Local Integration Accounts for Weak Selectivity of Mouse Neocortical Parvalbumin Interneurons

Adolescent neurocognitive development and decision-making abilities regarding gender-affirming care

Recently, politicians and legislative bodies have cited neurodevelopmental literature to argue that brain immaturity undermines decision-making regarding gender-affirming care (GAC) in youth. Here, we review this literature as it applies to adolescents' ability to make decisions regarding GAC. The research shows that while adolescence is a time of peak risk-taking behavior that may lead to impulsive decisions, neurocognitive systems supporting adult-level decisions are available given deliberative processes that minimize influence of short-term rewards and peers. Since GAC decisions occur over an extended period and with support from adult caregivers and clinicians, adolescents can engage adult-level decision-making in this context. We also weigh the benefits of providing GAC access during adolescence and consider the significant costs of blocking or delaying GAC. Transgender and non-binary (TNB) adolescents face significant mental health challenges, many of which are mitigated by GAC access. Further, initiating the GAC process during adolescence, which we define as beginning at pubertal onset, leads to better long-term mental health outcomes than waiting until adulthood. Taken together, existing research indicates that many adolescents can make informed decisions regarding gender-affirming care, and that this care is critical for the well-being of TNB youth. We highlight relevant considerations for policy makers, researchers, and clinicians.

Population coding of time-varying sounds in the nonlemniscal inferior colliculus

The inferior colliculus (IC) of the midbrain is important for complex sound processing, such as discriminating conspecific vocalizations and human speech. The IC's nonlemniscal, dorsal "shell" region is likely important for this process, as neurons in these layers project to higher-order thalamic nuclei that subsequently funnel acoustic signals to the amygdala and nonprimary auditory cortices, forebrain circuits important for vocalization coding in a variety of mammals, including humans. However, the extent to which shell IC neurons transmit acoustic features necessary to discern vocalizations is less clear, owing to the technical difficulty of recording from neurons in the IC's superficial layers via traditional approaches. Here, we use two-photon Ca2+ imaging in mice of either sex to test how shell IC neuron populations encode the rate and depth of amplitude modulation, important sound cues for speech perception. Most shell IC neurons were broadly tuned, with a low neurometric discrimination of amplitude modulation rate; only a subset was highly selective to specific modulation rates. Nevertheless, neural network classifier trained on fluorescence data from shell IC neuron populations accurately classified amplitude modulation rate, and decoding accuracy was only marginally reduced when highly tuned neurons were omitted from training data. Rather, classifier accuracy increased monotonically with the modulation depth of the training data, such that classifiers trained on full-depth modulated sounds had median decoding errors of ∼0.2 octaves. Thus, shell IC neurons may transmit time-varying signals via a population code, with perhaps limited reliance on the discriminative capacity of any individual neuron.NEW & NOTEWORTHY The IC's shell layers originate a "nonlemniscal" pathway important for perceiving vocalization sounds. However, prior studies suggest that individual shell IC neurons are broadly tuned and have high response thresholds, implying a limited reliability of efferent signals. Using Ca2+ imaging, we show that amplitude modulation is accurately represented in the population activity of shell IC neurons. Thus, downstream targets can read out sounds' temporal envelopes from distributed rate codes transmitted by populations of broadly tuned neurons.

Emergence of co-tuning in inhibitory neurons as a network phenomenon mediated by randomness, correlations, and homeostatic plasticity

Cortical excitatory neurons show clear tuning to stimulus features, but the tuning properties of inhibitory interneurons are ambiguous. While inhibitory neurons have been considered to be largely untuned, some studies show that some parvalbumin-expressing (PV) neurons do show feature selectivity and participate in co-tuned subnetworks with pyramidal neurons. In this study, we first use mean-field theory to demonstrate that a combination of homeostatic plasticity governing the synaptic dynamics of the connections from PV to excitatory neurons, heterogeneity in the excitatory postsynaptic potentials that impinge on PV neurons, and shared correlated input from layer 4 results in the functional and structural self-organization of PV subnetworks. Second, we show that structural and functional feature tuning of PV neurons emerges more clearly at the network level, i.e., that population-level measures identify functional and structural co-tuning of PV neurons that are not evident in pairwise individual-level measures. Finally, we show that such co-tuning can enhance network stability at the cost of reduced feature selectivity.

Functional specificity of recurrent inhibition in visual cortex

In the neocortex, neural activity is shaped by the interaction of excitatory and inhibitory neurons, defined by the organization of their synaptic connections. Although connections among excitatory pyramidal neurons are sparse and functionally tuned, inhibitory connectivity is thought to be dense and largely unstructured. By measuring in vivo visual responses and synaptic connectivity of parvalbumin-expressing (PV+) inhibitory cells in mouse primary visual cortex, we show that the synaptic weights of their connections to nearby pyramidal neurons are specifically tuned according to the similarity of the cells' responses. Individual PV+ cells strongly inhibit those pyramidal cells that provide them with strong excitation and share their visual selectivity. This structured organization of inhibitory synaptic weights provides a circuit mechanism for tuned inhibition onto pyramidal cells despite dense connectivity, stabilizing activity within feature-specific excitatory ensembles while supporting competition between them.

Mammals Achieve Common Neural Coverage of Visual Scenes Using Distinct Sampling Behaviors

Most vertebrates use head and eye movements to quickly change gaze orientation and sample different portions of the environment with periods of stable fixation. Visual information must be integrated across fixations to construct a complete perspective of the visual environment. In concert with this sampling strategy, neurons adapt to unchanging input to conserve energy and ensure that only novel information from each fixation is processed. We demonstrate how adaptation recovery times and saccade properties interact and thus shape spatiotemporal tradeoffs observed in the motor and visual systems of mice, cats, marmosets, macaques, and humans. These tradeoffs predict that in order to achieve similar visual coverage over time, animals with smaller receptive field sizes require faster saccade rates. Indeed, we find comparable sampling of the visual environment by neuronal populations across mammals when integrating measurements of saccadic behavior with receptive field sizes and V1 neuronal density. We propose that these mammals share a common statistically driven strategy of maintaining coverage of their visual environment over time calibrated to their respective visual system characteristics.

Widespread and Multifaceted Binocular Integration in the Mouse Primary Visual Cortex

The brain combines two-dimensional images received from the two eyes to form a percept of three-dimensional surroundings. This process of binocular integration in the primary visual cortex (V1) serves as a useful model for studying how neural circuits generate emergent properties from multiple input signals. Here, we perform a thorough characterization of binocular integration using electrophysiological recordings in the V1 of awake adult male and female mice by systematically varying the orientation and phase disparity of monocular and binocular stimuli. We reveal widespread binocular integration in mouse V1 and demonstrate that the three commonly studied binocular properties-ocular dominance, interocular matching, and disparity selectivity-are independent of each other. For individual neurons, the responses to monocular stimulation can predict the average amplitude of binocular response but not its selectivity. Finally, the extensive and independent binocular integration of monocular inputs is seen across cortical layers in both regular-spiking and fast-spiking neurons, regardless of stimulus design. Our data indicate that the current model of simple feedforward convergence is inadequate to account for binocular integration in mouse V1, thus suggesting an indispensable role played by intracortical circuits in binocular computation.SIGNIFICANCE STATEMENT Binocular integration is an important step of visual processing that takes place in the visual cortex. Studying the process by which V1 neurons become selective for certain binocular disparities is informative about how neural circuits integrate multiple information streams at a more general level. Here, we systematically characterize binocular integration in mice. Our data demonstrate more widespread and complex binocular integration in mouse V1 than previously reported. Binocular responses cannot be explained by a simple convergence of monocular responses, contrary to the prevailing model of binocular integration. These findings thus indicate that intracortical circuits must be involved in the exquisite computation of binocular disparity, which would endow brain circuits with the plasticity needed for binocular development and processing.

