The Basic Human Tastes

Our society has traditionally recognized four tastes: sweet, salty, bitter and sour. In modern times however we’re better able to get to the root of taste, seeing what molecules are detected by our taste buds, and it has become apparent that there are more basic tastes than we once thought. The most well known one is umami, a savory taste, but LiveScience brings us some other candidates for basic tastes, excerpted below:

1. Calcium

The element calcium is critical in our bodies for muscle contraction, cellular communication and bone growth. Being able to sense it in our chow, therefore, would seem like a handy tool for survival.

Mice seem to have it figured out, kind of. Recent research has revealed that the rodents’ tongues have two taste receptors for calcium. One of those receptors has been found on the human tongue, though its role in directly tasting calcium is not yet settled, said Tordoff.

Calcium clearly has a taste, however, and counterintuitively most mice (and humans) don’t like it. People have described it as sort of bitter and chalky – even at very low concentrations. Tordoff thinks our calcium taste might actually exist to avoid consuming too much of it…

2. Kokumi

That calcium receptor might also have something to do with an unrelated sixth-taste candidate called kokumi, which translates as “mouthfulness” and “heartiness.” Kokumi has been promulgated by researchers from the same Japanese food company, Ajinomoto, who helped convince the taste world of the fifth basic taste, umami, a decade ago.

Ajinomoto scientists published a paper in early 2010 suggesting that certain compounds, including the amino acid L-histidine, glutathione in yeast extract and protamine in fish sperm, or milt – which, yes, they do eat in Japan, and elsewhere – interact with our tongue’s calcium receptors.

The result: an enhancement of flavors already in the mouth, or perhaps a certain richness. Braised, aged or slow-cooked foods supposedly contain greater levels of kokumi…

3. Piquance

Spicy-food lovers delight in that burn they feel on their tongues from peppers. Some Asian cultures consider this sensation a basic taste, known in English as piquance (from a French word). Historically, however, food scientists have not classified this undeniable oral sensation as a taste.

That’s because certain piquant compounds, such as capsaicin from peppers, directly activate our tongue’s touch, rather than taste-bud, receptors. The key piquancy receptor is called TRPV1, and it acts as a “molecular thermometer,” said John E. Hayes, a professor of food science at Penn State….

4. Coolness

At the opposite end of taste sensation from piquance’s peppers is that minty and fresh sensation from peppermint or menthol. The same trick of sensory perception is at work here – activated touch receptors, called TPRM8 in this case, fool the brain into sensing coldness at normal oral temperatures, said Hayes.

As touch sensations, both piquance and coolness are transmitted to the brain via the trigeminal nerve, rather than the three classical nerves for taste. “The set of nerves that carry the burn and cooling sensation are different than from taste sensation,” said Hayes.

Still, there is an argument that temperature sensation, both in the genuine sense and in the confused-brain phenomenon of piquance and coolness, deserves to be in the pantheon of basic tastes. Interestingly, Germanic people dating back to 1500 had considered heat sensation as a taste, Hayes said, and the modern debate over temperature’s status is far from over.

5. Metallicity

… Some Asian cultures place gold and silver leaf, as it’s called, atop curry dishes and candies, while Europeans fancy a bit of these metallic foils on pastries…

Although usually tasteless, such garnishes are sometimes reported as having a distinctive flavor. Researchers have shown that this sensation might have something to do with electrical conductivity, in effect giving the tongue a little zap…

Lab tests have failed to turn up a metallic-taste receptor, Lawless said, and it remains unclear if electrical conductivity or something more is going on for those shiny culinary embellishments. “We’re leaving the door open,” Lawless said.

6. Fat

The jury is still out on whether our tongues can taste fat, or just feel its creamy texture…

Mice can taste fat, research has shown, and it looks like humans can too, according to a 2010 study in the British Journal of Nutrition. The study revealed varying taste thresholds for fatty acids – the long chains that along with glycerol comprise fats, or lipids – in participants.

Intriguingly, the subjects with the higher sensitivities to fat ate fewer fatty menu items and were less likely to be overweight than those with low sensitivity…

7. Carbon Dioxide

Yet another strong sixth taste candidate: carbon dioxide (CO2). When dissolved in liquids, this gas gives soda, beer, champagne and other carbonated beverages their zingy fizz.

That familiar tingling was thought to result from bubbles bursting on the tongue, and had therefore been consigned to the touch category. “It’s tricky because CO2 was always considered a trigeminal stimulus,” said Tordoff.

Researchers presented a strong case for dedicated, taste bud-based carbon dioxide sensors in a Science paper in 2009. They found that an enzyme called carbonic anhydrase 4, which appears on sour taste-sensing cells, specifically detects carbon dioxide in mice…

Pretty interesting stuff. This is another scientific topic where it pays not to get emotionally attached to convention – the fact that there aren’t 5 senses or 9 planets or 4 tastes should be exciting new developments, not scary challenges to our worldview.

