Squealer – the silenced pig
This is a blog post about a character close to my heart: the pig.
During most of my childhood I collected pigs. My whole room was filled with little statues, pictures, books, cups and stickers of pigs. I had fallen in love with their intelligence, cute ugliness and laissez-faire attitude. Little did I know that my favorite childhood animal provided a huge insight into my adult career field!
Two hundred years A.C., the Greeks were strongly divided over where our thoughts, feelings and behaviors lie: was it in the heart or the brain? Most followed Aristotle, who had the following strong arguments for why the heart must be the site1:
- The heart connects to all sense organs through blood vessels, whereas the brain does not. (Actually it does, but nerves can be hard to see compared to blood vessels)
- In embryos, the heart develops before the brain.
- All animals have a heart but not all animals have a brain. (Most animals do have brains, although some can be difficult to recognize. And indeed there are some animals without brains.)
- The heart is sensitive to touch and responds to emotions, whereas the brain does not.
- You need blood for sensation (everyone who has slept on their arm can vouch for this!), and this is provided by the heart.
These were indeed fair points. Proponents of the brain as the site of our mind used medical cases, showing that brain injury can change behavior to argue their case, but they lacked any direct evidence. This changed when medical doctor Galen silenced a squealing pig…
Galen was born in 129 A.C. in a town called Pergamon and quickly grew up to a successful physician, serving gladiators and many famous people like Marcus Aurelius’s son, the emperors Commodus and Septimus Severus2. His interactions with gladiators brought him a variety of interesting medical injuries – including brain injuries – and strengthened his belief that the brain is controlling our thoughts and behaviors.
Alongside his work as a physician, Galen also did a lot of research. He mostly studied anatomy, with a particular interest in the nervous system. He used oxes and macaque monkeys to study the brain and nerve cord. In addition to that he did vivisections (dissections of living animals) on pigs, “to avoid seeing the unpleasant expression of the ape when it is being vivisected”3. Galen was the first to do vivisections. They allowed him to pinpoint the specific functions of the different nerves he found.
One day, during the vivisection of a badly struggling pig –which, since it is now one of our brain celebrities, I will call Squealer – Galen accidentally cut the recurrent laryngeal nerves. To his surprise, the pig stopped squealing, but continued to struggle. This was the first causal evidence that nerves, coming from the brain, control specific behaviors. Without intact nerves, the muscles in the larynx (voice box) could not move the vocal cords of the pig, making him unable to squeal. The heart and blood vessels were clearly intact, showing that it was undeniably the brain that was making Squealer scream for his life.
Many other pigs followed Squealer’s fate in public demonstrations that Galen did to prove to his opponents that the brain controls our behavior. Quickly after Galen’s death, the Middle Ages kicked in and scientific debates silenced. However, during the Renaissance, his writings were found, and the silenced pig once again became one of the most famous examples of the link between brain and behavior.
Image from the bottom panel of the title page to the 1541 Junta edition of Galen’s Works. Figure taken from https://commons.wikimedia.org/wiki/File:Galen-Pig-Vivisection.jpg
Directly damaging the nervous system is still an often-used technique in neuroscience. By killing or silencing the activity of specific neurons or brain regions we can figure out their function in a circuit or a behavior. A complex part of these experiments is often that changes in behavior are difficult to interpret.
If damage to an area X leads to change in behavior, the most straightforward conclusion would be that the area X is necessary for this behavior. However, it could also be that this area is in close contact with another brain region (Y) that actually controls the behavior. Area X changes the activity of area Y, which leads to a behavioral change, but area X actually does not control the behavior.
On the other hand, damage to area X could lead to no notable change. It would be reasonable to then conclude that area X is not involved in this behavior. However, it could also be that area X plays a major role, but that there are other area’s involved that can take over area X’s function when it is damaged.
A lab in our department found a very strong example of this. Many studies have shown that the motor cortex (including the laryngeal motor cortex) is responsible for controlling motor actions. However, they found that rats could execute a learned complex motor task perfectly after the complete motor cortex had been lesioned4. The lab also recently published a study showing that the specific way in which you silence an area can strongly influence your conclusions5.
Thus, although Galen’s strategy of damaging the nervous system directly is effective, the brain is usually too complex to draw simple conclusions from this type of experiment!
Squealer taught the Greeks that the brain controls our actions, and served as an inspiration for the field of experimental neuroscience. Although nowadays the pig is not a frequently used model organism in neuroscience, pigs are crucial to other medical sciences. Since the body of a pig is in many ways the closest you can get to a human body, pigs were for a long time a crucial source of insulin, and pig organs have been thoroughly studied to understand human ones. In addition, we will probably be able to transplant genetically adapted pig organs into humans in the near future, saving tens of thousands lives a year of people who die waiting for a suitable human donor6.
By stopping to squeal, Squealer was the first in a long row of pigs advancing neural and medical sciences. So say a little thank you to the Christmas ham this week!
- C. Gross. A hole in the head: more tales in the history of neuroscience. The Cambridge, Massachusetts, MIT Press, 2009
- Siegel, R.E. Galen’s system of physiology and medicine. New York: S.Karger, 1968