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Neil Burgess 談大腦如何告訴你身處何方

Neil Burgess: How your brain tells you where you are

 

Photo of three lions hunting on the Serengeti.

講者:Neil Burgess

2011年11月演講,2012年2月在TED上線

 

翻譯:TED

編輯:朱學恆、洪曉慧

簡繁轉換:洪曉慧

後製:洪曉慧

字幕影片後制:謝旻均

 

影片請按此下載

MAC及手持裝置版本請按此下載

閱讀中文字幕純文字版本

 

關於這場演講

你如何回憶起將車停在什麼地方?如何判斷自己是否走對了方向?神經科學家Neil Burgess研究神經建構我們周遭環境的機制,及它們如何與我們的記憶及想像產生關聯。

 

關於Neil Burgess

在倫敦大學學院中,Neil Burgess研究腦細胞中的電流活動模式如何引導我們尋找空間中的位置。

 

為什麼要聽他演講

Neil Burgessis是倫敦大學學院認知神經科學研究所副所長,研究海馬迴在空間導航和情節記憶中扮演的角色。他的研究直接回答了以下問題:大腦如何呈現、儲存及使用關於位置的記憶?大腦運用什麼過程及部位來記憶空間及日常事件的時空背景,及協助我們尋找方位?

 

為了探索這個領域,他和他的團隊使用一系列方法收集數據,包括率先使用虛擬現實科技及電腦模型,對大鼠海馬迴神經元的功能進行電生理分析,記錄人類導航的功能性影像,並進行空間和情節記憶的神經心理學實驗。

 

一舉數得的研究:依序探索人類的短期記憶,或我們如何瞭解認知的程序。

 

Neil Burgess的英語網上資料

Home: Neil Burgess at UCL

Interesting: "Imagine Being Somewhere Else"

TEDSalon: Full speaker program

 

[TED科技‧娛樂‧設計]

已有中譯字幕的TED影片目錄(繁體)(簡體)。請注意繁簡目錄是不一樣的。

 

Neil Burgess 談大腦如何告訴你身處何方

 

當我們把車停在一個大停車場後,要如何想起停車的位置?這就是現在困擾荷馬的問題,讓我們試著瞭解他的大腦裡發生了什麼事。

 

我們先從黃色標記的部分-海馬迴開始。它是記憶的源頭,如果這部分受到損傷,例如阿茲海默症,你會記不住事情,包括把車子停哪。這個部位以拉丁文「海馬」命名,因為形狀和海馬相似;如同腦內其他部位一樣,它由神經元構成。

 

所以人腦內大約有一千億個神經元,神經元之間的溝通是藉由神經元間的連結傳送脈衝或電波,海馬迴是由兩層細胞彼此緊密相連組成。近期科學家開始瞭解我們對空間的記憶如何運作-藉由當老鼠在一個空間中探索、尋找食物時,記錄牠們每一個獨立的神經訊號。

 

我們記錄這隻老鼠海馬迴裡的一個獨立神經元,當它發送電波時,會出現紅點和喀噠聲。我們觀察到,這個神經元能偵測出老鼠在環境中的特定位置,並藉由電波將訊號傳遞給大腦其餘部份。我們藉此得知,神經元的傳輸速率是動物位置的函數,如果我們紀錄很多不同的神經元,會發現當動物移到不同位置時,不同的神經會各自發出訊號,就像圖中方塊所顯示的,這所有的訊號會為大腦其餘部份勾勒出一張地圖,持續不斷地告訴大腦,「我現在環境中哪個位置?」

 

我們也記錄了人腦中的位置細胞。有時我們需要監測癲癇患者的腦電波活動情形,有些病患會玩一個電腦遊戲,讓他們在一個小鎮裡開車閒晃,當他們經過鎮裡某個特定地點時,海馬迴裡的位置細胞會被活化,開始傳送電脈衝訊號。

 

