A Step Further 6.2: Brainstem Systems Help Localize Sounds

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).

A diagram of a section of an owl brain shows an arrow pointing to a schematic depiction of the nucleus laminaris, represented by an oval with five neurons, numbered 1 through5, arrayed left to right.

A red line depicting an axon from the left cochlear nucleus runs from left to right, terminating at each of the five neurons in the nuclear laminaris. A blue line depicting an axon from the right cochlear nucleus runs from right to left, terminating at each of the neurons in the nucleus laminaris. On the far left, on the red line, a symbol representing an action potential is shown, with an arrow indicating it is moving along the axon toward the nucleus laminaris. Text explains that sound occurring to the left of the owl’s midline has been detected by the left cochlea slightly earlier than by the right cochlea. Monaural neurons of the left cochlear nucleus of the brainstem become active, sending action potentials along their axons toward the nucleus laminaris.

A second diagram shows the action potential on the red line further along the line toward the nucleus laminaris. An action potential also appears on the far right end of the blue line. Text explains that shortly after the events shown in the first diagram, the monaural neurons of the right cochlear nucleus send their own action potential toward the nucleus laminaris. Because they were fired earlier, the action potentials from the left side have travelled farther along their axons.

A third diagram shows both action potentials arriving at the fifth neuron on the right send of the nucleus laminaris. Text explains that the action potential from the left side and the one from the right side have arrived simultaneously at neuron 5, but not at any other binaural neuron. Neuron 5 is thus a coincidence detector that signals a particular location to the left of midline. While only five neurons are shown; in reality thousands of such neurons make up a detailed map of space. — **Figure 1 The Classic (Jeffress) Model of Sound Localization in the Auditory Brainstem of Birds**

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.

References

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.