Classic graphics #5: Brainbow

Is it presumptuous to award "classic" status to something that's less than a month old? Normally, yes. But sometimes, something is just so stunning that you strongly suspect it will be shown for years to come.

This essay is different from the previous ones, which each focused on a single image. This one pans out to review a veritable gallery of images that will surely just be the first of many galleries.

At the start of this month, Livet and colleagues published a paper in Nature that has arguably the most beautiful pictures of neurons ever taken. And that's a tall order, because most neurons are really beautiful in their own right, particularly when you get a good stain, and you're really able to see their structure in detail under a microscope. But these leave you open mouthed, gaping "The colours, man, check out the colouuuuurs..." like a hippie on an LSD trip in the Summer of Love.

The authors have created mice whose neurons glow a variety of colours. Hence, brain + rainbow = Brainbow.

Unfortunately, in contrast to the beauty of the pictures, the prose of the actual article is not accessible to anyone but real specialists. By specialist, I don't mean, "biologist" or "neurobiologist," I mean, "transgenic mouse neuroscientists." The paper is loaded with cryptic abbreviations ("XFP" means "fluorescent proteins" -- I get the FP, but the X?) and hinges on what the authors call the "widely used Cre/lox recombination system," which I had never heard of, and got sent to a 22 page review when I tried to make heads or tails of it. And even though the word that will probably stick in most peoples' heads when they sit down to search Google Scholar is the neologism "Brainbow," the word "Brainbow" is not in the title.

As far as I can tell, here's what they've done.

It's been a reasonably common trick in biology for some years now to be able to take a gene from one organism and put it into another. These are transgenic organisms, and when they're plants, they're also known as genetically modified (GM) crops. A fairly well known example is to take a gene from a jellyfish that makes then glow called green fluorescent protein (GFP) and introduce that into other animals (like mice), so now that other animal gains the ability to fluoresce, just like the jellyfish.

Now, how were Livet and colleagues able to get neurons to glow a bunch of different colours?

After people were able to put GFP into new organisms, people started tweaking the sequence and found they could make other colours -- like red fluorescent proteins. Other people took genes from other animals that glowed different colours. By doing so, researchers developed a palette of different colours. But as an artist knows, the trick is in combining the colours on the palette.

The authors introduced several of these fluorescent genes (up to four different ones) into mice, and found a way to get each neuron to activate a random selection of these genes using this Cre/lox system. If you remember colour theory, you can mix two colours together to create a third. If you mix three colours, the range of possible new colours is very large indeed. By having these multiple genes activating in unpredictable combinations, each cell glows a particular colour that is shared by few of its neighbours. The authors estimate there are at least 89 distinct colours that they can see.

Now, there is some more genetic trickery involved here that I don't pretend to understand fully. One is that the expression is not automatic in all cases -- it can be turned on in specific regions of the nervous system (Figure 3e in the paper shows neurons "lit" only in the retina of the eye). There's also some jiggery-pokery involving crossbreeding some of these genetically modified mice. Sometimes, the mice gave only the single "primary colours," indicating that only one protein was ever expressed. Some others showed the mixtures, giving many different colours.

The paper goes on to show that the colour of a neuron appears to be consistent throughout its length, an important consideration given that neurons have such long projecting branches. They also show the colours stay stable over time by tracking some neurons for 50 days.

As far as I can tell, this paper is a real technical tour de force. There are a lot of experiments compiled here, that appear to be very thorough. The authors did not just stop and publish when they had a few pretty pictures. ... Okay, make that breathtaking pictures.

It will be very interesting to see how this technology develops, and what it will reveal about neuronal wiring. Because so much research is driven by what we can see.

Meanwhile, here are some more pictures.

References

Livet, J., Weissman, T., Kang, H., Draft, R., Lu, J., Bennis, R., Sanes, J., & Lichtman, J. (2007). Transgenic strategies for combinatorial expression of fluorescent proteins in the nervous system Nature, 450 (7166), 56-62 DOI: 10.1038/nature06293

Supplemental info: http://www.nature.com/nature/journal/v450/n7166/suppinfo/nature06293.html

Classic graphics #4: Cortical wiring

In the last installment of this series, I talked about cortex. And here we are again at the cortex.

I got the idea for looking at this graphic from the recent Neuroscience meeting, where I saw this diagram in at least two of the featured late afternoon talks. So I reckoned that qualified it as a classic. Of course, it probably didn't hurt that one of the authors of the paper that featured this diagram was the current president of the Society, David Van Essen. I was able to track it down, and it's freely available online in the original paper (it's figure 4, on page 30).

It also didn't hurt the fame of this diagram that it was printed in the very first article of a brand new journal, I imagine.

And I'm pretty sure I saw this picture prominently featured in a commentary by Nobel laureate and DNA structure describer Francis Crick in Nature. He used it as an example of something we know in monkey, but we should know in humans. We discussed the Crick paper when I was a grad student at our weekly "neurolunch" seminar. (Checking this now, it was Crick and Jones, actually.)

