|Brain: Inferior colliculus|
|Transverse section of mid-brain at level of inferior colliculi.|
|Deep dissection of brain-stem. Lateral view. ("Inferior colliculus" is yellow, and is labeled at left.|
|Gray's||subject #188 806|
The inferior colliculi (Latin, lower hill) together with the superior colliculi form the eminences of the corpora quadrigemina, and also part of the tectal region of the midbrain. The inferior colliculus lies caudal to its counterpart - the superior colliculus - above the trochlear nerve, and at the base of the projection of the medial geniculate nucleus (MGN) and the lateral geniculate nucleus (LGN).
The inferior colliculus is the principal midbrain nucleus of the auditory pathway and receives input from several more peripheral brainstem nuclei in the auditory pathway, as well as inputs from the auditory cortex. The inferior colliculus has many subnuclei.
Relationship to auditory system
The inferior colliculi of the midbrain, are located just below the visual processing centers known as the superior colliculi. The inferior colliculus is the first place where vertically orienting data from the fusiform cells in the dorsal cochlear nucleus can finally synapse with horizontally orienting data. This homecoming of the aural dimensions puts these dual mesencephalic bumps in the position to fully integrate all the sound location data.
The inferior colliculus function as a master computer both in regard to its hardware (complex connections) and its software (internal organization). IC are large auditory nuclei on the right and left sides of the midbrain. It is divided into three part, the Central Nucleus of IC (CNIC), peicentral nucleus and external nucleus; however, CNIC is the principal way station for ascending auditory information in the IC.
1. Input and Output Connection of IC
The input connections to the inferior colliculus are composed of many brainstem nuclei. All nuclei except the contralateral ventral nucleus of the lateral leminiscus (LL) send projections to the central nucleus (CNIC) bilaterally. It has been shown that great majority of auditory fibers ascending in the lateral leminiscus terminate in the CNIC. In addition, the IC receives descending inputs from the auditory cortex, medial geniculate body (MGB), and superior colliculus (SC).
The inferior colliculus receives input from both the ipsilateral and contralateral cochlear nucleus and respectively the corresponding ears. Of course, there is some lateralization, the dorsal projections (containing vertical data) only project to the contralateral inferior colliculus. This inferior colliculus contralateral to the ear it is receiving the most information from, then projects to its ipsilateral medial geniculate nucleus.
-Deep layers of the SC
-Contralateral and other nuclei in the ipsilateral IC
The medial geniculate body (MGB) is the output connection from interior colliculus and the last subcortical way station. The MGB is composed of ventral, dorsal, and medial divisions, which are relatively similar in humans and other mammals. The ventral division receives auditory signals from the central nucleus of the IC (1).
CNIC: Central Nucleus of IC
AVCN: Anterior Ventral Cochlear Nucleus
PVCN: Posterior Ventral Cochlear Nucleus
SCN: Superior Colliculus Nucleus
LSO: Lateral Superior Olive
MSO: Medial Superior Olive
DNLL: Dorsal Nucleus of the Lateral Lemniscus
MGB: Medial Geniculate Body
SC: Superior Colliculus
TB: Trapezoid Body
2. Function of IC
The majority of the ascending fibers from LL project to IC, which means major ascending auditory pathway coverage in here. IC appears as an integrative station and switchboard as well. It is involved integration and routing of multi-model sensory perception, mainly startle reflex and ocular reflex. It is also responsive to specific amplitude modulation frequencies and this might be responsible for detection of pitch. In addition, spatial localization by binaural hearing is a related function of IC as well.
The inferior colliculus has a relatively high metabolism in the brain. The Conrad Simon memorial institute research measured the blood flow of the IC and put a number at 1.80 cc/g/min in the cat brain. For reference, the runner up in the included measurements was the somatosensory cortex at 1.53. This indicates that the inferior colliculus is metabolically more active than many other parts of the brain. The hippocampus, normally considered to use up a disproportionate amount of energy, was not measured or compared (6).
Skottun et al. ran a study of the neurons in the inferior colliculus, comparing the single unit information to the behavioral response in guinea pigs. They found that the interaural time delay discrimination in the neurons was comparable to the behavioral ability to detect the source of a sound. If these findings were not consistent, then the localization of sound must work through a different mechanism (7).
1) Stanley A. Gelfand "Hearing, an Introduction to Psychological and Physiological Acoustics", 4th Ed., Marcel Dekker, 2004 pp 71-75
2) Conrad Simon Memorial Research Initiative homepage <http://www.conradsimon.org/InferiorColliculus.shtml>
3) Skottun, Bernt C. et al. The ability of inferior colliculus neurons to signal differences in interaural delay. PNAS November 20, 2001 vol. 98 . no. 24 . pp 14050-14054