Investigating mouse superior colliculus function using two-photon calcium imaging

The superior colliculus occupies a privileged position in the hierarchy of sensorimotor circuits, sitting at the interface between sensory input and motor output. Its laminar organization mirrors this role: the superficial layers receive direct retinal input and construct a topographic map of visual space, while the intermediate and deep layers integrate visual, auditory, and somatosensory signals and issue commands for orienting movements of the eyes and head. This convergence of sensory channels onto a common motor map has made the structure a long-standing model for studying how the brain computes behavioral choices from multimodal evidence. Yet the same anatomical position that makes the colliculus functionally interesting also makes it one of the most surgically demanding targets for optical imaging in the mouse brain, and the technical narrative described in the source review reflects a decade of incremental problem-solving by individual laboratories.

Two-photon microscopy itself deserves brief consideration as the method of choice for this structure. Unlike single-photon or confocal approaches, two-photon excitation relies on the near-simultaneous absorption of two infrared photons by a fluorophore, an event that only occurs with appreciable probability at the focal point of the objective. The use of longer-wavelength light reduces scattering and phototoxicity in biological tissue, allowing imaging several hundred micrometers below the surface with cellular resolution. For the superior colliculus, this depth range is well matched to the structure’s dimensions, since the superficial layers span roughly the first 300 micrometers below the dorsal surface and the intermediate layers extend somewhat deeper. Calcium indicators, whether synthetic dyes loaded into cells or genetically encoded proteins expressed through viral vectors, translate neuronal activity into fluorescence changes by binding calcium ions that enter during action potentials. The resulting signals provide an indirect but reliable readout of spiking activity, with genetically encoded indicators offering the additional advantage of cell-type specificity through Cre-dependent viral strategies, as reflected in the studies targeting Vgat-positive, Gad2-positive, and Ntsr1-positive neuronal populations listed in the review’s summary table.

The surgical challenge posed by the transverse sinus is worth examining in more detail, because it illustrates why the displacement method has become so widely adopted. The transverse sinus is a large venous channel running within the dura mater along the posterior aspect of the skull, and it drains blood from the brain toward the jugular system. Puncturing or rupturing this vessel during surgery would cause substantial bleeding that could obscure the imaging field, compromise the health of the animal, and potentially damage the underlying collicular tissue. The insight that the sinus lacks firm adhesion to the brain surface, and can therefore be gently pushed aside without tearing, transformed the field’s approach to accessing the caudal colliculus. The mechanical displacement must be performed with care, since excessive pressure could compress the underlying neural tissue or impair venous drainage, but when done properly it preserves both the overlying cortex and the vascular integrity of the preparation. This stands in contrast to the cortical aspiration approach, which necessarily severs corticocollicular projections and raises interpretive questions about how much of the observed collicular activity depends on this descending input.

The functional significance of corticocollicular projections is not trivial. The mouse visual cortex sends substantial projections to the superior colliculus, originating from layer 5 neurons in primary visual cortex and higher visual areas. These descending inputs modulate collicular responses and are thought to contribute to the selection of behaviorally relevant stimuli, particularly in contexts requiring learned or attention-dependent modulation of orienting behavior. The finding cited in the review, that optogenetic silencing of visual cortex reduces stimulus-evoked responses in collicular neurons by roughly half while preserving their tuning properties, suggests that cortical input amplifies collicular responses without fundamentally reshaping their feature selectivity. This observation has practical implications for imaging experiments: acute preparations that remove cortex may still yield meaningful maps of stimulus tuning, but absolute response magnitudes and any cortical-dependent modulation of those responses will be altered. Chronic preparations using sinus displacement, by contrast, retain the full corticocollicular loop and are therefore better suited for studying how cortical and subcortical circuits interact during behavior.

The distinction between acute and chronic preparations carries additional weight beyond the preservation of specific circuits. Acute preparations, in which imaging is performed in a single session following surgery, offer the advantage of controlled conditions and the ability to use synthetic calcium dyes that can be loaded directly into tissue. These dyes, such as Cal-520 AM and OGB-1 AM, typically produce large fluorescence transients and can be used without the time required for viral expression. Chronic preparations, by contrast, allow the same neurons to be tracked across days or months, opening the door to longitudinal studies of learning, adaptation, and plasticity. The durations reported in the review’s table, ranging from several weeks to six months, indicate that stable optical access to the colliculus can be maintained for substantial periods, provided the imaging window remains clear and the animal tolerates the implantation. The choice between acute and chronic approaches therefore depends on the scientific question: population mapping of functional organization may be accomplished in a single session, whereas questions about how collicular representations change with experience require longitudinal access.

