Superior Colliculus Function
Superior Colliculus: The superior colliculus refers to the rostral (front) bump on the lateral (side) part of the midbrain. It is, in fact, a pair of two colliculi, superior and inferior, on either side of the midbrain that together constitutes the tectum.
Part of the colliculus sticks out in the direction of the spinal cord region. This key projection helps the head to respond to different sensory stimuli. Movement of the eyes is also connected with the cells present in the lower layers of the colliculus. In mammals and other higher animals, the brain processes sensory inputs in correlation with the superior colliculus. In lower animals, it helps to perceive any sudden movement that occurs before the retina.
Superior Colliculus Function
In mammals, the superior colliculus forms a major component of the midbrain. It is a paired structure and together with the paired inferior colliculi from the corpora quadrigemina. The superior colliculus is a layered structure, with a number of layers that vary by species. The layers can be grouped into the superficial layers (stratum opticum and above) and the deeper remaining layers. Neurons in the superficial layers receive direct input from the retina and respond almost exclusively to visual stimuli. Many neurons in the deeper layers also respond to other modalities, and some respond to stimuli in multiple modalities. The deeper layers also contain a population of motor-related neurons, capable of activating eye movements as well as other responses.
The general function of the tectal system is to direct behavioral responses toward specific points in egocentric (“body-centered”) space. Each layer contains a topographic map of the surrounding world in retinotopic coordinates, and activation of neurons at a particular point in the map evokes a response directed toward the corresponding point in space. In primates, the superior colliculus has been studied mainly with respect to its role in directing eye movements. Visual input from the retina, or “command” input from the cerebral cortex, create a “bump” of activity in the tectal map, which, if strong enough, induces a saccadic eye movement. Even in primates, however, the superior colliculus is also involved in generating spatially directed head turns, arm-reaching movements, and shifts in attention that do not involve any overt movements. In other species, the superior colliculus is involved in a wide range of responses, including whole-body turns in walking rats. In mammals, and especially primates, the massive expansion of the cerebral cortex reduces the superior colliculus to a much smaller fraction of the whole brain. It remains nonetheless important in terms of function as the primary integrating center for eye movements.
In non-mammalian species, the optic tectum is involved in many responses including swimming in fish, flying in birds, tongue-strikes toward prey in frogs, and fang-strikes in snakes. In some species, including fish and birds, the optic tectum, also known as the optic lobe, is one of the largest components of the brain.
Note on terminology: This article follows terminology established in the literature, using the term “superior colliculus” when discussing mammals and “optic tectum” when discussing either specific non-mammalian species or vertebrates in general.
Superior Colliculus Location
The superior colliculus in all mammals consists of seven layers, six of which are labeled in Fig. 25.3. The projection from the retina in the mouse terminates in the three superficial layers – the stratum zonale, stratum griseum superficiale, and the stratum opticum (Drager and Hubel, 1975). The relatively small ipsilateral projection to the superior colliculus terminates in a series of patches in the rostromedial portion of the superior colliculus (Drager and Hubel, 1975).
The retinotopic organization of the superior colliculus is consistent among mammals: the zero vertical meridians is represented rostrally and the most peripheral part of the contralateral visual field is represented caudally; the upper visual field is represented medially in the superior colliculus and the lower visual field is represented laterally (Drager and Hubel, 1975, 1976). As noted above, the representation of the ipsilateral hemifield is restricted to the most rostral area of the superior colliculus.
The retinal representation is the same in all retina recipient layers. The superior colliculus receives substantial input from the ipsilateral primary visual cortex (Stein and Meredith, 1991). In the deeper layers of the superior colliculus, many cells respond to somatosensory and auditory stimuli as well as visual input. The main somatosensory input to the superior colliculus is from the whiskers (Drager and Hubel, 1975).
Superior And Inferior Colliculus
There are two superior colliculi in the midbrain. They are symmetrically positioned, one on either side of the midline of the brainstem; they form two bumps on the posterior external surface of the brainstem. The superior colliculi are just below the thalamus and above the two inferior colliculi. The superior colliculus is often referred to as the tectum or optic tectum in non-human vertebrates.
