青光眼患者视觉皮层激活的fMRI评价

目的 应用功能磁共振成像(fMRI)评价青光眼患者视觉皮层的激活情况,分析视野改变与视觉皮层激活间的对应关系.资料与方法 30例青光眼患者和30名健康自愿者进行左、右眼单眼刺激组块设计的fMRI实验.刺激为全屏幕黑白翻转棋盘格,对照为中心带静止白色圆点的黑色屏幕.在1.5 T MR扫描仪行血氧水平依赖(BOLD)-fMRI,6个对照组块与5个刺激组块交替进行,每个组块20 s.采用SPM2软件进行数据处理.观察青光眼患者左、右眼单独刺激时视觉皮层的激活情况并与对照组比较.结果 青光眼和正常对照组左、右眼刺激各16例入组.(1)青光眼左、右眼刺激时双枕叶视觉皮层均激活,且均以右枕叶视觉皮层激活为主;(2)与正常对照组比较,青光眼左、右眼刺激时双枕叶视觉皮层激活强度和范围减低,且均以右枕叶视觉皮层激活减低为主;(3)9例中晚期青光眼患者,视野缺损主要表现为管状视野和视野颞侧视岛.左、右眼刺激时视觉皮层激活方式不同,左眼刺激以右枕叶视觉皮层激活为主,右眼刺激则以左枕叶视觉皮层激活为主.结论 fMRI可以反映青光眼患者视野缺损和视觉皮层激活的对应关系,进而推断视网膜病变的部位和程度,有助于指导临床诊断和治疗.     

弱视视觉运动功能的功能磁共振研究

目的 采用功能磁共振成像（fMRI）技术分别对屈光不正及斜视性弱视在视觉运动刺激时的皮层反应进行观察，探索不同弱视的视觉运动受影响机制。方法 采用旋转光栅／静止光栅为视觉运动刺激源，在1．5T的磁共振仪下分别对7例临床诊断为屈光不正型弱视及10例斜视型的弱视患者在单眼刺激下行功能磁共振检查，SPM（statistical parametric mapping）软件进行数据处理，结果 采用单样本数据分析。并设定双眼间反应体素差异水平指数，采用Mann-Whitney秩和检验，用以判断双眼分别刺激时皮层反应的差异水平。结果 两组样本中，双侧枕叶视觉运动功能区域有不同程度的反应，弱视眼刺激较正常眼有额外的额叶反应。以中颞（MT）区为感兴趣区进行分析，7例屈光不正型弱视患者中，弱视眼反应水平相比正常眼均下降，10例斜视性弱视，除1例无任何反应外，5例弱视眼反应水平较正常眼下降，4例较正常增高。两组数据体素差异水平指数（1AD）之间有统计学差异（Z=2．382，P=0．017）。结论 单眼视觉运动刺激时，弱视眼比正常眼有更多的脑区参与反应。屈光不正型弱视皮层可能存在视觉运动功能的下降，斜视性弱视功能受损情况可能与弱视的程度有关；提示两种弱视的视觉运动功能受损情况不一致。     

健康人视觉刺激的fMRI研究

目的：探讨健康人单眼及双眼刺激功能磁共振成像（fMRI）时双侧视觉皮层激活的特点及单、双眼刺激脑激活区的异同。方法：正常受试者25例,行单眼及双眼刺激fMRI检查。选择符合条件者16例。fMRI内容为全屏幕黑白翻转棋盘格刺激的Block设计。fMRI采用SPM2软件进行功能数据预处理、统计分析和结果显示,对单眼及双眼刺激行组间分析,并对三种刺激的激活两两配对进行SPM2基本模型配对t检验。结果：正常受试者左、右眼及双眼刺激纹状区、纹旁及纹周区（BA17,18,19）均见激活;单眼及双眼刺激组分析中引起视觉皮层反应的优势大脑半球均为右侧,同一个体的左眼、右眼及双眼刺激的优势半球一致;左、右眼刺激,双眼与左眼刺激枕区视觉皮层激活区未见显著性差异;双眼较右眼刺激仅有较小范围的视觉皮层激活明显增加。结论：健康人左、右眼刺激及双眼刺激引起的双侧视觉皮层激活不对称,存在右侧优势现象。     