Mammals achieve common neural coverage of visual scenes using distinct sampling behaviors

Most vertebrates use head and eye movements to quickly change gaze orientation and sample different portions of the environment with periods of stable fixation. Visual information must be integrated across several fixations to construct a more complete perspective of the visual environment. In concert with this sampling strategy, neurons adapt to unchanging input to conserve energy and ensure that only novel information from each fixation is processed. We demonstrate how adaptation recovery times and saccade properties interact, and thus shape spatiotemporal tradeoffs observed in the motor and visual systems of different species. These tradeoffs predict that in order to achieve similar visual coverage over time, animals with smaller receptive field sizes require faster saccade rates. Indeed, we find comparable sampling of the visual environment by neuronal populations across mammals when integrating measurements of saccadic behavior with receptive field sizes and V1 neuronal density. We propose that these mammals share a common statistically driven strategy of maintaining coverage of their visual environment over time calibrated to their respective visual system characteristics.

Correlated Somatosensory Input in Parvalbumin/Pyramidal Cells in Mouse Motor Cortex

In mammalian cortex, feedforward excitatory connections recruit feedforward inhibition. This is often carried by parvalbumin (PV+) interneurons, which may densely connect to local pyramidal (Pyr) neurons. Whether this inhibition affects all local excitatory cells indiscriminately or is targeted to specific subnetworks is unknown. Here, we test how feedforward inhibition is recruited by using two-channel circuit mapping to excite cortical and thalamic inputs to PV+ interneurons and Pyr neurons to mouse primary vibrissal motor cortex (M1). Single Pyr and PV+ neurons receive input from both cortex and thalamus. Connected pairs of PV+ interneurons and excitatory Pyr neurons receive correlated cortical and thalamic inputs. While PV+ interneurons are more likely to form local connections to Pyr neurons, Pyr neurons are much more likely to form reciprocal connections with PV+ interneurons that inhibit them. This suggests that Pyr and PV ensembles may be organized based on their local and long-range connections, an organization that supports the idea of local subnetworks for signal transduction and processing. Excitatory inputs to M1 can thus target inhibitory networks in a specific pattern which permits recruitment of feedforward inhibition to specific subnetworks within the cortical column.

Unitary synaptic responses of parvalbumin interneurons evoked by excitatory neurons in the mouse barrel cortex

The mammalian cortex integrates and processes information to transform sensory inputs into perceptions and motor outputs. These operations are performed by networks of excitatory and inhibitory neurons distributed through the cortical layers. Parvalbumin interneurons (PVIs) are the most abundant type of inhibitory cortical neuron. With axons projecting within and between layers, PVIs supply feedforward and feedback inhibition to control and modulate circuit function. Distinct populations of excitatory neurons recruit different PVI populations, but the specializations of these synapses are poorly understood. Here, we targeted a genetically encoded hybrid voltage sensor to PVIs and used fluorescence imaging in mouse somatosensory cortex slices to record their voltage changes. Stimulating a single visually identified excitatory neuron with small-tipped theta-glass electrodes depolarized multiple PVIs, and a common threshold suggested that stimulation elicited unitary synaptic potentials in response to a single excitatory neuron. Excitatory neurons depolarized PVIs in multiple layers, with the most residing in the layer of the stimulated neuron. Spiny stellate cells depolarized PVIs more strongly than pyramidal cells by up to 77%, suggesting a greater role for stellate cells in recruiting PVI inhibition and controlling cortical computations. Response half-width also varied between different excitatory inputs. These results demonstrate functional differences between excitatory synapses on PVIs.

Neural circuits for binocular vision: Ocular dominance, interocular matching, and disparity selectivity

The brain creates a single visual percept of the world with inputs from two eyes. This means that downstream structures must integrate information from the two eyes coherently. Not only does the brain meet this challenge effortlessly, it also uses small differences between the two eyes' inputs, i.e., binocular disparity, to construct depth information in a perceptual process called stereopsis. Recent studies have advanced our understanding of the neural circuits underlying stereoscopic vision and its development. Here, we review these advances in the context of three binocular properties that have been most commonly studied for visual cortical neurons: ocular dominance of response magnitude, interocular matching of orientation preference, and response selectivity for binocular disparity. By focusing mostly on mouse studies, as well as recent studies using ferrets and tree shrews, we highlight unresolved controversies and significant knowledge gaps regarding the neural circuits underlying binocular vision. We note that in most ocular dominance studies, only monocular stimulations are used, which could lead to a mischaracterization of binocularity. On the other hand, much remains unknown regarding the circuit basis of interocular matching and disparity selectivity and its development. We conclude by outlining opportunities for future studies on the neural circuits and functional development of binocular integration in the early visual system.

Strong tuning for stereoscopic depth indicates orientation-specific recurrent circuitry in tree shrew V1

Neurons in the primary visual cortex (V1) are tuned to specific disparities between the two retinal images, which form the neural substrate for stereoscopic vision. We show that V1 neurons in tree shrews, but not in mice, display highly selective responses to narrow ranges of disparity in random-dot stereograms. Surprisingly, V1 neurons in both species show similarly strong tuning to gratings of varying interocular phase differences. This stimulus-dependent dissociation of disparity tuning can be explained by a network model that combines both feedforward and recurrent connections. The features of the model connections are supported by cortical organizations specific to each species. We validate this model by identifying putative inhibitory neurons and confirming their predicted disparity tuning in both species. Together, our studies establish a foundation for using tree shrews in studying binocular vision and raise an exciting possibility of how cortical columns could be uniquely important in computing stereoscopic depth.

Activating parvalbumin-expressing interneurons produces iceberg effects in mouse primary visual cortex neurons

In the primary visual cortex (V1) inhibitory interneurons form a local circuit with excitatory pyramidal cells to produce distinct receptive field properties. Parvalbumin-expressing interneurons (Pvalb+) are the most common subclass of V1 interneurons, and studies of orientation tuning indicate they shape pyramidal stimulus selectivity by balancing excitation with inhibition relative to the spike threshold. The iceberg effect, where subthreshold responses have broader tuning than spiking responses, predicts that other receptive field properties besides orientation tuning should also be affected by this balance mediated by Pvalb+ cells. To test this, we measured receptive field size and visual latency of pyramidal cells while Pvalb+ activity was optogenetically increased. We found that amplifying Pvalb+ input to pyramidal cells significantly increased their latency and decreased their receptive field size, which corroborates the proposed role of Pvalb+ interneurons in sculpting pyramidal tuning by controlling cortical gain.

Development of visual response selectivity in cortical GABAergic interneurons

The visual cortex of carnivores and primates displays a high degree of modular network organization characterized by local clustering and structured long-range correlations of activity and functional properties. Excitatory networks display modular organization before the onset of sensory experience, but the developmental timeline for modular networks of GABAergic interneurons remains under-explored. Using in vivo calcium imaging of the ferret visual cortex, we find evidence that before visual experience, interneurons display weak orientation tuning and widespread, correlated activity in response to visual stimuli. Robust modular organization and orientation tuning are evident with as little as one week of visual experience. Furthermore, we find that the maturation of orientation tuning requires visual experience, while the reduction in widespread, correlated network activity does not. Thus, the maturation of inhibitory cortical networks occurs in a delayed, parallel process relative to excitatory neurons.

