The newest machines use an array of sound and graphic effects, latching onto pop culture themes meant to draw both older and younger customers.

As at Maryland Live, Horseshoe Casino visitors wandering among the slots are apt to bump into characters from "The Wizard of Oz," "Willy Wonka & the Chocolate Factory," "Sex and the City," "Grease," and an array of superheroes zipping about on the Super Team game made by WMS Gaming that runs on a new, more powerful digital processor.

While conventional slot machines have one screen that shows the game symbols rolling by, machines featuring movie and TV characters usually include a second screen above that, where characters and video clips appear.

The Willy Wonka machine, made by WMS, features a chair that vibrates and moves with the action of the game, part of what the company calls Sensory Immersion 2.0. The gambler starts the game by using a touch screen to select "chair intensity," low medium or high. The company's Aladdin machine features a chair that follows the movement of a magic carpet ride.

Matt Wilson, vice president of marketing for game maker Aristocrat Americas, said the Horseshoe will have the Australian company's new machines based on two TV shows, one vintage, one current.

Batman 1966, based on the TV series that ran in the late 1960s, is pitched to the person he considers the typical slots player: a baby boomer, and likely a woman. The Walking Dead machine, based on a show in its fifth season on AMC, is meant to appeal to a younger crowd.

The Batman and Walking Dead machines represent two broad categories of slots. Batman is the sort of machine sometimes called a "dribbler," meaning it pays out more often, but in small amounts, and is designed for gamblers who want more playing time and entertainment value for their money. The Walking Dead, Wilson said, is more "volatile," paying less frequently and wired for gamblers seeking the big payoff.

Wilson and Tom Shortall, regional vice president for IGT, which makes one-third of the machines found in Maryland casinos, said their companies don't see a market shift in one direction or another between the entertainment machines and the bigger-payout machines.

Bradley Rose, vice president of game development for WMS, said his company has in the past few years moved toward the "volatile" side and now builds 70 percent of its machines that way.

"We do tons and tons of player research; we are constantly talking to players," said Rose, whose Chicago-based company is one of five whose machines are most prevalent in Maryland. He said his company's research shows that "the majority of people who come to a casino, they want to win money ... they want the chance to win big jackpots."

Rose said gamblers also get a thrill when the machine gives the appearance that they have just missed a hit — one winning symbol missing in a row, say, or a wheel that moves just past the sweet spot. Some of those methods have been outlawed as deceptive, but others are legal and they take up lots of designers' time.

"All of that is where we spend the most time," Rose said. "If they have that rush, it's almost as good as a win."

Nigel Turner, a psychologist who has studied and written extensively on slot machines as part of his work with the Centre for Addiction and Mental Health in Toronto, said slot-machine designers have two very powerful elements working in their favor: fundamental psychology concepts and weak math skills among the general public.

The basic psychology says that behavior is learned best when rewarded intermittently: Sometimes you get the reward and sometimes not, so you keep trying. The concept is exemplified by laboratory rat experiments by behavioral psychologist B.F. Skinner, who wrote about the slot machine in the 1950s as a powerful example of the same idea.

The effect is enhanced in the contemporary slot machine. While yesterday's slots player hoped to get three cherries to stop evenly on one line, today's gambler can choose machines with a dizzying array of "pay lines": straight, diagonal and zigzagging across the screen.

"The primary mechanism for holding the gambler's attention comes in the prize structure," said Turner, citing the small rewards along the way such a free spins and bonus credits that keep the player spending.

A recent study of one IGT game, Money Storm, published in January by three scholars in the Journal of Gambling Studies, found that after 50 hours of play and betting on 20 lines, only three of 10,000 players were ahead, and by less than 25 percent. The odds of winning the top prize were 1 in 215,702.

"The chances of getting that symbol all across five [columns] is very, very small ... but people like fooling themselves," Turner said. "Part of the enjoyment of the game is fooling yourself."