A metaphorical tour of the brain

Albert Einstein once said: “The only source of knowledge is experience.” Not to undermine Einstein’s authority, but there are many phenomena, especially in science, that we just can’t experience directly. We can make diagrams, but we can’t actually see or touch things like dopamine or gravity. And although we’re constantly experiencing both of those things, we’re not aware that we’re experiencing them in the way that we know we’re experiencing blueness when we look at the ocean. If experience is the source of knowledge and we’re unable to experience so many scientific phenomena, how, then, do we know and communicate about them? Metaphor provides one way of doing so: we talk about abstract concepts in terms of more concrete ones that we do have experience with as a way of making sense of intangible ideas.

Neuroscience is filled with metaphors, many of which are hardly recognizable as such. The brain is commonly referred to as a machine or a computer, a metaphor that can be traced back at least as far as Descartes. Many people find this metaphor compelling because both brains and computers receive input from the world, process that information, and produce outputs. A human hears a sentence, comprehends it, and responds. A computer receives a command, processes it, and executes the command. We talk about online (quicker, in-the-moment thinking) and offline cognition (slower, more deliberate thought), and we talk about both brains and computers storing information in memory. There are many critics of the brain as a computer metaphor, though. They point out, for example, that many parts of the brain are active at once, engaging in parallel processing, while traditional computers are serial processors that do one thing at a time. Further, computers are deterministic: a single input will always produce the same output, so a computer’s response to 3 x 5 won’t vary from day to day. Brains are not deterministic, so just because tomato soup is delicious one day, the same soup might be offputting the next. Brains are also situated in bodies, and bodies play a crucial role in cognition, even for thinking about abstract ideas that don’t seem to be related to the body at all, such as the past and future or morality (in fact, there is a whole subfield, embodied cognition, that investigates the body’s role in cognition).

Beyond the brain as a whole, metaphors also pervade our language about the dynamics of neurons. Physically, we often talk about neurons as trees because there are many analogous-looking parts. The roots, branches, and trunk of a tree can be compared to the dendrites, axon terminals, and axon of a neuron. In fact, the term for dendrite itself has a metaphorical history, coming from Greek roots for treelike and tree.

braintree

Thinking about neurons as trees is not just a practice for introductory students, but also appears in sophisticated research, as this Cell cover illustrates.

A neuron’s function is also typically described metaphorically. Neurons send and receive messages from each other, which allows them to communicate information. When a neuron at rest becomes excited, it fires an action potential, releasing its message (neurotransmitters) into the synaptic gap so that it can be received by another neuron. Much of the language we use to describe this process draws on everyday experiences we have when communicating with other people. Even though neuronal and human communication look very different, the overlapping language that we use for both processes sheds light on their functional similarities, making the complex topic of neuronal communication more understandable.

We also use metaphors to talk about individual brain areas. The frontal lobe, especially, is often referred to as an executive controller because of its crucial role in reasoning, decision making, problem solving, planning, and many other processes considered to be higher level. This metaphor is clearly illustrated by the new and popular movie Inside Out, where personified emotions control Riley’s thoughts and behaviors by manipulating a control panel in the headquarters (also see this NeuWrite post for a more in-depth discussion of the brain science behind Inside Out). We often think of the frontal lobe’s role in cognition as akin to that of an orchestra conductor, air traffic controller, or company manager: it coordinates incoming information and controls much of our behavior.

It’s difficult to find an aspect of neuroscience that’s not described metaphorically. A fundamental feature of the brain is its plasticity, which refers to its ability to change as a response to changes in things like the environment, behavior, experience, or physical injury. The term plasticity highlights the similarity of this process to the substance plastic, which by definition is “easily shaped or molded.” Another fundamental feature of the brain is that it contains mirror neurons, another metaphorically inspired name. If you imagine making a face in the mirror, you have the unique opportunity to do an action and simultaneously see what that action looks like. Mirror neurons function similarly. They are most famous for producing the same patterns of activity when a person is doing an action as when they are watching someone else do that action, which doesn’t require any actual mirrors at all.

Although the metaphors discussed so far are subtle, others are deliberately creative. Dr. Maiken Nedergaard, a professor of neurosurgery at the University of Rochester, has described his research as demonstrating that during sleep, the brain is like a dishwasher: it clears away harmful toxins. Similarly, Patricia Bauer, a memory researcher at Emory University, has compared memory to a colander and memories as the pasta (which might go well with the secret meatball recipe of life). When you pour a pot of fettucine into a colander, the pasta stays in. These are the memories we tend to make as adults. But when we’re younger, our memories are more like orzo, and they slip through the colander’s holes. The image of orzo slipping through the colander’s holes illustrates the gist of the phenomenon of childhood amnesia, the common inability to remember much from our early childhood. Of course the orzo metaphor also oversimplifies the concept of childhood amnesia – if the phenomenon really arose from little pastas slipping through holes, many psychologists would probably be out dining at fancy Italian restaurants right now, instead of grinding away in their labs.

Amidst vivid metaphors, it’s important for us to remember that the brain is still a 3-pound organ of tissue. It doesn’t contain trees that can be literally excited or communicate with each other, and it definitely doesn’t contain plastic, mirrors, or pasta. Losing sight of the aspects of those metaphors that don’t map to the brain might lead to misunderstandings that keep us from making informed decisions about science problems in society or might even misinform lines of research. But when taken with a grain of salt, metaphors can provide a powerful way of thinking and talking about the complex structures and processes hidden inside our skull.

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