Excitatory neurons in the carnivore and primate visual cortex display orientation selectivity arranged in a modular fashion before the onset of visual experience, but the developmental timeline for visual response selectivity of inhibitory neurons is unknown. Using in vivo calcium imaging in ferret visual cortex, the authors show that experience-dependent emergence of orientation selectivity develops in a modular fashion for inhibitory interneurons over the first week of visual experience.

Feedforward mechanisms of cross-orientation interactions in mouse V1

Sensory neurons are modulated by context. For example, in mouse primary visual cortex (V1), neuronal responses to the preferred orientation are modulated by the presence of superimposed orientations ("plaids"). The effects of this modulation are diverse; some neurons are suppressed, while others have larger responses to a plaid than its components. We investigated whether this diversity could be explained by a unified circuit mechanism. We report that this masking is maintained during suppression of cortical activity, arguing against cortical mechanisms. Instead, the heterogeneity of plaid responses is explained by an interaction between stimulus geometry and orientation tuning. Highly selective neurons are uniformly suppressed by plaids, whereas the effects in weakly selective neurons depend on the spatial configuration of the stimulus, transitioning systematically between suppression and facilitation. Thus, the diverse responses emerge as a consequence of the spatial structure of feedforward inputs, with no need to invoke cortical interactions.

Tightly coupled inhibitory and excitatory functional networks in the developing primary visual cortex

Intracortical inhibition plays a critical role in shaping activity patterns in the mature cortex. However, little is known about the structure of inhibition in early development prior to the onset of sensory experience, a time when spontaneous activity exhibits long-range correlations predictive of mature functional networks. Here, using calcium imaging of GABAergic neurons in the ferret visual cortex, we show that spontaneous activity in inhibitory neurons is already highly organized into distributed modular networks before visual experience. Inhibitory neurons exhibit spatially modular activity with long-range correlations and precise local organization that is in quantitative agreement with excitatory networks. Furthermore, excitatory and inhibitory networks are strongly co-aligned at both millimeter and cellular scales. These results demonstrate a remarkable degree of organization in inhibitory networks early in the developing cortex, providing support for computational models of self-organizing networks and suggesting a mechanism for the emergence of distributed functional networks during development.

Visual pursuit behavior in mice maintains the pursued prey on the retinal region with least optic flow

Mice have a large visual field that is constantly stabilized by vestibular ocular reflex (VOR) driven eye rotations that counter head-rotations. While maintaining their extensive visual coverage is advantageous for predator detection, mice also track and capture prey using vision. However, in the freely moving animal quantifying object location in the field of view is challenging. Here, we developed a method to digitally reconstruct and quantify the visual scene of freely moving mice performing a visually based prey capture task. By isolating the visual sense and combining a mouse eye optic model with the head and eye rotations, the detailed reconstruction of the digital environment and retinal features were projected onto the corneal surface for comparison, and updated throughout the behavior. By quantifying the spatial location of objects in the visual scene and their motion throughout the behavior, we show that the prey image consistently falls within a small area of the VOR-stabilized visual field. This functional focus coincides with the region of minimal optic flow within the visual field and consequently area of minimal motion-induced image-blur, as during pursuit mice ran directly toward the prey. The functional focus lies in the upper-temporal part of the retina and coincides with the reported high density-region of Alpha-ON sustained retinal ganglion cells.

Optogenetic Activation of Interneuron Subtypes Modulates Visual Contrast Responses of Mouse V1 Neurons

Interneurons are critical for information processing in the cortex. In vitro optogenetic studies in mouse primary visual cortex (V1) have sketched the connectivity of a local neural circuit comprising excitatory pyramidal neurons and distinct interneuron subtypes that express parvalbumin (Pvalb+), somatostatin (SOM+), or vasoactive intestinal peptide (VIP+). However, in vivo studies focusing on V1 orientation tuning have ascribed discrepant computational roles to specific interneuron subtypes. Here, we sought to clarify the differences between interneuron subtypes by examining the effects of optogenetic activation of Pvalb+, SOM+, or VIP+ interneurons on contrast tuning of V1 neurons while also accounting for cortical depth and photostimulation intensity. We found that illumination of the cortical surface produced a similar spectrum of saturating additive photostimulation effects in all 3 interneuron subtypes, which varied with cortical depth rather than light intensity in Pvalb+ and SOM+ cells. Pyramidal cell modulation was well explained by a conductance-based model that incorporated these interneuron photostimulation effects.

Natural binocular depth discrimination behavior in mice explained by visual cortical activity

In mice and other mammals, forebrain neurons integrate right and left eye information to generate a three-dimensional representation of the visual environment. Neurons in the visual cortex of mice are sensitive to binocular disparity,^1-3 yet it is unclear whether that sensitivity is linked to the perception of depth.^4-8 We developed a natural task based on the classic visual cliff and pole descent tasks to estimate the psychophysical range of mouse depth discrimination.^5,9 Mice with binocular vision descended to a near (shallow) surface more often when surrounding far (deep) surfaces were progressively more distant. Occlusion of one eye severely impaired their ability to target the near surface. We quantified the distance at which animals make their decisions to estimate the binocular image displacement of the checkerboard pattern on the near and far surfaces. Then, we assayed the disparity sensitivity of large populations of binocular neurons in primary visual cortex (V1) using two-photon microscopy² and quantitatively compared this information available in V1 to their behavioral sensitivity. Disparity information in V1 matches the behavioral performance over the range of depths examined and was resistant to changes in binocular alignment. These findings reveal that mice naturally use stereoscopic cues to guide their behavior and indicate a neural basis for this depth discrimination task.

Impact of Perineuronal Nets on Electrophysiology of Parvalbumin Interneurons, Principal Neurons, and Brain Oscillations: A Review

Perineuronal nets (PNNs) are specialized extracellular matrix structures that surround specific neurons in the brain and spinal cord, appear during critical periods of development, and restrict plasticity during adulthood. Removal of PNNs can reinstate juvenile-like plasticity or, in cases of PNN removal during early developmental stages, PNN removal extends the critical plasticity period. PNNs surround mainly parvalbumin (PV)-containing, fast-spiking GABAergic interneurons in several brain regions. These inhibitory interneurons profoundly inhibit the network of surrounding neurons via their elaborate contacts with local pyramidal neurons, and they are key contributors to gamma oscillations generated across several brain regions. Among other functions, these gamma oscillations regulate plasticity associated with learning, decision making, attention, cognitive flexibility, and working memory. The detailed mechanisms by which PNN removal increases plasticity are only beginning to be understood. Here, we review the impact of PNN removal on several electrophysiological features of their underlying PV interneurons and nearby pyramidal neurons, including changes in intrinsic and synaptic membrane properties, brain oscillations, and how these changes may alter the integration of memory-related information. Additionally, we review how PNN removal affects plasticity-associated phenomena such as long-term potentiation (LTP), long-term depression (LTD), and paired-pulse ratio (PPR). The results are discussed in the context of the role of PV interneurons in circuit function and how PNN removal alters this function.

