Eine Landkarte im Gehirn

Edvard Moser fand ihn unter seinen entgangenen Anrufen: den Anruf des Stockholmer Nobelpreiskommitees. Erst als er in München aus dem Flugzeug stieg, erfuhr der norwegische Forscher, dass er einer der Nobelpreisträger 2014 für Physiologie sei. Gemeinsam mit seiner Ehefrau und Ko-Gruppenleiterin May-Britt Moser und dem britisch-amerikanischen Forscher John O’Keefe, bei dem die Mosers eine kurze Zeit lang forschten, erhält er den Nobelpreis für die Entdeckung der “Landkarte im Gehirn”.

Bereits 1971 entdeckte John O’Keefe Nervenzellen, die Ratten als Kompass dienen. Er taufte sie “Platzzellen” (englisch: place cells). Läuft die Ratte durch den Käfig, sendet eine Platzzelle ein Signal, wenn die Ratte beim Futterspender steht, während eine andere Platzzelle sendet, wenn die Ratte in der hinteren Käfigecke hockt. Eine Platzzelle feuert also immer dann ein Signal ab, wenn sich die Ratte an einem bestimmten Punkt befindet. An jedem Ort feuert nur eine geringe Zahl an Platzzellen, die sich in einer speziellen Hirnregion, dem Hippocampus, befinden. So kodieren die Signale von nur wenigen Nervenzellen jeden Punkt in der Umgebung.

Nur eine Schaltstelle zwischen Nervenzellen, Synapse genannt, trennt Platzzellen und “Koordinaten-Zellen”. Zwischen der Entdeckung von Platzzellen durch John O’Keefe und der ersten Beschreibung von Koordinaten-Zellen (englisch: grid cells) durch May-Britt und Edvard Moser verstrichen aber über 30 Jahre. 2005 entdeckten die beiden Norweger ein Koordinatensystem im Gehirn von Ratten. Sie zeichneten dafür, ähnlich wie O’Keefe, die Signale auf, die Nervenzellen senden. Anders als O’Keefe konzentrierten sie sich auf Signale von Nervenzellen im sogenannten entorhinalen Kortex. Das “Feuern” von Nervenzellen klingt beim Aufzeichnen wie das Poppen von Popcorn in der Mikrowelle: jedes Signal ist ein “Pop”. Wenn eine Ratte durch eine experimentelle Umgebung läuft, das ist meistens einfach eine Kiste, hörten die Forscher immer wieder ein Pop. An manchen Orten aber “poppte” es ganz besonders häufig, wie wenn das meiste Popcorn im Sackerl aufspringt.

Zeichnen wir an jedem Ort, an dem sich die Ratte befindet, wenn eine Koordinatenzelle am meisten “poppt”, einen Punkt, entsteht rasch ein faszinierendes Muster: die “Pops” konzentrieren sich auf Sechsecke, die sich in regelmäßigen Abständen wiederholen. Alle Koordinatenzellen feuern in diesem sechseckigen Muster. Bei manchen Koordinatenzellen sind die Sechsecke aber größer oder kleiner, nach links oder rechts verdreht, oder näher oder weiter von einander entfernt. Dieses Koordinatensystem legt sich über den ganzen Raum der Ratte: wie der Längen- und Breitengrad jeden Ort der Welt bestimmen, definieren eine Handvoll Koordinatenzellen jeden Punkt in der Umgebung der Ratte.

Ratten haben noch mehr Nervenzellen, die nur an bestimmten Orten oder Positionen Signale senden: “Kopfrichtungszellen” feuern, wenn die Ratte den Kopf in eine bestimmte Himmelsrichtung hält, “Grenzzellen” senden Signale, wenn die Ratte in einem bestimmten Abstand von einer Wand läuft. Platzzellen und Koordinatenzellen bilden, zusammen mit den Kopfrichtungszellen und Grenzzellen, eine “Landkarte” im Gehirn der Ratte. Mit dieser Landkarte hat die Ratte auch im Dunkeln und in einer neuen Umgebung immer eine Darstellung davon, wo sie ist und wohin sie sich bewegt.

Ein ähnliches GPS befindet sich vermutlich auch in unserem menschlichen Gehirn. Edvard Mosers Platzzellen und Koordinatenzellen, die gerade feuerten, als er von seinem Nobelpreis erfuhr, sind wahrscheinlich untrennbar mit Freude verbunden.

Paper von John O’Keefe und Jonathan Dostrovsky:

Paper von May-Britt Moser und Edvard Moser

Podcast Love

I spend 45 minutes driving to work in each direction every day. Podcasts are what keep my brain busy and happy while I drive. My recommendation for this week – a Radiolab podcast dedicated to the Galapagos islands. It’s classic Radiolab style, taking you on a journey through what many of us probably still think of as Darwin’s evolutionary garden of Eden. The reality is different: many species, including the iconic Galapagos tortoises, are on the brink of extinction. This episode includes one of the craziest stories I’ve ever heard about conservation efforts – Project Isabela, during which helicopters were used to hunt for goats on the island of Isabela.


Cognitive research – coming to a smartphone near you

“Citizen science” projects have successfully gotten people to classify galaxy shapes, find optimal protein folding structures or decipher manuscripts. But is it possible to use online participation not just to harness users problem-solving abilities but for actual experiments and data gathering in cognitive research? There has been much furore recently about a study on the spread of emotion that used data not only gathered through Facebook without users’ consent, but for which users’ newsfeed was actively manipulated. In contrast, researchers from the University of Oxford and the University of Birmingham now show that data for cognitive science experiments can be crowdsourced with informed consent, and results from the mass data collection replicate results which researchers got when they carried out experiments with the same aim in a carefully controlled lab setting.

