睡覺的時候，動物們都做什麼夢？

"Almost all other Animals are clearly observed to partake in sleep, whether they are aquatic, aerial, or terrestrial," wrote Aristotle in his work On Sleep and Sleeplessness. But do other animals dream? On that the Greek philosopher also had an opinion. In The History of Animals, he wrote: "It would appear that not only do men dream, but horses also, and dogs, and oxen; aye, and sheep, and goats, and all viviparous quadrupeds; and dogs show their dreaming by barking in their sleep." His research methods may lack sophistication, but Aristotle may not have been too far off the mark.

亞里士多德在文章《論睡和醒》中寫道：“人們清楚地觀察到，幾乎所有的動物都會睡覺，不論是水生、陸生還是在空中飛翔的動物。”但是動物會做夢嗎？在這個問題上，這個希臘哲學家也有自己的看法。在專著《動物志》中，他寫道：“看來不只人會做夢，馬、狗、牛也都會做夢；同樣，綿羊、山羊以及所有的胎生四足動物也都會做夢；並且，狗在睡眠中吠叫就是做夢的表現。”亞里士多德的研究方法也許不夠完善成熟，但和事實或許也已經相去不遠了。

We certainly can't ask animals if they dream, but we can at least observe the evidence that they might. There are two ways in which scientists have gone about this seemingly impossible task. One is to look at their physical behaviour during the various phases of the sleep cycle. The second is to see whether their sleeping brains work similarly to our own sleeping brains.

我們當然不能直接問動物它們是否做夢，但我們至少可以觀察到它們可能做夢的證據。對於這個看似不可能的任務，科學家們已經嘗試了兩種方法。一種是觀察動物在睡眠週期各個階段中的身體行爲，另一種是觀測動物和人類在睡眠時的大腦狀態是否相似。

The story of how we worked out how to peer into the minds of sleeping animals begins in the 1960s. Back then, scattered reports began to appear in medical journals describing people acting out movements in their dreams. This was curious, because during so-called REM sleep (rapid eye movement), our muscles are usually paralysed.

人們從上世紀60年代開始研究如何窺探動物睡眠時的大腦活動。那時，醫學雜誌上開始零星出現描述人在睡夢中“表演”動作的報告。這在當時是很新奇的，因爲在所謂的REM（rapid eye movement快速眼球運動）* 睡眠中，我們的肌肉通常是處於麻痹狀態的。

Researchers realised that inducing a similar state in animals could allow them to probe how they dream. In 1965, French scientists Michel Jouvet and J F Delorme found that removing a part of the brainstem, called the pons, from a cat's brain prevented it becoming paralysed when in REM. The researchers called the condition "REM without atonia" or REM-A. Instead of lying still, the cats walked around and behaved aggressively.

研究者們意識到，如果能引導動物產生和人類相似的狀態，那麼他們就可以探查動物是如何做夢的了。在1965年，法國科學家米歇爾·朱費（Michel Jouvet）和J·F·德羅爾姆（J F Delorme）發現，如果把貓腦中腦幹的腦橋部分移除，就可以防止其在REM睡眠中出現麻痹狀態。研究者把這種情況稱作“REM肌張力缺失現象消失（REM without atonia）”或REM-A。在這種情況下，貓不會靜靜地躺着，反而會四處走動並表現出攻擊性。

This hinted they were dreaming of activities from their waking hours. And studies since have revealed similar behaviour. According to veterinary neurologist Adrian Morrison, who has written a review of this research, cats in REM-A will move their heads as if following stimuli. Some cats also show behaviour identical to predatory attacks, as if they were chasing mice in their dreams. Similar dream activity has been seen in dogs.

這表明，貓正在做醒時活動的夢。自那以後的研究也都揭示了相似的行爲表現。根據獸神經學家阿德里安·莫里森（Adrian Morrison）對這項研究的評述，處於REM-A狀態中的貓會像有刺激物一樣搖晃腦袋。一些貓還會表現出等同於捕食性攻擊的行爲，就像它們在夢境中追逐老鼠一樣。而狗也表現出了相似的夢中活動。

Some humans have been found to ‘act out’ their dreams too – if they suffer from a condition called REM Sleep Behaviour Disorder. "Punching, kicking, leaping, and running from the bed during attempted dream enactment are frequent manifestations and usually correLate with the reported imagery," according to the International Classification of Sleep Disorders (ICSD). Injuries are common among these people and those sleeping with them, the ICSD adds.

