Human language in dog brains

Spoken language conveys meaning in two ways: the meaning of the words (semantics or lexical knowledge) and the intonation that the speaker uses. We can sense questions by the rising pitch at the end of the sentence. Likewise, we can tell if someone is upset or being sarcastic based on how they say the words. The patterns of intonation in language is known as prosody. There are areas of the brain that are specialized for decoding the semantic meaning of language and different areas for interpreting prosody. In fact, you can have damage to one area during a stroke, while the other area remains intact. There are great examples of this in “The President’s Speech” in Oliver Sacks' book The Man who Mistook his Wife for a Hat.

In most people, word meanings are processed by the left side of the brain and prosody is localized to the right side of the brain. Some animals also use the left side of their brains to understand meaningful and familiar sounds of their species (like alert calls or bird songs). What about for animals, like dogs, which can understand the sounds of another species (i.e. commands from humans). Is the dog brain really processing the intonations of praise “good dog!” or are they responding to the words? Do they process meaning and intonation separately like humans do?

Dogs in MRI machines

In the latest issue of Science, Andies et al. published their studies of language processing in dog brains. My first thought when I read the abstract was “how do you get a dog into an MRI machine?” We commonly study which areas of human brains are active during different tasks using a technique called functional MRI (or fMRI). fMRI was done on these dogs while they listened to their trainers speak. If you have ever had an MRI scan, you know they strap you in and you cannot move your head at all. Same thing with these dogs. Needless to say, they were very well trained dogs. If you still can’t believe it, check out this video the researchers made and the cute photo of dogs in an MRI machine below.

Really well trained dogs lying still before their MRIs. (Image from phys.org)

Dogs process language like humans

Okay, so they got the dogs in the MRI machine and scanned their brains while they heard their trainer say different things. The trainer would either say words of praise, like “good boy” (in Hungarian), or neutral words. And they used either a neutral, flat intonation or they raised the pitch of their voice to create a praising intonation. This created four possibilities:

• Praise words with praising intonation
• Praise words with neutral intonation
• Neutral words with praising intonation
• Neutral words with neutral intonation

They compared the brain responses to each combination and found that the left side of the brain responded to words of praise regardless of the intonation. This is amazing, right? The dogs have heard “good boy” enough times that their brains responded specifically to that phrase regardless of how it was said. It’s like they sort of know what it means. It would be interesting to see if they respond to the same phrase spoken by a stranger.

The researchers also found that the right side of the brain had active areas when praising intonation was used, regardless of the word meaning. So dogs also understand how our voices change when we praise them.

Finally, the researchers looked at areas of the brain associated with reward. These areas are active in a variety of animals when they receive natural rewards like food or during sex, but the reward pathways are also active if the animal is given an addictive drug like cocaine. Alternatively, you can put an electrode into a mouse brain that stimulates the reward pathway and the mouse will push a lever to receive an electrical shock in this area of the brain over and over until it starves.

Andies et al. found that praising words spoken in a praising intonation activated the reward pathway in the dogs. Praise words alone and praise intonation alone had no effect. So dogs really do feel good when you say “good dog” in a high pitched voice.

Notice the organization of language processing in the dog brain. Just like in humans, language semantics (praise vs neutral words) was processed on the left side and prosody (praise vs neutral intonation) was processed on the right side. What does this tell us about the evolution of language? Language lateralization has likely been around a long time and is not uniquely human. The authors end the article with this gem: “What makes lexical items uniquely human is thus not the neural capacity to process them, but the invention of using them.”

Bigger brains with Frizzled HARE

We have all heard that the sequence of human DNA differs from chimpanzee DNA by only about 1%.  Yet humans are capable of building complex civilizations while the chimps are still eating bugs in the forest.  If you compare the human brain to the brain of any other primate, it’s easy to see where our sophisticated cognitive abilities come from. 

From thebrain.mcgill.ca

DNA is the blueprint for making proteins, cells and organs, so is there something special hidden in that 1% sequence difference that gives humans bigger brains?  In particular, scientists have focused on regions in the human genome that have undergone rapid sequence changes in the human lineage, but not in other primates.  Besides looking for differences in genes that make proteins, we can also look for changes in regulatory regions, like enhancers, that control when and where the genes are expressed.

