Subthreshold polarization:

The cochlear implant plays the role of the deficient hair cells, presents in the cochlea: pulses evoked by the electrodes are able to polarize or depolarize the neural membrane of auditory neurons. As a first approximation, the membrane can be modeled as a leaky integrator of charge. Once the trans-membrane potential has been depolarized beyond a certain threshold, an action potential is generated and propagates along the nerve. However, this threshold is not constant across the whole population of neurons and different neurons across the array receive different contributions depending on how far they are from the stimulating electrode. Therefore, in response to a single pulse,, some neurons will fire while others will remain in a subthreshold state. Because the membrane needs some time to return to its resting potential, a second pulse presented shortly after the first (as in CI stimulation) will affect theses “subthreshold-state” neuron differently depending on their degree of hyperpolarization.

Neural refractoriness and adaptation:

After neurons fire, they enter a period of absolute refractoriness of about 400 µs followed by a progressive recovery during which the probability of them firing to a subsequent pulse is reduced. Furthermore, in response to a high-rate stimulus, the amplitude to the whole response of the auditory nerve decreases. This “neural adaptation” consists of several decaying components which may have different time constants, ranging from a few milliseconds to several seconds. More recently, studies showed that adaptation could also arise from subthreshold electrical stimulation.