Advertising firm, WPP, announced it had invested US$3m for a minority stake in Affectiva, a technology firm that specialises in measuring the emotional responses of consumers. WPP said its latest investment had been made through its wholly-owned information consultancy group, Kantar.The company said that Kantar and Affectiva would be able to take advantage of their combination of survey-based and technology enabled observational data."This investment continues WPP's strategy of investing in fast-growing markets and sectors and further strengthening its capabilities in measurable marketing disciplines," WPP said.WPP is interested in two pieces of Affectiva's technology, the first being 'Q Sensor', a wearable biometric device used to measure emotional states such as excitement, attention, anxiety and relaxation. The second is 'Affdex', a piece of software that monitors and codes viewers' facial expressions and interprets how they are responding to what they see. This is designed to give marketers faster, more accurate insight into the emotional impact of brands and media.WPP has been on a buying spree of late; it's last purchase was at the start of July when it bought a majority stake in Indian advertising agency, Mindset Advertising.WPP's shares were up almost 3% in late afternoon trading in London.