Cortical response selectivity derives from strength in numbers of synapses

Single neocortical neurons are driven by populations of excitatory inputs, which form the basis of neuronal selectivity to features of sensory input. Excitatory connections are thought to mature during development through activity-dependent Hebbian plasticity1, whereby similarity between presynaptic and postsynaptic activity selectively strengthens some synapses and weakens others2. Evidence in support of this process includes measurements of synaptic ultrastructure and in vitro and in vivo physiology and imaging studies3-8. These corroborating lines of evidence lead to the prediction that a small number of strong synaptic inputs drive neuronal selectivity, whereas weak synaptic inputs are less correlated with the somatic output and modulate activity overall6,7. Supporting evidence from cortical circuits, however, has been limited to measurements of neighbouring, connected cell pairs, raising the question of whether this prediction holds for a broad range of synapses converging onto cortical neurons. Here we measure the strengths of functionally characterized excitatory inputs contacting single pyramidal neurons in ferret primary visual cortex (V1) by combining in vivo two-photon synaptic imaging and post hoc electron microscopy. Using electron microscopy reconstruction of individual synapses as a metric of strength, we find no evidence that strong synapses have a predominant role in the selectivity of cortical neuron responses to visual stimuli. Instead, selectivity appears to arise from the total number of synapses activated by different stimuli. Moreover, spatial clustering of co-active inputs appears to be reserved for weaker synapses, enhancing the contribution of weak synapses to somatic responses. Our results challenge the role of Hebbian mechanisms in shaping neuronal selectivity in cortical circuits, and suggest that selectivity reflects the co-activation of large populations of presynaptic neurons with similar properties and a mixture of strengths.

How many neurons are sufficient for perception of cortical activity?

Many theories of brain function propose that activity in sparse subsets of neurons underlies perception and action. To place a lower bound on the amount of neural activity that can be perceived, we used an all-optical approach to drive behaviour with targeted two-photon optogenetic activation of small ensembles of L2/3 pyramidal neurons in mouse barrel cortex while simultaneously recording local network activity with two-photon calcium imaging. By precisely titrating the number of neurons stimulated, we demonstrate that the lower bound for perception of cortical activity is ~14 pyramidal neurons. We find a steep sigmoidal relationship between the number of activated neurons and behaviour, saturating at only ~37 neurons, and show this relationship can shift with learning. Furthermore, activation of ensembles is balanced by inhibition of neighbouring neurons. This surprising perceptual sensitivity in the face of potent network suppression supports the sparse coding hypothesis, and suggests that cortical perception balances a trade-off between minimizing the impact of noise while efficiently detecting relevant signals.

Long-Range Interhemispheric Projection Neurons Show Biased Response Properties and Fine-Scale Local Subnetworks in Mouse Visual Cortex

Integration of information processed separately in distributed brain regions is essential for brain functions. This integration is enabled by long-range projection neurons, and further, concerted interactions between long-range projections and local microcircuits are crucial. It is not well known, however, how this interaction is implemented in cortical circuits. Here, to decipher this logic, using callosal projection neurons (CPNs) in layer 2/3 of the mouse visual cortex as a model of long-range projections, we found that CPNs exhibited distinct response properties and fine-scale local connectivity patterns. In vivo 2-photon calcium imaging revealed that CPNs showed a higher ipsilateral (to their somata) eye preference, and that CPN pairs showed stronger signal/noise correlation than random pairs. Slice recordings showed CPNs were preferentially connected to CPNs, demonstrating the existence of projection target-dependent fine-scale subnetworks. Collectively, our results suggest that long-range projection target predicts response properties and local connectivity of cortical projection neurons.

Disparity Sensitivity and Binocular Integration in Mouse Visual Cortex Areas

Binocular disparity, the difference between the two eyes' images, is a powerful cue to generate the 3D depth percept known as stereopsis. In primates, binocular disparity is processed in multiple areas of the visual cortex, with distinct contributions of higher areas to specific aspects of depth perception. Mice, too, can perceive stereoscopic depth, and neurons in primary visual cortex (V1) and higher-order, lateromedial (LM) and rostrolateral (RL) areas were found to be sensitive to binocular disparity. A detailed characterization of disparity tuning across mouse visual areas is lacking, however, and acquiring such data might help clarifying the role of higher areas for disparity processing and establishing putative functional correspondences to primate areas. We used two-photon calcium imaging in female mice to characterize the disparity tuning properties of neurons in visual areas V1, LM, and RL in response to dichoptically presented binocular gratings, as well as random dot correlograms (RDC). In all three areas, many neurons were tuned to disparity, showing strong response facilitation or suppression at optimal or null disparity, respectively, even in neurons classified as monocular by conventional ocular dominance (OD) measurements. Neurons in higher areas exhibited broader and more asymmetric disparity tuning curves compared with V1, as observed in primate visual cortex. Finally, we probed neurons' sensitivity to true stereo correspondence by comparing responses to correlated RDC (cRDC) and anticorrelated RDC (aRDC). Area LM, akin to primate ventral visual stream areas, showed higher selectivity for correlated stimuli and reduced anticorrelated responses, indicating higher-level disparity processing in LM compared with V1 and RL.SIGNIFICANCE STATEMENT A major cue for inferring 3D depth is disparity between the two eyes' images. Investigating how binocular disparity is processed in the mouse visual system will not only help delineating the role of mouse higher areas for visual processing, but also shed light on how the mammalian brain computes stereopsis. We found that binocular integration is a prominent feature of mouse visual cortex, as many neurons are selectively and strongly modulated by binocular disparity. Comparison of responses to correlated and anticorrelated random dot correlograms (RDC) revealed that lateromedial area (LM) is more selective to correlated stimuli, while less sensitive to anticorrelated stimuli compared with primary visual cortex (V1) and rostrolateral area (RL), suggesting higher-level disparity processing in LM, resembling primate ventral visual stream areas.

Sparse Representation in Awake Auditory Cortex: Cell-type Dependence, Synaptic Mechanisms, Developmental Emergence, and Modulation

Sparse representation is considered an important coding strategy for cortical processing in various sensory modalities. It remains unclear how cortical sparseness arises and is being regulated. Here, unbiased recordings from primary auditory cortex of awake adult mice revealed salient sparseness in layer (L)2/3, with a majority of excitatory neurons exhibiting no increased spiking in response to each of sound types tested. Sparse representation was not observed in parvalbumin (PV) inhibitory neurons. The nonresponding neurons did receive auditory-evoked synaptic inputs, marked by weaker excitation and lower excitation/inhibition (E/I) ratios than responding cells. Sparse representation arises during development in an experience-dependent manner, accompanied by differential changes of excitatory input strength and a transition from unimodal to bimodal distribution of E/I ratios. Sparseness level could be reduced by suppressing PV or L1 inhibitory neurons. Thus, sparse representation may be dynamically regulated via modulating E/I balance, optimizing cortical representation of the external sensory world.

Interocular velocity cues elicit vergence eye movements in mice

We stabilize the dynamic visual world on our retina by moving our eyes in response to motion signals. Coordinated movements between the two eyes are characterized as version when both eyes move in the same direction and vergence when the two eyes move in opposite directions. Vergence eye movements are necessary to track objects in three dimensions. In primates they can be elicited by intraocular differences in either spatial signals (disparity) or velocity, requiring the integration of left and right eye inputs. Whether mice are capable of similar behaviors is not known. To address this issue, we measured vergence eye movements in mice using a stereoscopic stimulus known to elicit vergence eye movements in primates. We found that mice also exhibit vergence eye movements, although at a low gain and that the primary driver of these vergence eye movements is interocular motion. Spatial disparity cues alone are ineffective. We also found that the vergence eye movements we observed in mice were robust to silencing visual cortex and to manipulations that disrupt the normal development of binocularity in visual cortex. A sublinear combination of motor commands driven by monocular signals is sufficient to account for our results.NEW & NOTEWORTHY The visual system integrates signals from the left and right eye to generate a representation of the world in depth. The binocular integration of signals may be observed from the coordinated vergence eye movements elicited by object motion in depth. We explored the circuits and signals responsible for these vergence eye movements in rodent and find these vergence eye movements are generated by a comparison of the motion and not spatial visual signals.

