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81.
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83.
The majority (217/325, 66%) of the neurons in the middle temporal (MT)
area/V5 show strong antagonistic surrounds, defined here by a decrease of
at least 50% in the summation curve. We mapped the antagonistic surround in
145 such cells, using eight circularly distributed surround stimulus
patches (Surround Asymmetry Test, SAT) and also mapped the surround in 51
of these 145 cells using a grid consisting of 25 square patches (Surround
Mapping Test, SMT). Both tests showed that the angular surround
distribution was non-uniform in the majority of these neurons. In half the
neurons, the antagonistic surround was asymmetric, and arose from a single
region on one side of the excitatory receptive field (ERF). In another
quarter of the sample the surround was bilaterally symmetric, and arose
from a pair of regions on opposite sides of the ERF. Only the remaining 20%
showed a circularly symmetric surround distribution. These three groups
differed in their laminar distribution. The SMT showed that, radially, the
surround antagonism reached a maximum, on average, at 1.5 times the ERF
radius. Detailed comparisons of the spatial relationships of excitatory and
inhibitory regions of the RF components shows that non-homogeneity of the
surround influence appears to be an intrinsic property of the surround.
Such a property may underly the extraction of the surface orientation and
curvature from speed patterns.
相似文献
84.
Intracranial vascular stenosis and occlusion: evaluation with three-dimensional time-of-flight MR angiography. 总被引:7,自引:0,他引:7
To assess the usefulness of magnetic resonance (MR) angiography in the characterization of intracranial arterial stenosis and occlusion, a three-dimensional time-of-flight method was compared with conventional angiography in 214 vessels in 29 patients. Studies were independently interpreted by two neuroradiologists who scored each vessel as normal, narrowed, or occluded. Overall, 97% of normal vessels and 100% of occlusions were correctly graded. Sixty-one percent of stenoses were graded correctly; the remainder were graded as normal. The portions of the intracranial vessels near the skull base and especially the paracavernous and supraclinoid segments of the internal carotid arteries were areas of frequent over- and underestimation of stenosis due to the presence of dephasing artifacts. In patients with stenosis or occlusion, MR angiography also provided information regarding the presence of collateral flow in the circle of Willis. When used in conjunction with MR imaging of the brain and MR angiography of the extracranial carotid arteries, intracranial MR angiography allows a more complete evaluation of the patient with symptoms of cerebral ischemia or infarction. 相似文献
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J E Heiserman B L Dean J A Hodak R A Flom C R Bird B P Drayer E K Fram 《AJNR. American journal of neuroradiology》1994,15(8):1401
PURPOSETo examine the incidence of neurologic complications associated with modern cerebral angiography and to assess patient characteristics associated with an increased risk of complications.METHODSOne thousand consecutive cerebral angiographic procedures were evaluated prospectively. Examinations were performed using transfemoral catheterization and film-screen methods. For purposes of this trial, a neurologic complication was defined as any new focal neurologic deficit or change in mental status occurring during the angiogram or within the following 24 hours. Patients were evaluated during and at the completion of angiography. Follow-up evaluations were performed on the day of and the day after angiography.RESULTSThere were a total of 10 neurologic complications within 24 hours of angiography, 5 of which were persistent. Onset of 5 of the deficits occurred during angiography, the other 5 (3 persistent) were delayed. All complications occurred in patients being evaluated for stroke/transient ischemic attack or (in one case) asymptomatic bruit. A higher average age, longer average procedure time, and greater volume of radiographic contrast was noted in these patients than in the study population.CONCLUSIONCerebral angiography was associated with a 1% overall incidence of neurologic deficit and a 0.5% incidence of persistent deficit. All complications occurred in patients presenting with a history of stroke/transient ischemic accident or carotid bruit, which may reflect the difficulty of performing angiography in this population at risk for atherosclerotic changes. 相似文献
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88.
MR imaging of pericallosal lipoma 总被引:4,自引:0,他引:4
Early pathologic reports of corpus callosal lipoma described a consistent relationship between the lipoma and the dorsal surface of the corpus callosum, particularly when the lipoma is not associated with corpus callosal agenesis. MR imaging, especially T1-weighted sagittal acquisitions, exquisitely demonstrated this anatomic relationship in three relatively asymptomatic patients. Therefore, in most cases, a lipoma of the corpus callosum is more accurately described as a pericallosal lipoma. In one individual, common associated findings (partial agenesis of the corpus callosum and choroid plexus lipoma) were also noted. Surgical therapy is usually not indicated because symptoms are generally not related and the anterior cerebral artery is often encased by the lipoma. 相似文献
89.
Digital subtraction angiography (DSA) of the spinal cord was performed in 6 patients using selective intra-arterial injections of contrast material. Two arteriovenous malformations of the spinal cord, 1 dural fistula, and 1 case of multiple hemangioblastomas were studied. Contrast and spatial resolution were satisfactory for defining normal and abnormal vascularity while reducing examination time, contrast dosage, patient discomfort, and film cost. The only significant limitation was misregistration artifacts seen on lateral views encompassing the diaphragm. 相似文献
90.