– That lottery fever that swept the country in anticipation of Saturday night's record $900 million Powerball drawing? Scientists have a name for it: dopamine.

It's the brain chemical associated with reward, pleasure and addiction. And it was digging into pockets at a maddening pace last week.

Thanks a lot, dopamine. Whether the jackpot is $8 million or $900 million, the odds of winning are stuck at a buzz-killing 1 in 292 million.

"I don't want to miss a big chance," said Javier Berec as he paid $10 for five tickets at Hana's Bottle Shop Liquors in Santa Clara, billed as a lucky retailer because it sold two $1 million tickets in 2011.

With $900 million up for grabs, it doesn't take a neuroscientist like Howard Fields to explain how people may have ignored probability this week because of the way the brain processes risk and reward.

"In the brain stem of a gambler, dopamine neurons are firing very high, pushing them to put out the money, to go and buy the ticket," said Fields, a neurology professor at the University of California, San Francisco. During pleasurable experiences, dopamine floods the brain and urges humans to repeat the behavior.

Overestimating rewards

But, Fields added, the brain tends to overestimate the possibility of reward.

If you program a computer to make the same calculations, it "would never do what a person does," Fields said. "It would say, OK, I'm not going to buy a ticket until I have at least a good chance of winning."

And, of course, like any lab rat, we are more likely to take that chance as the reward — in this case, a $900 million piece of cheese — gets bigger.

"Some people will just religiously play scratchers because it's more instant gratification," said Alex Traverso, spokesperson for the California lottery. But last week's Powerball lured a horde of new customers. "When you get to $900 million, you assume everyone's playing."

Tony DeMola is one of the uninitiated who stopped by Cork'n Bottle Liquor in Concord to buy $10 in tickets. Dopamine wasn't his motivation — his wife was: He decided to add a few tickets to her birthday gift.

At Hana's Bottle Shop Liquors, store manager Ravee Patel heard all the stories from a front-row seat at the dopamine parade as a steady stream of customers tried their luck. The shop issues about $500 in scratch lottery winnings a day, he said, but the main attraction was clear. "A guy just bought $200 in Powerball," Patel said.

What drives customers to dig so deep in their wallets while standing at the counter? Stanford University psychology professor Brian Knutson and his colleagues recently discovered a connection between an area of the brain related to excitement, the nucleus accumbens, and another area related to caution known as the anterior insula.

Motivated to gamble

The weaker the connection between the two regions, the more likely people were to take a gamble similar to buying a lottery ticket.

When customers are at the cash register, "there's something about that display that is getting them motivated and psyched up," Knutson said. "And when that happens, we think dopamine is released."

All that dopamine might tip the balance in favor of buying a ticket — or two — overruling the more cautious part of the brain. And once people decide to play, they often spend more than they anticipated.

"Give me $15," customer Marinko Balde said at the register. As Patel handed him the change, Balde hesitated. "Make it $20."