The inferior colliculus (IC) (

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

for ''lower hill'') is the principal

midbrain The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum. It is functionally associated with vision, hearing, mo ...

nucleus of the

auditory pathway The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, increas ...

and receives input from several peripheral

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...

nuclei in the auditory pathway, as well as inputs from the

auditory cortex The auditory cortex is the part of the temporal lobe that processes auditory information in humans and many other vertebrates. It is a part of the auditory system, performing basic and higher functions in hearing, such as possible relations to ...

.Shore, S. E.: ''Auditory/Somatosensory Interactions''. In: Squire (Ed.): ''Encyclopedia of Neuroscience'', Academic Press, 2009, pp. 691–695 The inferior colliculus has three subdivisions: the central nucleus, a dorsal cortex by which it is surrounded, and an external cortex which is located laterally. Its bimodal neurons are implicated in auditory-

somatosensory The somatosensory system, or somatic sensory system is a subset of the sensory nervous system. The main functions of the somatosensory system are the perception of external stimuli, the perception of internal stimuli, and the regulation of bod ...

interaction, receiving projections from somatosensory nuclei. This

multisensory integration Multisensory integration, also known as multimodal integration, is the study of how information from the different sensory modality, sensory modalities (such as sight, sound, touch, smell, self-motion, and taste) may be integrated by the nervous sy ...

may underlie a filtering of self-effected sounds from vocalization, chewing, or respiration activities. The inferior colliculi together with the superior colliculi form the eminences of the corpora quadrigemina, and also part of the midbrain tectum. 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 The medial geniculate nucleus (MGN) or medial geniculate body (MGB) is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus (IC) and the auditory cortex (AC). It is made up of a number of sub-nuclei that ...

and the

lateral geniculate nucleus In neuroanatomy, the lateral geniculate nucleus (LGN; also called the lateral geniculate body or lateral geniculate complex) is a structure in the thalamus and a key component of the mammalian visual pathway. It is a small, ovoid, Anatomical ter ...

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 has three subdivisions – the central nucleus, the dorsal cortex by which it is surrounded, and an external cortex which is located laterally. The inferior colliculus is the first place where vertically orienting data from the fusiform cells in the

dorsal cochlear nucleus The dorsal cochlear nucleus (DCN, also known as the "tuberculum acusticum") is a cortex-like structure on the dorso-lateral surface of the brainstem. Along with the ventral cochlear nucleus (VCN), it forms the cochlear nucleus (CN), where all a ...

can finally synapse with horizontally orienting data. Sound location data thus becomes fully integrated by the inferior colliculus. IC are large auditory nuclei on the right and left sides of the midbrain. Of the three subdivisions the central nucleus of IC (CNIC) is the principal way station for ascending auditory information in the IC.

Input and output connections

The input connections to the inferior colliculus are composed of many brainstem nuclei. All nuclei except the contralateral ventral nucleus of the

lateral lemniscus The lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the contralateral inferior colliculus of the midbrain. Three distinct, primarily ...

send projections to the central nucleus (CNIC) bilaterally. It has been shown that great majority of auditory fibers ascending in the lateral lemniscus terminate in the CNIC. In addition, the IC receives inputs from the auditory cortex, the medial division of the medial geniculate body, the posterior limitans, suprapeduncular nucleus and subparafascicular intralaminar nuclei of the thalamus, the substantia nigra pars compacta lateralis, the dorsolateral periaqueductal gray, the nucleus of the brachium of the inferior colliculus (or inferior brachium) and deep layers of the superior colliculus. The inferior brachium carries auditory

afferent fibers Afferent nerve fibers are axons (nerve fibers) of sensory neurons that carry sensory information from sensory receptors to the central nervous system. Many afferent projections ''arrive'' at a particular brain region. In the peripheral nervou ...

from the inferior colliculus of the

mesencephalon The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum. It is functionally associated with vision, hearing, mo ...

to the

. The inferior colliculus receives input from both the ipsilateral and contralateral cochlear nucleus and respectively the corresponding ears. 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. The inferior colliculus also receives descending inputs from the

and auditory

thalamus The thalamus (: thalami; from Greek language, Greek Wikt:θάλαμος, θάλαμος, "chamber") is a large mass of gray matter on the lateral wall of the third ventricle forming the wikt:dorsal, dorsal part of the diencephalon (a division of ...

(or medial geniculate nucleus). The medial geniculate body (MGB) is the output connection from inferior 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.

Function

The majority of the ascending fibers from the lateral lemniscus project to IC, which means major ascending auditory pathways converge here. IC appears as an integrative station and switchboard as well. It is involved in the integration and routing of multi-modal sensory perception, mainly the

startle response In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening Stimulus (physiology), stimuli, such as sudden noise or sharp movement, and is associated with negative Affect (psychology), af ...

and

vestibulo-ocular reflex The vestibulo-ocular reflex (VOR) is a reflex that acts to stabilize Gaze (physiology), gaze during head movement, with eye movement due to activation of the vestibular system, it is also known as the cervico-ocular reflex. The reflex acts to im ...

. 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 Research Initiative 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. Skottun ''et al.'' measured the

interaural time difference The interaural time difference (or ITD) when concerning humans or animals, is the difference in arrival time of a sound between two ears. It is important in the Sound localization, localization of sounds, as it provides a cue to the direction or ...

sensitivity of single neurons in the inferior colliculus, and used these to predict behavioural performance. The predicted just noticeable difference was comparable to that achieved by humans in behavioral tests.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. This suggested that by the level of the inferior colliculus, integration of information over multiple neurons is unnecessary (see

population code Neural coding (or neural representation) is a neuroscience field concerned with characterising the hypothetical relationship between the Stimulus (physiology), stimulus and the neuronal responses, and the relationship among the Electrophysiology, e ...

). Axiomatically determined functional models of spectro-temporal receptive fields in inferior colliculus have been determined by Lindeberg and Friberg T. Lindeberg and A. Friberg "Idealized computational models of auditory receptive fields", PLOS ONE, 10(3): e0119032, pages 1-58, 2015
/ref> in terms of derivatives of Gaussian functions over the log-spectral domain and either Gaussian kernels over time in the case of non-causal time or first-order integrators (truncated exponential kernels) coupled in cascade in the case of truly time-causal operations, optionally in combination with local glissando transformations to account for variations in frequencies over time. The shapes of the receptive field functions in these models can be determined by necessity from structural properties of the environment combined with requirements about the internal structure of the auditory system to enable theoretically well-founded processing of sound signals at different temporal and log-spectral scales. Thereby, the receptive fields in inferior colliculus can be seen as well adapted to handling natural sound transformations.

Additional images

File:Inferior_colliculus_of_the_human_midbrain.svg, Inferior colliculus (K) from a cerebellar perspective. Sagittal view. File:Gray682.png, Superficial dissection of brain-stem. Lateral view. File:Gray685.png, Deep dissection of brain-stem. Lateral view. File:Gray691.png, Dissection of brain-stem. Dorsal view. File:Gray719.png, Hind- and mid-brains; postero-lateral view. File:Human brainstem-thalamus posterior view description.JPG, Posterior dissection of brainstem with colliculi visible. File:Slide2PITER.JPG, Fourth ventricle. Posterior view. Deep dissection. File:Slide2BRA.JPG, Brainstem. Posterior view.

References

External links

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NIF Search - Brachium of inferior colliculus
via the

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