视觉皮层分区的功能MRI视网膜脑图

目的探讨功能MRI（fMRI）视网膜脑图描绘人视觉皮层各视区的技术方法和生理意义。方法采用1．5T超导MR设备，运用2种具有相位编码信息的投影刺激：离心外散和向心内聚的光环刺激，以及顺时针和逆时针方向旋转的极角刺激，对6名正常人（受试者）进行血氧水平依赖（BLOD）的fMRI。将检测数据进行头动校正、配准等空间预处理，再根据刺激视野的时间相位编码，经傅立叶变换计算出视野特征，得到完整的视网膜脑图，并以二维和三维可视化方式显示。结果成功得到6名受试者初级视觉皮层的个体化分区图，描绘出视区V1、V2（V2腹侧和背侧）、V3和VP、V3A和V4v、V7和V8等视区的精细分区，各视区以一定的拓扑结构分布于枕叶皮层。结论fMRI视网膜脑图技术可以无创、精确地显示人脑视觉皮层功能分区。     

屈光参差性弱视皮层功能损害fMRI视网膜脑图研究

目的 利用血氧水平依赖性功能磁共振成像(BOLD fMRI)视网膜脑图技术(retinotopic mapping),研究屈光参差性弱视皮层功能损害的发生机制. 资料与方法 以1.5T MRI系统采集8例单眼屈光参差性弱视患者枕叶视觉皮层对于视网膜脑图刺激和6 cpd空间频率黑白光点刺激的功能数据,比较弱视眼和健眼在初、高级视觉皮层功能区功能反应;对初、高级皮层间功能损害进行回归分析. 结果 弱视眼在V1、V2、V3、VP、V7区反应T值低于健眼( P＜0.05 ),在V3a、V4、V8区弱视眼与健眼T值无统计学差异;V1区与V2、V3、VP、V7区之间反应减低相关分析无统计学意义( P＞0.05 ). 结论 弱视患者初、高级视觉皮层均存在功能损害,两者之间无明显因果关系;在高级视觉皮层,背、腹侧传导通路均有功能损害;fMRI视网膜脑图技术为深入研究弱视初、高级视觉皮层功能损害的发病机制提供了新的方法.     

聋人枕叶视皮层对频率刺激反应规律的事件相关fMRI研究

目的:探讨聋人枕叶视皮层对视觉频率刺激的反应规律.材料和方法:采用事件相关fMRI技术,对8例聋人和正常人分别给予不同频率的刺激,通过MR采集其脑部数据,运用SMPM99统计软件进行处理分析.结果:聋人与正常人的枕叶视皮层在不同频率刺激下均产生兴奋;随着频率的增加,聋人与正常人枕叶视皮层的兴奋区兴奋强度(intensity)呈现先增加后下降并略有反弹的趋势,二者相似;聋人与正常人的最佳刺激频率分别为4Hz和6Hz.结论:聋人与正常人的枕叶视皮层对频率刺激的基本反应规律是相似的,但最佳的刺激频率是不同的.     

7.0T功能MR波谱动态定量检测视觉皮层代谢物对视觉刺激的反应

目的 采用超高场强(7.0T)功能MR波谱(fMRS),动态定量检测人脑枕叶视觉皮层代谢物对视觉刺激的反应,探讨人脑在功能活动状态下的能量代谢和神经递质传递的活动情况.方法 9名健康志愿者参与本次试验.采用人体7.0 T MR扫描仪及16通道敏感相位编码技术(SENSE)头线圈.首先进行fMRI检查,明确视觉皮层的最大激活区.然后进行1H-MRS检查,ROI为2cm×2cm×2 cm,定位于视觉皮层最大激活层面.采用短TE激励回波采集模式(STEAM)序列采集波谱数据,1 H-MRS扫描过程中同时给予受试者与fMRI检查相同的视觉刺激.运用线性拟合模型( LCModel)以水为内标准,分别测定各代谢物在静息期和刺激期的绝对浓度.采用配对设计数据的Wilcoxon符号秩和检验比较各代谢物浓度在刺激前后差异是否有统计学意义.结果 视觉刺激状态下,天冬氨酸、谷氨酰胺及甘氨酸的浓度分别为(3.20±0.28)、(2.07 ±0.10)和(1.65±0.11) μmol/g,较静息期[分别为(3.52±0.28)、(2.25 ±0.10)和(1.85±0.11) μmol/g]降低,差异有统计学意义(Z值分别为-2.073、-2.073、- 2.429,P值均＜0.05).谷氨酸、谷胱甘肽及乳酸的浓度分别为(11.50±0.11)、(2.45±0.10)和(0.89±0.05) μmol/g,较静息期[分别为(11.28±0.11)、(2.28±0.10)和(0.79±0.05)μmol/g]升高,差异有统计学意义(Z值分别为2.521、2.310、2.016,P值均＜0.05).葡萄糖浓度[(1.54±0.23) μmol/g]在刺激状态下有降低趋势,但与静息期[(1.78±0.28) μmol/g]比较差异无统计学意义(Z=-1.897,P＞0.05).结论 7.0 T fMRS结合LCModel可以定量检测人脑视觉皮层多种代谢物对视觉刺激的反应变化.视觉刺激状态下,激活区乳酸、葡萄糖和天冬氨酸的变化提示脑能量代谢水平的升高;谷氨酸升高伴随谷氨酰胺下降可能提示兴奋性神经递质传递活动的改变;谷胱甘肽升高则提示脑激活状态下抗氧自由基活动增强.     

