Reiterating what Darwin said, Panksepp has argued that only when we understand the emotional systems of our fellow creatures will we begin to understand the origins of human feelings. This is a compelling argument. When we look at the brains of animals, it is immediately apparent that there are many structures in common. The commonalities have traditionally been called primitive, reflecting scientists’ belief that they must have an old evolutionary origin. In the 1960s, the neuroscientist Paul MacLean used the evolutionary analogy to divide the brain into three parts: the reptilian brain (the basal ganglia), the paleomammalian brain (the limbic system), and the neomammalian brain (the neocortex). Although these divisions are overly simplistic, it is clear that only the neocortex is substantially different in humans and other mammals. The other two divisions—the basal ganglia and the limbic system—are largely the same from rats to humans. It is in these systems that Berridge and Panksepp believe that emotions originate.
The first difficulty in studying animal emotions lies in describing what an emotion is. Humans have a rich language for emotion, but even if you take something as basic and universal as love, you quickly realize the vast nuances that that word contains. There are so many different types of love that the word itself is inadequate. Assuming, for the moment, that our dogs love us, what kind of love would that be?
To proceed scientifically, we must set aside such subtleties. It helps to break emotion down to fundamental components: valence and arousal. Valence is simply goodness or badness, while arousal describes the level of excitement, ranging from calm to maximum excitement. Many human emotions can be plotted on a graph as a function of the combination of valence and arousal. Because the graph forms a circle, it is called the circumplex model of emotion. Positive emotions are plotted to the right, while negative ones are on the left. In the vertical direction, high-arousal emotions are at the top, while low-arousal ones are at the bottom.
Many psychologists have argued that the two-factor model is too simplistic. However, it provides an excellent starting point to understanding which parts of the brain give rise to the different emotions. As it turns out, the reptilian part of the brain, which we now call the basal ganglia, is closely associated with positive valence, while the limbic system is associated with arousal. By examining the relationship of activity in these different brain systems to the emotions experienced by human subjects, we can build an emotional brain map.
Because dogs have basal ganglia and limbic systems that look almost the same as ours, such a map could be applied to dog brains to help determine what a dog is feeling.
In the upper left portion of the circumplex are the emotions with high arousal and negative valence. The usual behavioral manifestation of these emotions is avoidance or retreat from whatever caused them. However, the close proximity of emotions like fear, anger, and frustration in the circumplex make that quadrant difficult to map in the brain. Despite having similar levels of valence and arousal, those emotions feel quite different from one another. Although much is known about the fear system of the brain, almost nothing is known about rage or frustration.
The upper left quadrant would remain uncharted territory in the Dog Project because of the ethical problems with inducing those types of emotions in our dogs.
In contrast, the upper right quadrant of the circumplex model is well understood and seemed like an excellent place to begin mapping the dog brain. These are the emotions that are maximally enjoyable: very good and very exciting. These positive emotions are also associated with a specific behavior seen in all animals. If something is good and exciting, every animal—dog, rat, human—will approach it. Panksepp calls this the seeking system. In the brain, we know that approach behavior, as well as the corresponding positive emotions, is associated with activity in a tiny part of the basal ganglia called the nucleus accumbens. In humans, when we observe activation in this region we can deduce that the person is experiencing a positive emotion and very likely wants whatever is making them excited.
Although I couldn’t know for sure, when Callie saw me with the bag of hot dogs, her nucleus accumbens was probably lighting up like a Christmas tree. Wagging her tail and running toward me was a classic approach behavior. This led me to assume that she was experiencing joy and excitement. But only through fMRI would we know what she truly felt.
13
The Lost Wedding Ring
UNLESS WE CAME UP WITH a solution for the head movement problem, the Dog Project would grind to a halt. We had two choices: either train the dogs to hold their heads still or come up with a better chin rest.
Mark was confident we could train the dogs. Considering that we needed the dogs to move less than two millimeters in every direction, this struck me as a significant obstacle. As I worked with Callie, I couldn’t even discern movements that small. She could easily move a few millimeters while I was looking away, and I wouldn’t even know. The alternative, a more restrictive chin rest, didn’t appeal either. I didn’t want to encase the dogs’ heads in plastic like what Andrew and I saw at Yerkes.
We were at an impasse.
Movement during a scan causes ghosting, but movement between scans causes a different problem. At the beginning of a series of scans, we have to set the boundaries of the scan, called the field of view (FOV). As Callie demonstrated during the dress rehearsal, she didn’t always place her head in the same position in the head coil. Because of the inconsistency of her head placement, sometimes she was in the FOV, but most of the time she wasn’t. All but one of the images were empty.
We had a few tricks we could do with the functional scans that would help with the movement problems. An easy fix for ghosting is to shorten the time it takes to acquire an image of the brain. By shortening the scan time, it makes it less likely the subject will move during that period. But the only way to shorten the scan is to take fewer slices. Each slice through the brain takes roughly sixty milliseconds. For humans, it usually takes thirty slices to cover the whole brain, for a total of two seconds. Fortunately, dogs have smaller brains than humans, so we don’t need as many slices. But if we get fewer slices, the FOV shrinks, and the between-scan movement becomes a bigger problem. With a small FOV, we might miss the brain entirely if the dog didn’t put her head in the correct location.
We needed a solution to both the within-scan and between-scan motion.
The problem was the chin rest. The foam bar provided feedback to the dogs about where to place their heads in the up-down direction, but left them free in the left-right and forward-backward directions. We needed something that would guide the dogs to place their heads in exactly the same position every time they went into the coil and would keep them there for the duration of the scan.
Atlanta had caught a week of unseasonably warm weather that January. It was a good opportunity to get outside, and I figured a change of scenery might generate some new ideas about the movement problem. So Kat and I piled the girls and the dogs into the minivan and headed down to the river for some hiking.
The Chattahoochee River originates in the northeast corner of Georgia, in the foothills of the Blue Ridge Mountains. From there, it flows southwest toward Atlanta, picking up volume along the way. Eventually, the “Hooch” flows all the way to the Gulf of Mexico. Much of the area around the Chattahoochee is national forest, and we were fortunate to live only a mile from the river. It was a great place to hike or mountain bike or just relax on the banks and watch wildlife.