所以,位置細胞如何知道人或老鼠在環境中的某個位置?圖中這兩個細胞告訴我們,環境邊界是極重要的資訊。上方那張圖顯示,當老鼠在盒子的兩面牆之間時,這個神經元會產生訊號;當盒子的空間擴大時,細胞被活化的區域也隨之擴大。下方的圖顯示,當南方有一面牆接近時,這個神經元會被活化。如果你在盒中放置另一面牆-只要動物在箱子裡探索時,南方出現一面牆,細胞這兩個相應位置都會被活化。所以我們推測,偵測周遭環境的邊界距離及方向,例如建築物等資訊,對海馬迴來說極為重要。事實證明,當老鼠探索周遭環境時,外界訊號刺激海馬迴後,有些細胞會偵測特定距離與方向外的邊界或端點,將此資訊輸入海馬迴中。

 

你可以看見,當動物接近東邊的牆或障礙物時,左側的細胞會被活化-不論是方盒或圓盒的邊界,或是動物在桌上遊盪時碰到桌緣。當南方有障礙物時,右側的細胞會被活化-無論動物碰的是桌緣或障礙物,還是兩張桌子間的縫隙。因此,我們推斷這是位置細胞在動物探索環境時,決定牠位置的方式之一。

 

我們也可以測試如何給簡單環境中的物體定位-例如這根目標旗桿,或是你車子的位置。所以我們讓人們探索一個環境,記下物體所在位置後,如果讓他們回到那個環境,通常他們都能準確標出之前目標旗桿或車輛擺放的位置。但在一些試驗中,我們會改變環境的形狀或大小,如我們在位置細胞實驗中所做的。

 

在這個例子中,我們發現,他們所認為的旗幟位置,會隨著環境的形狀和大小而發生改變。例如現在你所看到的,如果旗幟在圖中小四方形內的「X」位置,當你問人們旗幟在哪裡時-但其實你已擴大了環境範圍-他們所認為的旗幟位置會如同位置細胞被活化的區域般向外擴張。這就像你對旗幟位置的記憶,是藉由儲存所有位置細胞對那個位置發出的訊號模式,當你回到那個地點,四處打量後,便可將目前腦中位置細胞的訊號模式與之前儲存的模式進行比對,使你能找到記憶中目標的位置。

 

我們也可藉由移動來定位,因此當我們走其他路線時-也許是停車後四處走走-我們可藉由移動來得知自己的位置,因為我們可將移動路線整合,粗略地得知返回方向,位置細胞也能從所謂的網格細胞中獲得此類路線整合的資訊。

 

網格細胞同樣能將訊息輸入海馬迴中,它有點類似位置細胞,當老鼠四處探索時,每個細胞會被不同位置所激發的一連串訊號活化,在整個空間中呈現出令人驚訝的規律三角形網格。如果你紀錄幾個網格細胞的模式,以不同顏色標記,每個細胞在環境中都呈現網格狀活化模式,每個細胞的網格狀活化模式與其他細胞相較下都有些許位移,因此紅色的會在這個網格中活化、綠色的在這個、藍色的在這個網格中。

 

因此整體看來,就像老鼠可以在環境中建立一個虛擬的位置訊號。網格有點像地圖上的經線和緯線,只是這裡用的是三角形。當老鼠四處移動時,這些電訊號可由其中一個細胞傳遞給下一個細胞,使老鼠得以追蹤自己的位置,因此,牠可藉由移動得知自己在環境中的位置。

 

人類有網格細胞嗎?因為所有網格狀活化模式都有相同的對稱軸、相同的網格相位,即以橘色標出的部分。這意味著大腦中特定部位、所有網格細胞的整體行為,其變化取決於我們是沿這六個方向移動,或是沿這六個方向之間的某個方向移動,所以我們可為人們做核磁共振掃描,讓他們玩之前提過的那個電腦遊戲,追蹤所產生的訊號。事實證明,你可以在人腦中的內鼻皮層看見這些訊號,這與老鼠網格細胞在大腦中的位置相同。

 

我們回頭來看荷馬的例子。他可能藉由停車位置的周邊建築,及四周邊界的距離與方向來記憶車子的位置,這個過程由偵測邊界的細胞所發出的訊號來執行。他也記得自己從停車場走出的路線,這有賴於網格細胞所發出的訊號。現在,這兩種細胞都可使位置細胞活化,讓他可藉由移動找到當初車子停放的位置,方法是在腦海中尋找與停車時儲存的訊號模式最匹配的當前訊號模式,這可引導他找到停車位置。這跟視覺上的線索,例如車子是否確實在那裡無關。也許車子已被拖吊,但他仍知道車子原來的位置,而回到那裡取車。