聋人与正常人枕叶视皮层对亮度敏感性的功能磁共振成像研究

目的　利用血氧水平依赖性功能磁共振成像 (BOLD fMRI)技术 ,探讨聋人在固定频率、不同亮度的点光源刺激下 ,枕叶视皮层的兴奋情况 ,并比较兴奋区的分布、范围、强度与正常人是否一致。方法　对 12例聋人和 15例正常人给予闪烁频率为 8Hz、亮度为 0 7、2 2、5 0 0、180 0lm的点光源刺激 ,同时应用GE 1 5TSignaHorizonLX超导型磁共振成像系统 ,采集其脑部的BOLD fMRI数据。通过AFNI软件进行统计分析得到脑功能活动的图像。结果　在 4种不同亮度的点光源刺激下 ,聋人与正常人的枕叶视皮层均有明显兴奋区 ,且主要兴奋区位于枕叶矩状裂周围的视皮层内 ;随着亮度的增加 ,聋人与正常人视皮层兴奋区内的活动像素数 (numberofactivatedpixels)显著增加 (F正常人 =4 2 7,P <0 0 5 ;F聋人 =6 41,P <0 0 5 ) ,且在较高亮度刺激下 ,聋人枕叶视皮层的兴奋性高于正常人 ,但两者差异无显著性意义 (F =2 17,P >0 0 5 ) ;聋人与正常人视皮层内所有兴奋像素的平均活动水平(activationlevel)随亮度的增加变化不大 ,而在 4种亮度刺激下均兴奋的像素的平均活动水平却随着亮度的增加而有较明显的增加 ,但无显著性差异 (F正常人 =0 79,P >0 0 5 ;F聋人 =1 6 ,P >0 0 5 )。结论聋人与正常人对亮度的基本反应特性是相     

The fMRI Study of Visual Cortex Activation in Patients with Primary Angle -Closure Glaucoma

目的 应用功能磁共振成像(fMRI)研究原发性慢性闭角型青光眼患者视觉皮层激活情况.资料与方法 20例原发性慢性闭角型青光眼患者(青光眼组)和20名健康成人(对照组)左、右眼分别行3.0T MR血氧水平依赖(BOLD)功能磁共振成像(fMRI),将BOLD-fMRI原始数据经基于Matlab平台的SPM2软件进行数据的预处理和统计分析,分别对青光眼组、对照组左、右眼进行组内分析,获得组内平均激活图;病变组与对照组的左、右眼进行组间分析,获得组间比较激活图.结果 (1)与对照组比较,青光眼组左、右眼刺激时双侧视皮层激活范围、激活强度都明显减小,以右侧视皮层为著.(2)与对照组比较,青光眼组左、右眼刺激时大脑皮层均发现异常激活区域.结论 fMRI可以反映青光眼患者视觉皮层功能活动,能为青光眼发病机制的研究提供重要信息.     