Using psychophysical performance to predict short-term ocular dominance plasticity in human adults

Binocular rivalry has become an important index of visual performance, both to measure ocular dominance or its plasticity, and to index bistable perception. We investigated its interindividual variability across 50 normal adults and found that the duration of dominance phases in rivalry is linked with the duration of dominance phases in another bistable phenomenon (structure from motion). Surprisingly, it also correlates with the strength of center-surround interactions (indexed by the tilt illusion), suggesting a common mechanism supporting both competitive interactions: center-surround and rivalry. In a subset of 34 participants, we further investigated the variability of short-term ocular dominance plasticity, measured with binocular rivalry before and after 2 hours of monocular deprivation. We found that ocular dominance shifts in favor of the deprived eye and that a large portion of ocular dominance variability after deprivation can be predicted from the dynamics of binocular rivalry before deprivation. The single best predictor is the proportion of mixed percepts (phases without dominance of either eye) before deprivation, which is positively related to ocular dominance unbalance after deprivation. Another predictor is the duration of dominance phases, which interacts with mixed percepts to explain nearly 50% of variance in ocular dominance unbalance after deprivation. A similar predictive power is achieved by substituting binocular rivalry dominance phase durations with tilt illusion magnitude, or structure from motion phase durations. Thus, we speculate that ocular dominance plasticity is modulated by two types of signals, estimated from psychophysical performance before deprivation, namely, interocular inhibition (promoting binocular fusion, hence mixed percepts) and inhibition for perceptual competition (promoting longer dominance phases and stronger center-surround interactions).

Cholinergic potentiation of visual perception and vision restoration in rodents and humans

BACKGROUND

The cholinergic system is a potent neuromodulator system that plays a critical role in cortical plasticity, attention, and learning. Recently, it was found that boosting this system during perceptual learning robustly enhances sensory perception in rodents. In particular, pairing cholinergic activation with visual stimulation increases neuronal responses, cue detection ability, and long-term facilitation in the primary visual cortex. The mechanisms of cholinergic enhancement are closely linked to attentional processes, long-term potentiation, and modulation of the excitatory/inhibitory balance. Some studies currently examine this effect in humans.

OBJECTIVE

The present article reviews the research from our laboratory, examining whether potentiating the central cholinergic system could help visual perception and restoration.

METHODS

Electrophysiological or pharmacological enhancement of the cholinergic system are administered during a visual training. Electrophysiological responses and perceptual learning performance are investigated before and after the training in rats and humans. This approach's ability to restore visual capacities following a visual deficit induced by a partial optic nerve crush is also investigated in rats.

RESULTS

The coupling of visual training to cholinergic stimulation improved visual discrimination and visual acuity in rats, and improved residual vision after a deficit. These changes were due to muscarinic and nicotinic transmissions and were associated with a functional improvement of evoked potentials. In humans, potentiation of cholinergic transmission with 5 mg of donepezil showed improved learning and ocular dominance plasticity, although this treatment was ineffective in augmenting the perceptual threshold and electroencephalography.

CONCLUSIONS

Potential therapeutic outcomes ought to facilitate vision restoration using commercially available cholinergic agents combined with visual stimulation in order to prevent irreversible vision loss in patients. This approach has the potential to help a large population of visually impaired individuals.

Functional Access to Neuron Subclasses in Rodent and Primate Forebrain

Viral vectors enable foreign proteins to be expressed in brains of non-genetic species, including non-human primates. However, viruses targeting specific neuron classes have proved elusive. Here we describe viral promoters and strategies for accessing GABAergic interneurons and their molecularly defined subsets in the rodent and primate. Using a set intersection approach, which relies on two co-active promoters, we can restrict heterologous protein expression to cortical and hippocampal somatostatin-positive and parvalbumin-positive interneurons. With an orthogonal set difference method, we can enrich for subclasses of neuropeptide-Y-positive GABAergic interneurons by effectively subtracting the expression pattern of one promoter from that of another. These methods harness the complexity of gene expression patterns in the brain and significantly expand the number of genetically tractable neuron classes across mammals.

Fast and reversible neural inactivation in macaque cortex by optogenetic stimulation of GABAergic neurons

Optogenetic techniques for neural inactivation are valuable for linking neural activity to behavior but they have serious limitations in macaques. To achieve powerful and temporally precise neural inactivation, we used an adeno-associated viral (AAV) vector carrying the channelrhodopsin-2 gene under the control of a Dlx5/6 enhancer, which restricts expression to GABAergic neurons. We tested this approach in the primary visual cortex, an area where neural inactivation leads to interpretable behavioral deficits. Optical stimulation modulated spiking activity and reduced visual sensitivity profoundly in the region of space represented by the stimulated neurons. Rebound firing, which can have unwanted effects on neural circuits following inactivation, was not observed, and the efficacy of the optogenetic manipulation on behavior was maintained across >1000 trials. We conclude that this inhibitory cell-type-specific optogenetic approach is a powerful and spatiotemporally precise neural inactivation tool with broad utility for probing the functional contributions of cortical activity in macaques.

Functional Logic of Layer 2/3 Inhibitory Connectivity in the Ferret Visual Cortex

Understanding how cortical inhibition shapes circuit function requires identifying the connectivity rules relating the response properties of inhibitory interneurons and their postsynaptic targets. Here we explore the orientation tuning of layer 2/3 inhibitory inputs in the ferret visual cortex using a combination of in vivo axon imaging, functional input mapping, and physiology. Inhibitory boutons exhibit robust orientation-tuned responses with preferences that can differ significantly from the cortical column in which they reside. Inhibitory input fields measured with patterned optogenetic stimulation and intracellular recordings revealed that these inputs originate from a wide range of orientation domains, inconsistent with a model of co-tuned inhibition and excitation. Intracellular synaptic conductance measurements confirm that individual neurons can depart from a co-tuned regime. Our results argue against a simple rule for the arrangement of inhibitory inputs supplied by layer 2/3 circuits and suggest that heterogeneity in presynaptic inhibitory networks contributes to neural response properties.

Mice Discriminate Stereoscopic Surfaces Without Fixating in Depth

Stereopsis is a ubiquitous feature of primate mammalian vision, but little is known about if and how rodents such as mice use stereoscopic vision. We used random dot stereograms to test for stereopsis in male and female mice, and they were able to discriminate near from far surfaces over a range of disparities, with diminishing performance for small and large binocular disparities. Based on two-photon measurements of disparity tuning, the range of disparities represented in the visual cortex aligns with the behavior and covers a broad range of disparities. When we examined their binocular eye movements, we found that, unlike primates, mice did not systematically vary relative eye positions or use vergence eye movements when presented with different disparities. Nonetheless, the representation of disparity tuning was wide enough to capture stereoscopic information over a range of potential vergence angles. Although mice share fundamental characteristics of stereoscopic vision with primates and carnivores, their lack of disparity-dependent vergence eye movements and wide neuronal representation suggests that they may use a distinct strategy for stereopsis.SIGNIFICANCE STATEMENT Binocular vision allows us to derive depth information by comparing right and left eye information. We characterized binocular integration in mice because tools exist in these animals to dissect the underlying neural circuitry for binocular vision. Using random dot stereograms, we find that behavior and disparity tuning in the visual cortex share fundamental characteristics with primates, but we did not observe any evidence of disparity-dependent changes in vergence angle. We propose that mice use a distinct strategy of stereopsis compared with primates by using a broad range of disparities to encode depth over a large field of view and to compensate for nonstereoscopic changes in vergence angle that arise during natural behavior.

