The superior olivary nucleus is a collection of nuclei in the human auditory system. Also known as the superior olivary complex (SOC), it is a processing station in the brain’s auditory pathway. It is one of the first locations where information from both ears is combined, setting the stage for more complex auditory processing.
Anatomical Position in the Brainstem
The superior olivary complex is situated within the brainstem, which connects the brain to the spinal cord. It resides in the caudal, or lower, portion of the pons in an area known as the pontine tegmentum. The SOC is found near the facial nucleus and is associated with a bundle of nerve fibers called the trapezoid body. Many of the nuclei that form the complex are located just dorsal to this fiber bundle.
Its position within the brainstem is distinct from another similarly named structure, the inferior olivary nucleus. The inferior olivary nucleus is located lower in the brainstem, within the medulla oblongata, and is primarily involved in motor control and learning, rather than hearing. This placement allows it to receive and integrate auditory signals early in their journey through the brain.
Key Subdivisions of the Superior Olivary Complex
The superior olivary complex is not a single entity but is composed of several distinct groups of neurons, or nuclei. The two principal subdivisions are the medial superior olive (MSO) and the lateral superior olive (LSO).
Closely associated with these is the medial nucleus of the trapezoid body (MNTB). The MNTB plays a supportive role in the function of the SOC. It is composed of a uniform population of cells necessary for the processing that occurs in the LSO.
Function in Sound Localization
A primary function of the superior olivary complex is to determine the location of a sound source in space, a process known as sound localization. The subdivisions of the SOC are specialized to detect different cues the brain uses to pinpoint a sound’s origin.
The medial superior olive (MSO) is specialized for processing low-frequency sounds. It detects interaural time differences (ITDs), which are the minuscule differences in the arrival time of a sound at each ear. Neurons in the MSO act as coincidence detectors, firing most strongly when signals from both ears arrive simultaneously. The specific neuron that fires indicates the time difference, and thus the sound’s location.
The lateral superior olive (LSO) is specialized for localizing high-frequency sounds. It detects interaural level differences (ILDs), which are the differences in the intensity, or loudness, of a sound as it reaches each ear. A sound coming from the left will be louder in the left ear because the head creates a “sound shadow” that reduces its intensity at the right ear. The LSO receives excitatory input from the ipsilateral (same side) ear and inhibitory input from the contralateral (opposite side) ear, via the MNTB, allowing it to compute this difference in intensity.
Connections within the Auditory Pathway
The superior olivary complex is a hub in the auditory pathway, receiving information from and sending it to other auditory centers. Its primary input comes from the cochlear nuclei, the first stop for auditory information after leaving the cochlea of the inner ear. The SOC receives projections from the cochlear nucleus on both sides of the brainstem, a bilateral input that allows it to compare sounds from both ears.
After processing, neurons in the SOC project to higher auditory centers. The main output target is the inferior colliculus, an auditory processing center in the midbrain. These projections travel through a fiber bundle known as the lateral lemniscus. The LSO sends both excitatory and inhibitory signals to the inferior colliculus, further refining the auditory information as it ascends through the brain. The SOC also sends descending projections to the outer hair cells of the cochlea, allowing it to modulate the amplification of sound at its earliest stage.