脑室旁白质软化症患儿脑性视觉损伤的fMRI 和DTI研究

目的:探讨联合应用fMRI和DTI评价PVL患儿的脑性视觉损伤的价值.材料和方法:选取24名PVL患儿及24名无窒息史的健康儿童志愿者进行镇静的状态下fMRI及DTI检查,视觉刺激采用2HZ闪烁光,fMRI数据采用SPM2软件进行后处理(ROI),计算患儿组与对照组的激活像素数目.DTI数据采用MedINRIA软件进行后处理,根据矢量元素和部分各向异性值(FA值)生成彩色弥散张量图(DTI图),计算患儿与对照组视觉相关主要白质纤维束的FA值.将视觉激活图与彩色弥散张量图融合.结果:正常对照组的激活区位于初级视觉皮层,PVL患儿的激活区部分移位.PVL患儿枕叶平均激活像素数目较正常对照显著减少,视觉相关主要白质纤维束FA值较正常对照显著降低.PVL患儿视觉区激活像素数目与视觉相关主要白质纤维束FA值呈正相关.结论:联合应用BOLD-fMRI和DTI在评价缺氧缺血性脑损伤后视觉功能变化方面有较高的应用价值.     

人脑视皮质功能MRI的初步研究及临床应用

目的：本实验应用功能磁共振成像（fMRI）的回波平面（EPI）技术,着重研究了人脑视皮质的血氧水平依赖（BOLD）的功能磁共振成像。方法：本实验包括对36名正常健康志愿者及17例具有枕区病变患者的fMRI研究。在刺激和静止2种对比条件下采集枕区的回波平面图像。结果：主要视皮质位于双侧距状裂两侧 ,属于布劳德曼（Brodmann)17区。3例具有距状裂附近病变的患者的视皮质由于病变的占位效应而发生移位。结论：fMRI可用于在活体上脑上研究各功能区活动。视觉刺激下的fMRI可对人脑视皮质初步定位。由于病变占位效应的影响,病变周围的功能活动区常发生变形或移位。     

The role of functional MR imaging in patients with ischemia in the visual cortex.

BACKGROUND AND PURPOSE: Functional MR (fMR) imaging is based on changes in regional blood flow. The purpose of this study was to evaluate the role of fMR imaging for detection of a vascular compromised status in the occipital lobe in patients with ischemia in the visual cortex. METHODS: We performed fMR imaging in seven control subjects and seven patients with symptoms and signs of visual cortical transient ischemia and/or infarct. fMR imaging was performed using a gradient-echo sequence with the 2D fast low-angle shot technique. An axial slice including both visual cortices was selected, and stimulation of the visual cortex was performed using a red photostimulator. The number of activated pixels in each primary visual cortex area were counted and an asymmetry ratio [AR (%) = 100 x (R-L)/(R+L)/2] was calculated. Patients and control subjects underwent visual field examination, conventional MR imaging, and vascular imaging (MR angiography in all patients and control subjects, conventional catheter angiography in two patients). fMR imaging results were compared with the results of a visual field examination, conventional MR imaging, and vascular imaging. RESULTS: fMR imaging of the patients showed significant activation asymmetry (P <.05) compared with that of control subjects. Vascular abnormalities in the posterior circulation were found in all seven patients. By conventional MR imaging, five patients were found to have infarction in the occipital lobe and the remaining two patients showed no abnormality. In visual field examination, six of the seven patients showed homonymous hemi- or quadrantanopsia suggesting postchiasmic abnormalities, and the remaining patient had normal findings. fMR imaging showed decreased activity in the visual cortices corresponding to vascular abnormalities (seven of seven patients), permanent infarction (five of seven patients), or visual field defect (six of seven patients). Two patients with normal conventional MR imaging had vascular lesions in the posterior circulation, and fMR imaging showed decreased activity in the corresponding visual cortices. One patient with normal visual field examination had multifocal stenosis in the posterior cerebral artery without infarction, and fMR imaging showed decreased activity in the corresponding visual cortex. CONCLUSION: fMR imaging of the visual cortex may be a sensitive method for the detection of vascular-compromised status in the occipital lobe.     

Color-specific effects of intense laser exposure on visual evoked potentials in rhesus monkeys

Color-specific visual impairments following exposure to intense laser flashes were investigated using visual evoked potentials (VEPs) recorded from four anesthetized rhesus monkeys. Steady-state VEPs were recorded from the primary visual cortex in response to counterphasing high-contrast sine-wave gratings composed of either luminance contrast (red-black, green-black, and blue-black) or chromatic contrast (red-green). The effects of laser flashes in the red (647 nm), green (514 nm), and blue (476 nm) regions of the visible spectrum were assessed. Only slight wavelength-specific effects were demonstrated, primarily for the red-black grating relative to the other two luminance gratings. Recovery functions for VEPs to the red-green chromatic grating were similar to those of the three luminance gratings. The results are discussed in terms of nonopponent (luminance) and opponent (chromatic) processing.     

