6.4 ILDs in the LSO

The lateral superior olive takes a different approach. It is excited by the ipsilateral ear (via the AVCN on the same side) and inhibited by the contralateral ear (via an intermediate nucleus, the medial nucleus of the trapezoid body, which converts the excitatory contralateral signal into a sign-flipped inhibitory one).

If a sound is on the right, it is louder at the right ear than the left. The right LSO receives strong excitation (from the right cochlear nucleus) and weak inhibition (from the left, attenuated by the head shadow), so it fires strongly. The left LSO receives weak excitation and strong inhibition, and fires weakly. The difference in firing rate between the two LSOs encodes the ILD.

The LSO operates predominantly at high frequencies, where head-shadow ILDs are large (movement 3). At low frequencies, where the head does not shadow well, the ILD code is weak and the MSO’s ITD code carries the localization burden.