12Two Ears Are Better: Bilateral, Bimodal and SSD Outcomes

FThe binaural mechanisms

The head-shadow effect is purely physical: when noise sits opposite the target, the ear nearer the talker enjoys a better SNR, and a second device guarantees there is always an ear on the better side; this is the largest and most reliable binaural advantage. Binaural summation gives a roughly 3 dB threshold improvement when the same sound reaches both ears, reflected clinically in lower comfortable-level current settings under bilateral listening. Binaural squelch is true central processing: the brainstem uses interaural time, level and spectral differences to suppress a spatially separate masker; it appears in only some users and depends on hearing history. Binaural redundancy adds a further small gain, on the order of 1 to 2 dB SNR with target and noise in front, because both ears carry overlapping, complementary information. Localisation is overwhelmingly a binaural function, restored only when two inputs allow the auditory system to compare interaural time and level differences.[2014][2009]

TBilateral implants

Two implants restore horizontal localisation far better than one, cutting localisation error from the tens of degrees seen with a single device toward the range achievable with two matched inputs. Bilateral users gain head-shadow benefit reliably and summation in almost all cases; squelch is present in a subset, and the head-shadow component generally contributes the most SNR gain. Speech-in-noise improves with the second implant, especially when noise and talker are spatially separated, recovering some of the spatial release a single implant cannot provide. A second implant also provides redundancy: device failure no longer means a return to deafness, an argument with particular force for children, in whom head trauma is a leading cause of device failure. Loudness balancing between the two devices is a prerequisite for the brain to use interaural level differences, so bilateral fitting is not simply two independent maps.[2014][2020]

CBimodal hearing: implant plus contralateral aid

Bimodal listening pairs an implant in one ear with a hearing aid in the other, combining the implant's high-frequency speech coding with the residual low-frequency acoustic hearing the aided ear still has. The acoustic ear supplies low-frequency fine-structure and voice-pitch cues the implant codes poorly, which is why bimodal users often gain in noise, talker tracking and music appreciation over the implant alone. Measured benefit is most consistent for speech in noise and for the subjective quality of voices and music, and tends to track the amount and quality of residual contralateral hearing. Bimodal hearing is the default first step for many recipients with usable contralateral residual hearing, and a candidate for a second implant if that hearing later declines.[2020][2009]

CSingle-sided deafness

In single-sided deafness one ear hears normally and the other is profoundly deaf; an implant in the deaf ear aims to restore binaural input rather than to provide the only hearing. The principal audiological gains are improved sound localisation and improved speech understanding in noise, because the restored ear re-enables head-shadow and binaural comparison the normal ear alone cannot deliver. Single-sided-deafness implantation is also an established treatment for ipsilateral tinnitus, with many recipients reporting substantial reduction or suppression of the tinnitus when the implant is active. Outcomes depend on a relatively short duration of deafness in the implanted ear, since a long-deprived auditory pathway integrates poorly with the normal-hearing side. Single-sided-deafness and bimodal candidacy share a logic: both add a second, complementary input to recover the binaural benefits a single hearing channel cannot provide.[2009][2020]