一些人類也被發現有“表演”夢境的行爲——如果它們患上了“REM睡眠行爲障礙”。根據國際睡眠障礙分類（ICSD）描述，“這種情況的常見表現是表演出夢境中的行爲，如拳打腳踢、跳躍奔跑，並且通常能與報告中描繪的畫面對應。”ICSD還補充道，這些人常會自傷或傷及同牀者。

Physical movement is not the only way of peering into dreams, though. Researchers can now humanely peer into the electrical and chemical activities of brain cells in animals while they sleep. In 2007, MIT scientists Kenway Louise and Matthew Wilson recorded the activity of neurons in a part of the rat brain called the hippocampus, a structure known to be involved in the formation and encoding of memories. They first recorded the activity of those brain cells while the rats ran in their mazes. Then they looked at the activity of the very same neurons while they slept. Louise and Wilson discovered identical patterns of firing during running and during REM. In other words, it was as if the rats were running the maze in their minds while they were snoozing. The results were so clear that the researchers could infer the rats' precise location within their mental dream mazes and map them to actual spots within the actual maze.

然而，身體行爲並不是窺探夢境的唯一方法。現在，研究者們可以在不傷害動物的情況下探查其睡眠時腦細胞內的電流和化學活動。2007年，麻省理工大學的科學家肯威·路易斯（Kenway Louise）和馬修·威爾森（Matthew Wilson）記錄下了老鼠腦內“海馬體”部位的神經細胞活動，而“海馬體”是與記憶的形成轉換有關的結構。當老鼠在迷宮裏奔跑時，他們第一次記錄下了它們腦細胞內的活動。之後在老鼠睡眠時，他們又觀察相同神經細胞的活動。路易斯和威爾森發現，在奔跑和睡眠時，老鼠腦細胞內的活動模式是相同的。也就是說，當老鼠在酣睡時，它們腦內活動就像還在迷宮裏奔跑一樣。結果是如此明確，以至於研究者們可以推測出老鼠在腦內夢中迷宮裏的準確位置，並且能在現實迷宮中把它們的實際位置標出來。

University of Chicago biologists Amish Dave and Daniel Margoliash looked into the brains of zebra finches and discovered something similar. These birds are not born with the melodies of their songs hardwired into the brains; instead, they have to learn to sing their songs. When they're awake, the neurons in part of the finches' forebrain called the robutus archistriatalis fire following their singing of particular notes. Researchers can determine which note was sung based on the firing patterns of those neurons. By piecing together the electrical patterns in those neurons over time, Dave and Margoliash can reconstruct the entire song from start to finish.

芝加哥大學的生物學家阿米什·戴夫（Amish Dave）和丹尼爾·馬格賴許（Daniel Margoliash）觀察了斑胸草雀的大腦，並有了類似的發現。這些鳥並不是天生腦子裏就有歌曲的旋律，相反，它們要通過學習才能唱歌。當斑胸草雀醒着的時候，在它們唱到某些特定音符時，其前腦中古紋狀體粗核部位的神經細胞就會開始活動。研究者們基於這些神經細胞的活動模式就可以判定斑胸草雀唱到了哪個音符。通過把這些神經細胞中的電流模式拼合，戴夫和馬格賴許就能把整首歌從頭到尾地重現了。

Later, when the birds were asleep, Dave and Margoliash looked again at the electrical activity in that part of their brains. The firing of those neurons wasn't entirely random. Instead, the neurons fired in order, as if the bird was audibly singing the song, note for note. It might be said that the zebra finches were practising their songs while they slumbered.

隨後，在這些鳥睡眠時，戴夫和馬格賴許再一次觀察其腦內相同部位的電流活動。這些神經細胞的活動並不是完全隨機的。相反，這些神經細胞在有秩序地活動，就像這些鳥把歌曲一個音符一個音符地唱出聲來一樣。這可能說明，斑胸草雀在睡眠中也在練習唱歌。

Does the behaviour of cats in science experiments actually qualify as dreaming? Do rats have any subjective awareness that they're running their mazes in their minds while they nap? Do the songbirds realise that they're singing in their sleep? These questions are as hard to answer as the question of consciousness. It's tricky. We humans do not usually realise we're dreaming while we're dreaming, but it becomes clear as soon as we wake up. Do zebra finches remember their dreams as dreams when they're shaken out of their sleep? Can they distinguish the real world from the one in their dreams? We can say with a reasonable amount of certainty that the physiological and behavioural features of dreaming in humans have now been observed in cats, rats, birds, and other animals. Yet what it’s actually like to experience a dream if you’re not human remains a mystery.

科學實驗中貓的行爲表現是否可看作是真正在做夢呢？對於睡眠時腦內在迷宮中奔跑的狀態，老鼠自身有客觀意識嗎？這些唱歌的鳥又是否意識到自己在睡眠中唱歌呢？這些問題和與意識相關的問題一樣難以回答。這很棘手。我們人類在做夢時通常都不能意識到自己是在做夢，但是在醒來後就能馬上清楚地意識到這件事。那麼，斑胸草雀從夢中醒來後是否記得自己的夢呢？它們能把現實世界和夢境區分開嗎？我們可以非常確定地說，如今在貓、鼠、鳥等動物中都能觀察到與人類相同的做夢時的身體行爲特徵。然而，非人類在做夢時是怎樣的感覺，這依然是個謎。