Emotions are messy, complicated phenomena—not just for lovers, but for neuroscientists as well, because they combine cognition with physiology. Scientists once thought of emotion as a purely mental activity which elicited bodily responses, but they now see the mind and body as equally responsible for creating the experiences of fear, joy, and anger. Despite this complexity, science is beginning to understand emotion by examining one emotional pathway at a time, with the hope of some day combining them into a comprehensive understanding.

Fear is the most researched and best understood of all emotions. It's the easiest emotion to study because it has the most measurable physiological response, but it's also the most important emotion we have from an evolutionary standpoint. Learning to fear something dangerous that caused pain in the past, like a snake or a spider, most likely helped our ancestors to survive. And proving its evolutionary importance, the fear circuit in our brains is actually a shortcut that allows dangerous stimuli to bypass parts of the brain normally involved in sensory processing, as NYU neuroscientist Joseph LeDoux explains in the video below.

During normal perception, our sense organs transmit stimuli to part of the brain called the thalamus en route to the sensory cortex, where perceptions and thoughts are generated. To have a visual perception, for instance, "information has to be transmitted from the eye, from the retina, through the optic nerve, into the visual thalamus, and from the visual thalamus to the visual cortex, where the processing continues and you can ultimately have the perception," says LeDoux. "The visual cortex connects directly with the amygdala, and so that one route by which the information can get in: retina, thalamus, cortex, amygdala." 

But LeDoux, one of the foremost researchers of emotion and fear in the brain, discovered early in his research that fear, despite being reliant on sensory stimuli to trigger the amygdala, was not processed via the normal sensory pathways, or at least not entirely. "What we found was that if the cortical pathway was blocked completely. Rats could still form a memory about a sound." Instead of traveling from ear to thalamus to cortex to amygdala, the loud sound could bypass the cortex altogether, going directly from thalamus to amygdala. "This was really important because we generally think that the cortex is required for any kind of conscious experience," LeDoux says. In other words, a frightening stimulus can trigger emotions and fears unconsciously—without our even realizing it. 

To illustrate this point, LeDoux proposes the following scenario: "Let’s say we were having lunch one day and there’s a red-and-white checkered table cloth, and we have this argument. And the next day I see somebody coming down the street and I say, I have this gut feeling about this guy, he’s an SOB and I don’t like him. And maybe what’s going on there is that he’s got a red-and-white checkered necktie on. Consciously, I’m saying it’s my gut feeling because I don’t like the way he looks, but what’s happened is that the necktie has triggered the activation of the amygdala through the thalamus, the so-called low road, triggered a fear response in me, which I now consciously interpret as this gut feeling about not liking the guy. But in fact, it’s being triggered by external stimuli that I’m not processing consciously." 

And this unconscious processing of frightening stimuli happens much more quickly than our conscious processing of it. To become consciously aware of a particular stimulus takes 250-300 milliseconds, says LeDoux. But a fear-evoking stimulus can reach the amygdala in a mere 12 milliseconds, which is evolutionarily advantageous if one's response time means the difference between life and death.