Perceptual learning of vocoded speech with and without contralateral hearing: Implications for the rehabilitation of cochlear implant subjects

An increasing number of cochlear implant (CI) candidates show residual acoustic hearing, usually in their contralateral ear. A small proportion of them even has normal or quasi-normal hearing. This patient population may react very differently to CI implantation compared to bilaterally-deaf patients, mainly because they may not rely mostly on their CI to understand speech. From their first activation, they are receiving very different information in each ear, and there is currently no data on the type of training or rehabilitation strategy they should follow: do they need to spend some time listening to their CI alone, without contralateral hearing to maximize their speech intelligibility? Or, on the contrary, does their residual hearing help them train to recognize speech with their CI more quickly?

To get a first insight into this question, we tested 60 normal-hearing listeners in an auditory perceptual learning experiment. Each subject was randomly assigned to one of three groups of 20 referred to as NORMAL, LOWPASS and NOTHING. The experiment consisted of two test phases separated by a training phase. In the test phases, all subjects were tested on recognition of monosyllabic words passed through a six-channel “PSHC” vocoder presented to a single ear. In the training phase, all subjects were also presented with the same vocoded speech in one ear but the signal they received in their other ear differed across groups. The NORMAL group was presented with the unprocessed speech signal; the LOWPASS group with a low-pass filtered version of the speech signal (filter cut-off of 250 Hz) and the NOTHING group with no sound at all. These three groups aim to simulate groups of CI subjects having normal contralateral hearing, residual low-frequency hearing and no residual hearing, respectively. The training phase consisted of listening to a 30-minute audio book with subtitles displayed on a computer screen.

A mixed-effect ANOVA showed a significant effect of training. All subject groups performed better after than before the training phase. Furthermore, there was a significant interaction between training and group. Further analysis showed that the amount of improvement was significantly smaller for the NORMAL than for the LOWPASS and NOTHING groups.

This shows that having normal contralateral hearing reduces or slows down perceptual learning of vocoded speech but that having instead an unintelligible contralateral signal does not have any effect. Potential implications for the rehabilitation of CI patients with partial or full contralateral hearing will be discussed.