Although the complete scope of functions that can be attributed to the superior colliculi has not been fully delineated, the superior colliculi are understood to be important in directing behavioral responses toward stimuli in the environment. In other words, the superior colliculus seems to be able to receive information from the environment and then use that information to initiate a behavioral response appropriate to the current environmental context. For example, if you were sitting in the stands at a baseball game and someone hit a home run, you would follow the ball with your head and eyes.
This behavioral response to an environmental stimulus would involve the superior colliculi. In fact, eye and head movements like this are the most-studied functions of the superior colliculus, but the structure is thought to be involved in a variety of other responsive movements as well.
Function Of Superior Colliculus
The cerebral hemispheres consist of an inner core of myelinated nerve fibers, the white matter, and an outer cortex of gray matter. The cerebral cortex is responsible for integrating sensory impulses, directing motor activity, and controlling higher intellectual functions. The human cortex is several centimeters thick and has a surface area of about 2,000 square cm (310 square inches), largely because of an elaborate series of convolutions; the extensive development of this cortex in humans is believed to distinguish the human brain from those of other animals. Nerve fibers in the white matter primarily connect functional areas of the cerebral cortex. The gray matter of the cerebral cortex usually is divided into four lobes, roughly defined by major surface folds. The frontal lobe contains control centers for motor activity and speech, the parietal for somatic senses (touch and position), the temporal for auditory reception and memory, and the occipital for visual reception. Sometimes the limbic lobe, involved with smell, taste, and emotions, is considered to be the fifth lobe.
white matter; cerebrum© Alex Orellana, Rokers Vision Laboratory, University of Wisconsin – Madison
Numerous deep grooves in the cerebral cortex, called cerebral fissures, originate in the extensive folding of the brain’s surface. The main cerebral fissures are the lateral fissure, or fissure of Sylvius, between the frontal and temporal lobes; the central fissure, or fissure of Rolando, between the frontal and parietal lobes, which separates the chief motor and sensory regions of the brain; the calcarine fissure on the occipital lobe, which contains the visual cortex; the parieto-occipital fissure, which separates the parietal and occipital lobes; the transverse fissure, which divides the cerebrum from the cerebellum; and the longitudinal fissure, which divides the cerebrum into two hemispheres.
Brachium Of Superior Colliculus
The superior colliculus is made up of several layers of cells, which anatomists have divided into what are called superficial and deep layers. The superficial layers seem to primarily receive visual information from the retina and the visual cortex, while the deep layers receive information from the auditory, visual, and somatosensory systems. The deep layers also appear to be where the motor functions of the superior colliculus originate, as stimulation of neurons in these layers can produce a variety of movements.
The different layers of the superior colliculi contain what are known as topographic maps for the sense modalities they process information from. The term topographic map in neuroscience is used to refer to an organization where sensory input from a particular region of the body is sent to a specific area of the central nervous system. For example, information from a particular part of the visual field is sent to a corresponding region of the superficial layers of the superior colliculus. Because all of the layers of the superior colliculus have a similar topographic arrangement, it allows for the rapid integration and enhancement of signals that arrive via multiple sense modalities (e.g. vision and hearing). Additionally, because the motor areas of the superior colliculus have the same topographic arrangement as the sensory areas, it allows for the rapid initiation of motor responses to incoming sensory information.
In many other vertebrates (e.g. fish, birds), the superior colliculus is one of the largest brain regions. In humans this is not the case, as it is dwarfed by a number of other structures. Despite its relatively small size, however, the superior colliculus plays a very important role in integrating sensory information and quickly triggering behavioral reactions to it.
Superior Colliculus And Inferior Colliculus
A thick band of white matter that connects the two hemispheres, called the corpus callosum, allows the integration of sensory input and functional responses from both sides of the body. Other cerebral structures include the hypothalamus, which controls metabolism and maintains homeostasis, and the thalamus, a principal sensory relay center. These structures surround spaces (ventricles) filled with cerebrospinal fluid, which helps to supply the brain cells with nutrients and provides the brain with shock-absorbing mechanical support.
