Multisensory integration-attention trade-off in cochlear-implanted deaf individuals

The cochlear implant (CI) allows profoundly deaf individuals to partially recover hearing. Still, due to the coarse acoustic information provided by the implant, CI users have considerable difficulties in recognizing speech, especially in noisy environments, even years after implantation. CI users therefore rely heavily on visual cues to augment speech comprehension, more so than normal-hearing individuals. However, it is unknown how attention to one (focused) or both (divided) modalities plays a role in multisensory speech recognition. Here we show that unisensory speech listening and speech reading were negatively impacted in divided-attention tasks for CI users - but not for normal-hearing individuals. Our psychophysical experiments revealed that, as expected, listening thresholds were consistently better for the normal-hearing, while lipreading thresholds were largely similar for the two groups. Moreover, audiovisual speech recognition for normal-hearing individuals could be described well by probabilistic summation of auditory and visual speech recognition, while CI users were better integrators than expected from statistical facilitation alone. Our results suggest that this benefit in integration, however, comes at a cost. Unisensory speech recognition is degraded for CI users when attention needs to be divided across modalities, i.e. in situations with uncertainty about the upcoming stimulus modality. We conjecture that CI users exhibit an integration-attention trade-off. They focus solely on a single modality during focused-attention tasks, but need to divide their limited attentional resources to more modalities during divided-attention tasks. We argue that in order to determine the benefit of a CI for speech comprehension, situational factors need to be discounted by presenting speech in realistic or complex audiovisual environments.