to a new theory of cortical underconnectivity in autism, which posits a deficit in integration of information at the neural and cognitive levels.     

Neural correlates of the spontaneous phase transition during bimanual coordination

Repetitive bimanual finger-tapping movements tend toward mirror symmetry: There is a spontaneous transition from less stable asymmetrical movement patterns to more stable symmetrical ones under frequency stress but not vice versa. During this phase transition, the interaction between the signals controlling each hand (cross talk) is expected to be prominent. To depict the regions of the brain in which cortical cross talk occurs during bimanual coordination, we conducted event-related functional magnetic resonance imaging using a bimanual repetitive-tapping task. Transition-related activity was found in the following areas: the bilateral ventral premotor cortex, inferior frontal gyrus, middle frontal gyrus, inferior parietal lobule, insula, and thalamus; the right rostral portion of the dorsal premotor cortex and midbrain; the left cerebellum; and the presupplementary motor area, rostral cingulate zone, and corpus callosum. These regions were discrete from those activated by bimanual movement execution. The phase-transition-related activation was right lateralized in the prefrontal, premotor, and parietal regions. These findings suggest that the cortical neural cross talk occurs in the distributed networks upstream of the primary motor cortex through asymmetric interhemispheric interaction.     

The size of corpus callosum correlates with functional activation of medial motor cortical areas in bimanual and unimanual movements

Effects of the size of corpus callosum measured from in vivo magnetic resonance imaging (MRI) recordings on cortical activations evaluated using functional MRI (fMRI) were analyzed during motor tasks. Twelve right-handed men performed unilateral finger movements and bilateral movements either with or without a temporal delay between left and right fingers. The size of the rostral part of corpus callosum and the anterior and posterior callosal truncus explained 11.9 and 15.2% of activation in the mesial frontal cortex in unimanual left and right finger movements, respectively. In bimanual simultaneous movements, 34.2% of the activated voxels in the mesial frontal cortex were related to the size of corpus callosum. In bimanual movements in which left finger movement preceded the onset of the right finger movement, the callosal size accounted for 88.7% of activation in the mesial frontal cortex. In contrast, when the right finger movement preceded the left, callosal size accounted for only 31.3% of the mesial frontal cortex activation. The correlations between callosal parameters and activation over the lateral cortex were sparse and occurred only in bimanual movements. The results suggest that corpus callosum modulates the activity of the supplementary motor and cingulate cortical areas depending on temporal complexity of bimanual movements.     

Distinctive neural processes during learning in autism

This functional magnetic resonance imaging study compared the neural activation patterns of 18 high-functioning individuals with autism and 18 IQ-matched neurotypical control participants as they learned to perform a social judgment task. Participants learned to identify liars among pairs of computer-animated avatars uttering the same sentence but with different facial and vocal expressions, namely those that have previously been associated with lying versus truth-telling. Despite showing a behavioral learning effect similar to the control group, the autism group did not show the same pattern of decreased activation in cortical association areas as they learned the task. Furthermore, the autism group showed a significantly smaller increase in interregion synchronization of activation (functional connectivity) with learning than did the control group. Finally, the autism group had decreased structural connectivity as measured by corpus callosum size, and this measure was reliably related to functional connectivity measures. The findings suggest that cortical underconnectivity in autism may constrain the ability of the brain to rapidly adapt during learning.     

Functional magnetic resonance imaging of somatosensory cortex activity produced by electrical stimulation of the median nerve or tactile stimulation of the index finger

OBJECT: Functional magnetic resonance (fMR) imaging was used to determine patterns of cerebral blood flow changes in the somatosensory cortex that result from median nerve stimulation (MNS). METHODS: Ten healthy volunteers underwent stimulation of the right median nerve at frequencies of 5.1 Hz (five volunteers) and 50 Hz (five volunteers). The left median nerve was stimulated at frequencies of 5.1 Hz (two volunteers) and 50 Hz (five volunteers). Tactile stimulation (with a soft brush) of the right index finger was also applied (three volunteers). Functional MR imaging data were transformed into Talairach space coordinates and averaged by group. Results showed significant activation (p < 0.001) in the following regions: primary sensorimotor cortex (SMI), secondary somatosensory cortex (SII), parietal operculum, insula, frontal cortex, supplementary motor area, and posterior parietal cortices (Brodmann's Areas 7 and 40). Further analysis revealed no statistically significant difference (p > 0.05) between volumes of cortical activation in the SMI or SII resulting from electrical stimuli at 5.1 Hz and 50 Hz. There existed no significant differences (p > 0.05) in cortical activity in either the SMI or SII resulting from either left- or right-sided MNS. With the exception of the frontal cortex, areas of cortical activity in response to tactile stimulation were anatomically identical to those regions activated by electrical stimulation. In the SMI and SII, activation resulting from tactile stimulation was not significantly different (p > 0.05) from that resulting from electrical stimulation. CONCLUSIONS: Electrical stimulation of the median nerve is a reproducible and effective means of activating multiple somatosensory cortical areas, and fMR imaging can be used to investigate the complex network that exists between these areas.     

