Processing a visual stimulation involves multiple areas of the brain. One important component is the superior colliculus. This structure—located deep within the brainstem—helps process both visual and somatic tactile sensations. It is involved in specific eye movements called “saccades,” head movements, and in interpreting body position relative to somatic sensation.
Studying this structure across different species reveals varying functions of this structure. Certain snakes possess the ability to sense infrared signals. This sense is aided by neurons from the trigeminal nerve (cranial nerve V) in contrast to the optic nerve (cranial nerve II) in primates or humans. The trigeminal nerve senses stimulations such as touch, temperature, and pain for the face and helps to explain how these reptiles are able to detect heat signals.
The superior colliculus is located on the uppermost part of the brainstem in the posterior aspect. Surrounding structures include the pineal gland, thalamus, and cerebellum. Piercing through this structure is the cerebral aqueduct responsible for connecting the flow of cerebrospinal fluid from the ventricles to the exterior of the cortex and spinal cord. Within the structure of the superior colliculus, there exist multiple individual structures. Surrounding the cerebral aqueduct is the periaqueductal gray. This area plays a role in the sensation and modulation of pain signals from the periphery. Additionally, the oculomotor nucleus and the Edinger-Westphal nucleus are present. The oculomotor nucleus is responsible for multiple eye movements facilitated by the medial, superior, and inferior rectus muscles. Also, this area controls the opening and closing of the eyelid. The Edinger-Westphal nucleus is responsible for constricting the eye and aiding in accommodation of the lens (seeing close) and convergence of the eye (ensuring both eyes are pointed in similar directions).
Directly inferior to this structure is the inferior colliculus. It is similar in structure but different in function. The major function is processing of auditory information. The location of a sound source can be triangulated utilizing the shorter relay time from the side a sound originates compared to the longer relay time on the opposite side. Taken together, these two structures are known as the tectum or roof of the midbrain.
The visual input from the retina is rapidly conducted via the optic nerve to the occipital cortex, the frontal eye field, and subsequently the superior colliculus. This frontal eye field is specifically responsive to new visual stimulus. For example, consider what happens when another person walks into a room at the periphery of your right visual field. Your brain will give more attention to this new stimulus with the signal being transmitted to neurons within the superior colliculus.
Before the signal arrives, it first had to cross to the contralateral side via the optic chiasm. Similar to the left occipital cortex processing information from the right visual field, the left superior colliculus reflects a map of the right visual field.
The signal from the person walking into the right visual field is processed in the left superior colliculus. At this point, the position of the new stimulus is compared to the location of the fovea in the retina. A calculation of direction and distance is made in order to align the fovea and the new visual stimulus. The next step is to initiate rapid movement of both eyes known as a saccade and head positioning to facilitate this alignment.
In order for the rightward saccade to occur in a horizontal manner, the paramedian pontine reticular formation (PPRF) is engaged. This structure relays a signal to the right eye lateral rectus muscle to contract, allowing for a deviation to the right. Simultaneously, the left eye quickly moves left via a signal relayed by the median longitudinal fasciculus. This structure signals the left oculomotor nerve to facilitate a contraction of the left medial rectus muscle. Ultimately, this coordinated effort brings the new visual stimulus into the focus of both foveas.
Interestingly, the superior colliculus also directs rapid movement of the head and neck. This process takes place via the tectospinal tract. Nerve cell bodies responding to the visual or somatosensory signal on the right will again initiate in the left within the superior colliculus. These tectospinal neurons will decussate (or cross over) at an area called the medulla pyramid in the brainstem. The motor signal will exit at the level of the cervical spinal cord toward musculature in the right periphery. At this point, neck muscles quickly contract to help adjust the visual field to the new stimulus.
Taken together, the superior colliculus is a region within the brainstem responsible for reflexive adjustments to new visual or tactile stimulation, arising from an evolutionarily old portion of the brain (the brainstem itself). It is clear that an animal’s survival can be dependent on quickly processing and adjusting to new information.
See also: Accommodation; Inferior Colliculus; Occipital Lobe; Optic Nerve; Retina; Saccades; Senses of Animals; Thalamus; Visual Fields; Visual System
Dragoi, Valentin, & Chieyeko Tsuchitani. (1997). Visual processing: Cortical processing. In Neuroscience Online, an electronic textbook for the neurosciences (Chap. 15). Retrieved from http://neuroscience.uth.tmc.edu/s2/chapter15.html
Wisconsin University. (2006). Unit No. 2, brain stem: Superior colliculus. Retrieved from http://www.neuroanatomy.wisc.edu/virtualbrain/BrainStem/23Colliculus.html