The superior and inferior colliculi are four small bumps on the back surface of the midbrain that serve as the brain’s primary relay centers for vision and hearing, respectively. Together they form the tectum, a structure so ancient that some version of it exists in every vertebrate animal alive today. The superior pair handles visual processing, eye movements, and spatial attention; the inferior pair processes sound frequency, timing, and location. Despite their modest size, these structures sit at a crossroads of sensory and motor information that makes them far more influential than most people realize.
Where They Sit and What They Look Like
If you could look at the midbrain from behind, you would see four rounded elevations arranged in two rows. The upper two are the superior colliculi, and the lower two are the inferior colliculi. This quartet is collectively called the corpora quadrigemina, and together with a thin layer of overlying tissue they make up almost the entire tectum of the midbrain.1ScienceDirect / Practical Surgical Neuropathology: A Diagnostic Approach. Midbrain Tectum – Section: Normal Brain Histopathology Each colliculus is roughly the size of a small pea, but packed with layered circuits that perform surprisingly sophisticated computations before information ever reaches the cerebral cortex.
The superior colliculi sit just behind the thalamus, near the pineal gland, while the inferior colliculi protrude slightly farther back and lower. A ridge of fibers called the brachium connects each inferior colliculus to the medial geniculate body of the thalamus (the auditory relay station), and a separate brachium links each superior colliculus to the lateral geniculate body (the visual relay). These fiber bundles are the highways that carry processed sensory signals upward toward the cortex.
The Superior Colliculus and Vision
The superior colliculus is built in layers, and those layers divide neatly into two functional zones. The superficial layers sit near the surface and deal almost exclusively with visual information, receiving direct input from the retina. The deeper layers handle motor commands and also receive signals from other senses, including hearing and touch.2Current Biology. The superior colliculus – Section: Anatomical blueprint of the superior colliculus This layered design means visual information arrives at the top and gets progressively combined with other sensory and motor signals as it filters downward.
In many animals, the superior colliculus is the dominant visual center. In mice, for example, it is the most prominent visual processing area in the brain, surpassing even the visual cortex in its role for guiding visually driven behavior.3Annual Reviews. Visual Function, Organization, and Development of the Mouse Superior Colliculus In humans, the visual cortex has taken on a much larger share of conscious visual perception, but the superior colliculus still plays an essential role in rapid, reflexive visual processing, including the kind that happens before you are consciously aware of what you are looking at.
Eye Movements and Gaze Shifts
One of the superior colliculus’s best-studied jobs is controlling saccades, the rapid eye movements you make several times per second to shift your gaze from one point to another. The superior colliculus and a cortical area called the frontal eye field both project to brainstem premotor circuits where saccades are ultimately generated.4PubMed. Interaction of the frontal eye field and superior colliculus for saccade generation The two structures work in parallel rather than in a strict hierarchy: either one can drive a saccade on its own, but together they produce the precise, rapid gaze shifts you use to read a page, scan a crowd, or track a moving ball.
Inside the superior colliculus, saccade targets are laid out as a map. Imaging studies in humans have confirmed that eye movements along the up-down visual axis map across the side-to-side anatomy of the colliculus, and this motor map lines up with the visual map of the world that the superficial layers maintain. Saccade-related activity also occurs deeper in the colliculus than purely visual responses, consistent with the layered architecture described above.5PubMed Central. Polar-angle representation of saccadic eye movements in human superior colliculus
Beyond eye movements alone, the deeper layers of the superior colliculus help coordinate whole-body orienting responses. In cats allowed to move their heads freely, the vast majority of tested neurons in the pathway running from the tectum through the brainstem increased their firing before gaze shifts that matched each cell’s preferred direction. The response was strongest for visually triggered movements and weaker when the animal oriented toward a predicted target.6American Physiological Society / Journal of Neurophysiology. Control of orienting gaze shifts by the tectoreticulospinal system in the head-free cat. III. Spatiotemporal characteristics of phasic motor discharges So the superior colliculus is not just an eye-movement generator; it orchestrates the coordinated turning of eyes, head, and sometimes the whole body toward something worth looking at.