A recent paper in Current Biology by Boyd et al. explores these questions by studying a human-accelerated regulatory enhancer (HARE5), which differs significantly between humans and chimps.

Enhancer activity of HARE5

How do you study enhancers?  One way is to use a reporter gene.  Enhancers drive expression of nearby genes, so what if you swapped out a nearby gene and replaced it with a gene for a fluorescent protein?  Then you can look at your organism and wherever you see the fluorescent protein, the enhancer is active, meaning that the normal “nearby gene” is normally expressed in those cells.  Instead of doing these experiments with chimps and humans, which would take forever and be unethical in some cases, the authors put these reporter constructs into mice.  The enhancers from the chimps and humans drove expression of the reporter gene in the embryonic mouse brains.  The gene adjacent to the human enhancer was expressed earlier in development and more strongly than when placed next to the chimp enhancer (in other words, a lot more protein is being made).

Reporter gene experiment.  The mouse brain images are actual results from Figure 2 in Boyd et al. (2015).

This tells us that whatever normal gene is near HARE5, it is probably expressed earlier and way more in humans than in chimps.  There are just 10 sequence differences in the human HARE5 (i.e. mutations), which is enough to affect the way the enhancer functions and activates expression of genes. 

Frizzled expression is regulated by HARE5

So which genes are near the HARE5 sequence?  The closest gene is called Frizzled 8 and it is a receptor that responds to signals sent by other cells.  Frizzled 8 (FZD8) is a well known component of the Wnt signaling pathway that regulates many aspects of embryonic development, including neurogenesis (formation of new neurons).  The authors demonstrate that the mouse HARE5 physically interacts with Fzd8, which is a necessary  first step of gene expression, so Fzd8 is likely affected by the HARE5 sequence differences in humans and chimps.

The authors wanted to see what would happen to development of the mouse brain when Fzd8 is expressed in the same pattern as in humans or chimps.  They repeated the earlier experiments, but this time instead of using a reporter gene, they put the mouse Fzd8 gene next to the chimp or human HARE5 sequence.  They injected these DNA constructs into mice and waited to see what would happen to embryonic brain development.  When the chimp-HARE5 was driving expression of Fzd8, not much changed in terms of mouse brain development.  However, when the human-HARE5 sequence was activating the mouse Fzd8 gene, the mouse brain grew 12% bigger!! 

Let me be clear here-- they are not expressing the human Fzd gene in mice.  No, they are using the human enhancer to drive expression of the mouse Fzd8 gene, so presumably it is expressed more and earlier in development (like they saw in the reporter gene experiment).  The neural progenitor cells (pre-neurons) divided faster than in a normal mouse, leading to formation of more neurons, and a bigger brain! 

10 sequence changes in an enhancer may be one reason why I am able to write and you are able to read and understand this blog.  Whoa.  No news yet about whether these mice with bigger brains are also able to read and write… I’m sure they’re saving that for another paper.

I should say too, that there are probably a number of other similar changes to other enhancers and genes that all led to the rapid development of the big ol’ human brain.

Here's another blogger's take on this paper 

Ctenophores come before

Three months ago, if I had seen this article about the ctenophore genome, I would have moved right passed it without a second look.  What is a ctenophore and why would I care about the sequence of its DNA?  But then I taught Bio 2 this spring and learned about animal diversity and the evolutionary tree (a day before I taught it).  This is a great example that the more you learn, the more interested you become in the subject.  Today’s article by Moroz et al. was published recently online in Nature (this one is open access, so take a look).  The results totally change the roots of the animal tree and invalidate what we taught to our students this semester.  Before we get into the paper, let me answer my own questions:

What is a ctenophore?

Ctenophores are also known as comb jellies, because they look like jellyfish and have a comb-like structure that they wiggle around to move through the water.  They have sensory organs to sense light and gravity.  They have tentacles that they move with their nervous system in order to catch prey.

