神經(jīng)活動(dòng)能夠增加大腦皮層的血流，但是一項(xiàng)新的研究發(fā)現(xiàn),，這些血流在神經(jīng)細(xì)胞持續(xù)工作時(shí)會(huì)發(fā)生變化。
（圖片提供:Yevgeniy Sirotin,，Aniruddha Das）
在研究每件事物——從人們?nèi)绾蚊鎸?duì)親人的離去到我們?yōu)槭裁慈绱丝释承┦澄?mdash;—的神經(jīng)學(xué)機(jī)制時(shí),，科學(xué)家越來(lái)越多地傾向于使用功能性磁共振成像技術(shù)(fMRI)。在大多數(shù)情況下,，這項(xiàng)技術(shù)能測(cè)量大腦中的血氧水平,，并通常假設(shè)大腦中血氧水平越高的區(qū)域，其神經(jīng)細(xì)胞的活動(dòng)也就越頻繁,。然而,，事實(shí)果真如此嗎？一項(xiàng)新的研究對(duì)這種假設(shè)提出了質(zhì)疑,，進(jìn)而在這一萌芽中的研究領(lǐng)域激起了一抹意想不到的波瀾,。
這一驚人發(fā)現(xiàn)始于一項(xiàng)有關(guān)兩只猴子的試驗(yàn)。美國(guó)哥倫比亞大學(xué)的神經(jīng)科學(xué)家Yevgeniy Sirotin和Aniruddha Das訓(xùn)練每只猴子觀察來(lái)自另一間暗室的微光,。當(dāng)這些光線以有規(guī)律且可預(yù)見(jiàn)的時(shí)間間隔變紅后,，每只猴子只需凝視幾秒鐘的光線便能得到一杯作為獎(jiǎng)勵(lì)的果汁。研究人員在猴子的初級(jí)視覺(jué)皮層——視覺(jué)信息進(jìn)入大腦皮層的第一座“驛站”——中植入了微電極,。當(dāng)兩只猴子完成這項(xiàng)試驗(yàn)時(shí),，微電極僅僅獲得了神經(jīng)細(xì)胞活動(dòng)的一組穩(wěn)定且無(wú)噪音的信號(hào)。（Das表示,，微光只提供了很小的視覺(jué)刺激,，就像夜空中的一顆星一樣。）然而對(duì)血流和血氧水平進(jìn)行的光學(xué)測(cè)量卻得出了不同的結(jié)論。研究人員在最近出版的英國(guó)《自然》雜志上報(bào)告了這一研究成果,。
在整個(gè)試驗(yàn)過(guò)程中,，這兩項(xiàng)針對(duì)視覺(jué)皮層的血液動(dòng)力學(xué)測(cè)試結(jié)果起起落落，并分別在猴子凝視光線的幾秒鐘之前達(dá)到了峰值,。Das認(rèn)為,，這一發(fā)現(xiàn)表明，在響應(yīng)神經(jīng)細(xì)胞活動(dòng)的過(guò)程中,，特定大腦區(qū)域的血氧水平并非只是簡(jiǎn)單地升高,，而是會(huì)搶在一項(xiàng)預(yù)期任務(wù)之前作出響應(yīng)——即便周圍的神經(jīng)細(xì)胞相對(duì)平靜時(shí)依然如此。這意味著神經(jīng)細(xì)胞運(yùn)行與血液動(dòng)力學(xué)之間的關(guān)系并非像許多學(xué)者認(rèn)為的那樣簡(jiǎn)單,。
Das表示,，盡管這些發(fā)現(xiàn)“并不會(huì)對(duì)整個(gè)fMRI研究領(lǐng)域造成麻煩”，但會(huì)讓fMRI的研究人員重新思考應(yīng)該如何設(shè)計(jì)以及解釋他們的試驗(yàn),。Das認(rèn)為可能需要改變大多數(shù)試驗(yàn)的設(shè)計(jì),，從而扣除他和Sirotin所發(fā)現(xiàn)的提前發(fā)生的血液動(dòng)力學(xué)變化，并使研究人員能更緊密地追蹤神經(jīng)細(xì)胞活動(dòng)產(chǎn)生的變化,。
美國(guó)加利福尼亞大學(xué)伯克利分校的神經(jīng)科學(xué)家Ralph Freeman認(rèn)為,，這是一個(gè)“非常令人吃驚的”研究成果。Freeman說(shuō)：“直覺(jué)告訴我,，它將開(kāi)啟一片相關(guān)研究的新領(lǐng)域,。”（生物谷Bioon.com）
生物谷推薦原始出處：
Nature，457, 475-479,，Yevgeniy B. Sirotin,，Aniruddha Das
Anticipatory haemodynamic signals in sensory cortex not predicted by local neuronal activity
Yevgeniy B. Sirotin1 & Aniruddha Das1,2,3,4,5,6
1 Department of Neuroscience,
2 Department of Psychiatry,
3 W. M. Keck Center on Brain Plasticity and Cognition,
4 Mahoney Center for Brain and Behavior,
5 Department of Biomedical Engineering, Columbia University, New York, New York 10027, USA
6 New York State Psychiatric Institute, 1051 Riverside Drive, Unit 87, New York, New York 10032, USA
Haemodynamic signals underlying functional brain imaging (for example, functional magnetic resonance imaging (fMRI)) are assumed to reflect metabolic demand generated by local neuronal activity, with equal increases in haemodynamic signal implying equal increases in the underlying neuronal activity1, 2, 3, 4, 5, 6. Few studies have compared neuronal and haemodynamic signals in alert animals7, 8 to test for this assumed correspondence. Here we present evidence that brings this assumption into question. Using a dual-wavelength optical imaging technique9 that independently measures cerebral blood volume and oxygenation, continuously, in alert behaving monkeys, we find two distinct components to the haemodynamic signal in the alert animals' primary visual cortex (V1). One component is reliably predictable from neuronal responses generated by visual input. The other component—of almost comparable strength—is a hitherto unknown signal that entrains to task structure independently of visual input or of standard neural predictors of haemodynamics. This latter component shows predictive timing, with increases of cerebral blood volume in anticipation of trial onsets even in darkness. This trial-locked haemodynamic signal could be due to an accompanying V1 arterial pumping mechanism, closely matched in time, with peaks of arterial dilation entrained to predicted trial onsets. These findings (tested in two animals) challenge the current understanding of the link between brain haemodynamics and local neuronal activity. They also suggest the existence of a novel preparatory mechanism in the brain that brings additional arterial blood to cortex in anticipation of expected tasks.