This figure shows the wiring diagram of the part of the brain responsible for visual processing in macaques -- which, because primates are visual animals, and because it's easy to control visual stimuli, is one of the best understood regions of the brain. 32 areas, 10 hierarchical, levels, and 187 linkages, most two-way between connected areas.

I should say, though, that the original was published in colour, based on comments in the text. That the PDF online now is in black and white is probably an oversight.

This diagram is clearly not famous because of its elegance. It's very hard to interpret and looks like the electrical wiring from the Chilton's manual of the car you hope you never own. Heck, even the authors write, "The sheer complexity of Figure 4 makes it difficult in many places to trace the lines representing specific pathways." (They go on to describe a computer representation that allows you to highlight specific connections. Sadly, that diagram does not appear to have made its way online, though I haven't looked hard).

But then, that's the point. It's considered a classic, not despite its complexity and difficulty in interpretation, but because of it. It emphasizes the tremendous complexity of the cortex and how different areas are connected to others.

And make no mistake: this diagram certainly represents a nearly heroic compilation of experimental results. And perhaps that admirable feature has helped people view it favourably over time.

Looking at the text, though, I'm struck by the several qualifiers, provisions, caveats, and tentative interpretations about the information that went into making this figure. The diagram, in a way, is often shown as factual, but is in fact somewhat hypothetical. Something which is not often mentioned when this picture is shown. It's possible, I suppose, that all the hypothesis have been shown correct in the following 16 years of research -- though I doubt that.

Next in this series, I will probably be looking at some graphics by Jerison on brain size.

References

Felleman DJ, Van Essen DC. 1991. Distributed hierarchical processing in the primate cerebral cortex. Cerebral Cortex 1: 1-47.
http://cercor.oxfordjournals.org/cgi/content/abstract/1/1/1-a

Crick F, Jones E. 1993. Backwardness of human neuroanatomy. Nature 361: 109-110. http://dx.doi.org/10.1038/361109a0

Classic graphics #3: The somatosensory cortex

When I lived in Montreal, I had the unusual experience of living in a city where a scientist was not just known (unusual enough), but frankly, revered. That scientist was Wilder Penfield.

I worked at McGill University, I worked on Dr. Wilder Penfield Avenue. And when I went to some talks at the Montreal Neurological Institute, I heard a speaker talk about having the chance to meet with "the great man," with no hint of irony.

Although it was probably Penfield's reputation as a surgeon that cemented him in the minds of Montrealers as a great individual, one of his most lasting scientific contributions is the map of the somatosensory cortex. This is truly a classic figure, that has been reproduced many times, and is virtually a standard in any textbook that even mentions brains.

Indeed, this picture has been used to study how textbooks copy from one another. In one redrawing of this diagram, a right hand got drawn onto the left side of a body (or vice versa). The error was then propagated in textbook after textbook.

I think this diagram comes from The Cerebral Cortex of Man: A Clinical Study of Localization of Function, co-authored with Ted Rasmussen and published in 1950.

The one up top shows half the story, namely the motor cortex. An equivalent diagram is available showing the sensory side.

What makes this figure a classic is that it uses images so successfully to tell the story. Wrapping the images of the highly distorted body takes something that is actually very abstract and makes it much more easy to relate to. If I am not mistaken, Penfield made these maps during open brain surgery, often with the patient awake. He would stimulate these regions, and observed what the patient did in response to the stimulation (on the motor side), or what the patient reported.

The diagram compiles all those observations of various body movements and miscellaneous reports of sensations.

This edited version of the figure has all the same information as the top one, only using text labels. Indeed, the text labels on the top diagram are almost redundant. But it is clearly nowhere near as compelling as the one showing all the bits of body, with some regions enlarged and some shrunk.

Some people have gone the other way, and created representations of a body sized proportionately to the area of the brain that represents them. In my estimation, these are not as powerful, either.

The distorted body (sometimes called a homunculus, which means little man) is now isolated from the brain. You lose that sense that this is something tied to how the brain works. Such representations have a certain interest, but I don't think they make the scientific point about how our body is represented in the brain anywhere near as well as the original.

Additional, 15 February 2015: Ran across this academic article describing how this diagram changed over time. The first effort, in 1937, was confusing compared to the famous 1950 version.

Schott GD. 1993. Penfield’s homunculus: a note on cerebral cartography. Journal of Neurology, Neurosurgery, and Psychiatry 56 329-333. http://jnnp.bmj.com/content/56/4/329.full.pdf

Classic graphics #2: Crayfish tailflips

This particular picture is not famous outside of my own field. But it has been reproduced many times, and as such, I think warrants the classic designation.

Crayfish tailflipping was one of the first natural animal behaviours that neuroethology really cracked. It was one of the first cases where we could explain how the connections between the neurons explained a lot about how the behaviour of the animal worked.

