Much of what we don't understand about being human is simply in our heads. The brain is a befuddling organ, as are the very questions of life and death, consciousness, sleep, and much more. Here's a heads-up on what's known and what's not understood about your noggin. -Jeanna Bryner

Sweet Dreams

If you were to ask 10 people what dreams are made of, you'd probably get 10 different answers. That's because scientists are still unraveling this mystery. One possibility: Dreaming exercises brain by stimulating the trafficking of synapses between brain cells. Another theory is that people dream about tasks and emotions that they didn't take care of during the day, and that the process can help solidify thoughts and memories. In general, scientists agree that dreaming happens during your deepest sleep, called Rapid Eye Movement (REM).

Slumber Sleuth

Fruit flies do it. Tigers do it. And humans can't seem to get enough of it. No, not that. We're talking about shut-eye, so crucial we spend more than a quarter of our lives at it. Yet the underlying reasons for sleep remain as puzzling as a rambling dream. One thing scientists do know: Sleep is crucial for survival in mammals. Extended sleeplessness can lead to mood swings, hallucination, and in extreme cases, death. There are two states of sleep - non-rapid eye movement (NREM), during which the brain exhibits low metabolic activity, and rapid eye movement (REM), during which the brain is very active. Some scientists think NREM sleep gives your body a break, and in turn conserves energy, similar to hibernation. REM sleep could help to organize memories. However, this idea isn't proven, and dreams during REM sleep don't always correlate with memories.

Phantom Feelings

It's estimated that about 80 percent of amputees experience sensations, including warmth, itching, pressure and pain, coming from the missing limb. People who experience this phenomenon, known as "phantom limb," feel sensations as if the missing limb were part of their bodies. One explanation says that the nerves area where the limb severed create new connections to the spinal cord and continue to send signals to the brain as if the missing limb was still there. Another possibility is that the brain is "hard-wired" to operate as if the body were fully intact - meaning the brain holds a blueprint of the body with all parts attached.

Mission Control

Residing in the hypothalamus of the brain, the suprachiasmatic nucleus, or biological clock, programs the body to follow a 24-hour rhythm. The most evident effect of circadian rhythm is the sleep-wake cycle, but the biological clock also impacts digestion, body temperature, blood pressure, and hormone production. Researchers have found that light intensity can adjust the clock forward or backward by regulating the hormone melatonin. The latest debate is whether or not melatonin supplements could help prevent jet lag - the drowsy, achy feeling you get when "jetting" across time zones.

Memory Lane

Some experiences are hard to forget, like perhaps your first kiss. But how does a person hold onto these personal movies? Using brain-imaging techniques, scientists are unraveling the mechanism responsible for creating and storing memories. They are finding that the hippocampus, within the brain's gray matter, could act as a memory box. But this storage area isn't so discriminatory. It turns out that both true and false memories activate similar brain regions. To pull out the real memory, some researchers ask a subject to recall the memory in context, something that's much more difficult when the event didn't actually occur.

Brain Teaser

Laughter is one of the least understood of human behaviors. Scientists have found that during a good laugh three parts of the brain light up: a thinking part that helps you get the joke, a movement area that tells your muscles to move, and an emotional region that elicits the "giddy" feeling. But it remains unknown why one person laughs at your brother's foolish jokes while another chuckles while watching a horror movie. John Morreall, who is a pioneer of humor research at the college of William and Mary, has found that laughter is a playful response to incongruities - stories that disobey conventional expectations. Others in the humor field point to laughter as a way of signaling to another person that this action is meant "in fun." One thing is clear: Laughter makes us feel better.

Nature vs. Nurture

In the long-running battle of whether our thoughts and personalities are controlled by genes or environment, scientists are building a convincing body of evidence that it could be either or both! The ability to study individual genes points to many human traits that we have little control over, yet in many realms, peer pressure or upbringing has been shown heavily influence who we are and what we do.

Mortal Mystery

Living forever is just for Hollywood. But why do humans age? You are born with a robust toolbox full of mechanisms to fight disease and injury, which you might think should arm you against stiff joints and other ailments. But as we age, the body's repair mechanisms get out of shape. In effect, your resilience to physical injury and stress declines. Theories for why people age can be divided into two categories: 1) Like other human characteristics, aging could just be a part of human genetics and is somehow beneficial. 2) In the less optimistic view, aging has no purpose and results from cellular damage that occurs over a person's lifetime. A handful of researchers, however, think science will ultimately delay aging at least long enough to double life spans.