Comb jelly (from Wikimedia Commons)

Although they look like jellyfish, they are in a totally different phylum, or branch of the evolutionary tree (also known as a phylogenetic tree).  Jellyfish are in the phylum Cnidaria, along with sea anemones, coral and hydras.  Comb jellies are in their own phylum known as Ctenophora. 

Why should we care about ctenophores? 

Ctenophores, along with cnidarians and sponges, represent some of the most ancient lineages of animals.  Studying them can give us a clue about how animals evolved.  All the rest of the animals are in the large taxonomic group called Bilateria, because they have bilateral symmetry (which is symmetry across a single axis).  This includes humans, insects, crustaceans, worms, fish, molluscs, etc.  Think about a jellyfish or a sea anemone; they have radial symmetry, which means they can be bisected in lots of different axes and you would still have symmetrical halves.  The bilateral animals are more complicated in lots of other ways, such as having a greater variety of tissues and more complex physiology.

The phylogenetic tree according to the biology textbook

Sponges don’t have organized tissues and they don’t have a nervous system, so based on that, researchers have considered them to be the most ancient lineage (i.e. the “basal” animals).  So if we are building a phylogenetic tree based on morphological characteristics, we are going to put them as the first branch.

Cnidarians and ctenophores look very similar, so it would make sense to put them right next to each other, followed by the bilateral animals.  Thus, based on morphological observations, the tree should look something like this:

No one ever explained to me why cnidarians get to be closer to the bilaterals than ctenophores.  Perhaps this is because the cnidarians come in so many different body plans, so maybe they are considered to be more complex and thus, an evolutionarily “newer” animal.

We told our students over and over: “Sponges are the most basal animals”.  But like many phylogenetic theories that have come before, new DNA sequencing data is challenging this view.

What does the ctenophore genome tell us?

First off, for the non-biologists out there, you need to understand one fundamental thing about gene evolution.  Two species that are highly related will have very similar DNA sequences.  The further apart two species are in evolutionary time, the more time there is for mutations to change the DNA sequences and the gene functions.

Moroz et al. sequenced one of the ctenophore genomes and then compared it with the genomes of sponges and cnidarians.  One of the main findings was that there are many missing animal-specific genes that are involved in animal development (Hox genes), regulating gene expression (no miRNAs!) and innate immunity.  Although some animal-specific genes are absent, the ctenophores have many unique genes that are not found in other animals, indicating that these genes evolved independently in the ctenophore lineage. 

The researchers devoted a lot of the paper to looking at genes involved in nervous system function.  Ctenophores, like cnidarians, have neural nets, as opposed to organized bundles of nerves.  Bilateral animals have many different neurotransmitters, which are the signals that get sent between nerve cells.  The ctenophores only have genes for making the neurotransmitter glutamate (and GABA), but they have a ton of glutamate receptors, more than other animals.

All of these findings led the authors to conclude that ctenophores are the most basal animals, not sponges.  Alternatively, it is still possible to keep the same phylogenetic tree, but there would need to have been massive gene loss in the ctenophore lineage.  The most parsimonious explanation is shown here:

The main difference between these two trees is that the sponges and ctenophores have swapped positions.  Note that this would require a nervous system to have developed twice independently.  That’s totally insane.  The ctenophores and the cnidarians “needed” a method of controlling their body to capture prey and both lineages “came up” with the same solution (of course evolution is random and doesn’t have a particular goal in mind).  When similar structures evolve independently, this is known as convergent evolution.

Next year, instead of devoting half a lecture to sponges and tossing in a single slide on ctenophores, I think I’ll have to give ctenophores their due, as potentially the most ancient lineage of animals still in existence. 

Carl Zimmer always beats me to the punch, so here’s his take on the same article.  Better writing, but fewer trees!

(Insert mildly provocative title here)

Ever seen a pair of pigeons going at it?  And did you notice a penis on the male pigeon?  The answer is no, because most birds do not have external genitalia large enough for penetration.  And yet birds reproduce via internal fertilization.  Why would evolution favor male genitalia too small to actually enter into the female?  This just seems so inefficient. 