Long-term potentiation of polysynaptic responses in layer V of the sensorimotor cortex induced by theta-patterned tetanization in the awake rat

Although the neocortex in awake, adult animals is resistant to the induction of long-term potentiation (LTP), synaptic potentiation may be enhanced by rhythmic patterns of activation that evoke short- term synaptic facilitation effects. The effectiveness of stimulation patterned after the theta (4-12 Hz) EEG rhythm for the induction of LTP of sensorimotor cortex responses to corpus callosum stimulation was assessed in vivo by inducing LTP using either high- frequency (300 Hz) trains or paired trains delivered at a 100 ms (10 Hz) interval. High-frequency trains caused a reduction of the early field potential component, reflecting a potentiation of direct layer V activation, and a potentiation of the late component, reflecting enhanced polysynaptic activation in layer V. Paired trains resulted in a much larger potentiation of polysynaptic responses than was observed following 300 Hz trains. To determine if short-term facilitation effects contributed to the enhanced LTP induction by theta-patterned trains, facilitation effects induced by the trains were challenged with NMDA receptor antagonists. NMDA-receptor antagonism reduced responses to single pulses, and also reduced facilitated responses evoked by theta-patterned stimulation. The effectiveness of theta-patterned stimulation for the induction of LTP of layer V polysynaptic responses is therefore likely due to frequency-dependent synaptic facilitation effects that enhance NMDA receptor activation.     

Microsurgical anatomy of the distal anterior cerebral artery

The microsurgical anatomy of the distal anterior cerebral artery (ACA) has been defined in 50 cerebral hemispheres. The distal ACA, the portion beginning at the anterior communicating artery (ACoA), was divided into four segments (A2 through A5) according to Fischer. The distal ACA gave origin to central and cerebral branches. The central branches passed to the optic chiasm, suprachiasmatic area, and anterior forebrain below the corpus callosum. The cerebral branches were divided into cortical, subcortical, and callosal branches. The most frequent site of origin of the cortical branches was as follows: orbitofrontal and frontopolar arteries, A2; the anterior and middle internal frontal and callosomarginal arteries, A3; the paracentral artery, A4; and the superior and inferior parietal arteries, A5. The posterior internal frontal artery arose with approximately equal frequency from A3 and A4 and callosomarginal artery. All the cortical branches arose more frequently from the pericallosal than the callosomarginal artery. Of the major cortical branches, the internal frontal and paracentral arteries arose most frequently from the callosomarginal artery. The distal ACA of one hemisphere sent branches to the contralateral hemisphere in 64% of brains. The anterior portions of the hemisphere between the 5-cm and 15-cm points on the circumferential line showed the most promise of revealing a recipient artery of sufficient size for an extracranial-intracranial artery anastomosis. The distal ACA was the principal artery supplying the corpus callosum. The recurrent artery, which arose from the A2 segment in 78% of hemispheres, sent branches into the subcortical area around the anterior limb of the internal capsule.     

Surgical removal of corpus callosum infiltrated by low-grade glioma: functional outcome and oncological considerations

OBJECT: Although still controversial, many authors currently advocate extensive resection in the treatment of low-grade gliomas (LGGs). Because these tumors usually migrate along white matter pathways, the corpus callosum is often invaded. Nevertheless, there is evidently no specific study featuring resection of the corpus callosum infiltrated by glioma, despite abundant literature concerning callosotomy in epilepsy surgery or transcallosal ventricular approaches. The aim of this paper was to analyze functional outcome following removal of corpus callosum invaded by LGG and to analyze the impact of this callosectomy on the quality of resection. METHODS: Between 1996 and 2002, a total of 32 patients harboring an LGG involving part of the corpus callosum and having no or only a mild preoperative deficit underwent surgery aided by intraoperative electrical mapping to preserve eloquent structures identified on stimulation and to perform the most extensive resection possible. Preoperatively, no clinical response was elicited on stimulation of the corpus callosum; thus, the part of this structure that was invaded by LGG was removed. Despite immediate postoperative neurological worsening, all patients but one recovered within 3 months and returned to a normal socioprofessional life. The additional callosectomy allowed for nine total resections, 18 subtotal resections, and five partial resections. Furthermore, only two cases of contralateral hemispherical migration occurred during a median follow up of 3 years. CONCLUSIONS: Resection of the corpus callosum infiltrated by glioma improves the quality of tumor removal without increasing the risk of sequelae.     

