What brain systems analyze binaural cues? Researchers have found that although both birds and mammals are highly talented at localizing sound sources, they accomplish this feat in quite different ways. Birds have a neural map, located in the brainstem (in a structure called the nucleus laminaris), that associates specific spatial locations with specific latency differences between the two ears; neurons that behave this way are termed coincidence detectors (see Figure 1) (Agmon-Snir et al., 1998; Jeffress, 1948).
By contrast, mammals don’t bother with a map in the brainstem; instead, sound location is encoded by comparing the activity of the entire left medial superior olive (MSO)—a division of the superior olivary nucleus that is concerned with calculating latency differences—with the entire right MSO (Grothe, 2003; McAlpine et al., 2001). So, for example, a sound on the midline would activate the left and right MSO equally, and the two signals would effectively cancel each other out. But a sound on the right would produce proportionally more excitation of the left MSO than the right MSO, and vice versa.
In both birds and mammals, the output of sound localization systems is transmitted to higher levels of the auditory system for further processing.
Agmon-Snir, H., Carr, C. E., and Rinzel, J. (1998). The role of dendrites in auditory coincidence detection. Nature 393: 268–272.
Grothe, B. (2003). New roles for synaptic inhibition in sound localization. Nature Reviews. Neuroscience 4: 540–550.
Jeffress, L. A. (1948). A place theory of sound localization. Journal of Comparative and Physiological Psychology 41: 35–39.
McAlpine, D., Jiang, D., and Palmer, A. R. (2001). A neural code for low-frequency sound localization in mammals. Nature Neuroscience 4: 396–401.