The world's biggest companies have got a new way of convincing you to buy their products – by getting inside your head. Brands including Google, Facebook and ITV are turning to mind-reading technology to help them develop products and create adverts that people like.

Traditionally, focus groups have been used to tell marketeers what they think of adverts. Unfortunately for advertisers, some people don't tell the truth.

Faced with the prospect of consumers hiding their emotions – perhaps a middle-aged man reluctant to reveal that he shed a tear at a sentimental John Lewis Christmas advert – a new breed of "neuromarketer" has emerged, armed with medical technology to probe consumers' brains for genuine responses.

"We put a cap on your head that measures your brain impulses," said AK Pradeep, a pioneer of neuromarketing science and chief executive of NeuroFocus, one of the biggest players in a booming industry. "We measure all parts of your brain continuously. Second by second, we measure how much attention you're paying. We get [to learn] what emotions you're experiencing and what memories you're memorising."

Pradeep says watching people's brains via caps covered in electrodes or magnetic scanners that are normally used by hospitals to detect cancer is better than direct questioning because, "when you ask people to tell you how they feel, the very act of thinking about a feeling changes the feeling".

NeuroFocus grades adverts against the emotions the advertisers wished to evoke on a scale of 10 (perfect; don't change a thing) to zero (requires major surgery). Low-scoring ads are sent back with suggestions of changes to make them more appealing.

A spokesman for NeuroFocus, which was bought last year by the $5bn global measurement and analytics firm Nielsen, said the company has worked with Google, Microsoft, Intel, Facebook, PayPal, Hewlett-Packard and Citigroup, but refused to provide details of adverts or products involved. "It's not just one company and one advertiser; it is all sorts of companies and brands around the world," Pradeep said. "They come to us and say, 'Please run our campaign through the neural lab.'"

Gemma Calvert, a former Oxford University neurologist who founded rival company Neurosense, said neuromarketing has "completely changed our understanding of the brain" and is now so advanced that she is "able to predict how customers will behave".

She said Neurosense, which monitors blood flow to detect activity levels in different parts of the brain, worked with GMTV to provide evidence that consumers were more receptive to advertising early in the morning. The company's website also lists clients including McDonald's, Unilever, Procter & Gamble, and GlaxoSmithKline.

"Neuroscience has completely changed our understanding of the brain. This information is not a flash-in-the-pan," Calvert said. "We are trying to find out what aspects of the images [in adverts] are having effect on the reward system – and making them [the brand] more likeable." For example, she said, if research showed a chocolate bar's crunchiness made it appealing, she would advise manufacturers to make it more crunchy.

Calvert said one restaurant asked her to determine the best aroma to tempt customers. "One client was interested in pumping a fragrance into a restaurant to change the way people perceived the food they were going to eatthere. Certain fragrances encourage different eating habits; we were trying to determine which are the most effective."

She said the research has led to brands changing their logos, packaging and even theme tunes: "We are changing the way brands understand themselves so they can better understand their audiences." The techniques are also used in the development of new products: "There are lots of products that have been developed with knowledge about the brain and psychology that's been derived from this stuff," she said.

Duncan Stewart, a director of technology research at auditing firm Deloitte, estimates that "more than 10% of prime time TV adverts" have been developed using neuromarketing techniques. He said that neuromarketing was first used in the creation of adverts in 2006-7, before becoming more mainstream in 2009-10.

"The industry is growing well in excess of 100%," he said. "It is used in TV, film, advertising and social media. People have been creating iconic ads for decades and have always wanted to know what people really think of them. Now they can."

• This article was amended on 15-16 January 2012. A line saying "Neilsen bought NeuroFocus for $5bn" should have read "NeuroFocus, which was bought last year by the $5bn global measurement and analytics firm Nielsen".