Deep Freeze

Living forever may not be a reality. But a pioneering field called cryonics could give some people two lives. Cryonics centers like Alcor life Extension Foundation, in Arizona, store posthumous bodies in vats filled with liquid nitrogen at bone-chilling temperatures of minus 320 degrees Fahrenheit (78 Kelvin). The idea is that a person who dies from a presently incurable disease could be thawed and revived in the future when a cure has been found. The body of the late baseball legend Ted Williams is stored in one of Alcor's freezers. Like the other human popsicles, Williams is positioned head down. That way, if there were ever a leak in the tank, the brain would stay submerged in the cold liquid. Not one of the cryopreserved bodies has been revived, because that technology doesn't exist. For one, if the body isn't thawed at exactly the right temperature, the person's cells could turn to ice and blast into pieces.

Consciousness

When you wake up in the morning, you might perceive that the Sun is just rising, hear a few birds chirping, and maybe even feel a flash of Happiness as the fresh morning air hits your face. In other words, you are conscious. This complex topic has plagued the scientific community since antiquity. Only recently have neuroscientists considered consciousness a realistic research topic. The greatest brainteaser in this field has been to explain how processes in the brain give rise to subjective experiences. So far, scientists have managed to develop a great list of questions.

人類身體上的很多我們難以理解的謎存在于我們的大腦中。大腦是一個讓人迷惑的器官,就像生和死、意識、睡眠和其他更多的東西,這都是人類關于人腦至今也沒有解開的謎團。--Jeanna Bryner

1.夢境
如果問10個人同一個問題關于什么引起做夢,你可能會得到10種不同的答案。這是因為目前科學家還沒有揭開這個謎底。一種可能是:做夢過程中通過刺激大腦分子間的信息神經鍵對大腦進行鍛煉。另一個理論是,人們夢到白天不能顧及的任務和情感,這個過程可以幫助人們鞏固思想和記憶。一般而言,科學家贊同夢境會在淺睡時發生的觀點,他們稱這一時期為雷姆期睡眠。

2.睡眠
果蠅要睡覺,老虎也要,人類似乎永遠也睡不夠。這里討論的是睡眠的話題,人的一生要花費四分之一的時間睡覺。然而,睡覺的根本原因仍舊像天馬行空的夢境一樣讓人不得費解。但科學家也確實了解睡眠的一個重要方面:睡覺對哺乳動物的生存至關重要。長期失眠能導致精神恍惚、幻覺,并最終引起死亡。睡眠的兩種狀態——深睡期(眼球活動減慢),此時腦部代謝活動放慢;淺睡期(此時會做夢),這一時期大腦活動活躍。某些科學家認為深睡期睡眠能讓身體休息,保存精力,就像動物冬眠一樣。淺睡期睡眠有助于把記憶的東西組織起來。然而這種觀點沒有得到證實,淺睡期做夢不總是與記憶有關。

3.幻覺
估計80%的截肢者都體驗過來自斷肢的包括溫暖、渴望、壓力和痛苦等感覺,經歷這種現象(我們所知的“幻覺肢體”)的人,總是感覺到被截掉的肢體仍然存在。一種解釋認為,斷肢的神經區與脊髓重新建立了聯系,好像缺少的肢體依然存在一樣,繼續向大腦發送信號。另一個可能是,大腦是一條傳輸“硬線”,它就像對待完美無缺的身體一樣操縱殘體——這意味著大腦仍然保存著肢體健全時的操縱藍本。

4.任務控制
大腦丘腦下部的下丘腦視交叉上核或生物鐘保持身體隨著24小時的節奏運轉。生理節奏引起的一個最明顯的結果是:睡眠-醒來的循環,但是生物鐘還影響著消化力、體溫、血壓和激素的產生。研究人員發現,通過增強光亮調節褪黑激素可以將生物鐘向前或向后調整。最近人們不斷爭論,是否可以通過補充褪黑激素來幫助人們預防飛機時差——昏昏欲睡和飛機通過時區時產生的頭痛感。