Propagation of orientation selectivity in a spiking network model of layered primary visual cortex

Neurons in different layers of sensory cortex generally have different functional properties. But what determines firing rates and tuning properties of neurons in different layers? Orientation selectivity in primary visual cortex (V1) is an interesting case to study these questions. Thalamic projections essentially determine the preferred orientation of neurons that receive direct input. But how is this tuning propagated though layers, and how can selective responses emerge in layers that do not have direct access to the thalamus? Here we combine numerical simulations with mathematical analyses to address this problem. We find that a large-scale network, which just accounts for experimentally measured layer and cell-type specific connection probabilities, yields firing rates and orientation selectivities matching electrophysiological recordings in rodent V1 surprisingly well. Further analysis, however, is complicated by the fact that neuronal responses emerge in a dynamic fashion and cannot be directly inferred from static neuroanatomy, as some connections tend to have unintuitive effects due to recurrent interactions and strong feedback loops. These emergent phenomena can be understood by linearizing and coarse-graining. In fact, we were able to derive a low-dimensional linear dynamical system effectively describing stimulus-driven activity layer by layer. This low-dimensional system explains layer-specific firing rates and orientation tuning by accounting for the different gain factors of the aggregate system. Our theory can also be used to design novel optogenetic stimulation experiments, thus facilitating further exploration of the interplay between connectivity and function.

Understanding the precise roles of neuronal sub-populations in shaping the activity of networks is a fundamental objective of neuroscience research. In complex neuronal network structures like the neocortex, the relation between the connectome and the algorithm implemented in it is often not self-explaining. To this end, our work makes three important contributions. First, we show that the connectivity extracted by anatomical and physiological experiments in visual cortex suffices to explain important properties of the various sub-populations, including their selectivity to visual stimulation. Second, we introduce a novel system-level approach for the analysis of input-output relations of recurrent networks, which leads to the observed activity patterns. Third, we present a method for the design of future optogenetic experiments that can be used to devise specific stimuli resulting in a predictable change of neuronal activity. In summary, we introduce a novel framework to determine the relevant features of neuronal microcircuit function that can be applied to a wide range of neuronal systems.

Complementary networks of cortical somatostatin interneurons enforce layer specific control

The neocortex is functionally organized into layers. Layer four receives the densest bottom up sensory inputs, while layers 2/3 and 5 receive top down inputs that may convey predictive information. A subset of cortical somatostatin (SST) neurons, the Martinotti cells, gate top down input by inhibiting the apical dendrites of pyramidal cells in layers 2/3 and 5, but it is unknown whether an analogous inhibitory mechanism controls activity in layer 4. Using high precision circuit mapping, in vivo optogenetic perturbations, and single cell transcriptional profiling, we reveal complementary circuits in the mouse barrel cortex involving genetically distinct SST subtypes that specifically and reciprocally interconnect with excitatory cells in different layers: Martinotti cells connect with layers 2/3 and 5, whereas non-Martinotti cells connect with layer 4. By enforcing layer-specific inhibition, these parallel SST subnetworks could independently regulate the balance between bottom up and top down input.

The Physiology of Homeoprotein Transduction

The homeoprotein family comprises ~300 transcription factors and was long seen as primarily involved in developmental programs through cell autonomous regulation. However, recent evidence reveals that many of these factors are also expressed in the adult where they exert physiological functions not yet fully deciphered. Furthermore, the DNA-binding domain of most homeoproteins contains two signal sequences allowing their secretion and internalization, thus intercellular transfer. This review focuses on this new-found signaling in cell migration, axon guidance, and cerebral cortex physiological homeostasis and speculates on how it may play important roles in early arealization of the neuroepithelium. It also describes the use of homeoproteins as therapeutic proteins in mouse models of diseases affecting the central nervous system, in particular Parkinson disease and glaucoma.

Cell Type Specific Representation of Vibro-tactile Stimuli in the Mouse Primary Somatosensory Cortex

Although the processing of whisker deflections in the barrel area of the rodent primary somatosensory cortex (S1) has been studied extensively, how cutaneous vibro-tactile stimuli are processed in the rodent S1 outside the barrel area has not been fully examined. Particularly, the cell-type specific representation of multiple vibration frequencies in genetically identified inhibitory cells in the S1 has not been examined. Using two-photon calcium imaging, we examined the responses to vibration stimuli of excitatory and inhibitory neurons in the S1 hind limb area of male and female mice. The excitatory cells showed relatively sharp selectivity to vibration stimuli, whereas the inhibitory cells exhibited less selectivity. The excitatory and inhibitory cells with different preferred stimuli were intermingled in a “salt and pepper” manner. Furthermore, the noise correlation tended to be especially strong in excitatory-inhibitory and inhibitory-inhibitory cell pairs that have similar stimulus selectivity. These results suggest that excitatory cells tend to represent specific stimulus information and work together with similarly tuned inhibitory cells as a functionally connected network.

Adolescence as a neurobiological critical period for the development of higher-order cognition

The transition from adolescence to adulthood is characterized by improvements in higher-order cognitive abilities and corresponding refinements of the structure and function of the brain regions that support them. Whereas the neurobiological mechanisms that govern early development of sensory systems are well-understood, the mechanisms that drive developmental plasticity of association cortices, such as prefrontal cortex (PFC), during adolescence remain to be explained. In this review, we synthesize neurodevelopmental findings at the cellular, circuit, and systems levels in PFC and evaluate them through the lens of established critical period (CP) mechanisms that guide early sensory development. We find remarkable correspondence between these neurodevelopmental processes and the mechanisms driving CP plasticity, supporting the hypothesis that adolescent development is driven by CP mechanisms that guide the rapid development of neurobiology and cognitive ability during adolescence and their subsequent stability in adulthood. Critically, understanding adolescence as a CP not only provides a mechanism for normative adolescent development, it provides a framework for understanding the role of experience and neurobiology in the emergence of psychopathology that occurs during this developmental period.

Natural image and receptive field statistics predict saccade sizes

Humans and other primates sample the visual environment using saccadic eye movements that shift a high-resolution fovea toward regions of interest to create a clear perception of a scene across fixations. Many mammals, however, like mice, lack a fovea, which raises the question of why they make saccades. Here we describe and test the hypothesis that saccades are matched to natural scene statistics and to the receptive field sizes and adaptive properties of neural populations. Specifically, we determined the minimum amplitude of saccades in natural scenes necessary to provide uncorrelated inputs to model neural populations. This analysis predicts the distributions of observed saccade sizes during passive viewing for nonhuman primates, cats, and mice. Furthermore, disrupting the development of receptive field properties by monocular deprivation changed saccade sizes consistent with this hypothesis. Therefore, natural-scene statistics and the neural representation of natural images appear to be critical factors guiding saccadic eye movements.

Sensation, movement and learning in the absence of barrel cortex

For many of our senses, the role of the cerebral cortex in detecting stimuli is controversial^1–17. Here, we examine the effects of both acute and chronic inactivation of primary somatosensory cortex (S1) in mice trained to move their large facial whiskers to detect an object via touch and respond with a lever to obtain a water reward. Using transgenic animals, we expressed inhibitory opsins in excitatory cortical neurons. Transient optogenetic inactivation of S1, as well as permanent lesions, initially produced both movement and sensory deficits that impaired detection behavior, demonstrating the inextricable link between sensory and motor systems during active sensing. Surprisingly, lesioned mice rapidly recovered full behavioral capabilities by the subsequent session. Recovery was experience-dependent, and early re-exposure to the task after lesion facilitated recovery. Furthermore, primary sensory cortex ablation prior to learning did not affect task acquisition. This combined optogenetic and lesion approach suggests that manipulations of sensory cortex may be only temporarily disruptive to other brain structures, which are themselves capable of coordinating multiple, arbitrary movements with sensation. Thus, the somatosensory cortex may be dispensable for active detection of objects in the environment.

