A developmental critical period for ocular dominance plasticity of binocular neurons in mouse superior colliculus

Experience-dependent plasticity of multiple receptive field properties in lateral geniculate binocular neurons during the critical period

The visual thalamus serves as a critical hub for feature preprocessing in visual processing pathways. Emerging evidence demonstrates that experience-dependent plasticity can be revealed by monocular deprivation (MD) in the dorsolateral geniculate nucleus (dLGN) of the thalamus. However, whether and how this thalamic plasticity induces changes in multiple receptive field properties and the potential mechanisms remain unclear. Using in vivo electrophysiology, here we show that binocular neurons in the dLGN of 4-day MD mice starting at P28 undergo a significant ocular dominance (OD) shift during the critical period. This OD plasticity could be attributed to the potentiation of ipsilateral eye responses but not to the depression of deprived eye responses, contrasting with conventional observations in the primary visual cortex (V1). The direction and orientation selectivity of ipsilateral eye responses, but not of contralateral eye responses in these neurons, were dramatically reduced. Developmental analysis revealed pre-critical and critical period-associated changes in densities of both GABA positive neurons and GABAA receptor α1 subunit (GABRA1) positive neurons. However, early compensatory inhibition from V1 feedback in P18 MD mice maintained network stability with no changes in OD and feature selectivity. Mechanistically, pharmacological activation of GABAA receptors rescued the MD-induced OD shifts and feature selectivity impairments in critical period MD mice, operating independently of the V1 feedback. Furthermore, under different contrast levels and spatial frequencies, these critical period-associated changes in receptive field properties still indicate alterations in ipsilateral eye responses alone. Together, these findings provide novel insights into the developmental mechanisms of thalamic sensory processing, highlighting the thalamus as an active participant in experience-dependent visual plasticity rather than merely a passive relay station. The identified GABA-mediated plasticity mechanisms offer potential therapeutic targets for visual system disorders.

Evidence for a push-pull interaction between superior colliculi in monocular dynamic vision mode

Visual perception can operate in two distinct vision modes-static and dynamic-that have been associated with different neural activity regimes in the superior colliculus (SC). However, the associated pathway-wide mechanisms remain poorly understood, especially in terms of corticotectal and tectotectal feedback upon encoding the continuity illusion during the dynamic vision mode. Here, we harness functional MRI combined with rat brain lesions to investigate whole-pathway neural interactions in the dynamic vision mode. We find a push-pull mechanism embodying contralateral suppression of SC activity opposing positive ipsilateral neural activation upon monocular visual stimulation. Cortical amplification is confirmed through cortical lesions, while further lesioning the ipsilateral SC leads to a boost in the contralateral SC negative signals, suggesting a tectal origin for the push-pull interaction. These results highlight hitherto unreported frequency-dependent modulations in the tectotectal pathway and further challenge the notion that intertectal connections solely serve as reciprocal inhibitory mechanisms for avoiding visual blur during saccades.

Binocular processing facilitates escape behavior through multiple pathways to the superior colliculus

The superior colliculus (SC) is the main brain region regulating defensive behaviors to visual threats. Yet, how the SC integrates binocular visual information and to what extent binocular vision drives defensive behaviors remains unknown. Here, we show that SC neurons respond to binocular visual input with diverse synaptic and spiking responses, summating visual inputs largely sublinearly. Using pathway-specific optogenetic silencing, we find that contralateral and ipsilateral visual information is carried to binocular SC neurons through retinal, interhemispheric, and corticotectal pathways. These pathways carry binocular visual input to the SC in a layer-specific manner, with superficial layers receiving visual information through retinal input, whereas intermediate and deep layers rely on interhemispheric and corticotectal pathways. We further show that binocular vision facilitates visually evoked escape behavior. Together, our data shed light on the cellular and circuit mechanisms underlying binocular visual processing in the SC and its role in defensive behaviors to visual threats.

Genetically defined neuron types underlying visuomotor transformation in the superior colliculus

The superior colliculus (SC) is a conserved midbrain structure that is important for transforming visual and other sensory information into motor actions. Decades of investigations in numerous species have made the SC and its nonmammalian homologue, the optic tectum, one of the best studied structures in the brain, with rich information now available regarding its anatomical organization, its extensive inputs and outputs and its important functions in many reflexive and cognitive behaviours. Excitingly, recent studies using modern genomic and physiological approaches have begun to reveal the diverse neuronal subtypes in the SC, as well as their unique functions in visuomotor transformation. Studies have also started to uncover how subtypes of SC neurons form intricate circuits to mediate visual processing and visually guided behaviours. Here, we review these recent discoveries on the cell types and neuronal circuits underlying visuomotor transformations mediated by the SC. We also highlight the important future directions made possible by these new developments.

Gene-environmental regulation of the postnatal post-mitotic neuronal maturation

Embryonic neurodevelopment, particularly neural progenitor differentiation into post-mitotic neurons, has been extensively studied. While the number and composition of post-mitotic neurons remain relatively constant from birth to adulthood, the brain undergoes significant postnatal maturation marked by major property changes frequently disrupted in neural diseases. This review first summarizes recent characterizations of the functional and molecular maturation of the postnatal nervous system. We then review regulatory mechanisms controlling the precise gene expression changes crucial for the intricate sequence of maturation events, highlighting experience-dependent versus cell-intrinsic genetic timer mechanisms. Despite significant advances in understanding of the gene-environmental regulation of postnatal neuronal maturation, many aspects remain unknown. The review concludes with our perspective on exciting future research directions in the next decade.