One of the little oddities in the story of understanding the escape response of crayfish was that the major neurons involved were recognized back in the 1920s. These huge neurons became known as the lateral giant (LG) and medial giant (MG) neurons, but even in the flapper era, there was pretty strong evidence then these giant neurons were involved in tailflipping.

But it took over 40 years for people to explain why there were two sets of neurons involved in tailflipping. And they can be forgiven for not figuring this out quickly, because this behaviour is so blindingly fast, you can't make it out with the naked eye.

When people finally got around to using high speed film, the answer became clear. There wasn't one behaviour. There were two. There were two different kinds of tailflips, each triggered by a different set of neurons. (Actually, it turned out that there were three kinds of tailflips, but that's another story.)

What fascinates me about this particular figure (from Wine and Krasne, 1972) is that it was not the first to make this point. Priority is huge in science. Normally, the first to show anything gets priority, and hold a very strong sway on our imaginations. Ask someone who was the second person to climb Mount Everest, or the second person to fly solo across the Atlantic. Heck, I don't know.

Just a few months earlier, in the same journal, another group of researchers led by Jim Larimer (which included my intellectual predecessor, Don Kennedy) published a picture that shows the same thing: that stimulating the different neurons generates different movements.

In this case, I think it's pretty clear why the later picture became the canonical representation of crayfish escape behaviour: context. You see the stick tapping animal, and you see the whole animal, moving through space, rather than an isolated part of an animal being held static. The advantages of this are huge.

The top diagram makes it forcefully clear what advantage there is to having two tailflips: both tailflips take the animal away from the source of the offending stimulus, but do so by throwing the animal in different directions. The crayfish jackknifes into the water when the LGs fire, but heads straight back when the MGs fire. You can sort of extrapolate that from what's shown in the earlier figure, but since you can't see where the stimulus would be, you don't have that immediate "Aha!" insight.

Sometimes, it isn't better to be first. It's better to be better.

References

Larimer, J. L., Eggleston, A. C., Masukawa, L. M. and Kennedy, D. (1971). The different connections and motor outputs of lateral and medial giant fibres in the crayfish. The Journal of Experimental Biology 54, 391-404.

Wine, J. J. and Krasne, F. B. (1972). The organization of escape behaviour in the crayfish. The Journal of Experimental Biology 56, 1-18.

Classic graphics#1: The Cartesian reflex

One of my side interests is in graphics. And I'm curious about what makes an image famous. Particularly scientific images. So I'm going to try exploring that from time to time, focusing on my own field of interest, neurobiology.

Over at the left is an image that I've seen reproduced many times. So often, in fact, that I'm having a hard time finding exactly the original source, to my embarrassment. It's from the work of René Descartes, 17^th century French intellectual. I think it's from Treatise on Man, published around 1662 after Descartes' death.

I do not know if Descartes drew his own figures. I doubt it, because preparing woodcuts is a pretty specialized talent. And Descartes, I've heard, loved a life of luxury. I don't think he was the sort to handle woodcutting tools. Be that as it may...

What the figure is showing is Descartes' conception of how behaviour worked. It looks fairly modern. Even if you understand only a bit of modern biology, you might think, "You have a sensory stimulus, impulses travel up the spinal cord to the brain, are processed, and you get a withdrawal from the painful stimulus."

I think that's one reason why this particular diagram is famous: because it is almost eerily prescient.

And indeed, in broad strokes, that's what this is showing. Descartes was the first to develop a reflexive theory of behaviour, although I don't think he used the term "reflex." Because Descartes is so famous for his ideas on the soul, his arguments that much of behaviour -- even in humans -- is purely "push pull" sorts of automatic, reflexive responses is often overlooked.

That the picture looks quite modern in some ways actually sort of overshadows some important problems. Descartes had what I consider to be a pre-scientific concept of behaviour. The discovery of biological electricity was centuries away, so Descartes followed the ancients in thinking that behaviour was controlled through the movements of "animal spirits."

Is the picture at top famous because of the idea that it shows? I don't think it's just that. Over to the left is another picture showing the same basic idea, again from Descartes, and I think even from the same book. Regardless, you see the same basic elements: a fire, an outstretched limb, and "animal spirit vessels" leading to the brain.

Yet this one I have not reproduced seen over and over again like the top one. I would say the top one is a classic, but the bottom one not. And here, the actual rendering of the diagram might be important in why this one isn't reproduced that often.

I find the second one is kind of disturbing. The figure has sort of babyish proportions. The torso sort of floats out of a cloak, which no obvious indications of a bottom half. Plus, the "animal spirit vessels" don't go right out to the fingers, making the idea that there's a connection between the stimulus and the person's brain harder to understand.

Or maybe the reason the top one is more famous is that editors are always looking for excuses to put in more nudes in their books.