Neural correlates of bilateral summation between natural and vocoded speech

The normal auditory system integrates redundant information across ears, which aids listeners to detect weak sounds. For patients with single sided deafness (SSD), cochlear implants (CI) have been attempted expecting improved speech recognition in challenging listening scenarios. However, it is not yet known whether CI recovers the bilateral summation for weak sound detection in SSD. The binaural benefit of soft-speech recognition was tested in six SSD CI patients and thirty normal hearing (NH) listeners. The task was identifying quietly presented (i.e., RMS ∼6 dB SL) monosyllabic words. CI patients were tested in three conditions: acoustic-ear only (A1), electric-ear only (E1), or both ears (A+E). NH participants were tested in four conditions: monaural natural speech (A1), monaural vocoded speech (E1), dichotic natural and vocoded speech (A+E), or dichotic natural speech (A2). Electroencephalography was recorded during the task. Bilateral conditions (A+E and A2), even when one ear was vocoded, resulted better accuracy than monaural conditions (A1 and E1). However, A+E condition exhibited smaller early auditory cortical responses (i.e., N1P2 complex) than any natural-speech conditions (A2 and even A1). This result may imply that bilateral summation benefits in SSD-CI users are achieved by later cognitive processing rather than immediate early integration.