6.5 Convergence in the inferior colliculus

Gray's Anatomy plate 713: the fibers of the lemniscus, with the medial lemniscus in blue and the lateral lemniscus in red. — Gray's plate 713: the fibers of the lemniscus. The lateral lemniscus (red) carries the auditory pathway from cochlear nucleus and superior olive up to the inferior colliculus. Henry Gray & H. V. Carter · public domain · Wikimedia Commons

Outputs from both MSOs and both LSOs (plus several other brainstem nuclei) converge in the inferior colliculus (IC) of the midbrain. The IC is a large, conspicuous structure on the dorsal aspect of the midbrain; it is the obligatory waystation for essentially all ascending auditory information. Lesions here cause severe deafness; surgical implantation here is being investigated for patients in whom cochlear implants are insufficient.

The IC is where the various auditory-brainstem codes — left vs right ITD, left vs right ILD, plus a host of more specialized signals — are combined into a unified representation of where a sound is. Single IC neurons typically have spatial receptive fields: they fire preferentially for sounds at particular locations in azimuth and elevation. Some IC neurons code azimuth via ITD-derived signals; others via ILD; others via a combination.

By the time information leaves the IC, the brain has a robust spatial map of the auditory scene. This map is what the cortex uses (along with non-spatial information) to construct the conscious experience of where each sound source is.