正常男女性兴奋的磁共振功能成像研究

目的:利用磁共振功能成像技术探讨在色情录像刺激下正常男女性兴奋在中枢神经系统的差异。方法:健康男性和女性各20例,均为右利手,无性功能障碍史,无任何精神、心理及其他器质性疾病史。用色情录像和非色情录像刺激,每个录像片段60s,每个色情和非色情录像片段之间间隔12s。用GE1.5THD磁共振扫描系统进行血氧水平依赖的功能磁共振扫描,共扫描7.0min。用3DSPGR-T1WI序列进行全脑轴位扫描作为全脑解剖图。结果:在视听相关的性刺激下,男女激活不同的脑区:①女性双侧杏仁核激活,左侧激活体积为(220.52±17.09)mm3,右侧为(155.45±18.34)mm3,左侧明显大于右侧(P<0.05)。而男性仅左侧杏仁核被激活。②正常男性左前扣带回的激活较女性明显,男性左前扣带回激活体积为(420.75±19.37)mm3,女性为(310.67±10.53)mm3,两者差异有显著性(P<0.05);③女性胼胝体压部激活较男性明显(P<0.01),女性激活体积为(473.45±14.92)mm3,男性为(363.32±13.30)mm3。结论:男女性反应在中枢神经系统的激活存在差异,推测男女性脑结构和功能存在不同是这种差异的基础。     

Somatosensory representation in patients who have undergone hemispherectomy: a functional magnetic resonance imaging study

OBJECT: Removal or disconnection of an entire cerebral hemisphere is occasionally used to treat refractory seizures. Patients who have undergone a hemispherectomy provide useful models to study the reorganization of cortical somatosensory representation. This plasticity may be a consequence of the pathological lesion, the hemispherectomy itself, or both. METHODS: Three patients who had undergone hemispherectomy were studied with functional magnetic resonance (fMR) imaging. Responses to sensory stimulation in normal hands and hands opposite the lesioned hemisphere were studied. Multislice T2*-weighted gradient-echo echoplanar images were obtained using a 1.5-tesla MR imager. The activation condition consisted of somatosensory stimulation of the index finger. A T1-weighted anatomical MR image was acquired. The fMR and anatomical MR images were coregistered, and statistically significant activation foci (p < 0.01) were identified. Stimulation of the normal hand produced activation in the primary somatosensory cortex (SI) in all patients. Stimulation of the impaired hand resulted in activation of the ipsilateral parietal operculum (second somatosensory area [SII]) and posterior parietal lobe (Brodmann's Area 7) in all cases, but no activation was elicited in the SI in any patient. In addition, other areas within the ipsilateral frontal and parietal lobes were activated in some individuals. CONCLUSIONS: Residual somatosensory function in the hand opposite the lesioned hemisphere is mediated by the SII and other cortical regions in the intact hemisphere, without involvement of the SI.     

Diffuse axonal injury associated with multiple traumatic aneurysms of the distal anterior cerebral artery--case report.

A rare case of multiple traumatic aneurysms, arising from the distal anterior cerebral artery (ACA), associated with a diffuse axonal injury is presented. An 18-year-old male sustained a frontal impact injury in a traffic accident on February 19, 1988. He immediately lost consciousness and was transported to a local hospital where his Glasgow Coma Scale score was 6. A computed tomographic scan showed a traumatic subarachnoid hemorrhage extending from the corpus callosum to the left parietal lobe. With conservative treatment, he gradually regained consciousness and was referred to our hospital 12 days later. Skull x-rays revealed no fracture. A right common carotid angiogram revealed multiple aneurysmal dilatations on the right distal ACA. A left frontoparietal craniotomy was then performed to determine the nature of the aneurysmal dilatation, and to evacuate the intracerebral hematoma because his right hemiparesis persisted. Two aneurysmal dilatations on the distal ACA were tightly surrounded by clots and a hematoma extended from the corpus callosum to the parietal lobe. The ACA was trapped proximal and distal to the aneurysmal dilatations. The postoperative course was uneventful.     