Spatial Attention Without Moving the Eyes
You do not always move your eyes toward what you are paying attention to. You can attend to something in your peripheral vision while keeping your gaze fixed elsewhere. It turns out the superior colliculus is involved even in these covert shifts of attention. Research has shown that collicular activity tracks where attention is directed and is necessary for normal spatial attention during perceptual tasks, even when no eye movement occurs. The circuits involved operate at least partly independently of the better-known attention signals in visual cortex.7PubMed Central. Superior colliculus and visual spatial attention
Recent work has refined this picture further. Neurons in the superior colliculus that bridge visual and motor functions modulate their firing in ways that track an animal’s perceptual sensitivity to stimuli at a given location.8Nature Communications. Attention-related modulation in the superior colliculus encodes perceptual sensitivity, but not perceptual choice In plain terms, the colliculus seems to help determine how well you can detect something at a particular spot in the visual field. It shapes what you notice, not just where you look.
Threat Detection and Defensive Behavior
The superior colliculus has a special sensitivity to looming stimuli, objects that appear to be rapidly approaching on a collision course. This makes evolutionary sense: detecting an incoming predator or projectile fast enough to duck is a matter of survival. Brain imaging in humans has shown that the colliculus encodes both the looming motion of an object and the type of object independently, meaning it can distinguish “something is rushing toward me” from “what is rushing toward me” using partly separate coding schemes. Collicular responses to looming stimuli also correlate with the viewer’s emotional reaction, linking to lower feelings of pleasantness and higher feelings of fear and arousal.9Cell Reports. Superior colliculus responses encode looming motion and object category and covary with emotional experience
A related line of research has confirmed that an expansion-specific model based on collision detection accurately predicts human superior colliculus responses, supporting the idea that the colliculus retains an ancient, hardwired alarm system for approaching objects.10PubMed Central. Visual looming is a primitive for human emotion This system likely operates beneath conscious awareness in many situations, triggering a flinch or a freeze before you have time to think about what is happening.
Combining Senses in the Deep Layers
The deep layers of the superior colliculus do something remarkable: they combine information from different senses. A neuron there might respond to both a flash of light and a sound, and when both stimuli come from the same location at the same time, the response is not simply the sum of the two inputs. Instead, the interaction is often multiplicative, meaning a weak visual signal and a weak auditory signal together produce a much stronger response than either alone.11PubMed. Visual, auditory, and somatosensory convergence on cells in superior colliculus results in multisensory integration This principle, sometimes called “superadditive” enhancement, is most dramatic when each individual stimulus is barely detectable on its own.
Spatial alignment matters enormously for this process. When stimuli from different senses come from the same spot, about nine in ten multisensory neurons show enhanced responses. But when the stimuli are separated in space so that one falls outside its corresponding receptive field, the response either drops or shows no multisensory benefit at all.12PubMed. Spatial determinants of multisensory integration in cat superior colliculus neurons This is why a ventriloquist’s trick works: your brain merges the sight of moving lips with a voice that is actually coming from a slightly different location, because the spatial discrepancy is small enough that the colliculus treats them as one event.
The Inferior Colliculus and Sound
While the superior colliculus handles vision and multisensory integration, the inferior colliculus is the central hub of the auditory pathway. Nearly every ascending auditory signal passes through it before reaching the thalamus and cortex. The inferior colliculus is organized tonotopically, meaning different sound frequencies are processed in an orderly spatial arrangement, much like the keys on a piano. In humans, this frequency gradient runs through the depth of the structure, with gradients measured at roughly 0.3 to 1.2 octaves per millimeter.13PubMed Central. Tonotopic Organization in the Depth of Human Inferior Colliculus
Look more closely, and the tonotopic map is not a smooth ramp. In rats, recordings taken at very fine spatial intervals reveal a staircase pattern: frequency stays nearly constant across a stretch of tissue and then jumps about a third of an octave before leveling off again. These “steps” repeat in a regular pattern, suggesting the colliculus devotes defined slabs of neural tissue to each frequency band.14Journal of Neuroscience. A Discontinuous Tonotopic Organization in the Inferior Colliculus of the Rat In mice, the highest frequencies occupy the inner (medial) part of the central nucleus, with isofrequency planes tilting from the inside out and from back to front.15PubMed. Inferior colliculus of the house mouse. I. A quantitative study of tonotopic organization, frequency representation, and tone-threshold distribution
Locating Sound in Space
Your brain figures out where a sound is coming from by comparing subtle differences in what each ear receives. The inferior colliculus is where many of those comparisons converge. Individual neurons in the central nucleus are sensitive to multiple spatial cues at once, including differences in timing between the ears, differences in loudness between the ears, and the way the shape of the outer ear filters the sound spectrum. Neurons tuned to lower frequencies tend to rely on timing differences, while those tuned to higher frequencies rely more on loudness differences and spectral cues.16PubMed Central. Information conveyed by inferior colliculus neurons about stimuli with aligned and misaligned sound localization cues
The localization cues are encoded in several aspects of a neuron’s response. Firing rate carries the most information, but first-spike timing also contributes, especially for loudness differences between the ears.17PubMed. Cues for sound localization are encoded in multiple aspects of spike trains in the inferior colliculus The system is not perfectly compartmentalized; neurons often encode more than one cue type, particularly at higher frequencies, making the representation somewhat diffuse rather than cleanly segregated.