There are a few birds that do have well developed phalluses, such as the duck and goose.  What happened during evolution that caused some birds to retain a phallus, whereas most other birds lost it?  A paper appeared this week in Current Biology by Herrera et al., which addresses these questions from a developmental point of view.

Developmental arrest

The authors started this study by comparing the development of the phallus in embryos of two different birds.  They chose to look at (1) chick embryos, which are part of the galliformes group of birds and have reduced phalluses and (2) duck embryos, which are part of the anseriforms group, which have well developed, penetrating penises.  They followed the growth of the genital tubercle, the tissue that will form the penis.  As the duck and chick embryos grow, so do their genital tubercles, with no noticeable difference between the two species during the early stages of development.  At a later time period, though, the tubercle stops growing and regresses in the chicks, while the duck keeps on growing.  This shows that the tissue that makes the two different types of phalluses has the same developmental origin.

Why does the genital tubercle stop growing in the chick?

From a molecular stand point, the chick embryos could either lose the “growth” signal or they could gain expression of a “stop” signal not present in ducks.  From work in other animals, the authors knew that there are two major growth signals responsible for guiding the development of the external genitalia – Sonic Hedgehog (Shh) [see my other post about this protein] and Hox13.  These two genes are strongly expressed in the duck genital tubercle throughout embryonic development, as expected.  Surprisingly, though, they are also strongly expressed in the chick embryos.  This means that the chickens haven’t lost the growth signal.

The authors then investigated if there is some sort of a “stop” signal in the chicks.  They found that in chicks and quails, with reduced phalluses, there is a lot of cell death in the genital tubercle in the later stages of development.  This could account for the regression of the genital tubercle.  They then found that the chicks highly express a protein called BMP4 at the tip of the tubercle, which induces cell death, whereas ducks do not. 

In fact, by overexpressing BMPs in the duck, they induced cell death in the genital tubercle.  In the opposite experiment, they inhibited BMPs in the chick and their genital tubercles increased growth, as if they were ducks.

In summary:

            Chicken: + BMP --> increased cell death --> reduced phallus

            Duck:       - BMP --> no cell death, so continued tissue development --> large phallus

Evolution of reduced phallus

So what does this mean?  Chicks and quails have reduced phalluses, because during development, they express BMP4, which tells the developing cells of the penis to die off.  One really cool thing that the authors did next was to look at cell death in the closest relative to birds-- the alligator.  Yah, they got alligator embryos for this research!  Alligators have developed phalluses and they show hardly any cell death in the genital tubercle.  From this work, they could create an evolutionary tree, which shows that chicks and quails most likely evolved the BMP4 signal after their group separated from ducks.  Although the authors didn’t test any members (ha ha) from the neoaves group, which includes most other birds, we can presume that they also have a similar cell death mechanism to reduce the development of their phalluses.

Phylogenetic tree of birds, showing when the BMP signal evolved. (Adapted from Herrera et al., 2013)

This still begs the question of why would natural selection favor a reduced phallus so much so that it evolved independently in different lineages?  The authors propose two different theories, both of which may have occurred:

1) Sexual selection – sure, it may not be favorable for the males to have reduced phalluses, but it might be advantageous for the females.  In order for insemination to occur in these species, the female has to be a willing participant to allow the male to shimmy up next to her and release the sperm in very close proximity.  This gives the females the power to select their mates.  As opposed to species with large penises, where the male could basically rape the female and still successfully pass on his genes to the next generation.

2) Pleiotropy – this term refers to when a single gene mutation can lead to multiple noticeable changes in the body.  BMPs are a major signal during development of animals.  BMPs are involved in a number of bird-only innovations such as feathers and beaks.  Maybe increased BMP expression gave an advantage to these birds, but also lead to reduced phalluses, as a secondary effect.  This may have occurred first in evolution, but sexual selection may have stabilized this characteristic in the population.

This article was so clear and interesting.  I’m sure it will catch people’s attention because of the subject matter, but it’s a great example of using development to solve an evolutionary question.  Plus it gives reviewers and bloggers a great opportunity to think up clever titles and puns for their articles.  The review that was published alongside this article was titled “Cock-a-doodle-don’t”. How can I compete with that?