Calpain inhibitor MDL-28170 reduces the functional and structural deterioration of corpus callosum following fluid percussion injury

It is known that calpain activation is involved in human traumatic brain injury (TBI) and that calpain inhibition can have neuroprotective effects on both gray matter and white matter injury of TBI models. However, the role of calpain activation in the corpus callosum remains unclear and requires elucidation given its potential clinical relevance. We evaluated the neuroprotective effects of calpain inhibitor MDL-28170 on corpus callosum function and structural destruction using a fluid percussion injury (FPI) model. The therapeutic time window for a single administration of MDL-28170 was up to 4 h post injury in protecting the corpus callosum structural integrity, and up to 30 min in protecting the axonal function evaluated 1 day following injury. When given 30 min prior injury, MDL-28170 showed neuroprotective effects that lasted up to 7 days. However, 30 min post injury administration of the drug afforded neuroprotection only up to 3 days. In contrast, two additional reinforcement injections at 24 and 48 h in addition to 30 min post FPI significantly protected both axonal function and structural integrity that lasted 14 days following FPI. Our data indicated that calpain inhibitor MDL-28170 is an effective neuroprotectant for axonal injury in corpus callosum following FPI with a therapeutic time window up to 4 hours. Although delayed treatment (2 or 4 h post FPI) was effective in protecting the axonal structure, the axons saved may not be as functional as normal fibers. Multiple drug administrations may be necessary for achieving a persisting effectiveness of this compound.     

Regional size reduction in the human corpus callosum following pre- and perinatal brain injury

This morphometric study examined two aspects of corpus callosum development: pediatric cortico-callosal topography and developmental neuroplasticity subsequent to perinatal brain injury. In vivo magnetic resonance imaging was used to quantify the total midsagittal cross-sectional area and five anterioposterior subregions of the callosum in 10 children with focal lesions and 86 healthy volunteer control subjects. Nine of the ten children with early injury showed a reduction in the total area of the callosum relative to matched controls. The area of the total callosum cross-section was inversely proportional to the size of lesion. All patients displayed region-specific size reduction. This regional thinning bore a topographical relationship to the lesion sites. Reduction in anterior subregions 1, 2 and 3 was respectively associated with lesions in the anterior inferior frontal area, the middle and superior frontal region, and the precentral area. Attenuation of subregion 4 corresponded to anterior parietal lesions, and thinning of subregion 5 occurred with posterior parietal injury. This cortical-callosal pattern coincides with adult and nonhuman primate mappings. Callosal thinning despite the early onset of the lesions suggests limits to developmental neuroplasticity.     

Structure-function correlates of cognitive decline in aging

To explore neural correlates of cognitive decline in aging, we used longitudinal behavioral data to identify two groups of older adults (n = 40) that differed with regard to whether their performance on tests of episodic memory remained stable or declined over a decade. Analysis of structural and diffusion tensor imaging (DTI) revealed a heterogeneous set of differences associated with cognitive decline. Manual tracing of hippocampal volume showed significant reduction in those older adults with a declining memory performance as did DTI-measured fractional anisotropy in the anterior corpus callosum. Functional magnetic resonance imaging during incidental episodic encoding revealed increased activation in left prefrontal cortex for both groups and additional right prefrontal activation for the elderly subjects with the greatest decline in memory performance. Moreover, mean DTI measures in the anterior corpus callosum correlated negatively with activation in right prefrontal cortex. These results demonstrate that cognitive decline is associated with differences in the structure as well as function of the aging brain, and suggest that increased activation is either caused by structural disruption or is a compensatory response to such disruption.     

