Humans, and other mammals, make use of three cues to localise sound sources. Two of these are binaural, involving a comparison of the level and/or timing of the sound at each ear. For high frequencies, level differences result from shadowing by the head. For low-frequencies, localisation relies on the time differences between the signals at the ears that result from different sound paths to the ears. The third cue depends on sensitivity to the elevation-dependent pattern of spectral peaks and troughs that result from multiple sound waves interfering at the tympanic membrane. Different physiological mechanisms process these different localisation cues. Neurons in the dorsal cochlear nucleus are selectively sensitive to the spectral notches that result from interference between sound waves at the ear. Interaural level differences are initially processed in the lateral superior olive by neurons receiving inhibition from one ear and excitation from the other. Interaural time differences are converted into discharge rate by neurons in the medial superior olive with excitatory inputs from both ears and that only fire when their inputs are coincident. The contribution of such coincidence detectors to sound-source localisation is discussed in the light of recent observations.
- Binaural hearing
- Interaural differences