"Ekman now admits it was a mistake to define micro-expressions by duration, and describes them now as any facial expression that is very difficult to detect in

"Ekman now admits it was a mistake to define micro-expressions by duration, and describes them now as any facial expression that is very difficult to detect in real time." I can see why he regrets having made it--it's the only concrete, measurable claim he made (that's mentioned in the article).