Functional convergence of on-off direction-selective ganglion cells in the visual thalamus

Experience influences the refinement of feature selectivity in the mouse primary visual thalamus

Neurons exhibit selectivity for specific features: a property essential for extracting and encoding relevant information in the environment. This feature selectivity is thought to be modifiable by experience at the level of the cortex. Here, we demonstrate that selective exposure to a feature during development can alter the population representation of that feature in the primary visual thalamus. This thalamic plasticity is not due to changes in corticothalamic inputs and is blocked in mutant mice that exhibit deficits in retinogeniculate refinement, suggesting that plasticity is a direct result of changes in feedforward connectivity. Notably, experience-dependent changes in thalamic feature selectivity also occur in adult animals, although these changes are transient, unlike in juvenile animals, where they are long lasting. These results reveal an unexpected degree of plasticity in the visual thalamus and show that salient environmental features can be encoded in thalamic circuits during a discrete developmental window.

Retinal ganglion cells encode the direction of motion outside their classical receptive field

Retinal ganglion cells (RGCs) typically respond to light stimulation over their spatially restricted receptive field. Using large-scale recordings in the mouse retina, we show that a subset of non- direction-selective (DS) RGCs exhibit asymmetric activity, selective to motion direction, in response to a stimulus crossing an area far beyond the classic receptive field. The extraclassical response arises via inputs from an asymmetric distal zone and is enhanced by desensitization mechanisms and an inherent DS component, creating a network of neurons responding to motion toward the optic disc. Pharmacological manipulations revealed the necessity of glycinergic amacrine cells for this response. Using in vivo recordings, we identified similar extraclassical responses in lateral geniculate nucleus neurons, suggesting such non conventional DS information is transferred to downstream structures. Our results suggest a complex integration of motion direction processing across the visual field, which arises beyond the classical receptive field boundaries.

Retinal direction of motion is reliably transmitted to visual cortex through highly selective thalamocortical connections

Motion perception is crucial to animal survival and effective environmental interactions. In mammals, detection of movement begins in the retina. Directionally selective (DS) retinal ganglion cells were first discovered in the rabbit eye,1 and they have since been found in mouse,2,3 cat,4 and monkey.5,6 These DS retinal neurons contact a small population of neurons in the visual thalamus (dorsal lateral geniculate nucleus [LGN]) that are highly DS.7,8,9,10 The primary visual cortex (V1) also contains DS neurons, but whether directional selectivity in V1 emerges de novo11,12,13 or is inherited from DS thalamic inputs14,15,16 remains unclear. We previously found that LGN-DS neurons generate strong and focal synaptic currents in rabbit V1, similar to those generated by LGN concentric cells.17 Thus, the synaptic drive generated by LGN-DS neurons in V1 is spatially well situated to influence the firing of layer 4 (L4) simple cells, most of which show strong directional selectivity.18 However, two important questions remain: do LGN-DS neurons synaptically target DS simple cells in L4, and, if so, do they contribute to the directional preferences of these V1 DS neurons? We used spike-train cross-correlation analysis of pairs of LGN-DS and L4 simple cells to address these questions. We found that LGN-DS neurons do target L4 DS simple cells and that the targeting is highly selective, largely following a simple set of "connectivity rules." We conclude that this highly selective thalamocortical connectivity of LGN-DS neurons contributes to the sharp directional selectivity of cortical simple cells.

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.

Specific retinal neurons regulate context-dependent defensive responses to visual threat

While encountering a visual threat, an animal assesses multiple factors to choose an appropriate defensive strategy. For example, when a rodent detects a looming aerial predator, its behavioral response can be influenced by a specific environmental context, such as the availability of a shelter. Indeed, rodents typically escape from a looming stimulus when a shelter is present; otherwise, they typically freeze. Here we report that context-dependent behavioral responses can be initiated at the earliest stage of the visual system by distinct types of retinal ganglion cells (RGCs), the retina's output neurons. Using genetically defined cell ablation in mature mice, we discovered that some RGC types were necessary for either escaping (alpha RGCs) or freezing (intrinsically photosensitive RGCs) in response to a looming stimulus but not for both behaviors; whereas other RGC types were not required for either behavior (direction-selective RGCs preferring vertical motion). Altogether, our results suggest that specific RGC types regulate distinct behavioral responses elicited by the same threatening stimulus depending on contextual signals in the environment. These findings emphasize the unique contribution of early visual pathways to evolutionally conserved behavioral reactions.

Individual thalamic inhibitory interneurons are functionally specialized toward distinct visual features

Inhibitory interneurons in the dorsolateral geniculate nucleus (dLGN) are situated at the first central synapse of the image-forming visual pathway, but little is known about their function. Given their anatomy, they are expected to be multiplexors, integrating many different retinal channels along their dendrites. Here, using targeted single-cell-initiated rabies tracing, we found that mouse dLGN interneurons exhibit a degree of retinal input specialization similar to thalamocortical neurons. Some are anatomically highly specialized, for example, toward motion-selective information. Two-photon calcium imaging performed in vivo revealed that interneurons are also functionally specialized. In mice lacking retinal horizontal direction selectivity, horizontal direction selectivity is reduced in interneurons, suggesting a causal link between input and functional specialization. Functional specialization is not only present at interneuron somata but also extends into their dendrites. Altogether, inhibitory interneurons globally display distinct visual features which reflect their retinal input specialization and are ideally suited to perform feature-selective inhibition.