Temporal relationship between action and visual outcome modulates brain activation: an fMRI study.

Monitoring the visual outcome of our actions is critical to our visuo-motor behavior. To investigate the neural basis of monitoring visual change produced by self-movement, we examined the temporal relationship between manual depression of a button and visual feedback on activation of the brain. Six neurologically normal subjects participated in 3 experiments (synchronous, delayed, and visual [control]). Magnetic resonance (MR) images of their brains were acquired during the experiments using a scanner operating at 3T. In the synchronous experiment, subjects pressed a button at self-paced intervals and received synchronous visual stimuli in response. In the delayed experiment, visual stimuli were presented with a delay after subjects pressed a button at self-paced intervals. In the control experiment (visual experiment), subjects did not press the button, but viewed visual stimuli generated by a computer at random intervals. In the synchronous experiment, activation in the cerebellum and right parietal lobe was stronger than in the delayed experiment, whereas activation in the primary visual cortex was weaker than in the delayed and visual experiments. These results suggest that visual outcomes produced synchronously with action are processed in the cerebellum and the parietal area for the organization of optimal motor behavior, rather than in the primary visual area that is known to process the visual properties of external objects. The cerebellar signal related to visuo-motor contingency may modulate the cortical processing of visual input that is synchronous with action.     

Quantitative functional MR imaging of the visual cortex at 1.5 T as a function of luminance contrast in healthy volunteers and patients with multiple sclerosis

BACKGROUND AND PURPOSE: In patients with multiple sclerosis (MS), a few preliminary functional MR (fMR) imaging studies of the visual cortex reveal information about magnitude differences between healthy individuals and patients with MS at only a single luminance level. We therefore investigated whether varying luminance contrast levels can help uncover subtle changes in fMR imaging characteristics of the visual cortex in healthy volunteers and patients with MS. METHODS: Blood oxygenation level-dependent fMR imaging signal changes in the primary visual cortex were examined as a function of luminance contrast at 1.5 T in 10 healthy volunteers and nine patients with MS. Ten axial sections through the calcarine fissure were obtained with an echo-planar T2*-weighted imaging sequence (4000/54/1 [TR/TE/excitation]; field of view, 220 mm; voxel size, 1.72 x 1.72 x 5 mm). The imaging series consisted of an alternating 20-second rest epoch (black screen) with a 20-second activation epoch (flickering checkerboard) repeated six times. Each imaging series used a graded increase of eight luminance contrast levels. A paired t test between rest and activation images was used to analyze significant (P <.001) contiguous voxels in the region of interest (primary visual cortex). RESULTS: A progressive increase in fMR imaging activation across all luminance contrast levels in healthy controls and patients with MS was shown. The patients with MS had a significantly lower magnitude in the number of fMR imaging activated voxels at all luminance contrast levels (P <.001). A statistically significant increase in fMR imaging activation (activation threshold) was seen at the second luminance contrast level in controls and at the seventh level in patients with MS. CONCLUSION: Quantifiable changes in blood oxygenation level-dependent signal and a progressive increase in activated voxels within the primary visual cortex with increasing luminance contrast were demonstrated at 1.5 T in controls. The patients with MS showed a significant decrease in the number of activated voxels and an increase in activation threshold compared with healthy controls.     

Material-dependent activation in prefrontal cortex: working memory for letters and texture patterns--initial observations

