The inferior colliculus, Latin for “little hills,” is a midbrain nucleus or a complex cluster of neurons consisting of two small rounded elevations situated on the back aspect of the midbrain, just below the superior colliculus. The inferior colliculi are relay centers for auditory (sense of hearing) fibers and serve as a principal auditory center for the body as well as for both auditory and visual reflexes.
There are four physical bumps located on the posterior surface of the midbrain. The upper two bumps are the superior colliculi, which are involved in visual processing and the control of eye movements. The lower two bumps are the inferior colliculi. Together, the two superior and two inferior colliculi form the corpora quadrigemina or quadruplet bodies. The inferior colliculus is organized into three distinct parts: the central nucleus, the external nucleus, and the dorsal cortex.
The incoming auditory data to the inferior colliculus involve links to many brainstem nuclei. All the connections between the central nucleus of the inferior colliculus and the brainstem nuclei have connections on both sides of the brain, with the exception of one. It is the nucleus of the lateral lemniscus, a tract of axons that carry sound information. In addition, the inferior colliculus receives input from three sources: the auditory cortex, the medial division of the medial geniculate body, and deep layers of the superior colliculus.
The inferior colliculus consists of a compact cluster of gray matter nerve cells. These clusters are made up of large and small multipolar nerve cells. They are almost completely surrounded by nerve fibers from the lateral lemniscus. Most of these fibers end in the gray nucleus of the same side, but some cross over the middle line and end in the opposite side. From the gray nucleus cells, fibers stretch through the brachium of the inferior colliculi into the midbrain tectum. From here, they are carried to the thalamus and then to the cortex of the temporal lobe. Other fibers cross the middle line and end in the opposite inferior colliculus.
The inferior colliculus is a part of the midbrain that serves as a principal auditory center for the body; each inferior colliculus receives auditory signals from both ears. Most signals sent by the right ear cross over to the left inferior colliculus of the human brain, and vice versa for the left ear. Like a radar system, one colliculus compares a given signal with its counterpart on the opposite side. The colliculi then determine if there are differences in the signal such as time lag and find the source of the sound within three-dimensional space.
The inferior colliculus receives input from somatosensory nuclei (neurons that contain sensory information of the body), which are involved in auditory-somatosensory interaction. This multisensory interaction leads to the ability to ignore “self-induced” sounds that are caused by vocalization, chewing, or breathing. Furthermore, the function of the inferior colliculus can be compared to a computer, acting to unify all the data about sound location. It is also responsible for pitch discrimination, ramping frequency recognition, and the startle reflex. Because of its complex functions in both vision and hearing, this part of the brain shows a higher rate of metabolic activity than other areas of the brainstem.
Since the main function of the inferior colliculus is to process sound information, one of the main effects of damage to this structure is hearing loss. Hearing loss can be categorized by what part of the auditory system is damaged. There are three basic types of hearing loss: sensorineural (problems with the vestibulocochlear nerve, the inner ear, or central processing centers of the brain), conductive (problems with sound getting into the ear), and mixed (both sensorineural and conductive) hearing loss. There is also a fourth, less common type referred to as a central hearing loss, which involves damage to the inferior colliculus.
In central hearing loss, the ears work just fine but some function in the central nervous system (mainly the brainstem, including the inferior colliculus) is lost so that the person cannot mentally understand what is being heard. There are four subtypes of central hearing disorders or central deafness: central auditory processing disorder, auditory agnosia, pure word deafness, and cortical deafness. In central auditory processing disorder, the person finds it hard to understand or interpret what is being said, making communication very difficult. Because this condition involves a mental aspect, people tend to be diagnosed as having a learning disorder. They are thought to have trouble understanding people, ideas, and concepts, but actually they have central auditory processing disorder. Learning disorder is an incorrect diagnosis because the person’s hearing works just fine; he or she just cannot make any sense of what is being heard because of some loss of function in the auditory cortex.
Mario J. Perez
See also: Auditory System; Cochlea; Deafness; Vestibulocochlear Nerve
MedlinePlus. (2012). Hearing disorders and deafness. Retrieved from http://www.nlm.nih.gov/medlineplus/hearingdisordersanddeafness.html