The researchers got an app produced for smartphones, which allows users to play four short games that are based on classic experimental paradigms of cognitive science. These tested decision-making, short-term memory, perception and action inhibition but were packaged as competitive games. In one of them, users tapped their screen when fruit fell from a tree, but not when the fruit turned brown while falling. When the users tapped correctly and quickly enough, they progressed to the next level, where fruits turned brown later in their fall. Before playing, users filled out a questionnaire about themselves and gave informed consent for the use of their data. Interestingly, when analyzing the data, the researchers got the same results that other researchers got when they carried out experiments with the same goal in a lab setting, with only a few participants. One advantage of mass data collection over the internet is that the participants come from a wider demographic than the usual lab rats for cognitive science, cash-strapped university students. Soon, there will be two billion people using smartphones worldwide. That’s quite a wide pool of potential participants for online experiments, if done correctly. And, Facebook – no, informed consent is not optional.

Original research paper in PLOS One

How do we store memories?

If you are a football fan, you probably remember Götze’s title-scoring goal for Germany in Sunday’s world-championship final. If you are not a football fan, you probably remember the pleasant evenings you spent before the football craze set in. The memories are clear before your inner eye, but how do you store them in your brain?

Neuroscientists know that one brain region, called the hippocampus, is our memory storage. However, they have three different theories of how the brain cells, or neurons, in this region can store the memory of Götze’s goal. According to the first theory, one neuron encodes this memory. So this neuron, and only this neuron, sends a signal when you think of Götze scoring – you have a “goal neuron”. The second theory states that many neurons together send signals in a pattern, and this pattern is typical only of your goal memory. But each neuron also contributes to many other memories, like that of the cool beer you drank alongside. Basically, you have a “goal pattern”. The third theory falls in the middle: only a few neurons signal when you think of Götze’s goal, and each neuron also stores a few other memories. But which theory is true?

Psychologists in the US tested this by looking at the brain activity of people recognizing familiar versus new words. The participants in the study were epilepsy patients who wore wire electrodes in preparation for possible surgery, with the aim to find out where in the brain seizures took place. To test how their brains store memory, the researchers gave participants a list of 32 words, which they studied. In the memory test, they were shown 64 words – the 32 from the original list, and 32 new words. The participants were asked to say which ones were “old” words and which ones “new”. Using the electrodes, the research could see the areas of the hippocampus in which neurons sent signals when a new or old word was shown. They found that neurons in some areas signal more often when the participant sees an “old” word rather than a “new” word. For each area, neurons only signal when the participant sees a few “old” words.

This supports the third theory: it is likely that in the hippocampus our memory of Götze’s goal is stored by a group of a few neurons. Each of these neurons, together with a different group of neurons, also stores a few other memories – maybe that of Argentina’s very near misses.

Orginal research paper in PNAS: www.pnas.org/content/early/2014/06/11/1408365111.abstract

DNA – it’s not just information, duh

In 1953, James Crick and Francis Watson discovered the source of information in our cells – DNA. This spiral of bases attached to a sugar-phosphate backbone is the ultimate information storage: it is stable, trustworthy, easy to repair and is divided up so that a cell’s offspring contains the same information as the original cell. But it’s not just information storage that DNA is apparently good for: researchers report in PLOS ONE that some bacteria like to eat it too.

The greedy DNA eater is Haloferax volcanii, an archeabacteria. Haloferax volcanii (Hfx. volcanii for short) is not just interesting because it can live in very salty conditions; it also has several copies of each chromosome, composed of DNA and backbone proteins. That is not unique – we humans are diploid, as we have two copies of each chromosome. Hfx. volcanii can have more than twenty copies of each chromosome. Now, why is that a good idea? One theory is that the additional copies are redundant copies for safety, similar to making copies of your files on several external hard drives. But for repairing damaged DNA, cells need to have already quite evolved DNA repair mechanisms, the researchers argue. Looking for an alternative hypothesis, they investigate whether DNA is used not just for what it represents – information, but also for what it is made of – bases, sugar and phosphate.

They grew bacteria in a broth with all nutrients they need for growing, except for phosphate. Within 140 hours, the bacteria had grown, and the number of bacteria in the broth doubled three times. The researchers then measured the number of chromosomes per bacteria in the stationary phase, when the bacteria have stopped doubling quickly. Bacteria that grew with plenty of phosphate in the broth had 24 copies of each chromosome, but bacteria without any phosphate only had 2 copies of each. Phosphate in the DNA could be used to fuel the growth without external phosphate, especially as other sources of phosphate in the cell, like ribosomal RNA, is not reduced in these bacteria.

When the researchers then added phosphate to the flask of bacteria that previously were without phosphate, the bacteria managed to accumulate 40 chromosome copies per cell within just 23 hours. So when the bacteria have an external source of phosphate, they take it up quickly and replenish their phosphate storage as DNA, accruing chromosome copies.

DNA as food? It does sound a bit outlandish, the quintessential code for “information” reduced to mere nosh. But the researchers suggest this as a hypothesis for the origin of DNA. Most theories on the origin of life suggest that RNA, a similar molecule to DNA but made of just one strand, was the first molecule to store information in organisms. DNA is more stable than RNA, and eventually replaced it as information storage. The authors suggest that the first cell with several chromosome copies actually did not use DNA as genetic storage. Rather, it was used for food first, and for information storage later, because of its stability. This is an intriguing hypothesis, offering an alternative view to the “RNA world”. As the researchers themselves admit, this view “might not be true and cannot be proven”. But different it certainly is.

Original research paper on PLOS ONE: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0094819