Metabolic activation of intercortical and corticothalamic pathways during enflurane anesthesia in rats

The purpose of this study was to examine the effects of enflurane on local cerebral glucose utilization (LCGU), and to provide further insight into the mechanism of the epileptogenic properties of enflurane. Twenty-four male Wistar rats were divided into four groups; three groups with intact cortex received 0.5, 2, or 4% enflurane, and one group with unilateral cortex excised received 4% enflurane. LCGU was measured at each anesthetic concentration using the autoradiographic 2-[14C]deoxyglucose method. LCGU in ten of 33 structures examined during 2% enflurane decreased by 19-33%, and LCGU in 22 structures during 4% enflurane decreased by 19-65%, when compared with that during 0.5% enflurane. While LCGU, in most structures, decreased in a dose-related manner, LCGU in the corpus callosum, thalamic ventrobasal complex, and hippocampal CA3 field during 4% enflurane increased by 31-70%, compared with that during 0.5% and/or 2% enflurane. With unilateral cortical excision during 4% enflurane, the increase in LCGU in the ventrobasal complex was obliterated in the excision side, and the increase in the corpus callosum was attenuated. High LCGU in the hippocampal CA3 field and contralateral ventrobasal complex was not affected with cortical excision. These results indicate that intercortical and corticothalamic pathways are metabolically activated during deep enflurane anesthesia, suggesting that the epileptogenic property of enflurane is related to activation of these pathways.     

Dominance for vestibular cortical function in the non-dominant hemisphere

The aim of this (15)O-labelled H(2)O bolus positron emission tomography (PET) study was to analyse the hemispheric dominance of the vestibular cortical system. Therefore, the differential effects of caloric vestibular stimulation (right or left ear irrigation with warm water at 44 degrees C) on cortical and subcortical activation were studied in 12 right-handed and 12 left-handed healthy volunteers. Caloric irrigation induces a direction-specific sensation of rotation and nystagmus. Significant regional cerebral blood flow increases were found in a network within both hemispheres, including the superior frontal gyrus/sulcus, the precentral gyrus and the inferior parietal lobule with the supramarginal gyrus. These areas correspond best to the cortical ocular motor centres, namely the prefrontal cortex, the frontal eye field and the parietal eye field, known to be involved in the processing of caloric nystagmus. Furthermore, distinct temporo-parietal activations could be separated in the posterior part of the insula with the adjacent superior temporal gyrus, the inferior parietal lobule and precuneus. These areas fit best to the human homologues of multisensory vestibular cortex areas identified in the monkey and correspond to the parieto-insular vestibular cortex (PIVC), the visual temporal sylvian area (VTS) and areas 7 and 6. Further cortical activations were seen in the anterior insula, the inferior frontal gyrus and anterior cingulum. The subcortical activation pattern in the putamen, thalamus and midbrain is consistent with the organization of efferent ocular motor pathways. Cortical and subcortical activation of the described areas was bilateral during monaural stimulation, but predominant in the hemisphere ipsilateral to the stimulated ear and exhibited a significant right hemispheric dominance for vestibular and ocular motor structures in right-handed volunteers. Similarly, a significant left hemispheric dominance was found in the 12 left-handed volunteers. Thus, this PET study showed for the first time that cortical and subcortical activation by vestibular caloric stimulation depends (i) on the handedness of the subjects and (ii) on the side of the stimulated ear. Maximum activation was therefore found when the non-dominant hemisphere was ipsilateral to the stimulated ear, i.e. in the right hemisphere of right-handed subjects during caloric irrigation of the right ear and in the left hemisphere of left-handed subjects during caloric irrigation of the left ear. The localization of handedness and vestibular dominance in opposite hemispheres might conceivably indicate that the vestibular system and its hemispheric dominance, which matures earlier during ontogenesis, determine right- or left-handedness.     

Brain activity during stimulation of the trigeminal nerve with noxious heat.

OBJECTIVE: The aim of this study was to observe areas of brain activation with painful hot stimulation to the trigeminal nerve. STUDY DESIGN: Nine healthy pain-free women (mean age 26.2 +/- 6.9 yrs) with a natural, regular menstrual cycle participated in the study. Whole-brain functional magnetic resonance imaging (fMRI) data were acquired for each participant on day 2 or 3 after the onset of menses using echo-planar imaging at 1.5T with near-isotropic spatial resolution and a temporal resolution of 4 s. RESULTS: Whole-brain fMRI with a Peltier thermode inside the head coil yielded a feasible imaging protocol with little disturbance from the thermode. Painful thermal stimulation of the left trigeminal system activated discrete brain regions within the insula, cingulate gyrus, thalamus, inferior parietal lobe/postcentral gyrus, right middle and inferior frontal gyri, cuneus, precuneus, and precentral gyrus. CONCLUSION: Painful stimulation of the trigeminal nerve resulted in activation of similar brain areas generally known for pain processing of painful peripheral stimulation.