The superior colliculus (Fig. 16.1) is essential for the generation of an orienting response to an object of visual or auditory interest (Grantyn et al., 1993). Pathways from the superior colliculus to the saccade generator and reticulospinal cells groups generate the eye and head movements, and have been documented in many studies (Harting, 1977; Harting et al., 1980; Moschovakis, 1996; Moschovakis et al., 1996; Olivier et al., 1993; Isa and Sasaki, 2002). Electrical stimulation and recording studios in the superior colliculus demonstrate a topographical motor map for the saccadic eye movements (Robinson, 1972). Large saccades are represented caudally and small saccades rostrally, and rostrolaterally lies the “rostral pole of the saccadic motor map,” a region described as the fixation zone in behavioral experiments (Munoz and Guitton, 1991; Munoz and Wurtz, 1993; Paré et al., 1994). These physiological studies in cat and monkey report regional specializations in the rostral pole considered to promote gaze fixation, and there is some neuroanatomical evidence supporting this (Büttner-Ennever et al., 1999; Sato and Ohtsuka, 1996; Ohtsuka and Nagasaka, 1999).
The superior colliculus of monkeys and humans contains numerous neurons immunoreactive for the calcium-binding proteins calretinin, calbindin, and parvalbumin, but with different distribution patterns throughout the layers (Leuba and Saini, 1996; McHaffie et al., 2001; Soares et al., 2001). Whereas the small calbindin-positive neurons represent predominantly interneurons, some of which are GABAergic, many of the large and medium-sized parvalbumin-containing neurons in the deep layers of the superior colliculus are projection neurons including predorsal bundle fibers, which innervate neurons of the saccadic premotor network in the pontine reticular formation (Mize, 1992, 1996). The deep layers of the superior colliculus contain fiber patches seen with acetylcholinesterase histochemistry (see Paxinos and Huang, 1995, figure 58). These patches overlap with neuron clusters that give rise to the tectoreticular pathways controlling the orientation of the eye and head to alerting stimuli (Jeon and Mize, 1993; Mize, 1996).
These neuron clusters were shown to receive transmitter specific afferents from the pedunculopontine tegmental nucleus (PPT) associated with attention (Zweig et al., 1987), which contain acetylcholine and nitric oxide, and from the GABAergic cells in the substantia nigra pars reticulata (Harting et al., 1991, 1992, 1997), which impose a tonic inhibition on the superior colliculus that pauses during saccades (Hikosaka and Wurtz, 1983). In the cat, equal numbers of tectotectal fibers are GABAergic or glutamatergic with identical topographical distribution within the contralateral SC (Olivier et al., 1993; Hardy,
The intermediate and deep layers of the superior colliculus are the origin of the tectospinal system. The descending collicular fibers decussate below the oculomotor nucleus in the “dorsal tegmental decussation” and run close to the midline in the “predorsal bundle.” Collicular terminals were found in the nucleus reticularis tegmenti pontis (RtTg), the caudal pontine reticular nucleus (PnC), and the medullary reticular formation mainly ipsilaterally (Sparks and Hartwich-Young, 1989), and they project directly to upper cervical motoneurons in the spinal cord (May and Porter, 1992). Ascending projections target the interstitial nucleus of Cajal (InC) and the rostral interstitial nucleus of the mlf (RI).
The premotor neurons of the levator palpebrae muscle in the M-group receive afferents only from the medial part of the SC, which mediates upgaze (Horn and Büttner-Ennever, 1998b). Afferents to the superior colliculus arise from many regions: cortical areas, thalamus, sensory trigeminal nucleus (including afferents related to the blink system), pedunculopontine tegmental nucleus, and substantia nigra. The afferents terminate in specific tiers, or sublayers, of the superior colliculus intermediate layer, and also are arranged in discrete clusters, whereby the sensory- and motor-related terminal clusters are interdigitated.
What is the function of the inferior colliculus?
Inferior colliculus. The inferior colliculus is a part of the midbrain that serves as the main auditory (sound) center for the body. It acts as the channel for almost all auditory signals in the human body. Its primary roles are signal integration, frequency recognition, and pitch discrimination.
Where are the superior Colliculi located?
The two superior colliculi sit below the thalamus and surround the pineal gland in the vertebrate midbrain. It comprises the dorsal aspect of the midbrain, posterior to the periaqueductal gray and immediately superior to the inferior colliculus.