Speech perception as a function of electrical stimulation rate: Using the nucleus 24 cochlear implant system

Objective: To investigate the effect of varying electrical stimulation rate on speech comprehension by cochlear implant users, while keeping the numbe r of stimulated channels constant. Design: Three average rates of electrical stimulation, 250, 807, and 1615 p ulses per second per channel (pps/ch), were compared using a speech process ing strategy that employed an electrode selection technique similar to that used in the Spectral Maxima Sound Processor strategy (McDermott, McKay, & Vandali, 1992; McDermott & Vandali, Reference Note 1; McKay, McDermott, Van dali, & Clark, 1991) and the Spectral Peak strategy (Skinner et al., 1994; Whitford et al., 1995). Speech perception tests with five users of the Nucl eus 24 cochlear implant system were conducted over a 21-wk period. Subjects were given take-home experience with each rate condition. A repeated ABC e valuation protocol with alternating order was employed so as to account for learning effects and to minimize order effects. Perception of open-set mon osyllabic words in quiet: and open-set sentences at signal to noise ratios ranging from +20 to 0 dB, depending on the subject's ability, were tested, A comparative performance questionnaire was also administered. Results: No statistical differences in group performance between the 250 an d 807 pps/ch rates were observed in any of the speech perception tests. How ever, significantly poorer group performance was observed for the 1615 pps/ ch rate for some tests due predominantly to the results of one subject. Ana lysis of individual scores showed considerable variation across subjects. F or some subjects, one or more of the three rate conditions evaluated provid ed benefits on some speech perception tasks. The results of the comparative performance questionnaire indicated a preference for the 250 and 807 pps/c h rates over the 1615 pps/ch rate for most listening situations. Conclusions: For the speech processing strategy, implant system, and subjec ts evaluated in this study, the group results indicated that the use of ele ctrical stimulation rates higher than 250 pps/ch (up do 1615 pps/ch) genera lly provided no significant improvement to speech comprehension. However, i ndividual results indicated that perceptual benefits could be obtained by a djusting rate of stimulation optimally to suit each subject. Results from o ne subject, together with tinnitus problems arising from high-rate stimulat ion for another subject, indicated that high rates of stimulation may in fa ct be undesirable for some subjects.