No bears in this house

Laurence Gonzales in Deep Survival discusses adaptation. Page 63.

Only in recent years has neuroscience begun to understand the detailed physiology of emotional states such as fear. The neocortex is responsible for your IQ, your conscious decisions, your analytical abilities. But the amygdala stands as a sort of watchdog for the organism. Amelia, who is the younger of my two daughters, has a chocolate Lab, Lucy. Lucy sometimes reminds me of the amygdala: When anyone comes to the door, she barks before I even hear it.

Perceptions from the world around us (sight, for example) reach the the thalamus first. In the case of vision, axons from the retina go to the visual thalamus (there are two, one in each side of the brain, receiving information from each side of the body). From there, the sight signals travel by way of axons from the visual thalamus to the middle layer of the neocortex and from there are sent out to the other five layers for processing. What emerges is a perception of sight. But before all that can be completed, a rough form of the same sensory information reaches the amygdala by a faster pathway. The amygdala screens that information for signs of danger. Like Lucy, the amygdala isn't very bright, but it detects a hazard, or anything remotely resembling one, before you're even conscious of the stimulus, it initiates a series of emergency reactions. The approach is: Better safe than sorry. (Unlike Lucy, the amygdala also is capable of ignoring a a lot of information as irrelevant.) It is a primitive but effective survival system that causes the rabbit that visits our backyard every morning to freeze and then run when she sees Amelia let Lucy out. Like Lucy, the amygdala is wrong a lot of the time: There is no danger. But in the long course of evolution, it has been a successful strategy.

So information from the senses takes a neural route that splits, one part reaching the amygdala first, the other arriving at the neocortex milliseconds later. Rational (or conscious) thought always lags behind the emotional reaction. Anyone can demonstrate this at home: Everyone has been startled by someone. It's a powerful response, marked by the familiar rocket rush of adrenaline (actually catecholamines), increased heart rate, flushing and panting. Then, as soon as you realize the person is someone you know, the response deescalates. But it takes a while to metabolize all those chemicals. It's a powerful emergency reaction and completely illogical, because you know the person and are not in any danger. But the reason you can't think of that logically before reacting is because visual signals reach the amygdala first. It's a big shadowy form: It could be a spouse, it could be a bear - you don't know. Only later (in milliseconds) does the visual cortex piece together an accurate picture that let's you in on who it is. Only later can you reason: No bears in this house.