The lateral superior olive as a time machine toward the spatial localization of transient sounds.

This project addresses a puzzle posed by a neural circuit in the brainstem. Traditionally, this circuit is thought to compute the differences in sound intensity at the two ears to aid localization of sounds in space. A sound to our left is louder at the left than at the right ear and excites neurons on the left side; and vice versa for sounds on our right. The puzzle is that this circuit shows several features that do not fit its sensitivity to intensity differences. The most spectacular feature is the calyx of Held; one of the largest synapses (i.e. structural connections through which neurons communicate) in the brain. It makes signal transmission in the circuit fast and reliable, but it is unclear why that is important for this circuit. We hypothesize that the specializations enable mammals to reliably localize high-frequency transient sounds. We use a new recording technique to obtain responses to brief clicks played to the two ears with small time differences. We predict that neurons will show congruent sensitivity to both intensity and time differences between the ears. We will also attempt to record from other neurons, found intermingled in this circuit, that are thought to provide control of the brain over the output of the cochlea but that have been impossible to study with previous methods. The general characterization of these circuits is critical to gain insight into the working of artificial ears for the profoundly deaf, in particular bilateral cochlear implants.