Temporal Patterns and Sound Texture
Beyond frequency and location, the inferior colliculus is heavily involved in processing the temporal texture of sound: how quickly a sound’s loudness fluctuates over time and how those fluctuations change across different frequencies. Neurons in the inferior colliculus respond selectively to specific rates of amplitude modulation, typically peaking at modulation rates below about 100 cycles per second.18PubMed. Auditory temporal processing: responses to sinusoidally amplitude-modulated tones in the inferior colliculus This temporal tuning is what helps you perceive the difference between a buzzing insect and a purring cat, even if the fundamental pitch is similar.
There is also an interesting tradeoff along the tonotopic axis. Neurons tuned to low frequencies tend to prefer fast temporal modulations and broad spectral patterns, while neurons tuned to high frequencies tend to prefer slower temporal modulations but finer spectral detail.19PubMed Central. Spectral and temporal modulation tradeoff in the inferior colliculus This gradient may reflect an optimization for the kinds of sound features that are most informative at different frequency ranges in natural environments.
Sending Signals Up and Receiving Orders Down
The inferior colliculus does not simply pass information upward to the thalamus in a single stream. Its subdivisions maintain parallel pathways to different parts of the auditory thalamus. The central nucleus projects mainly to one thalamic region, the dorsal cortex to another, and a third subdivision feeds yet another thalamic target. Each pathway also receives some cross-talk from the “wrong” collicular subdivision, so the channels are parallel but not perfectly isolated.20Frontiers in Neuroanatomy. Excitatory and inhibitory projections in parallel pathways from the inferior colliculus to the auditory thalamus
Just as interesting is the traffic going in the opposite direction. The auditory cortex sends signals back down to the inferior colliculus and can reshape how midbrain neurons respond to frequency, loudness, duration, and spatial cues.21Frontiers in Neural Circuits. Cortical modulation of auditory processing in the midbrain – Section: Corticofugal modulation of auditory processing in the inferior colliculus When the auditory cortex is experimentally silenced, nearly half of inferior colliculus neurons change their sensitivity to the loudness differences between the ears that underlie sound localization. Some shift from a smoothly increasing response to one that peaks and then drops off, and vice versa.22PubMed. Descending projections from auditory cortex modulate sensitivity in the midbrain to cues for spatial position In other words, your cortex constantly tunes the midbrain’s listening parameters, likely as part of how you focus on one voice in a noisy room or track a moving sound source.
Novelty Detection and Neurochemistry
The inferior colliculus also performs a kind of novelty filtering. When the same sound repeats over and over, collicular neurons gradually reduce their response, but they fire vigorously again when a different sound appears. This stimulus-specific adaptation helps the brain flag unexpected sounds. The process depends on a balance of brain chemicals: blocking inhibitory receptors causes an overall increase in neural activity, blocking excitatory receptors causes an overall decrease, and the neurotransmitter acetylcholine selectively affects the response to the repeated sound rather than the novel one.23PubMed. Stimulus-specific adaptation in the inferior colliculus: The role of excitatory, inhibitory and modulatory inputs Each of these chemical systems sculpts a different aspect of how the colliculus separates “same old sound” from “something new.”