The range of imaging depths achieved across studies, from roughly 10 to 400 micrometers below the collicular surface, reflects both the optical properties of the preparation and the biological questions being pursued. Superficial layer imaging, at depths under 100 micrometers, primarily captures neurons receiving direct retinal input and is well suited for studying retinotopic maps, direction selectivity, and the responses of specific retinal boutons or axons that have been labeled through viral strategies targeting the eye. Deeper imaging, extending into the intermediate layers, accesses neurons involved in sensorimotor integration, including those receiving convergent inputs from multiple modalities and those projecting to motor circuits. The ability to target specific cell types through Cre-dependent viral expression, combined with depth-resolved imaging, allows researchers to dissect how different collicular populations contribute to behaviors such as prey capture, predator avoidance, and visually guided navigation.

The viral vectors used across the studies compiled in the review also merit attention. Adeno-associated virus serotypes 1, 2, and 9, and combinations thereof, are the workhorses of collicular gene delivery, chosen for their efficiency of transduction, their relatively low immunogenicity, and their ability to support cell-type-specific expression through the use of Cre-responsive promoters such as FLEX or fDIO. The choice of promoter, whether the broadly active human synapsin promoter or cell-type-specific Cre-dependent constructs, determines which neurons express the calcium indicator. The choice of indicator itself, spanning the GCaMP6 family, the faster GCaMP7 and GCaMP8 variants, and the red-shifted jRGECO1a, reflects trade-offs between signal-to-noise ratio, kinetics, and spectral properties. Faster indicators such as jGCaMP8m are better suited for resolving the timing of individual spikes or high-frequency firing, while earlier-generation indicators may offer larger signals per spike at the cost of slower decay kinetics. Red-shifted indicators provide the additional advantage of imaging in a spectral channel distinct from green fluorophores, enabling simultaneous two-color imaging or reducing hemoglobin absorption at greater depths.

Looking forward, the review’s authors point toward continued refinement of these methods, with the goal of making collicular imaging less invasive and more broadly accessible. Several directions seem plausible based on the trajectory of the field. Improvements in viral vector design and indicator sensitivity may reduce the amount of light needed to detect activity, allowing imaging at greater depths or with lower laser power. Advances in surgical technique, perhaps including refined tools for sinus displacement or alternative optical access routes, may reduce the invasiveness of the preparation. The development of miniature head-mounted two-photon microscopes, while still technically demanding, could eventually allow collicular imaging in freely moving animals, extending the range of naturalistic behaviors that can be studied. Finally, the combination of calcium imaging with other techniques, such as optogenetic perturbation of specific inputs or simultaneous electrophysiological recording, will continue to deepen the mechanistic understanding of how this midbrain structure transforms sensory signals into behavioral commands.

The broader significance of these technical advances extends beyond the colliculus itself. The methods developed to access this deep brain structure, including cortical aspiration, sinus displacement, and chronic window implantation, provide templates that can be adapted to other subcortical targets previously considered inaccessible to optical imaging. As these techniques mature and disseminate, the population of brain regions amenable to cell-resolved functional imaging in vivo continues to expand, bringing within reach a more complete understanding of how circuits distributed across the neuraxis cooperate to generate perception and action.

Subject of Research: Investigating mouse superior colliculus function using two-photon calcium imaging

Article Title: Investigating mouse superior colliculus function using two-photon calcium imaging

Article References: Lee, H., & Stuart, G. J. (2026). Investigating mouse superior colliculus function using two-photon calcium imaging. Frontiers in Cellular Neuroscience, 20, Article 1922414. https://doi.org/10.3389/fncel.2026.1922414

Image Credits: AI Generated

DOI: 10.3389/fncel.2026.1922414

Keywords: Investigating, mouse, superior, colliculus, function, two-photon, calcium, imaging, scientific research