 

所以,除了空間記憶之外,如果我們觀察整個大腦裡的網格狀活化模式,當我們回憶過去經歷時,會發現有許多位置總是處於活化狀態,例如回想上次參加婚禮的情況。因此,也許記憶周遭環境的神經元運作機制也用於產生視覺影像,至少能讓我們在希望這麼做時,於腦海裡重現空間中的場景,使曾經發生過的景像重現。

 

如果確實如此,你的記憶開始於位置細胞藉由彼此間緊密相連的交互作用而活化,然後重新活化邊界細胞,產生視野中整個空間結構的場景,網格細胞可使視野在這個空間中移動。另一種我尚未提過的細胞-頭向細胞(Head Direction cells)就像指南針一樣,根據你面對的方向產生訊號,它可藉由你希望在視覺記憶中形成的景像來定義視野方向。舉例來說,你可藉此想像在婚禮中發生了什麼事。

 

所以,這只是認知神經科學中一個嶄新領域的例子,我們藉此開始瞭解心理活動的過程,例如如何記憶、想像,甚至思考,這都是由數十億個組成大腦的獨立神經元之運作所達成。

 

感謝聆聽。

 

(掌聲)

 

以下為系統擷取之英文原文

About this Talk

How do you remember where you parked your car? How do you know if you're moving in the right direction? Neuroscientist Neil Burgess studies the neural mechanisms that map the space around us, and how they link to memory and imagination.

About the Speaker

At University College in London, Neil Burgess researches how patterns of electrical activity in brain cells guide us through space. Full bio »

Transcript

When we park in a big parking lot, how do we remember where we parked our car?Here's the problem facing Homer. And we're going to try to understand what's happening in his brain.

So we'll start with the hippocampus, shown in yellow, which is the organ of memory. If you have damage there, like in Alzheimer's, you can't remember things including where you parked your car. It's named after Latin for "seahorse," which it resembles. And like the rest of the brain, it's made of neurons.

So the human brain has about a hundred billion neurons in it. And the neurons communicate with each other by sending little pulses or spikes of electricity via connections to each other. The hippocampus is formed of two sheets of cells, which are very densely interconnected. And scientists have begun to understand how spatial memory works by recording from individual neurons in rats or mice while they forage or explore an environment looking for food.

So we're going to imagine we're recording from a single neuron in the hippocampus of this rat here. And when it fires a little spike of electricity, there's going to be a red dot and a click. So what we see is that this neuron knows whenever the rat has gone into one particular place in its environment. And it signals to the rest of the brain by sending a little electrical spike. So we could show the firing rate of that neuron as a function of the animal's location. And if we record from lots of different neurons, we'll see that different neurons fire when the animal goes in different parts of its environment, like in this square box shown here. So together they form a map for the rest of the brain, telling the brain continually, "Where am I now within my environment?"

Place cells are also being recorded in humans. So epilepsy patients sometimes needthe electrical activity in their brain monitoring. And some of these patients played a video game where they drive around a small town. And place cells in their hippocampi would fire, become active, start sending electrical impulses whenever they drove through a particular location in that town.

So how does a place cell know where the rat or person is within its environment? Well these two cells here show us that the boundaries of the environment are particularly important. So the one on the top likes to fire sort of midway between the walls of the box that their rat's in. And when you expand the box, the firing location expands. The one below likes to fire whenever there's a wall close by to the south. And if you put another wall inside the box, then the cell fires in both place wherever there's a wall to the south as the animal explores around in its box. So this predicts that sensing the distances and directions of boundaries around you -- extended buildings and so on -- is particularly important for the hippocampus. And indeed, on the inputs to the hippocampus, cells are found which project into the hippocampus, which do respond exactly to detecting boundaries or edges at particular distances and directions from the rat or mouse as it's exploring around.