PURPOSE: To prospectively evaluate whether a distinction between verbal and nonverbal short-term memory systems, as predicted with the multicomponent working memory model, is reflected in the material-specific patterns of activation in the prefrontal cortex. MATERIALS AND METHODS: Informed written consent was obtained from all participants, and the institutional review board approved the study protocol. Echo-planar MR imaging was performed in 12 healthy subjects (five female and seven male subjects), with a mean age of 23.52 years +/- 2.52 (standard deviation) and a range of 20-29 years. A two-back task was used in the verbal and nonverbal versions. In the first version, letters were used as stimuli, and in the second version, the stimuli were abstract texture patterns. Timing parameters for both versions were the same. Statistical analysis of the functional data involved a fixed-effects general linear model. Regions of activation were identified from specific t-statistic contrasts between baseline and active tasks (corrected for whole-brain multiple comparisons). RESULTS: The following suprathreshold voxels for the verbal condition were observed predominantly in the left hemisphere (middle frontal gyrus, precentral gyrus, middle temporal gyrus, and occipital cortex). Bilateral activations were in inferior frontal gyri, insulae inferior, superior parietal lobules, and cingulate gyri. In the nonverbal condition, suprathreshold voxels were located mostly bilaterally in the following regions: inferior, middle, and medial frontal gyri and inferior parietal lobules. Active regions were also found in the precentral gyrus and precuneate gyrus in the left parietal lobe and the occipital cortex in the right hemisphere. CONCLUSION: Results of this study are consistent with the multicomponent model of working memory.     

人脑视皮层定位、定量脑功能成像研究

目的 利用功能性磁共振成像(fMRI)定位、定量研究视皮层的位置、不同刺激强度的改变及不同颜色变化在视皮层的信号强度的反应.资料与方法 48名受试者均接受红、绿、蓝3种颜色光(每种颜色均有4000 mcd、6000 mcd、10000 mcd 3种强度)共9次刺激.运用功能成像分析AFNI软件对原始数据进行处理,计算机自动产生出所选层面的时间-信号强度相关图,最后将功能区与相应层面的T2WI解剖图像进行叠加显示活动区的位置、大小、分布,并用统计学方法对图像进行比较.结果 48名受试者接受刺激后在双侧枕叶视皮层均有兴奋区,绿光刺激的平均血氧水平依赖(BOLD)信号强度比红光刺激增强17.5%左右(P＜0.05),而蓝光刺激的平均BOLD信号强度比绿光刺激增强9.6%左右(P＜0.05).6000 mcd强度光刺激的平均BOLD信号强度比4000 mcd光刺激增强84%左右(P＜0.05),10000 mcd强度光刺激的平均BOLD信号强度比6000 mcd光刺激增强66%左右(P＜0.05).同时发现右侧光刺激的平均BOLD信号强度比左侧光刺激增强18%左右(P＜0.05),此外脑组织兴奋还见于外侧膝状体、上丘、脑干的顶盖前区、顶叶皮层等.结论 通过对fMRI图进行统计学分析,证实在光的刺激下枕叶视皮层血流信号增强,3种颜色刺激中对蓝光最敏感,并且右侧信号变化比左侧多.     

Comparison of the hemodynamic response to different visual stimuli in single-event and block stimulation fMRI experiments

Experiments with three different types of basic visual stimulation were performed to compare cortical activation in single-event and block trials. Independent of the stimulation paradigm, the single-event presentation leads to highly consistent signal responses regarding both the activated cortical areas and the dynamics of the signal time course. In contrast, signal time courses during block paradigms depend on the stimulus applied and are a complex and nonlinear function of the single-event responses. Additionally, the initial dip during the first 2 seconds after stimulus onset is consistently observed. However, the small amplitude change (-0.1% to -0.3%) requires signal averaging to establish statistical significance of the effect. Furthermore, different patterns of activation were observed within the primary visual cortex. In an anterior part of the primary visual cortex, activation was only observed at the onset and at the cessation of stimulation involving luminance changes.     

Alteration of blood oxygenation level-dependent signaling by local circulatory condition

PURPOSE: To determine regional differences in the relationship between neuronal activation and blood oxygenation level-dependent (BOLD) signal changes. MATERIALS AND METHODS: We performed BOLD and perfusion-based studies on healthy adult volunteers (40 for BOLD and 20 for perfusion-based studies) with visual stimulation of varied extent in the visual field of subjects. RESULTS: Regions with a significant negative correlation between the extent of visual stimuli and BOLD and perfusion responses were found bilaterally on the lateral surface of the occipital lobe just anterior to the occipital pole. These regions were located in the border zone between the territories of the middle and posterior cerebral arteries. CONCLUSION: Insufficient perfusion pressure in these regions may have led to attenuation of BOLD and perfusion responses to stimulation of a large portion of the visual field, or a condition in which a large extent of the visual areas was activated in the proximal regions of the territories perfused by the middle and posterior cerebral arteries. This finding supports the hypothesis that regional differences in circulatory condition can result in alterations of the relationship between neuronal activation and BOLD signal changes in the normal human brain.