If the Russian psychologist Ivan Pavlov were alive today, what would he say about smartphones? He might not think of them as phones at all, but instead as remarkable tools for understanding how technology can manipulate our brains.

Pavlov's own findings — from experiments he did more than a century ago, involving food, buzzers and slobbering dogs — offer key insights into why our phones have become almost an extension of our bodies, modern researchers say. The findings also provide clues to how we can break our dependence.

Pavlov originally set off to study canine digestion. But one day, he noticed something peculiar while feeding his dogs. If he played a sound — like a metronome or buzzer — before mealtimes, eventually the sound started to have a special meaning for the animals. It meant food was coming! The dogs actually started drooling when they heard the sound, even if no food was around.

Hearing the buzzer had become pleasurable.

That's exactly what's happening with smartphones, says David Greenfield, a psychologist and assistant clinical professor of psychiatry at the University of Connecticut.

When we hear a ding or little ditty alerting us to a new text, email or Facebook post, cells in our brains likely release dopamine — one of the chemical transmitters in the brain's reward circuitry. That dopamine makes us feel pleasure, Greenfield says.

"That ping is telling us there is some type of reward there, waiting for us," Greenfield says.

Over time, that ping can become more powerful than the reward itself. Research on animals suggests dopamine levels in the brain can be twice as high when you anticipate the reward as when you actually receive it.

In other words, just hearing the notification can be more pleasurable than the text, email or tweet. "Smartphone notifications have turned us all into Pavlov's dogs," Greenfield says.

Signs you might need to cut back

The average adult checks their phone 50 to 300 times each day, Greenfield says. And smartphones use psychological tricks that encourage our continued high usage — some of the same tricks slot machines use to hook gamblers.

"For example, every time you look at your phone, you don't know what you're going to find — how relevant or desirable a message is going to be," Greenfield says. "So you keep checking it over and over again because every once in a while, there's something good there." (This is called a variable ratio schedule of reinforcement. Animal studies suggest it makes dopamine skyrocket in the brain's reward circuity and is possibly one reason people keep playing slot machines.)

A growing number of doctors and psychologists are concerned about our relationship with the phone. There's a debate about what to call the problem. Some say "disorder" or "problematic behavior." Others think over-reliance on a smartphone can become a behavioral addiction, like gambling.

"It's a spectrum disorder," says Dr. Anna Lembke, a psychiatrist at Stanford University, who studies addiction. "There are mild, moderate and extreme forms." And for many people, there's no problem at all.

In this way, the phone is kind of like alcohol, Lembke says. Moderate alcohol consumption can be beneficial, for some people.

"You can make an argument that a temperate amount of smartphone or screen use might be good for people," Lembke says. "So I'm not saying, 'Everybody get rid of their smartphones because they're completely addictive,' But instead, let's be very thoughtful about how we're how we're using these devices, because we can use them in pathological ways."

Signs you might be experiencing problematic use, Lembke says, include these: