A sound coding strategy based on a temporal masking model for cochlear implants

Auditory masking occurs when one sound is perceptually altered by the presence of another sound. Auditory masking in the frequency domain is known as simultaneous masking and in the time domain is known as temporal masking or non-simultaneous masking. This works presents a sound coding strategy that incorporates a temporal masking model to select the most relevant channels for stimulation in a cochlear implant (CI). A previous version of the strategy, termed psychoacoustic advanced combination encoder (PACE), only used a simultaneous masking model for the same purpose, for this reason the new strategy has been termed temporal-PACE (TPACE). We hypothesized that a sound coding strategy that focuses on stimulating the auditory nerve with pulses that are as masked as possible can improve speech intelligibility for CI users. The temporal masking model used within TPACE attenuates the simultaneous masking thresholds estimated by PACE over time. The attenuation is designed to fall exponentially with a strength determined by a single parameter, the temporal masking half-life T(½). This parameter gives the time interval at which the simultaneous masking threshold is halved. The study group consisted of 24 postlingually deaf subjects with a minimum of six months experience after CI activation. A crossover design was used to compare four variants of the new temporal masking strategy TPACE (T(½) ranging between 0.4 and 1.1 ms) with respect to the clinical MP3000 strategy, a commercial implementation of the PACE strategy, in two prospective, within-subject, repeated-measure experiments. The outcome measure was speech intelligibility in noise at 15 to 5 dB SNR. In two consecutive experiments, the TPACE with T(½) of 0.5 ms obtained a speech performance increase of 11% and 10% with respect to the MP3000 (T(½) = 0 ms), respectively. The improved speech test scores correlated with the clinical performance of the subjects: CI users with above-average outcome in their routine speech tests showed higher benefit with TPACE. It seems that the consideration of short-acting temporal masking can improve speech intelligibility in CI users. The half-live with the highest average speech perception benefit (0.5 ms) corresponds to time scales that are typical for neuronal refractory behavior.