Practical model description of peripheral neural excitation in cochlear implant recipients: 3. ECAP during bursts and loudness as function of burst duration

In this, the third paper of the series, the loudness of low-rate bursts of electrical pulses was measured as a function of the burst duration, in subjects implanted with the Nucleus ® 24 cochlear implant system (three with straight and two with Contour™ electrode arrays). In order to help distinguish between the contributions of peripheral and more central effects, the ECAP was recorded to the individual pulses comprising the bursts, using the Neural Response Telemetry™ (NRT™) system. At a pulse rate of 250 pulses/s, the ECAP amplitude did not decrease greatly during the bursts: the mean reduction factor was 0.89. The time-constant for summation of the loudness contributions from the pulses comprising a burst was found to be larger than that associated with normal hearing. In addition, the first pulse of a pulse train was found to contribute much more to the overall loudness than did the subsequent pulses, although a corresponding difference was not observed in the ECAP recordings. These results establish a necessary connection between the essentially single-pulse model, developed in the fourth and fifth papers of the series, and the psychophysical data for pulse bursts, but they also have broader implications.