13Binaural hearing & localization

FWhy two ears

Two ears do two things one cannot. They let us localise sound — judge its direction — and they let us hear better in noise, by giving the brain two slightly different views of a scene to compare and separate. Both rest on the same foundation: the brain comparing the signals arriving at the two ears, and the comparison begins at the superior olive of Module 12.[2012]

TCThe duplex theory

A sound off to one side reaches the near ear slightly earlier and slightly louder than the far ear. These are the two cues to horizontal direction: the interaural time difference (ITD) and the interaural level difference (ILD). The classic duplex theory says they divide the frequency range between them — ITD dominates at low frequencies, ILD at high — for a principled reason explored in the dial below.[1969]

Why the split? At low frequenciesthe wavelength is long compared with the head, so the head casts little “shadow” and there is barely any level difference — but the waveform is slow enough that the brain can compare its timing at the two ears. At high frequencies the head does shadow the far ear, creating a usable level difference, while the waveform is too fast for reliable timing comparison. Each cue works precisely where the other fails. The whole maximum time difference is only about ±700 µs.

CThe MSO — computing timing

The medial superior olive (MSO) computes the ITD. It receives excitatory input from both ears (via the timing-faithful bushy cells of the cochlear nucleus) and acts as a coincidence detector: a neuron fires best when spikes from the two ears arrive together, which happens for a particular combination of sound direction and the neuron's internal delays. The MSO is biased toward low frequencies, matching the ITD cue.[2012]

CThe LSO — computing level

The lateral superior olive (LSO) computes the ILD by comparing excitation and inhibition. It is excited by the ipsilateral ear and inhibitedby the contralateral ear (through an inhibitory relay, the MNTB). A sound louder on the ipsilateral side produces strong excitation and weak inhibition, and vice versa — so the cell's output signals the interaural level difference. The LSO is biased toward high frequencies, matching the ILD cue.[1968]

TCBinaural benefits beyond localisation

Localisation is not the only payoff. Three binaural benefits matter clinically, especially for hearing in noise:

• Head-shadow / better-ear listening — for a given noise source, one ear usually has a better signal-to-noise ratio; with two ears the brain can use whichever ear is better.
• Binaural summation — the same sound at both ears is a little louder and clearer than at one.
• Spatial release from masking (“squelch”) — when speech and noise come from different directions, the binaural system can partially separate them using the interaural differences, improving speech understanding in noise.

[2009]

FTBilateral & bimodal cochlear implants

This module is the rationale for implanting both ears, or combining an implant with a hearing aid in the other ear (bimodal hearing). Two devices give the brain two streams to compare, and recipients reliably gain the more robust binaural benefits — head shadow and better-ear listening — improving localisation and speech in noise. The catch follows directly from Module 11: implants convey fine timing poorly, so the microsecond ITD cue is hard to deliver, while the ILD (level) cue survives better. Restoring the level-based and better-ear benefits is achievable today; restoring true fine-timing binaural hearing remains a research frontier.[2009]

From here, the Objective Measures chapter takes this same pathway and asks how to measure it electrically through an implant — and the clinical chapters take it into candidacy, surgery, and programming.