The micromechanical properties of the cochlea accounting for the exquisite properties of sensitivity and frequency selectivity depend on the integrity of an active biomechanism probably based upon a motile activity of outer hair cells (OHCs). Evoked oto-acoustic emissions (EOAEs), i.e. sounds emitted by the cochlea in response to a click, reflect this active biomechanism. We demonstrate here that a selective attention task in human subjects alters EOAEs. This means that the central nervous system can modify active cochlear micromechanics prior to the transduction process, probably by using the medial efferent system which, coming from the brainstem, innervates the OHCs.