Speech processing for multichannel cochlear implants: variations of the Spectral Maxima Sound Processor strategy

The Spectral Maxima Sound Processor (SMSP) incorporates a bank of 16 band-pass filters which are assigned to 16 intracochlear electrodes. In each stimulation period 6 electrodes are activated, based on the outputs of the filters with the largest amplitudes. The SMSP has previously been compared with the present MSP(MULTIPEAK) processor and found to improve speech comprehension results. The SMSP speech processing scheme has recently been implemented successfully in a new speech processor, also developed at the University of Melbourne, which utilises digital signal processing techniques. The programming flexibility of this processor has facilitated the investigation of variations of the SMSP strategy which might provide further enhancement of speech perception. Three variations have been investigated: firstly, increasing the constant pulse rate from the usual 250 Hz to 400 Hz; secondly, changing the number of electrodes selected in each stimulation period from 6 to numbers between 4 and 8; thirdly, sharpening the spectral peaks prior to selection of the active electrodes. The results of these studies showed that all three variations had minimal effect on speech perception in quiet, but that increasing the number of electrodes selected for stimulation to 8, or increasing the rate of stimulation, was advantageous for some subjects when listening in background noise.