The use of electric-acoustic stimulation (EAS) is an established treatment in patients with partial deafness and residual hearing in the low frequencies. Studies have demonstrated better speech perception in quiet and in noise and better sound quality compared to cochlear implant (CI) users with electrical stimulation only. The present study aimed to compare speech perception and sound localization abilities in EAS subjects with groups of normal hearing, bimodal hearing or electric stimulation (CI) only.

Freiburg monosyllabic scores in quiet were compared between ears with CI stimulation (n=1666) and ears with EAS stimulation (n=109). Speech reception thresholds (SRTs) in continuous noise using German Oldenburg matrix test (OLSA) were compared between ears with CI stimulation (n=880) and ears with EAS stimulation (n=85), also for binaural stimulation with either continuous or modulated noise. SRT scores in different spatial noise conditions and for different noise characteristics in free-field and under reverberation were evaluated for smaller subgroups of CI and EAS users. Additionally, mean error in sound localization was measured with an LED pointer method.

EAS users achieved significantly higher speech perception scores in quiet (5 percentage points) and significantly better SRTs in continuous noise (1.6 dB), while still more than 5 dB worse than normal hearing subjects. Under binaural test conditions, bilateral EAS users had better SRTs compared to bilateral CI users in continuous and modulated noise. Bimodal and bilateral EAS users showed a tendency of better SRTs in spatially separated speech in noise conditions (in free-field and reverberation) compared to bilateral CI users. There was a significant correlation between mean low-frequency residual hearing (PTA_low) and speech perception in modulated noise. EAS users with contralateral normal hearing showed localization abilities superior to all other CI or EAS groups with mean localization errors closest to normal hearing.

Even if the hearing performance among but also within various groups of cochlear implantees is very heterogeneous and varies greatly depending on the noise and spatial condition, EAS users show better speech perception than users of electric stimulation only in many listening conditions.