It’ll be interesting to see when our culture catches up to these realities; we already acknowledge foods as spicy or minty, but how long will it be before advertisers say their food is more umami or kokumi than their rivals’? 

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Small Spiders’ Brains Fill Up Most of Their Body Cavity

If spiders weren’t creepy enough… It turns out that the smaller a spider is, the larger its brain is in proportion to its body size. This is how a tiny spider and a huge spider can have equally complex behaviour. What this means anatomically is that in some spiders, the central nervous system takes up as much as 80% of the body cavity, with their brains literally spilling into their legs. 

More from National Geographic:

Taking up so much body space for a brain would seem to be a problem for a spider’s other organs, Eberhard said. “But [that aspect] hasn’t really been studied.”

Just by the way the spiders look, though, it would make sense that the arachnids are trading something for their big brains.

For instance, in the jumping spider Phidippus clarus, which the researchers examined in a separate study, the adult’s digestive system is in the spider’s cephalothorax—its head and body cavity.

But “in the young one, all that stuff is filled up with brain,” and the baby spider has a less developed digestive system. It’s still unclear, though, what impact this has on the developing spiders…

Presumably, large brains are necessary to spin webs, a behavior thought to be more complex that, say, “a larval beetle that simply eats its way through the fungus where it lives,” Eberhard wrote in an article describing the research…

It’s a weird concept, having so much of a body filled up with the central nervous system (brain and spinal cord). I wonder how or if it relates to spiders’ reaction times, if their sensory organs (including skin) are so very close to their central nervous system. Is there a difference in reflexes between small and large spiders, or young and adult, attributable to relative brain size? Maybe we’ll find out in the future!

Empathy in Rats

A new study has shown that rats will go out of their way to help another rat in distress even with no reward and even if it’s actually costly to them. 

From LiveScience:

In the new study, laboratory rats repeatedly freed their cage-mates from containers, even though there was no clear reward for doing so. The rodents didn’t bother opening empty containers or those holding stuffed rats.

To the researchers’ surprise, when presented with both a rat-holding container and a one containing chocolate — the rats’ favorite snack — the rodents not only chose to open both containers, but also to share the treats they liberated…

In previous studies, researchers found that rodents show the simplest form of empathy, called emotional contagion — a phenomenon where one individual’s emotions spread to others nearby. For example, a crying baby will trigger the other babies in a room to cry as well. Likewise, rats will become distressed when they see other rats in distress, or they will display pain behavior if they see other rats in pain.

For the new study, Mason and her colleagues wanted to see if rats could go beyond emotional contagion and actively help other rats in distress. To do so, the rats would have to suppress their natural responses to the “emotions” of other rats, the result of emotional contagion. “They have to down-regulate their natural reaction to freeze in fear in order to actively help the other rat,” Mason explained…

“When the free rat opens the door, he knows exactly what he’s doing — he knows that the trapped rat is going to get free,” Mason said. “It’s deliberate, purposeful, helping behavior.”

The researchers then conducted other tests to make sure empathy was the driving force in the rats’ behavior. In one experiment, they rigged the container so that opening the door would release the captive rat into a separate arena. The free rat repeatedly set its cage-mate free, even though there was no reward of social interaction afterwards.

That’s pretty fascinating. I think a lot of things are taken for granted as human-only when in fact the world is much more interesting than that, and this may be just such a case. Of course the more similar we find animals to be to humans, the more strict we may become as a society in their uses in research, so this has extra meta-relevance to science.

Anyway, anthropocentrism is a pretty real impediment to science – part of the controversy over evolution, for example – and I think studies like this, finding “human” traits in other animals, chip a bit off of that block and make us face reality a bit more, which is great. Reality is a pretty good place to live in. 

Implanted Neurons Integrate Into Brain Circuitry

Here is some awesome news via PhysOrg: neurons made from human embryonic stem cells in a lab were transplanted into mice and formed functional connections. They adopted the activity patterns of their area and could apparently modify their neighbouring cells through their behaviour too. 

Neurons are specialized, impulse conducting cells that are the most elementary functional unit of the central nervous system. The 100 billion or so neurons in the human brain are constantly sending and receiving the signals that govern everything from walking and talking to thinking. The work represents a crucial step toward deploying customized cells to repair damaged or diseased brains, the most complex human organ…

The Wisconsin team tested the ability of their lab grown neurons to integrate into the brain’s circuitry by transplanting the cells into the adult mouse hippocampus, a well-studied region of the brain that plays a key role in processing memory and spatial navigation. The capacity of the cells to integrate was observed in live tissue taken from the animals that received the cell transplants.