Emergent Orientation Selectivity from Random Networks in Mouse Visual Cortex

The connectivity principles underlying the emergence of orientation selectivity in primary visual cortex (V1) of mammals lacking an orientation map (such as rodents and lagomorphs) are poorly understood. We present a computational model in which random connectivity gives rise to orientation selectivity that matches experimental observations. The model predicts that mouse V1 neurons should exhibit intricate receptive fields in the two-dimensional frequency domain, causing a shift in orientation preferences with spatial frequency. We find evidence for these features in mouse V1 using calcium imaging and intracellular whole-cell recordings.

Distinct learning-induced changes in stimulus selectivity and interactions of GABAergic interneuron classes in visual cortex

How learning enhances neural representations for behaviorally relevant stimuli via activity changes of cortical cell types remains unclear. We simultaneously imaged responses of pyramidal cells (PYR) along with parvalbumin (PV), somatostatin (SOM), and vasoactive intestinal peptide (VIP) inhibitory interneurons in primary visual cortex while mice learned to discriminate visual patterns. Learning increased selectivity for task-relevant stimuli of PYR, PV and SOM subsets but not VIP cells. Strikingly, PV neurons became as selective as PYR cells, and their functional interactions reorganized, leading to the emergence of stimulus-selective PYR-PV ensembles. Conversely, SOM activity became strongly decorrelated from the network, and PYR-SOM coupling before learning predicted selectivity increases in individual PYR cells. Thus, learning differentially shapes the activity and interactions of multiple cell classes: while SOM inhibition may gate selectivity changes, PV interneurons become recruited into stimulus-specific ensembles and provide more selective inhibition as the network becomes better at discriminating behaviorally relevant stimuli.

Mild Traumatic Brain Injury Induces Structural and Functional Disconnection of Local Neocortical Inhibitory Networks via Parvalbumin Interneuron Diffuse Axonal Injury

Diffuse axonal injury (DAI) plays a major role in cortical network dysfunction posited to cause excitatory/inhibitory imbalance after mild traumatic brain injury (mTBI). Current thought holds that white matter (WM) is uniquely vulnerable to DAI. However, clinically diagnosed mTBI is not always associated with WM DAI. This suggests an undetected neocortical pathophysiology, implicating GABAergic interneurons. To evaluate this possibility, we used mild central fluid percussion injury to generate DAI in mice with Cre-driven tdTomato labeling of parvalbumin (PV) interneurons. We followed tdTomato+ profiles using confocal and electron microscopy, together with patch-clamp analysis to probe for DAI-mediated neocortical GABAergic interneuron disruption. Within 3 h post-mTBI tdTomato+ perisomatic axonal injury (PSAI) was found across somatosensory layers 2-6. The DAI marker amyloid precursor protein colocalized with GAD67 immunoreactivity within tdTomato+ PSAI, representing the majority of GABAergic interneuron DAI. At 24 h post-mTBI, we used phospho-c-Jun, a surrogate DAI marker, for retrograde assessments of sustaining somas. Via this approach, we estimated DAI occurs in ~9% of total tdTomato+ interneurons, representing ~14% of pan-neuronal DAI. Patch-clamp recordings of tdTomato+ interneurons revealed decreased inhibitory transmission. Overall, these data show that PV interneuron DAI is a consistent and significant feature of experimental mTBI with important implications for cortical network dysfunction.

Rapid Rebalancing of Excitation and Inhibition by Cortical Circuitry

Excitation is balanced by inhibition to cortical neurons across a wide range of conditions. To understand how this relationship is maintained, we broadly suppressed the activity of parvalbumin-expressing (PV+) inhibitory neurons and asked how this affected the balance of excitation and inhibition throughout auditory cortex. Activating archaerhodopsin in PV+ neurons effectively suppressed them in layer 4. However, the resulting increase in excitation outweighed Arch suppression and produced a net increase in PV+ activity in downstream layers. Consequently, suppressing PV+ neurons did not reduce inhibition to principal neurons (PNs) but instead resulted in a tightly coordinated increase in both excitation and inhibition. The increase in inhibition constrained the magnitude of PN spiking responses to the increase in excitation and produced nonlinear changes in spike tuning. Excitatory-inhibitory rebalancing is mediated by strong PN-PV+ connectivity within and between layers and is likely engaged during normal cortical operation to ensure balance in downstream neurons.

Mechanisms of Orientation Selectivity in the Primary Visual Cortex

The mechanisms underlying the emergence of orientation selectivity in the visual cortex have been, and continue to be, the subjects of intense scrutiny. Orientation selectivity reflects a dramatic change in the representation of the visual world: Whereas afferent thalamic neurons are generally orientation insensitive, neurons in the primary visual cortex (V1) are extremely sensitive to stimulus orientation. This profound change in the receptive field structure along the visual pathway has positioned V1 as a model system for studying the circuitry that underlies neural computations across the neocortex. The neocortex is characterized anatomically by the relative uniformity of its circuitry despite its role in processing distinct signals from region to region. A combination of physiological, anatomical, and theoretical studies has shed some light on the circuitry components necessary for generating orientation selectivity in V1. This targeted effort has led to critical insights, as well as controversies, concerning how neural circuits in the neocortex perform computations.

Nonuniform surround suppression of visual responses in mouse V1

Complex receptive field characteristics, distributed across a population of neurons, are thought to be critical for solving perceptual inference problems that arise during motion and image segmentation. For example, in a class of neurons referred to as "end-stopped," increasing the length of stimuli outside of the bar-responsive region into the surround suppresses responsiveness. It is unknown whether these properties exist for receptive field surrounds in the mouse. We examined surround modulation in layer 2/3 neurons of the primary visual cortex in mice using two-photon calcium imaging. We found that surround suppression was significantly asymmetric in 17% of the visually responsive neurons examined. Furthermore, the magnitude of asymmetry was correlated with orientation selectivity. Our results demonstrate that neurons in mouse primary visual cortex are differentially sensitive to the addition of elements in the surround and that individual neurons can be described as being either uniformly suppressed by the surround, end-stopped, or side-stopped. NEW & NOTEWORTHY Perception of visual scenes requires active integration of both local and global features to successfully segment objects from the background. Although the underlying circuitry and development of perceptual inference is not well understood, converging evidence indicates that asymmetry and diversity in surround modulation are likely fundamental for these computations. We determined that these key features are present in the mouse. Our results support the mouse as a model to explore the neural basis and development of surround modulation as it relates to perceptual inference.

GABAergic Neurons in Ferret Visual Cortex Participate in Functionally Specific Networks

Functional circuits in the visual cortex require the coordinated activity of excitatory and inhibitory neurons. Molecular genetic approaches in the mouse have led to the "local non-specific pooling principle" of inhibitory connectivity, in which inhibitory neurons are untuned for stimulus features due to the random pooling of local inputs. However, it remains unclear whether this principle generalizes to species with a columnar organization of feature selectivity such as carnivores, primates, and humans. Here we use virally mediated GABAergic-specific GCaMP6f expression to demonstrate that inhibitory neurons in ferret visual cortex respond robustly and selectively to oriented stimuli. We find that the tuning of inhibitory neurons is inconsistent with the local non-specific pooling of excitatory inputs and that inhibitory neurons exhibit orientation-specific noise correlations with local and distant excitatory neurons. These findings challenge the generality of the non-specific pooling principle for inhibitory neurons, suggesting different rules for functional excitatory-inhibitory interactions in non-murine species.