Differential Expression Analysis Identifies Candidate Synaptogenic Molecules for Wiring Direction-Selective Circuits in the Retina

An organizational feature of neural circuits is the specificity of synaptic connections. A striking example is the direction-selective (DS) circuit of the retina. There are multiple subtypes of DS retinal ganglion cells (DSGCs) that prefer motion along one of four preferred directions. This computation is mediated by selective wiring of a single inhibitory interneuron, the starburst amacrine cell (SAC), with each DSGC subtype preferentially receiving input from a subset of SAC processes. We hypothesize that the molecular basis of this wiring is mediated in part by unique expression profiles of DSGC subtypes. To test this, we first performed paired recordings from isolated mouse retinas of both sexes to determine that postnatal day 10 (P10) represents the age at which asymmetric synapses form. Second, we performed RNA sequencing and differential expression analysis on isolated P10 ON-OFF DSGCs tuned for either nasal or ventral motion and identified candidates which may promote direction-specific wiring. We then used a conditional knock-out strategy to test the role of one candidate, the secreted synaptic organizer cerebellin-4 (Cbln4), in the development of DS tuning. Using two-photon calcium imaging, we observed a small deficit in directional tuning among ventral-preferring DSGCs lacking Cbln4, though whole-cell voltage-clamp recordings did not identify a significant change in inhibitory inputs. This suggests that Cbln4 does not function primarily via a cell-autonomous mechanism to instruct wiring of DS circuits. Nevertheless, our transcriptomic analysis identified unique candidate factors for gaining insights into the molecular mechanisms that instruct wiring specificity in the DS circuit.

Implications of variable synaptic weights for rate and temporal coding of cerebellar outputs

Purkinje cell (PC) synapses onto cerebellar nuclei (CbN) neurons allow signals from the cerebellar cortex to influence the rest of the brain. PCs are inhibitory neurons that spontaneously fire at high rates, and many PC inputs are thought to converge onto each CbN neuron to suppress its firing. It has been proposed that PCs convey information using a rate code, a synchrony and timing code, or both. The influence of PCs on CbN neuron firing was primarily examined for the combined effects of many PC inputs with comparable strengths, and the influence of individual PC inputs has not been extensively studied. Here, we find that single PC to CbN synapses are highly variable in size, and using dynamic clamp and modeling we reveal that this has important implications for PC-CbN transmission. Individual PC inputs regulate both the rate and timing of CbN firing. Large PC inputs strongly influence CbN firing rates and transiently eliminate CbN firing for several milliseconds. Remarkably, the refractory period of PCs leads to a brief elevation of CbN firing prior to suppression. Thus, individual PC-CbN synapses are suited to concurrently convey rate codes and generate precisely timed responses in CbN neurons. Either synchronous firing or synchronous pauses of PCs promote CbN neuron firing on rapid time scales for nonuniform inputs, but less effectively than for uniform inputs. This is a secondary consequence of variable input sizes elevating the baseline firing rates of CbN neurons by increasing the variability of the inhibitory conductance. These findings may generalize to other brain regions with highly variable inhibitory synapse sizes.

Local interneurons in the murine visual thalamus have diverse receptive fields and can provide feature selective inhibition to relay cells

By influencing the type and quality of information that relay cells transmit, local interneurons in thalamus have a powerful impact on cortex. To define the sensory features that these inhibitory neurons encode, we mapped receptive fields of optogenetically identified cells in the murine dorsolateral geniculate nucleus. Although few in number, local interneurons had diverse types of receptive fields, like their counterpart relay cells. This result differs markedly from visual cortex, where inhibitory cells are typically less selective than excitatory cells. To explore how thalamic interneurons might converge on relay cells, we took a computational approach. Using an evolutionary algorithm to search through a library of interneuron models generated from our results, we show that aggregated output from different groups of local interneurons can simulate the inhibitory component of the relay cell's receptive field. Thus, our work provides proof-of-concept that groups of diverse interneurons can supply feature-specific inhibition to relay cells.

Implications of variable synaptic weights for rate and temporal coding of cerebellar outputs

Purkinje cell (PC) synapses onto cerebellar nuclei (CbN) neurons convey signals from the cerebellar cortex to the rest of the brain. PCs are inhibitory neurons that spontaneously fire at high rates, and many uniform sized PC inputs are thought to converge onto each CbN neuron to suppress or eliminate firing. Leading theories maintain that PCs encode information using either a rate code, or by synchrony and precise timing. Individual PCs are thought to have limited influence on CbN neuron firing. Here, we find that single PC to CbN synapses are highly variable in size, and using dynamic clamp and modelling we reveal that this has important implications for PC-CbN transmission. Individual PC inputs regulate both the rate and timing of CbN firing. Large PC inputs strongly influence CbN firing rates and transiently eliminate CbN firing for several milliseconds. Remarkably, the refractory period of PCs leads to a brief elevation of CbN firing prior to suppression. Thus, PC-CbN synapses are suited to concurrently convey rate codes, and generate precisely-timed responses in CbN neurons. Variable input sizes also elevate the baseline firing rates of CbN neurons by increasing the variability of the inhibitory conductance. Although this reduces the relative influence of PC synchrony on the firing rate of CbN neurons, synchrony can still have important consequences, because synchronizing even two large inputs can significantly increase CbN neuron firing. These findings may be generalized to other brain regions with highly variable sized synapses.

Vision: Rules of thalamic mixology

The retina generates rich feature representations of the visual world that pass through the thalamus on their way to cortex and perception. A new study reveals rules that govern the separation and combination of retinal inputs in the thalamus.