Effect of the Stimulation Rate on the Speech Comprehension in Patients with Cochlear Implants using the CIS Strategy

1 Aronson Leonor DSc.
2 3 Pallares Norma MA

1 Departamento de Implante Coclear - Fundación Arauz
2 Centro de Implante Coclear Prof. Dr. Diamante
3 Universidad del Salvador

Buenos Aires Argentina

Abstract
The speech perception of 10 post-lingual Spanish-speaking adult patients implanted with the Combi 40+ system and fitted with the CIS strategy was measured by decreasing the stimulation rate per channel (SRch) from 1515 to 200 pps/ch in steps of 100 pps/ch. A battery including consonant identification in /VCV/ context with V=/a/, monosyllabic words recognition, vowel transitions identification, everyday sentences comprehension and fluent speech was used. The aim of this work was to study the effect of the stimulation rate on the speech comprehension of the patients, while keeping the number of active electrodes constant. Due to the influence of the SRch on loudness, changes in loudness associated with changes in the SRch were also assessed.
Despite the variety of patients observed, a reduction of the SRch produces a decrease in the scores in all the tests conducted: one-way ANOVA for the /VCV/ test showed significant global differences between frequencies (F(11,78)=4.526, p<.001). However, in spite of this, high percentages of correct responses were obtained for low SRch values in all the tests administered. For all the patients, decreasing the SRch produced a decrease in loudness.
The results seem to indicate the need to look for the best SRch for each subject. Results also suggest that the maximum SRch provided by a system is not necessarily the best one for all the patients. Some individual features, such as survival neural tissue, years of deafness and etiology could influence the optimum value of SRch for each patient.

Patient Details

* patient fitted with 60µs/phase pulse width for whom tests were administered from 700 to 200 pps/ch .

Consonant identification as a function of the Stimulus Rate per channel (SRch) average values N = 10

Monosyllabic words recognition as a function of the SRch average values N = 10

Vowel transition identification as a function of the SRch average values N = 10

Every day sentences comprehension as a function of the SRch average values N = 10

Fluent speech as a function of the SRch average values N = 10

Loudness as a function of the SRch average values N = 10

Conclusions

The correct response percentages decrease when the SRch decreases, indicating the influence of this parameter on speech perception. This influence is related to the kind of tests administered.
The analysis of results would be indicating that in a test of a higher level of difficulty (consonant identification, monosyllabic word recognition, vowel transitions identification), the SRch plays a more important role than in a test of a lower level of difficulty, such as recognition of everyday sentences and fluent speech tests in which the semantic context compensates for the decrease in temporary clues provided by the system.
Individual data showed that the scores decrease monotonically together with decreases in the stimulation rate. There were significant subjects-by-rate interactions perhaps due to intersubjects’ etiologies, nerve survival differences and individual cognitive conditions. Differences among scores for different stimulation rates may have been masked by ceiling effects, specially in low difficulty tests.
It is remarkable that high levels of performance were obtained using 200 pps/ch in all the tests administered.
It could be stated that most of the subjects preferred 1200-1300 pps/ch to the maximum 1515 pps/ch: scores for 1300 pps/ch are better than those for 1515 pps/ch , indicating that the optimum stimulation rate could not always be the maximum provided by the system.
As a conclusion, we can assert that the maximum SRch provided by a cochlear implant is not necessarily the most appropriate for all the patients. Some individual features such as survival neural tissue, years of deafness, etiology, absolute and relative refractory periods of the eighth nerve could influence the optimum value of the SRch for each patient.
Considering that at present cochlear implant systems are able to provide different stimulation programs, an adequate policy might be to offer the patient the opportunity of experiencing maps with different stimulation rates per channel and to manipulate the stimulation rates in order to choose those ones that provide the best speech perception.