Joseph LeDoux is an American neuroscientist, whose research is primarily focused on survival circuits, including their impacts on emotions such as fear and anxiety. LeDoux is a professor of neural science and psychology at New York University, and director of the Emotional Brain Institute, a collaboration between NYU and New York state with research sites at NYU and the Nathan Kline Institute for Psychiatric Research in Orangeburg, New York. He is also the lead singer and songwriter in the band The Amygdaloids. His latest “extraordinary book,” as renowned neuroscientist Erik Kandel calls it, is “The Deep History of Ourselves: The Four-Billion-Year Story of How We Got Conscious Brains,” which sheds light on our universal instinct for survival.
Brain World: How did you get interested in the brain?
Joseph LeDoux: I was a business administration student at Louisiana State University, and was concerned with issues of consumer protection. That led me to consumer behavior, which took me to psychology. I ended up working in the laboratory of a researcher who was studying the brain and memory, and became fascinated with the brain. A light bulb went off in my head — that’s what I wanted to do. I then entered into a Ph.D. program at the State University of Stony Brook in 1974, and there I met Michael Gazzaniga, who researched the neural basis of the mind.
He and I went on to study a new group of patients that had their brains basically cut in half in an effort to control very serious epilepsy that couldn’t be controlled in any other way. One of the topics we got interested in was how the two hemispheres might interact. We looked into how the left hemisphere of the brain — which is the part of the brain you can have conversations with, because it’s where language resides — responds to behaviors that are produced by the right hemisphere.
So if we make a person laugh and ask “why are you laughing” (now we are talking to the left hemisphere), they’d say something like “well, you guys are really funny, etc.” What’s happening is that the left hemisphere is basically making up an explanation. Many observations like this led us to the idea that much of our behavior is produced nonconsciously, and what consciousness does is it generates a narrative of our behavior to make sense of it in the context in which it’s occurring. One of the things we proposed is that these narratives might be useful for explaining our emotional reactions to stimuli. That’s how I first got interested in the topic of emotion.
BW: What were your main findings?
JL: In the late ’70s there weren’t very good techniques to study the human brain. Once you’ve been studying split-brain patients, and you finish your degree, there aren’t a lot of other similar patients that you work with yourself. So I decided to turn to animal research to try and understand how basic primitive (subcortical) systems in the brain might be generating so-called emotional responses to threatening stimuli.
I studied rats, and I adopted a simple procedure called “fear conditioning,” in which you give the rat a tone paired with a shock. Upon hearing the tone, the rat freezes, blood pressure goes up, hormones are released etc. The same thing happens when a human encounters danger. So this seemed like a good way to study how the brain detects and responds to danger. I studied that for a long time. I showed that the brain area called the amygdala was an important part of the circuit that detects and responds to these kinds of danger stimuli in rats, and with my collaborator Elizabeth Phelps at NYU, we also implicated the human amygdala in these kinds of responses.
But lately I’ve been clarifying what I think the amygdala does. It is commonly through of as a “fear center.” But I think this gives the wrong impression. It implies that the amygdala gives rise to the conscious experience of fear. But I think the amygdala simply detects and responds to danger. The conscious experience of fear is produced by the cortex when you come to understand that it’s you who is in danger. My motto, is “no self, no fear.” You have to be personally involved in order to experience fear or other emotions, and that requires more complex cortical circuits.
BW: Then you started looking at the evolution of this ability to detect and respond to fear. How far does it go?
JL: I was well aware of the work of researchers such as Eric Kandel who had been studying invertebrate organisms — like the sea snail or the aplysia — using conditioning methods similar to those I used in rats. Kandel found certain molecules and genes that were required for the conditioning to occur. We then tested the same molecules and found that many were also involved in the amygdala of rats. This suggested that the ability to detect and respond to learned dangers, and the molecules involved, go back at least to the common ancestor of the invertebrates and vertebrates. That ancestor lived about 600 million years ago.
But amazingly, some of the same behaviors and molecules also exist in even more primitive animals, like jelly fish or sponges, and even in single-cell protozoa. Protozoa living about 800 or 900 million years ago are the branch point from which animals evolved. Despite not having neurons and a nervous system (they are just one cell) they behave and learn about danger. And they possess some of the same molecules that animals use. In fact, all organisms, even bacterial, have to detect and respond to danger. As Ralph Waldo Emerson said, “As soon as there is life, there is danger.” So behavior does not require a nervous system — and has no fundamental connection to psychology. It arose as part of what cells needed to do to stay alive. Behavior goes back to the beginning of life, almost four billion years ago.
BW: How do we explain our human experience of fear?
JL: To reiterate, the conclusion of all this evolutionary research is that detecting danger has nothing to do with psychology. It’s not about fear. It’s about survival. It’s how an organism survives in its environment.
But detecting danger is not all that early cells had to do. They also had to find and incorporate nutrients, balance fluids and ions, and reproduce. When us humans do these things we talk about fear, hunger, thirst, pleasure, etc. But none of these psychological states underlie why bacteria respond to danger. They don’t even underlie why we humans respond to danger.
The psychological states we experience happen in parallel to the more basic processes that generate survival reactions. For example, the amygdala, which is involved in detecting and responding to danger in mammals and other vertebrates, can be activated subliminally in people. So if I show you a stimulus signifying something dangerous (such as an image of a snake) very briefly, for just a few milliseconds, your conscious mind doesn’t know the stimulus is there.
The fast flash prevents you from consciously knowing that it’s there. But the amygdala is still activated. Your heartbeat begins to rise, your palms sweat. And yet you don’t feel fear. We so often associate these responses with fear. But fear itself is not what causes them. The brain mechanisms that control the responses are triggered in parallel with the mechanisms that underlie our conscious experience of fear.
