THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

234 10. Physiology of Hearing and Psychoacousticsthe ganglion that receives signals, the dendrite, may atrophy and the cells maydie. However, and fortunately, even in the case of complete hearing loss, someganglion cells survive and remain connected to the appropriate frequency receptorsin the cochlear nucleus. If the electrical action from implant electrodes can causeaction potentials among the remaining cells, then hearing can be restored. Also,if multiple groups of neurons can be compelled to respond to low-, middle-, andhigh-frequency components of the cochlea, then perception of speech will berestored.The next question is: how many channels of portions of the frequency spectrumare necessary to encode speech? Dorman, Louizon, and Rainey determined throughthe use of bandpass filters that as few as four channels of simplified audio signalsto normal-hearing listeners were required to achieve a 90% comprehension rateof words spoken in simple sentences (Dorman Wilson, 2004). Eight channelsallowed these listeners to identify 90% comprehension of spoken isolated words.With background noise, more channels were needed to retain this performance,and the more channels used, the better the comprehension.It is obvious that hair-cell failures engender a roadblock between the peripheraland central auditory systems, resulting in deafness. Cochlear implants restores thelink, though bypassing the hair cells to stimulate the direction of the cell bodies inthe spiral ganglion. A cochlear implant consists of five components, only two ofwhich are inside the body. In Figure 10.8, an external microphone picks up soundsand directs them to a sound processor that is enclosed in a case behind the ear. Theprocessed signals are sent on to a high-bandwidth RF transmitter, which then relaysthe information through a few millimeters of skin to a receiver/stimulator that hasbeen surgically implanted in the temporal bone above the ear. The signals pass onto an array of electrodes inside the cochlea. Target cells on the spiral ganglion aresegregated from the electrodes by a bony partition.Continuous interleaved sampling, or CIS, is a strategy used to covert signalsinto a code for stimulating the auditory nerve. It begins by filtering a signal intofrequency bands (16 bands or more). For each band, the CIS algorithm converts theslow changes of the sound envelope into amplitude-modulated groups of biphasic(i.e., having both positive and negative values) pulses at the electrodes. The processorsenses the information from low-frequency channels to the electrodes in theapex and information for high-frequency channels to electrodes in the base of thecochlea. Thus, this setup sustains the logic of the frequency mapping in a normalcochlea.The efficacy of the cochlea, however, depends on a number of factors, amongthem are the number and location of the surviving cells in the ganglion, the spatialpattern of current flow from the electrodes, and the manner in which the neurons inthe brainstem and cortex can encode frequency. If the period of deafness is a longone and only a few cells survive in the spiral ganglion, the electrode stimulation isless likely to convey frequency-specific information to the cochlear nucleon andcortex. Then there is a possibility of surviving cells clustered at one location in theganglion at the cortex, which results in the lack of range of frequencies necessaryfor speech understanding.