So the cell on the left, you can see, it fires whenever the animal gets near to a wall or a boundary to the east, whether it's the edge or the wall of a square box or the circular wall of the circular box or even the drop at the edge of a table, which the animals are running around. And the cell on the right there fires whenever there's a boundary to the south,whether it's the drop at the edge of the table or a wall or even the gap between two tables that are pulled apart. So that's one way in which we think place cells determine where the animal is as it's exploring around.

We can also test where we think objects are, like this goal flag, in simple environments --or indeed, where your car would be. So we can have people explore an environment and see the location they have to remember. And then, if we put them back in the environment,generally they're quite good at putting a marker down where they thought that flag or their car was. But on some trials, we could change the shape and size of the environment like we did with the place cell.

In that case, we can see how where they think the flag had been changes as a function of how you change the shape and size of the environment. And what you see, for example, if the flag was where that cross was in a small square environment, and then if you ask people where it was, but you've made the environment bigger, where they think the flag had been stretches out in exactly the same way that the place cell firing stretched out. It's as if you remember where the flag was by storing the pattern of firing across all of your place cells at that location, and then you can get back to that location by moving around so that you best match the current pattern of firing of your place cells with that stored pattern.That guides you back to the location that you want to remember.

But we also know where we are through movement. So if we take some outbound path --perhaps we park and we wander off -- we know because our own movements, which we can integrate over this path roughly what the heading direction is to go back. And place cells also get this kind of path integration input from a kind of cell called a grid cell.

Now grid cells are found, again, on the inputs to the hippocampus, and they're a bit like place cells. But now as the rat explores around, each individual cell fires in a whole array of different locations which are laid out across the environment in an amazingly regular triangular grid. And if you record from several grid cells -- shown here in different colors --each one has a grid-like firing pattern across the environment, and each cell's grid-like firing pattern is shifted slightly relative to the other cells. So the red one fires on this gridand the green one on this one and the blue on on this one.

So together, it's as if the rat can put a virtual grid of firing locations across its environment -- a bit like the latitude and longitude lines that you'd find on a map, but using triangles.And as it moves around, the electrical activity can pass from one of these cells to the next cell to keep track of where it is, so that it can use its own movements to know where it is in its environment.

Do people have grid cells? Well because all of the grid-like firing patterns have the same axes of symmetry, the same orientations of grid, shown in orange here, it means that the net activity of all of the grid cells in a particular part of the brain should change according to whether we're running along these six directions or running along one of the six directions in between. So we can put people in an MRI scanner and have them do a little video game like the one I showed you and look for this signal. And indeed, you do see it in the human entorhinal cortex, which is the same part of the brain that you see grid cells in rats.

So back to Homer. He's probably remembering where his car was in terms of the distances and directions to extended buildings and boundaries around the location where he parked. And that would be represented by the firing of boundary-detecting cells.He's also remembering the path he took out of the car park, which would be represented in the firing of grid cells. Now both of these kinds of cells can make the place cells fire.And he can return to the location where he parked by moving so as to find where it is that best matches the firing pattern of the place cells in his brain currently with the stored pattern where he parked his car. And that guides him back to that location irrespective of visual cues like whether his car's actually there. Maybe it's been towed. But he knows where it was, so he knows to go and get it.

So beyond spatial memory, if we look for this grid-like firing pattern throughout the whole brain, we see it in a whole series of locations which are always active when we do all kinds of autobiographical memory tasks, like remembering the last time you went to a wedding, for example. So it may be that the neural mechanisms for representing the space around us are also used for generating visual imagery so that we can recreate the spatial scene, at least, of the events that have happened to us when we want to imagine them.

So if this was happening, your memories could start by place cells activating each othervia these dense interconnections and then reactivating boundary cells to create the spatial structure of the scene around your viewpoint. And grid cells could move this viewpoint through that space. Another kind of cell, head direction cells, which I didn't mention yet, they fire like a compass according to which way you're facing. They could define the viewing direction from which you want to generate an image for your visual imagery, so you can imagine what happened when you were at this wedding, for example.

So this is just one example of a new era really in cognitive neuroscience where we're beginning to understand psychological processes like how you remember or imagine or even think in terms of the actions of the billions of individual neurons that make up our brains.

Thank you very much.

 


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