Weick and colleagues also reported that the human neurons adopted the rhythmic firing behavior of many brain cells talking to one another in unison. And, perhaps more importantly, that the human cells could modify the way the neural network behaved.

If human neurons could integrate in and work with mouse neurons, one would hope they can do the same in humans. There are plenty of diseases that involve the loss of neurons (neurodegenerative diseases), so this could possibly be a step towards better treatment for the many millions of people affected. 

They also describe how they modulated the implanted neurons’ behaviour using light, the basic mechanism of which I explained in the context of heart muscle cells earlier.

A critical tool that allowed the UW group to answer this question was a new technology known as optogenetics, where light, instead of electric current, is used to stimulate the activity of the neurons.

“Previously, we’ve been limited in how efficiently we could stimulate transplanted cells. Now we have a tool that allows us to specifically stimulate only the transplanted human cells, and lots of them at once in a non-invasive way,” says Weick.

Weick explains that the capacity to modulate the implanted cells was a necessary step in determining the function of implanted cells because previous technologies were too imprecise and unreliable to accurately determine what transplanted neurons were doing…

The new study opens the door to the potential for clinicians to deploy light-based stimulation technology to manipulate transplanted tissue and cells. “The marriage between stem cells and optogenetics has the potential to assist in the treatment of a number of debilitating neurodegenerative disorders,” notes Su-Chun Zhang, a UW-Madison professor of neuroscience and an author of the new PNAS report. “You can imagine that if the transplanted cells don’t behave as they should, you could use this system to modulate them using light.”

The idea of having brain cells implanted into me that can be externally manipulated by light is a bit eerie, but I guess flying through the air at super speeds in a big metal vehicle used to be weird too. 

My Thoughts on Free Will and Neuroscience

There have been a few articles written about neuroscience and its implications for free will in the last few days; here’s one in Salon, here in Scientific American, and here from Mind Hacks, which also links to an October article in the New York Times and August article from Nature

I’ve found that none of these reflect my own perspective, which is why I thought I’d add my thoughts to the mix just in case you wanted another opinion. Basically these articles portray a struggle over the definition of free will, and make it seem as if some people (namely neuroscientists) find free will soundly disproven while others (namely philosophers) think more evidence is needed. Philosophers argue that neuroscientists are seeing free will as something immaterial, a ghost in the machine, which is not how they see it – free will can coexist with a purely material brain.

My thought is that free will is irrelevant. Free will is an intuitive model for human thinking, a historical assumption. Now we have evidence for another model for human thinking, one based on physics via chemistry via biology. Is there any reason to continue to consider free will as a relevant way of thinking about thinking? I really don’t think so. It may be interesting to philosophers for the moment, but practically speaking what does it tell us? What reason is there for thinking it exists? Whether it exists by one or another definition is irrelevant if it’s a useless, unfalsifiable concept. 

The only point that’s brought up in these articles for the utility of free will is in courts, for assigning responsibility. However, we already acknowledge the flaws of free will in court, via the insanity plea. The idea is that insanity prevents a suspect from using their reason. I don’t think there’s any difference between that case and the case where reason prevents a suspect from using their insanity. There’s this common conception of a “normal” brain and a “defective” brain, instead of the more realistic acknowledgment that every brain is unique and has its own predispositions. Why would a brain we culturally consider defective not have free will, while an equally deterministic “normal” brain has free will? It doesn’t actually make any sense. 

So where does that leave us in terms of legal and moral responsibility? I think it leads us towards rehabilitation instead of pure punishment. The point of rehabilitation is to acknowledge and use determinism – change a convict’s circumstances, change their brain. We just have to realize that this applies to everyone, not just the people we currently don’t think have much free will, like children or the mentally ill.

I could probably ramble on, but I’ve probably made my point clear: free will is, as far as I can tell, a useless concept, and if anything it just obstructs a more effective justice system, as opposed to being the only thing between us and anarchy as some people would claim. I don’t think neuroscientists should be worrying about it at all, given what we know now, so I don’t. 

Mapping Genetic Expression in the Brain

In this TED Talk (which I found via Greg Laden’s Blog), Allan Jones explains how his team is finding out which genes are turned on and off everywhere in the human brain, and making that information freely accessible. My neuroscience bias aside, it’s pretty interesting and novel stuff, so take a peek. If you have a short attention span like me, there’s a cool animation starting at 2:55 that might suck you in more than the preamble does. 

How does this work? He hinted at it, but I don’t think he was perfectly clear, so I’ll fill in the steps to the best of my understanding. As you may know, RNA (specifically messenger RNA, mRNA) is an intermediate step between DNA and proteins, which, simply put, do just about everything in the cell. DNA is transcribed into mRNA, then mRNA is translated into proteins. mRNA is produced when the protein it codes for is needed, so measuring the presence of certain mRNA can tell you what proteins are being made in a cell. This is the point of this line of research.

