Standing in front of a mock-up shelf of crackers, Erik McMillan pointed a smartphone at a white, square contraption — a beacon — attached to the shelf. "Hold your phone here for savings," the sign above it said.

A "cha-ching" sound chirped from the phone, signaling that the consumer could see a range of content about those crackers that could include everything from a video about the product, customer reviews and perhaps most important, a coupon.

"Literally, you just walk around [the store] with your app and use it like a mouse — click, click," said McMillan, the exuberant CEO of the Austin, Texas-based start-up Shelfbucks.

For years, marketers have tried to tap into the human brain to better understand how colors, words and images might make consumers more inclined to buy a particular product.

Now they are looking for ways that technology can aid their efforts. They are experimenting with beacons that can send promotions to customers' smartphones when they're walking around a store. And they are tinkering with technology that can detect facial expressions to see whether a brand's message pleases or confuses someone.

Retailers and brands got a preview of such futuristic sales techniques this week at the Minneapolis Convention Center, where more than 100 exhibitors showed off the latest innovations in store displays. Items on exhibit ranged from traditional corrugated box-type displays to souped-up high-tech kiosks.

Some 4,000 people were in town for the event, the Shopper Marketing Conference & Expo, where vendors tried to convince retailers and brands that new technologies will boost sales.

In a nearby aisle from McMillan, Michael Garel explained to passersby how his start-up, EyeQ, uses sensors in its "intelligent end caps" to detect when a customer stands in front of them. The displays at the end of aisles can also determine gender and provide content on the screen accordingly, such as showing a bike made for women to a female customer.

One of Garel's kiosks is already in 10 Best Buy stores, including at the Mall of America. It's an HP display promoting some new notebook-tablet hybrids. When a customer steps in front of the display, the content on the touch screen changes from general promotional images to detailed product pages that include customer reviews and product specs.

The information helps customers make better buying decisions, Garel said. But it also can help HP better understand and improve its messages based on which ones seem to resonate with shoppers. The sensors can track how long people stand in front of the display and can detect facial expressions such as happy, angry, surprised or sad.

"You might have a naturally angry face," he noted, so the key is detecting the change in emotion as a way to gauge how the customer feels.

An aisle over, the sales staff in the Hypersound booth showcased targeted audio with a display for the new "Call of Duty" video game, set to be released next month. From about 10 feet away, an observer couldn't hear anything except the buzz of the expo floor. But from about 2 feet away, the sounds from the video on the screen were clearly audible.

The kiosk is being used by the game's publisher, Activision, and is being rolled out to more than 900 Best Buy stores.

"Best Buy is very strict with audio on its show floor," said Lyndsay Swann, Hypersound's marketing director. "They don't want anything to be super-intrusive."

This way Activision can add content for customers to encourage them to buy while also keeping Best Buy happy.

As for Shelfbucks, the beacon technology, it has been rolled out in about 36 GameStop locations in central Texas and should go live once GameStop updates its app, which McMillan said should be in the next week or so.

Of course, one of the biggest challenges is that people are likely to be annoyed by unsolicited messages that pop up on their smartphone, especially for items they have no interest in buying.

That's why Shelfbucks is selling itself as a "pull" instead of "push" technology. Customers have to ask for the content by swiping their phones.

Another issue is that not only does the retailer need to have Shelfbucks embedded into its app, but the customer has to have the app downloaded on his or her phone. And he or she has to opt into location services on the app, which can be a hurdle if someone is reluctant to be tracked.

But McMillan said younger people and active digital users are more open to opting in if it might give them more content or a coupon.

He added that he is talking to all of the top retailers and vowed that beacons won't fizzle out, the way scanning QR codes in stores did a few years ago.

"The problem with retailers is they move so slow," McMillan said. "If they're looking at it right now, it will start trickling in next year. And then in 2016, it's going to go"— he made an explosion noise. "It's going to blow up."