Hearing „Light“
 
 

Electrical stimulation of neuronal tissue is the adequate stimulation mode activating voltage-sensitive ionic channels in excitable membranes of neurons. Electrical stimulation has been successfully used for neuronal prosthetic devices since many years.


However, electrical stimulation is also difficult to focus. Electrical fields spread in a spherical shape in homogenous environments. To focus this field is possible, yet works only under certain preconditions (see here), and thus selective stimulation only partially feasible. Other means of neuronal stimulation are therefore under investigation in our lab.


The interaction of light and the cochlea has the main effect at the level of hair cells (Teudt et al., 2011): we could demonstrate that even at low energies used for cochlear stimulation, the laser evokes a pressure wave in humid air and water and this wave can activate the hair cells mechanically (Teudt et al., 2011). It is caused by stress-relaxation waves generated by absorption of pulsed lasers by water and hemoglobin (Schultz et al., 2012). A recent study described the physical mechanisms behind the response and demonstrated it both for stress and thermal confinement (Kallweit et al., 2016). Such an effect might be used for stimulation of a partially-hearing cochleae in combined, bimodal stimulation (Baumhoff et al., 2019). This may lead to a new generation of hearing aids.


In this project we collaborate with the Laser Center Hannover (Prof. W. Ertmer) and receive funding from the European Union (ACTION), German Science Foundation (Cluster of Excellence Hearing4all) and cochlear implant industry (MedEl Comp.).


















 

Laser „light“ activates the cochlea

Wavelength dependence of cochlear responses indicates that absorption of light by hemoglobin and water cause stress-relaxation waves in the fluids of the inner ear (Teudt et al., 2011, Schultz et al., 2012).

Top: Stimulation of the rodent  peripheral nerve by a pulsed laser light. Right: Distribution of response strengths (firing rates in the inferior colliculus, projected onto the cochlear location providing the main input to the given neurons) in response to pulsed laser light applied through the optical fiber (Baumhoff et al., 2018).

Cochlear compound action potentials elicited by a near infrared laser pulse of visible to near-infrared wavelengths (Schultz et al., 2012)

Below: Sound field generated by a 50 µs near-infrared laser pulse in humid air. The laser generates a sound along the laser beam at places where the lasers hits water molecules (Teudt et al., 2011).