5.記憶途徑
人生的某些經歷很難忘卻,就像你的初吻。但是一個人要怎樣把握這些私人情景呢?科學家正在利用大腦成像技術設法弄清楚創造記憶和儲存記憶的機械反映。他們發現大腦灰質內部的海馬體能充當記憶儲存箱的功能。但是這個儲存區域的分辨能力并不強。對相同的大腦區域的刺激,可以讓它產生真實的和虛假的記憶。為了把真實記憶從虛假記憶中脫離出來,研究人員提出根據背景回憶以加強記憶的方法,如果某些事情沒有真正發生過,就很難通過這種方法加強人腦對它的記憶。

6.取悅大腦
笑是人類最難理解的行為之一。科學家發現,當人們開懷大笑時,大腦內部有三個部位變的活躍起來,它們是:管轄思維的區域,它讓你獲得笑料;運動區域促使你的肌肉運動;情感區域引出“輕佻的”情緒,讓人露出笑容。但是為什么某人會因兄弟愚蠢的笑話John Morreall而發笑,而另一些人會在看恐怖影片時咯咯大笑。約翰·莫利爾是威廉與瑪麗學院 幽默研究的先驅,他發現,笑聲是對違反常規的不協調的故事的一個十分有趣的反映。幽默領域的另一個觀點把笑看作向其他人發出這種行為很“有趣”的信號的一條途徑。從而可以看出:笑讓我們感覺更好。

7.天生與營養
我們的思想和個性是否是由基因或環境控制的問題,長期以來一直爭論不休。科學家建立了一個讓人信服的證據體系,證明它可能是受其中之一控制或者是由兩方面同時控制。研究個體基因的能力顯示出我們對很多人類特性無法控制,然而在很多領域,同輩人面對的壓力或接受的教育會對我們是什么樣的人以及我們將做什么產生深遠影響。

8.死亡之謎
長命百歲是好萊塢電影中才會發生的事情。但是我們為什么會變老?你出生時像一個精力充沛的工具箱,體內充滿各種抵抗疾病和修復創傷的機制,也許你認為,僅憑這些就能讓你抵抗關節僵硬和其他疾病。但是隨著我們不斷變老,身體的修復機制漸漸失去往日的功效。事實上,你的身體恢復創傷和壓力的能力正隨著年齡的增加不斷下降。人的老化被分成兩個種類的學說:1)像人類的其他特征,變老可能是人類遺傳學的一部分,并且從某些方面來說對人類有益。2)以最不樂觀的觀點來看,變老不是有意圖的,人的一生中細胞不斷受到損害從而引起人體老化。大量研究人員認為,科學將最終推遲變老速度,使人類壽命至少可以達到預期生命期限的兩倍。

9. 人體冷凍
要求長命百歲可能并不現實。但是一個被稱作人體冷凍學的新領域將會讓某些人死而復生。人體冷凍學中心如亞利桑那州的Alcor生命延工基金會在充滿液態氮并且溫度為華氏零下320度(78絕對溫度)的刺骨的容器中儲藏死者的遺體。這個想法的目的是,死于目前無法治愈的疾病的人在將來發現治愈方法后可以通過解凍讓他們死而復生。最近, 棒球傳奇人物Ted Williams 被儲存在Alcor的一個冷藏柜中。像其他人體“冰棒”一樣,Williams被頭朝下放置。這種放置方式的好處是,如果容器發生泄漏,大腦仍能保存在冰冷的液態里。到目前為止還沒有一具被保存的尸體重獲新生,這是因為這種讓人重獲新生的技術還不存在。舉例來說,如果尸體沒在適宜的溫度下解凍,人體細胞就會結冰,導致受損破碎。

10.意識
清晨一覺醒來,你可能就已經意識到太陽剛剛升起,聽到一些鳥兒在枝頭歡快的鳴叫,甚至清新的空氣輕拂你的面頰,你會感到一有幸福感。換句話說,你是有意識的。這個復雜的話題從一開始就困擾著科學界。最近,神經學家才把意識作為一門現實的研究課題。此領域的最大謎團已被解析為大腦中的程序是如何引起自覺的經驗。至今為止,科學家們已經成功拓展出一大套相關問題的清單。