Contribution of Innate Cortical Mechanisms to the Maturation of Orientation Selectivity in Parvalbumin Interneurons

The maturation of cortical parvalbumin-positive (PV) interneurons depends on the interaction of innate and experience-dependent factors. Dark-rearing experiments suggest that visual experience determines when broad orientation selectivity emerges in visual cortical PV interneurons. Here, using neural transplantation and in vivo calcium imaging of mouse visual cortex, we investigated whether innate mechanisms contribute to the maturation of orientation selectivity in PV interneurons. First, we confirmed earlier findings showing that broad orientation selectivity emerges in PV interneurons by 2 weeks after vision onset, ∼35 d after these cells are born. Next, we assessed the functional development of transplanted PV (tPV) interneurons. Surprisingly, 25 d after transplantation (DAT) and >2 weeks after vision onset, we found that tPV interneurons have not developed broad orientation selectivity. By 35 DAT, however, broad orientation selectivity emerges in tPV interneurons. Transplantation does not alter orientation selectivity in host interneurons, suggesting that the maturation of tPV interneurons occurs independently from their endogenous counterparts. Together, these results challenge the notion that the onset of vision solely determines when PV interneurons become broadly tuned. Our results reveal that an innate cortical mechanism contributes to the emergence of broad orientation selectivity in PV interneurons.

SIGNIFICANCE STATEMENT

Early visual experience and innate developmental programs interact to shape cortical circuits. Visual-deprivation experiments have suggested that the onset of visual experience determines when interneurons mature in the visual cortex. Here we used neuronal transplantation and cellular imaging of visual responses to investigate the maturation of parvalbumin-positive (PV) interneurons. Our results suggest that the emergence of broad orientation selectivity in PV interneurons is innately timed.

Mild Traumatic Brain Injury Evokes Pyramidal Neuron Axon Initial Segment Plasticity and Diffuse Presynaptic Inhibitory Terminal Loss

The axon initial segment (AIS) is the site of action potential (AP) initiation, thus a crucial regulator of neuronal activity. In excitatory pyramidal neurons, the high density of voltage-gated sodium channels (NaV1.6) at the distal AIS regulates AP initiation. A surrogate AIS marker, ankyrin-G (ankG) is a structural protein regulating neuronal functional via clustering voltage-gated ion channels. In neuronal circuits, changes in presynaptic input can alter postsynaptic output via AIS structural-functional plasticity. Recently, we showed experimental mild traumatic brain injury (mTBI) evokes neocortical circuit disruption via diffuse axonal injury (DAI) of excitatory and inhibitory neuronal systems. A key finding was that mTBI-induced neocortical electrophysiological changes involved non-DAI/ intact excitatory pyramidal neurons consistent with AIS-specific alterations. In the current study we employed Thy1-yellow fluorescent protein (YFP)-H mice to test if mTBI induces AIS structural and/or functional plasticity within intact pyramidal neurons 2 days after mTBI. We used confocal microscopy to assess intact YFP+ pyramidal neurons in layer 5 of primary somatosensory barrel field (S1BF), whose axons were continuous from the soma of origin to the subcortical white matter (SCWM). YFP+ axonal traces were superimposed on ankG and NaV1.6 immunofluorescent profiles to determine AIS position and length. We found that while mTBI had no effect on ankG start position, the length significantly decreased from the distal end, consistent with the site of AP initiation at the AIS. However, NaV1.6 structure did not change after mTBI, suggesting uncoupling from ankG. Parallel quantitative analysis of presynaptic inhibitory terminals along the postsynaptic perisomatic domain of these same intact YFP+ excitatory pyramidal neurons revealed a significant decrease in GABAergic bouton density. Also within this non-DAI population, patch-clamp recordings of intact YFP+ pyramidal neurons showed AP acceleration decreased 2 days post-mTBI, consistent with AIS functional plasticity. Simulations of realistic pyramidal neuron computational models using experimentally determined AIS lengths showed a subtle decrease is NaV1.6 density is sufficient to attenuate AP acceleration. Collectively, these findings highlight the complexity of mTBI-induced neocortical circuit disruption, involving changes in extrinsic/presynaptic inhibitory perisomatic input interfaced with intrinsic/postsynaptic intact excitatory neuron AIS output.

Binocular Disparity Selectivity Weakened after Monocular Deprivation in Mouse V1

Experiences during the critical period sculpt the circuitry within the neocortex, leading to changes in the functional responses of sensory neurons. Monocular deprivation (MD) during the visual critical period causes shifts in ocular preference, or dominance, toward the open eye in primary visual cortex (V1) and disrupts the normal development of acuity. In carnivores and primates, MD also disrupts the emergence of binocular disparity selectivity, a cue resulting from integrating ocular inputs. This disruption may be a result of the increase in neurons driven exclusively by the open eye that follows deprivation or a result of a mismatch in the convergence of ocular inputs. To distinguish between these possibilities, we measured the ocular dominance (OD) and disparity selectivity of neurons from male and female mouse V1 following MD. Normal mouse V1 neurons are dominated by contralateral eye input and contralateral eye deprivation shifts mouse V1 neurons toward more balanced responses between the eyes. This shift toward binocularity, as assayed by OD, decreased disparity sensitivity. MD did not alter the initial maturation of binocularity, as disparity selectivity before the MD was indistinguishable from normal mature animals. Decreased disparity tuning was most pronounced in binocular and ipsilaterally biased neurons, which are the populations that have undergone the largest shifts in OD. In concert with the decline in disparity selectivity, we observed a shift toward lower spatial frequency selectivity for the ipsilateral eye following MD. These results suggest an emergence of novel synaptic inputs during MD that disrupt the representation of disparity selectivity.SIGNIFICANCE STATEMENT We demonstrate that monocular deprivation during the developmental critical period impairs binocular integration in mouse primary visual cortex. This impairment occurs despite an increase in the degree to which neurons become more binocular. We further demonstrate that our deprivation did not impair the maturation of disparity selectivity. Disparity selectivity has already reached a matured level before the monocular deprivation. The loss of disparity tuning is primarily observed in neurons dominated by the open eye, suggesting a link between altered inputs and loss of disparity sensitivity. These results suggest that new inputs following deprivation may not maintain the precise spatial relationship between the two eye inputs required for disparity selectivity.

An integrated calcium imaging processing toolbox for the analysis of neuronal population dynamics

The development of new imaging and optogenetics techniques to study the dynamics of large neuronal circuits is generating datasets of unprecedented volume and complexity, demanding the development of appropriate analysis tools. We present a comprehensive computational workflow for the analysis of neuronal population calcium dynamics. The toolbox includes newly developed algorithms and interactive tools for image pre-processing and segmentation, estimation of significant single-neuron single-trial signals, mapping event-related neuronal responses, detection of activity-correlated neuronal clusters, exploration of population dynamics, and analysis of clusters' features against surrogate control datasets. The modules are integrated in a modular and versatile processing pipeline, adaptable to different needs. The clustering module is capable of detecting flexible, dynamically activated neuronal assemblies, consistent with the distributed population coding of the brain. We demonstrate the suitability of the toolbox for a variety of calcium imaging datasets. The toolbox open-source code, a step-by-step tutorial and a case study dataset are available at https://github.com/zebrain-lab/Toolbox-Romano-et-al.