An Ancient Structure Across Species
The tectum is one of the oldest brain structures in vertebrate evolution. Every vertebrate from fish to humans has some version of it, though its relative importance has shifted over time. In fish and amphibians, the tectum (the equivalent of the superior colliculus) is the dominant visual processing center, handling most of what we would call “seeing.” As the cerebral cortex expanded in mammals, cortical visual areas took over much of conscious visual perception, and the superior colliculus became more specialized for rapid orienting, attention, and reflexive responses.24PubMed Central. The tectum/superior colliculus as the vertebrate solution for spatial sensory integration and action The underlying wiring plan, however, remains recognizable across all these species, which is why studies in cats, mice, and monkeys continue to illuminate how the human colliculi work.
How the Maps Get Wired During Development
The orderly maps in the superior colliculus, where neighboring points in the visual field are processed by neighboring neurons, do not happen by accident. During embryonic development, axons growing from the retina to the colliculus are guided by concentration gradients of signaling molecules on both surfaces. One family of these molecules, the Eph receptors and their ephrin partners, sets up the map along two axes. One set handles the nasal-to-temporal axis of the retina, and another set handles the dorsal-to-ventral axis. Because both the receptor and the signaling molecule are anchored on cell surfaces, they can push or pull growing axons in both directions, steering them through attraction and repulsion to land in the correct spot.25PubMed Central. Eph and ephrin signaling in the formation of topographic maps When these molecular gradients are experimentally disrupted, the visual map in the colliculus becomes scrambled.
When Things Go Wrong
Damage or dysfunction involving the colliculi shows up in distinct clinical ways depending on which structure is affected. Problems with the superior colliculus or its brainstem connections can produce abnormal eye movements. In progressive supranuclear palsy, a neurodegenerative disease, vertical saccades become slow, curved, and fragmented. The primary cause appears to be dysfunction of the burst neurons in the midbrain that generate the motor command for saccades, rather than a problem with the cells that normally keep unwanted saccades in check.26PubMed. Pathophysiology of slow vertical saccades in progressive supranuclear palsy Abnormal collicular activity then compounds the problem by producing saccades that veer off course mid-flight.27PubMed Central. Saccades in Progressive Supranuclear Palsy–Maladapted, Irregular, Curved, and Slow
On the auditory side, the inferior colliculus has been implicated in tinnitus, the perception of sound (often ringing or buzzing) without an external source. After noise exposure or certain drugs, the balance between excitation and inhibition in the inferior colliculus shifts.28PubMed Central. Tinnitus-related changes in the inferior colliculus In rats exposed to blast noise, those showing behavioral signs of tinnitus developed hyperactive neurons across the colliculus within a day. Over the following months, this hyperactivity shifted across frequency regions and was accompanied by increased burst firing, suggesting ongoing neural reorganization that may sustain the phantom sound.29PubMed. Blast-induced tinnitus and spontaneous activity changes in the rat inferior colliculus
The Inferior Colliculus and Seizures
A less well-known clinical connection involves audiogenic seizures, seizures triggered by loud sounds. The inferior colliculus is established as the initiation site for these seizures in susceptible animals. The central nucleus appears to be the most critical region for seizure initiation, while the external cortex of the inferior colliculus plays a larger role in propagation. The dorsal cortex, by contrast, seems relatively uninvolved.30PubMed. Differential roles in the neuronal network for audiogenic seizures are observed among the inferior colliculus subnuclei and the amygdala Blocking excitatory receptors in the central nucleus and external cortex can halt these seizures at identical drug doses, while the same treatment in the dorsal cortex has no effect.31PubMed. The role of the inferior colliculus in a genetic model of audiogenic seizures This selective vulnerability across subdivisions of a single structure highlights how functionally distinct the collicular subregions really are.
Echolocation and the Specialist Inferior Colliculus
Some of the most striking examples of collicular specialization come from echolocating bats. These animals emit ultrasonic pulses and listen for the returning echoes to map their surroundings in three dimensions. In the mustached bat, the inferior colliculus contains neurons tuned to specific delays between the outgoing pulse and the returning echo, effectively measuring target distance. These delay-tuned neurons are created through a series of integrative interactions within the colliculus itself, rather than simply inheriting the property from upstream stations. The colliculus then sends this distance information to premotor centers that help guide the bat’s flight.32PubMed Central. Neural processing of target distance by echolocating bats: functional roles of the auditory midbrain The basic architecture of the inferior colliculus is the same as in other mammals, but evolution has layered on a highly specialized computation that converts what is essentially a timing problem into a spatial map of the world.