And so it is with the brains of dogs and humans. Even though we don’t know the exact function of different parts of the canine brain, we can make educated guesses based on what we have learned about other brains. Using landmarks that are common to both brains, we can begin to construct a more precise functional map of the dog brain.
But where to begin?
At first glance, dog brains didn’t look much like human brains at all, so it wasn’t apparent how much of the vast human neuroscience literature we could use. As I lay awake, I pictured the basic divisions of the human brain and tried to imagine how these might look in a dog’s brain. It was very much like looking at a map of a foreign country.
If we think of the brain as a gigantic computer, information goes in, the brain does something with it, and an action is produced, often in the form of movement. In this manner, inputs and outputs form the first great divide in the brain.
Inputs are relatively easy to understand. All information that flows into our brains must come through the five senses: vision, hearing, touch, smell, and taste. From the scientist’s point of view, inputs can be controlled during an experiment. For the experiment we had just accomplished, we controlled the visual channel by the hand signals we gave, and we controlled smell and taste by giving either peas or hot dogs.
Outputs are also easy to understand, especially if we consider movement as the main output of the brain. The earliest fMRI experiments had human subjects lying in the MRI and tapping their fingers for periods of thirty seconds. When the subjects tapped their fingers, activity in the part of the brain that controlled the hand was plainly visible.
The central sulcus is a groove in the human brain that runs almost vertically down the outside of each hemisphere. Everything behind the central sulcus is broadly concerned with inputs and everything in front with outputs. It is a defining landmark that divides the frontal lobe in front of the groove from the parietal lobe behind. The frontal bank of the central sulcus, it’s important to note, contains the neurons that control movement of all the parts of the body. Toward the bottom of this groove, above the ear, we find neurons that control the hand and mouth, and as we move up toward the crown of the head, we find neurons that control the legs. The neurons found along the sulcus control the opposite side of the body. When you move your right hand, a portion of the left central sulcus will become active, and this can be seen easily with fMRI.
In contrast, the neurons behind the central sulcus respond when the corresponding parts of the body are touched. These are the primary sensory neurons. As you move farther toward the back of the head, the functions of the neurons become multimodal, meaning they integrate the inputs from many senses. At the very back of the head, we find the primary visual area, which receives inputs from the eyes.
Another obvious landmark of the human brain is the protuberance along the sides of the brain, just above the ear. This is the temporal lobe. Sitting directly next to the ear, parts of the temporal lobe are concerned with hearing. Other parts of the temporal lobe, along the inner crease next to the rest of the brain, contain structures critical for memory.
With the dog brain, the first thing you notice is that, apart from being smaller, it has a lot fewer folds. The massive amount of folding in the human brain is the solution that evolved to cram more brain into a small space. If you could flatten out the brain, you would find that all the neurons are contained in a thin sheet just a few millimeters thick. It’s like taking a very large sheet of paper and crumpling it up into a ball. Once crumpled, a very large area can be made to fit in a small space, like the skull.
The different amount of folding in the dog brain means that the usual landmarks, like the central sulcus, don’t exist. We can point to only the front and back of the brain and sort of make out the temporal lobe. The next thing you notice is that the dog doesn’t seem to have much of a frontal lobe at all. This is the area that really distinguishes humans from other primates. Humans have the largest frontal lobes of any animal. Because the frontal lobes of the brain are mostly concerned with outputs—in other words, doing things—we think that this part of the brain expanded in humans to accommodate higher-order cognitive functions. Uniquely human functions that reside in the frontal lobe include language and the related ability to think symbolically; the ability to think abstractly about the future and past, which leads to planning; and the ability to mentalize what other people might be thinking.
Although the dog brain looks, at first glance, like a scaled-down version of the human brain, there is one area that is noticeably larger in the dog. The part of the brain concerned with smell, called the olfactory bulb, is huge in the dog brain. When the dog brain is viewed in the dorsal plane at the level of the eyes, the olfactory bulb looks like a rocket ship. There is no human equivalent of this part of the brain. The dog’s olfactory bulb and the parts of the brain surrounding it compose almost a tenth of the total volume. Obviously, smell is important to dogs, but almost nothing is known about how this part of their brain works. That research would have to wait.
We had achieved the first milestone of success in the Dog Project by acquiring a sequence of functional images in both dogs. Over the next few days, we would match up the images with the timing data from the experiment. If everything worked, we would soon have a picture of the dogs’ brains that showed which parts responded to the signals for peas and hot dogs.
Dorsal plane view of the dog brain showing the olfactory bulb (left) and the corresponding view of the human brain (right). The arrows point to the caudate in both brains.
(Dog brain image by permission of Thomas Fletcher, University of Minnesota; human brain by Gregory Berns)
But what would that tell us?
The whole of the Dog Project hinged on the promise of figuring out what dogs think. Even if we succeeded in finding the parts of the brain that responded to different hand signals, that wouldn’t necessarily mean that we knew what the dogs were thinking. To answer this deeper question, we would have to interpret the patterns of activation based on similar patterns in humans. If we saw activity in parts of the dog brain that we could identify, and we knew what those parts did in humans, we could begin to build a functional map of the canine brain. Using the concept of homology, we could infer canine thought processes from their human equivalents.
This was a shaky premise.
In recent years, there has been a bit of a scientific backlash against neuroimaging. Functional MRI has made it easy to dream up poorly controlled experiments and have groups of undergraduates go into the scanner. Many scientists, eager to get a quick publication in a high-profile journal, overinterpreted the patterns of activity they found in the human brain. It became commonplace to point to activity in a particular brain region and interpret that as evidence for a particular emotion or other cognitive function. It was too easy to observe activation of a structure and conclude, for example, that the person was feeling happy or sad or fearful or some other emotional state based on the scientist’s assumptions of what different brain regions did. Eventually, neuroscientists termed this type of reasoning reverse inference, and it became a key factor in rejecting many fMRI papers.