In the movies and in detective shows on television, people fire guns all the time. They shoot and shoot and run after people, and sometimes they kill them, and when they do, they stand over the body and smoke a cigarette and then go and have a beer with their partner. To hear Hollywood tell it, shooting a gun is a fairly common and straightforward act. The truth is, though, that it isn’t. Most police officers—well over 90 percent—go their whole career without ever firing at anyone, and those who do describe the experience as so unimaginably stressful that it seems reasonable to ask if firing a gun could be the kind of experience that could cause temporary autism.

Here, for example, are excerpts of interviews that the University of Missouri criminologist David Klinger did with police officers for his fascinating book Into the Kill Zone. The first is with an officer who fired on a man who was threatening to kill his partner, Dan:

He looked up, saw me, and said, “Oh, shit.” Not like “Oh, shit, I’m scared.” But like “Oh, shit, now here’s somebody else I gotta kill”—real aggressive and mean. Instead of continuing to push the gun at Dan’s head, he started to try to bring it around on me. This all happened real fast—in milliseconds—and at the same time, I was bringing my gun up. Dan was still fighting with him, and the only thought that came through my mind was “Oh, dear God, don’t let me hit Dan.” I fired five rounds. My vision changed as soon as I started to shoot. It went from seeing the whole picture to just the suspect’s head. Everything else just disappeared. I didn’t see Dan anymore, didn’t see anything else. All I could see was the suspect’s head.

I saw four of my five rounds hit. The first one hit him on his left eyebrow. It opened up a hole and the guy’s head snapped back and he said, “Ooh,” like, “Ooh, you got me.” He still continued to turn the gun toward me, and I fired my second round. I saw a red dot right below the base of his left eye, and his head kind of turned sideways. I fired another round. It hit on the outside of his left eye, and his eye exploded, just ruptured and came out. My fourth round hit just in front of his left ear. The third round had moved his head even further sideways to me, and when the fourth round hit, I saw a red dot open on the side of his head, then close up. I didn’t see where my last round went. Then I heard the guy fall backwards and hit the ground.

Here’s another:

When he started toward us, it was almost like it was in slow motion and everything went into a tight focus. . . . When he made his move, my whole body just tensed up. I don’t remember having any feeling from my chest down. Everything was focused forward to watch and react to my target. Talk about an adrenaline rush! Everything tightened up, and all my senses were directed forward at the man running at us with a gun. My vision was focused on his torso and the gun. I couldn’t tell you what his left hand was doing. I have no idea. I was watching the gun. The gun was coming down in front of his chest area, and that’s when I did my first shots.

I didn’t hear a thing, not one thing. Alan had fired one round when I shot my first pair, but I didn’t hear him shoot. He shot two more rounds when I fired the second time, but I didn’t hear any of those rounds, either. We stopped shooting when he hit the floor and slid into me. Then I was on my feet standing over the guy. I don’t even remember pushing myself up. All I know is the next thing I knew I was standing on two feet looking down at the guy. I don’t know how I got there, whether I pushed up with my hands, or whether I pulled my knees up underneath. I don’t know, but once I was up, I was hearing things again because I could hear brass still clinking on the tile floor. Time had also returned to normal by then, because it had slowed down during the shooting. That started as soon as he started toward us. Even though I knew he was running at us, it looked like he was moving in slow motion. Damnedest thing I ever saw.

I think you’ll agree that these are profoundly strange stories. In the first instance, the officer appears to be describing something that is quite impossible. How can someone watch his bullets hit someone? Just as strange is the second man’s claim not to have heard the sound of his gun going off. How can that be? Yet, in interviews with police officers who have been involved with shootings, these same details appear again and again: extreme visual clarity, tunnel vision, diminished sound, and the sense that time is slowing down. This is how the human body reacts to extreme stress, and it makes sense. Our mind, faced with a life-threatening situation, drastically limits the range and amount of information that we have to deal with. Sound and memory and broader social understanding are sacrificed in favor of heightened awareness of the threat directly in front of us. In a critical sense, the police officers whom Klinger describes performed better because their senses narrowed: that narrowing allowed them to focus on the threat in front of them.

But what happens when this stress response is taken to an extreme? Dave Grossman, a former army lieutenant colonel and the author of On Killing, argues that the optimal state of “arousal”—the range in which stress improves performance—is when our heart rate is between 115 and 145 beats per minute. Grossman says that when he measured the heart rate of champion marksman Ron Avery, Avery’s pulse was at the top of that range when he was performing in the field. The basketball superstar Larry Bird used to say that at critical moments in the game, the court would go quiet and the players would seem to be moving in slow motion. He clearly played basketball in that same optimal range of arousal in which Ron Avery performed. But very few basketball players see the court as clearly as Larry Bird did, and that’s because very few people play in that optimal range. Most of us, under pressure, get too aroused, and past a certain point, our bodies begin shutting down so many sources of information that we start to become useless.

“After 145,” Grossman says, “bad things begin to happen. Complex motor skills start to break down. Doing something with one hand and not the other becomes very difficult. . . . At 175, we begin to see an absolute breakdown of cognitive processing. . . . The forebrain shuts down, and the mid-brain—the part of your brain that is the same as your dog’s (all mammals have that part of the brain)—reaches up and hijacks the forebrain. Have you ever tried to have a discussion with an angry or frightened human being? You can’t do it. . . . You might as well try to argue with your dog.” Vision becomes even more restricted. Behavior becomes inappropriately aggressive. In an extraordinary number of cases, people who are being fired upon void their bowels because at the heightened level of threat represented by a heart rate of 175 and above, the body considers that kind of physiological control a nonessential activity. Blood is withdrawn from our outer muscle layer and concentrated in core muscle mass. The evolutionary point of that is to make the muscles as hard as possible—to turn them into a kind of armor and limit bleeding in the event of injury. But that leaves us clumsy and helpless. Grossman says that everyone should practice dialing 911 for this very reason, because he has heard of too many situations where, in an emergency, people pick up the phone and cannot perform this most basic of functions. With their heart rate soaring and their motor coordination deteriorating, they dial 411 and not 911 because that’s the only number they remember, or they forget to press “send” on their cell phone, or they simply cannot pick out the individual numbers at all. “You must rehearse it,” Grossman says, “because only if you have rehearsed it will it be there.”