Firstly, they get a brain from a recently deceased individual, so that the RNA currently in the brain sample does not degrade before they get to it. They dice up the brain using a cryostat and then cut it even more precisely using a laser, mapping the anatomy as they go along. They purify the RNA from this now-tiny sample. This RNA they’ve collected and purified indicates which proteins are being made in the cells in this sample – the problem now is finding out what the RNA molecules they have actually are, what they code for.

To discover this, they attach a fluorescent probe, a little glowing molecule, to all of the RNA. They put this fluorescent RNA sample through a microarray. A microarray is basically a solid surface with up to tens of thousands of tiny areas delineated on it in a grid; each area will have a certain DNA sequence of interest bound to it. They put their RNA sample on the microarray, then wash the contents of the microarray off. If an RNA sequence is complementary to the DNA sequence in a given area, it will stick to that DNA sequence (which is stuck to the microarray) and not wash off – and you can see that it didn’t wash off because it’ll be fluorescent. 

Since you know which DNA is in which area, you can see which of them had complementary RNA in the sample, and therefore know which genes were being expressed (made into proteins) in the original sample. Ta da! By the relative brightness, you can also infer how much RNA there is – how much these genes are being expressed. 

The novelty of this TED Talk is that they’re doing this for every tiny bit of the human brain, and mapping it back onto the brain. He shows this at around 9:36 – in addition to an anatomical map of the brain, they’re making a gene expression map of the brain. 

It seems like this could be very useful in future neuroscience research, and since it’s a free resource it could become very common to refer to this to guide future experimentation. The biggest problem I imagine is the variety between people’s brains. Brains vary quite a bit between people, behaviorally and anatomically, so any map of gene expression is going to be difficult to generalize. I’m sure there are some regions that are more uniform than others though, for which this map can already prove useful even with its tiny sample size. 

If you’re still curious about the magic of microarrays, here‘s the National Center for Biotechnology Information’s primer on them and why they’re very, very important, although their example doesn’t exactly match ours. 

Genetic Changes in the Brain Over a Lifetime

Researchers from the Roslin Institute in Edinburgh have found that retrotransposons change the genetic structure of brain cells over time, which may be behind some neurological disorders. Retrotransposons are genes that can be multiplied and re-inserted into the genome. They can have effects on the cell if they’re inserted in or near other genes that encode proteins, since they can change how, when or if those proteins are made. Because retrotransposons can be multiplied, they make up a huge proportion of genomes – in our case, around half. 

Brain cells are probably the most interesting type of cells to study in this respect since they can generally survive for your entire lifetime, meaning that individual cells can accumulate quite a bit of genetic changes and become very genetically distinct from their neighbours. If they kept multiplying over our lifetimes like other kinds of cells, those changes would instead be passed on to new cells and the population would not end up being as different. 

From Medical Xpress:

The team, from the Roslin Institute in Edinburgh, studied the DNA structure from brain cells from three deceased people who died from non-brain related incidents and who were otherwise normal and healthy; focusing most specifically on the hippocampus and caudate nucleus. In so doing they were able to identify 25,000 areas where there was evidence that retrotransposons had inserted themselves. In addition, they found evidence of three distinct families of retrotransposons, one of which, the Alu family, had never before been seen in the brain…

They also found that they copied themselves into the genetic material in cells that make up some of the most important parts of the brain, such as chemical transporters. Also notably, some were found in the genes in some cells that are known to fight tumor growth, leading to speculation that they might in fact contribute to certain types of brain cancers. Adding fuel to that fire were retrotransposons found in cells that regulate proteins in the brain which of course have been linked to all manner of psychiatric ailments such as schizophrenia.

They also found a lot more copying went on in the hippocampus then in the caudate nucleus, something that could lead to speculation regarding the nature of memory and learning in general if the cells in that part of the brain have individualized DNA structures.

Genetics and neurobiology are both relatively new fields, so there have been and will be pretty awesome breakthroughs in both, of which this seems to be one. Things always end up being way more complicated than we thought, but these are also great opportunities for breakthroughs in medicine and technology. I look forward to seeing how this changes the field. 

The BBC also has an article on this, out of which I’d like to share just one gem:

They say their discovery completely overturns previous theories about how the brain works.

Wow. I guess we can throw out our biology textbooks then.

One would hope that this huge claim was put into some kind of context, but it wasn’t. This is a really silly statement that I would hope is a misrepresentation of what the authors actually may have said. This is why it’s generally better to read science news websites like PhysOrg or ScienceDaily than to read newspapers’ science sections, if you’re willing to wade deeper into science. 

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