For the first time we perform an experiment to test the possibility of Bell's inequality violation in mental states, during perception-cognition in humans. We study under a theoretical and experimental framework, a Bell-type test for human perception-cognition of ambiguous figures. It is performed a detailed analysis which demonstrates that, although we have not yet been able to violate Bell's inequality in the present performed experiment, there are strong theoretical arguments supporting our expectation to violate it by a simple articulation of the same experiment. In this framework we introduce for the first time what we retain to represent quantum cognitive observables. We consider that our analysis provides a solid ground for further investigations on quantum behaviour of cognitive systems. Therefore we reaffirm that quantum mechanics is a Giano Bifronte theory (two-faced Giano, a mythological God of the past), looking from one hand to physical reality and from the other hand to the sphere of mental reality and cognitive dynamics.