Apple Inc. has purchased Emotient Inc., a startup that uses artificial-intelligence technology to read people’s emotions by analyzing facial expressions.

It isn’t clear what Apple plans to do with Emotient’s technology, which was primarily sold to advertisers to help assess viewer reactions to their ads. Doctors also have tested it to interpret signs of pain among patients unable to express themselves, and a retailer used it to monitor shoppers’ facial expressions in store aisles, the company had said.

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