Auditory Neural Prostheses
Luke & Tom
Ear
Ear
• External Ear - Sound waves focused by
Pinna into the ear canal. • Middle Ear - The vibrations from the ear drum are passed onto three small bones (ossicles). These amplify sounds and pass them into the inner ear. • Inner Ear - Contains shell like structure called the cochlea. The nerve fibres at the entrance to the cochlea typically respond to high frequencies of sound, while those at the end of the cochlea respond to low frequencies. • Electrical signals decoded by the brain
Causes of deafness
• Age - Loss of hairs that convert sound into nerve
impulses; loss of nerve fibers from hairs (could administer neural growth factors) • Otosclerosis (hereditary) - Bone surrounding the inner ear sometimes affects nerves • Ménière’s Disease - Distention of the inner ear membranes caused by an excess accumulation of the inner ear fluids
Causes of deafness
• Conductive deafness - Sound is prevented from
reaching the inner ear (ear drum is perforated by an infection or the ossicles become stiff). Sound is prevented from getting to the cochlea. • Sensori-neural - Problem with either the cochlea or the transmission of the signals to the brain once they have left the cochlea (can be caused by exposure to loud sounds). • With severe or profound sensori-neural deafness hearing aids do not help. Use cochlear implant.
Cochlear - Nucleus® 24
• Electrical stimulation of
nerves inside the inner ear. • Electrode array is inserted into the shell-like structure in the inner ear known as the cochlea. • Ball electrode is placed under a muscle near the ear. • Maximum stimulation rate of 14400 pulses per second.
Nucleus® 24 - Electrode Array
• 22 half-banded platinum
electrodes spaced over 15 mm (0.5-0.8 mm tubing) • Independent electrodes • The electrodes deliver different signals that represent loudness and pitch of sound by stimulating the appropriate nerve fibres. • Bipolar, monopolar, common ground stimulation modes
Nucleus® 24 - External Bits
• The coil is held in position
against the skin by a magnet • microphone is worn behind the ear. The body-worn speech processor (filters out noise…) • The ear-level processor worn behind the ear. • Normally fitted two weeks after the operation. ear.
Post Operation/Side Effects
• Most successful for adults who have developed a
hearing loss (can recognise speech) • If person has been deaf for a long time, or has not learnt to speak, they may find it difficult to understand new signals -> Need an audiologist to assist them in recognising sounds around them and tune processor? • Occasional balance disorders after cochlear implantation (microscopic changes occur in the inner ear) • Ethical and cultural considerations?
Ongoing research aims
• • • • • • •
Improve electrode array and stimulation Better sound processing Noise reduction Stereo microphones Regeneration of auditory nerve Direct brainstem stimulation Microphone implant (sub-skin)
References
• • • • •
http://www.phm.gov.au/hsc/cochlear/ http://www.medoto.unimelb.edu.au/ http://www.hei.org/research/projects/ http://www.ninds.nih.gov/npp/ http://www.cochlear.com