3The Three Binaural Benefits: Head Shadow, Summation and Squelch
Hearing in noise with two ears comes from three measurable advantages. This module defines and quantifies head shadow, summation and squelch, and explains which ones bilateral implant users reliably obtain.
THead shadow: the better-SNR ear
When speech and noise come from different directions, the head physically attenuates the noise at the far ear, giving that ear a more favorable signal-to-noise ratio. Head shadow is purely acoustic: it requires only a second microphone on the far side of the head, not an intact binaural brainstem, which is why it is the easiest benefit to obtain. In bilateral implant users the head-shadow advantage is large, averaging about 6.8 dB of improvement in speech reception threshold in the study of Schleich and colleagues. Because the brain need only attend to whichever ear has the cleaner signal, almost every bilateral implant recipient gains from head shadow.[2004][2009]
TSummation: hearing the same thing twice
Binaural summation, also called redundancy, is the gain from receiving the same signal at both ears, which the central auditory system combines into a louder, more salient percept. In normal hearing, summation improves detection of low-level sounds by roughly 2 to 6 dB and contributes a modest boost to speech in noise. In bilateral implant users the summation benefit is smaller and more consistent, on the order of about 2 dB in speech-reception-threshold terms. Summation does not depend on speech and noise being spatially separated, so it can help even when everything comes from straight ahead.[2004][2020]
TSquelch: the brain cancels the noise
Binaural squelch is the central nervous system's use of interaural differences to suppress noise and pull out the target talker, the truest binaural processing of the three. Because squelch leans on interaural timing cues that implants convey poorly, it is the hardest benefit for bilateral implant users to obtain and the smallest in magnitude. Schleich and colleagues measured an average squelch benefit of only about 0.9 dB in bilateral implant users, far below the head-shadow effect in the same patients. The presence of any squelch at all is meaningful, however, because it indicates that the binaural brainstem is at least partly using the two electric inputs together.[2004][2003]
CWhat bilateral users reliably get, and what they do not
The clinical takeaway is a clear hierarchy: head shadow is large and almost universal, summation is modest and consistent, and squelch is small and unreliable. Most of the real-world advantage of a second implant in noise comes from the acoustic head-shadow effect and the guarantee that the better ear is always in play. Counseling should set expectations accordingly: bilateral implantation reliably improves hearing when noise and speech are separated, but does not restore the full noise-cancelling squelch of normal binaural hearing. These limits also motivate research into synchronized bilateral processors and timing-preserving strategies that aim to recover the squelch and fine localization that today's independent devices forfeit.[2009][2002]
Which binaural benefit explains his restaurant improvement, and why is it limited when speech and noise are colocated?
Which of the three binaural benefits is purely acoustic and obtained by nearly all bilateral implant users?
Which benefit is the smallest and least reliable in bilateral cochlear implant users, because it depends on interaural timing?
Approximately how large is the head-shadow benefit reported in bilateral implant users by Schleich et al. (2004)?