The imaging techniques — positron emission tomography scanning and magnetic resonance imaging, known as PET and functional M. R. I. scans — are used prominently in studies of brain activity. The most active brain areas appear to light up on the scans as specific tasks are performed. The two techniques do not measure nerve-cell activity directly; they measure the extra flow of blood that surges to the most active brain areas.
Researchers at the Weizmann Institute of Science in Rehovot, Israel, have monitored these changes in blood flow in anesthetized cats by removing parts of the skull and observing how the nerve cells in activated regions fuel their activities by rapidly removing oxygen from nearby red blood cells.
This rapid uptake of oxygen, made evident by visible changes in the color of the red cells, proves that early oxygen transfer gives these neurons the energy to do their work, the researchers said.
They also found that subtle changes in blood flow began significantly earlier than was detected by PET and functional M. R. I. scans, which lack sufficient resolution and do not form. their images quickly enough to follow such rapid changes. Dr. Amiram Grinvald published the findings in the Journal Science.
"The initial event is very localized and will be missed if you don't look for it soon enough and use the highest possible resolution," Dr. Grinvald said. "Now people are beginning to use our results with other imaging methods."
Working on the exposed brain lets researchers follow electrical activity and the accompanying blood flow in greater detail than is possible by using indirect imaging methods that track neural activity through the skull. However, opportunities for open-skull studies of humans are limited to some kinds of neurosurgery, and researchers must mostly rely on PET and functional M. R. I. images for studies linking behavior. with specific brain activity.
By directly observing exposed cat brains and in similar work with a few human cases, Dr. Grinvald and his associates have been able to observe the first evidence of electrical activity and other changes in brain cells after a light has been seen or a limb moved.
The newest research showed that it took three seconds or more after an event for the flow of blood to increase to an area of the brain dealing with a stimulus. That is the blood-flow increase usually pictured in brain-function studies with PET or functional M. R. I techniques, the Israeli researchers said. However, the initial reaction observed in the Weizmann research by directly imaging the exposed brain — the direct transfer of oxygen from blood cells to neurons — occurred in the first-tenth of a second and was lost to conventional imaging, they said.
The later increase in blood flow to the area, Dr. Grinvald said, was obviously an attempt by the body to supply more oxygen for brain activity. But the increase in blood was so abundant that it covered an area much larger than the region directly involved in the activity being studied, masking some of the subtle changes, he said.
The body's reaction, the researchers said in the paper, was like "watering the entire garden for the sake of one thirsty flower."
Dr. Kamil Ugurbil, said that the Israeli research provided clues that allowed the use of functional M. R. I. scans to picture earlier events in the activity of brain cells.
"Dr. Grinvald's observations are very important, and they have significant implications for functional imaging with high resolution," Dr. Ugurbil said in an interview. "We have actual
A.those changes are subtle and masked by some reactions
B.subtle changes in blood flow began earlier
C.the imaging techniques are out of place
D.the flow of blood to increase to an area of the brain is slow
第1题
进程是操作系统中的重要概念之一。在下列有关Window98的进程的叙述中,正确的是______。
A.进程是操作系统进行资源分配的单位
B.一个进程只可以产生一个线程
C.一种应用程序(如获至宝MS Word)同时处理多个文档时,系统中仅产生一个进程
D.用户不可以强制性地中止某个进程
第2题
A.进程是操作系统进行资源分配的单位
B.一种应用程序(如 MS WorA) 同时处理多个文档时,系统中仅产生一个进程
C.一个进程可以产生(包含)多个线程
D.用户可以强制性地中止某个进程
第3题
A.线程也称为“轻量级进程”,它表示进程中的一个控制点和一条执行途径
B.Win 32应用程序通常是多线程的,如Microsoft Word软件
C.利用Windows 98提供的“系统监视器”可以查看当前系统中线程的数目
D.线程是存储器、外设等资源的分配单位,也是处理器调度的对象
第4题
A.线程也称为“轻量级进程”,它表示进程中的一个控制点和一条执行途径
B.Win32应用程序通常是多线程的,如Microsoft Word软件
C.利用Windows 98提供的“系统监视器”可以查看当前系统中线程的数目
D.线程是存储器、外设等资源的分配单位,也是处理器调度的对象
第5题
在下列有关Windows XP中进程与线程的叙述中,错误的是______。
A) 进程是系统资源分配的基本单位,系统中通常有多个进程在进行
B) 利用“Windows任务管理器”可以查看当前运行的进程情况,但不能通过它选择并结束某进程的运行
C) 线程是处理器的调度对象,一个进程可以创建多个并发执行的线程
D) 同一个进程内各线程可以访问该进程拥有的所有资源
第6题
进程是操作系统中一个重要的概念。下列有关进程的叙述中,错误的是
A.进程是指程序处于一个执行环境中在一个数据集上的运行过程
B.系统资源的分配主要是按进程进行的
C.进程在执行过程中通常会不断地在就绪、运行和阻塞这3种状态之间进行转换
D.在Windows 98中,所有的进程均在各自的虚拟机中运行,即进程的数目等于虚拟机的数目
第7题
进程是操作系统中一个重要的概念。下列有关进程的叙述中,错误的是:
A.进程是指程序处于一个执行环境中在一个数据集上的运行过程
B.系统资源的分配主要是按进程进行的
C.进程在执行过程中通常会不断地在就绪、运行和阻塞这3种状态之间进行转换
D.在Windows 98中,所有的进程均在各自的虚拟机中进行,即进程的数目等于虚拟机的数目
第8题
下列是有关Windows 98/XP操作系统中进程与线程的叙述,其中错误的是
A.引入线程机制是为了提高进程内部的并发性,以进一步提高处理器的利用率
B.进程是存储器与外设等资源的分配单位
C.线程是处理器的调度对象
D.所有的Windows应用程序都是多线程的,即运行时线程数为2个或2个以上
第9题
下列是有关Windows 98/XP操作系统中进程与线程的叙述,其中错误的是
A.引入线程机制是为了提高进程内部的并发性,以进一步提高处理器的利用率
B.进程是存储器与外设等资源的分配单位
C.线程是处理器的调度对象
D.所有的Windows应用程序都是多线程的,即运行时线程数为2个或2个以上
第10题
A.在DOS中,该机制是EXEC函数
B.在Windows中启动进程的函数是CreateProcess
C.在OS/2中启动进程的函数是CreateProcess
D.在DOS中启动进程的函数是CreateProcess
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