High sensitivity and selectivity of hearing require an active cochlea. The 
cochlear sensory epithelium, the organ of Corti, vibrates because of external 
and internal excitations. The external stimulation is acoustic pressures 
mediated by the scala fluids, whereas the internal excitation is generated by a 
type of sensory receptor cells (the outer hair cells) in response to the 
acoustic vibrations. The outer hair cells are cellular actuators that are 
responsible for cochlear amplification. The organ of Corti is highly structured 
for transmitting vibrations originating from acoustic pressure and active outer 
hair cell force to the inner hair cells that synapse on afferent nerves. 
Understanding how the organ of Corti vibrates because of acoustic pressure and 
outer hair cell force is critical for explaining cochlear function. In this 
study, cochleae were freshly isolated from young gerbils. The organ of Corti in 
the excised cochlea was subjected to mechanical and electrical stimulation that 
are analogous to acoustic and cellular stimulation in the natural cochlea. Organ 
of Corti vibrations, including those of individual outer hair cells, were 
measured using optical coherence tomography. Respective vibration patterns due 
to mechanical and electrical stimulation were characterized. Interactions 
between the two vibration patterns were investigated by applying the two forms 
of stimulation simultaneously. Our results show that the interactions could be 
either constructive or destructive, which implies that the outer hair cells can 
either amplify or reduce vibrations in the organ of Corti. We discuss a 
potential consequence of the two interaction modes for cochlear frequency 
tuning.