Hearing Impairment Hair cells are responsible for translating mechanical information into neural information. Thus, with damaged hair cells, the auditory system has no way of transforming acoustic pressure waves (sound) to neural impulses which in turn leads to hearing impairment. Hair cells damage might cause by diseases (e.g meningitis, Meniere’s). If large number of hair cells or auditory neurons throughout the cochlea are damaged, then the person with such a loss is diagnosed as profoundly deaf. There is research which shows that the most common cause of deafness is the loss of hair cells rather than the loss of auditory neurons. So, the remaining neurons could be used by exciting it with electrical stimulation. This make way to the wide use of cochlear implants.

Cochlear Implant Cochlear implants are beneficial for people with profound sensorineural hearing loss, who do not benefit from conventional hearing aids. Basic components of cochlear implant : Microphone Speech Processor Transmitter Receiver Electrode Array External components : microphone, speech processor, transmitter. Implanted components : receiver (in a drilled indentation on the temporal bone), electrode array (inserted in the cochlear)

Cochlear Implant

Cochlear Implant System

Cochlear Implant Cochlear Implant System

Cochlear Implant Principle operation of Cochlear Implant Sound is detected by the microphone and converted into an electrical signal The signal is sent to the speech processor Input signal is analysed and certain features are extracted and coded in the speech processor These codes containing the extracted speech information are sent to the transmitter The coded signal is sent via the transmitting coil, through the skin, to the receiver as an RF signal The custom IC in the receiver decodes the RF signal to determine the electrode number, stimulation level and stimulation rate. The signal is transformed into the appropriate electrical pulses. The pulses are sent to the electrode array in the cochlea, which stimulates the nerves inside the cochlea.

Cochlear Implant Cochlear Implant System

Cochlear Implant Cochlear Implant Characteristics Electrode design (e.g., number of electrodes, electrode configuration) Type of stimulation - analog or pulsatile Transmission link - transcutaneous or percutaneous Signal processing - waveform representation or feature extraction. Electrode Design Issues associated with electrode design are: electrode placement, number of electrodes and spacing of contacts, orientation of electrodes with respect to the excitable tissue and electrode configuration. Electrodes may be placed near the round window of the cochlea (extracochlear), or in the scala tympani (intracochlear) or on the surface of the cochlear nucleus.

Cochlear Implant Most commonly, placed in the scala tympani because it brings the electrodes in close proximity with auditory neurons which lie along the length of the cochlea. It is preferred because it preserves the "place" mechanism used in a normal cochlear for coding frequencies. Number of electrodes and spacing between the electrodes affects the place resolution for coding frequencies. Larger number of electrodes, the finer the place resolution for coding frequencies. Single-channel implant: Only a single pair of electrodes stimulating a fixed site in, or near, the cochlea. Only transmits temporal information, loudness cues and rate information. Multiple-channel implant: Uses a number (4-24) of electrodes that stimulate different regions of the cochlea. Transmits place- pitchinformation as well as temporal information.

Cochlear Implant Single-Channel Implant

Cochlear Implant Multi-Channel Implant

Cochlear Implant Type of Stimulation Two types : analog and pulsatile. In analog stimulation, an electrical analog of the acoustic waveform itself is presented to the electrode. In multi-channel implants, the acoustic waveform is bandpass filtered, and the filtered waveforms are presented to all electrodes simultaneously in analog form. Nervous system will sort out and/or make use of all the information contained in the raw acoustic waveforms. Disadvantage of analog stimulation is the simultaneous stimulation may cause channel interactions. In pulsatile stimulation, the information is delivered to the electrodes using a set of narrow pulses. Advantage is that the delivered in a non-overlapping (i.e., non- simultaneous) fashion, thereby minimizing channel interactions.

Cochlear Implant Cochlear Implant System

Cochlear Implant Modes of stimulation : bipolar, common-ground, monopolar. Bipolar: All electrodes inside the cochlea; Stimulation takes place between two electrodes. Common-ground: All electrodes inside the cochlea; Stimulation takes place between one electrode and all other electrodes. Monopolar: One extra-cochlear electrode; Stimulation between an intracochlear and the extra-cochlear electrodes.

Cochlear Implant Transmission Link Two ways of transmitting the signals: through a transcutaneous connection and through a percutaneous connection. Transcutaneous system transmits the stimuli through a radio frequency link. External transmitter encode the stimulus information for radio- frequency transmission from an external coil to an implanted coil. Internal receiver decodes the signal and delivers the stimuli to the electrodes. Transmitter and the implanted receiver are held in place on the scalp by a magnet. Disadvantage: - Implanted electronics may fail, and require a surgery replacement. - Transcutaneous connector contains magnetic materials which are incompatible with MRI scanners.

Cochlear Implant Cochlear Implant System

Cochlear Implant Percutaneous system transmits the stimuli to the electrodes directly through plug connections. No implanted electronics except the electrodes. Advantages of percutaneous are flexibility and signal transparency.