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.

The superior colliculus is a layered, multi-sensory structure. Its upper layer receives visual signals from the retina of the eye, while the lower layers process multiple signals from various other parts of the brain. This colliculus is not restricted to a visual role alone. It also helps with the orientation of the eyes and head.

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; cerebrumBottom-up view of the skull and the white matter pathways of the visual system. Diffusion-weighted imaging allows those pathways to be identified noninvasively in humans. Their investigation may reveal neuroanatomical deficits associated with specific visual disorders.© 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.